The viability and performance under hot conditions of featherless broilers versus fully feathered broilers

Hot conditions decrease the difference between ambient temperature (AT) and the average temperature of the body surface. A smaller difference reduces the rate of sensible heat loss of excessive internal heat, elevates the body temperature (BT), and may lead to mortality during heat waves. Under conditions of chronic heat, broilers avoid lethal BT elevation by reducing their feed intake; consequently, growth rate and meat yield are lower. Practices to avoid hot conditions are costly, whereas breeding for heat tolerance offers a sustainable approach. Being featherless was shown to provide heat tolerance; this was reevaluated in experimental broilers with a growth rate similar to that of contemporary commercial broilers. In experiment 1, 26 featherless birds and 49 feathered siblings (sibs) were reared at warm AT and exposed to moderate and acute heat waves. The featherless birds maintained normal BT under a moderate heat wave, with a slight elevation under an acute heat wave, and only 1 bird died. In contrast, the heat waves led to a significant elevation in BT of the feathered sibs, and 34% of them died. In experiment 2, featherless broilers were compared with feathered sibs and commercial broilers at 2 AT treatments: a constant temperature of 25°C (control AT) or a constant temperature of 35°C (hot AT). The birds were reared to 46 or 53 d at the control and hot AT, respectively, and the measured traits included BT, growth, and weight of the whole body and carcass parts (breast meat, legs, wings, and skin). At the hot AT, only the featherless broilers maintained a normal BT; their mean d 46 BW (2,031g) was significantly higher than that of birds maintained at the control AT, and it increased to 2,400 g on d 53, much higher than the corresponding means of all feathered broilers (approximately 1,700 g only). Featherless broilers had significantly higher breast meat yield (approximately 20% in both AT), lower skin weight, and supposedly better wing quality. These results confirmed that being featherless improved the livability and performance of fast-growing broilers in hot conditions and suggests that introduction of the featherless phenotype into commercial broiler stocks would facilitate highly efficient yet low-cost production of broiler meat under hot conditions.

Yolk sac nutrient composition and fat uptake in late-term embryos in eggs from young and old broiler breeder hens

In the present study, we examined the composition, amount, and uptake of yolk nutrients [fat, protein, water, and carbohydrates (COH)] during incubation of eggs from 30- and 50-wk-old broiler breeder hens. Eggs were sampled at embryonic d 0 (fresh eggs), 13, 15, 17, 19, and 21 (hatch). Egg, embryo, yolk content, and yolk sac membrane were weighed, and the yolk sac (YS; i.e., yolk content + yolk sac membrane) composition was analyzed. From 30 to 50 wk of age, the albumen weight increased by 13.3%, whereas the yolk increased by more than 40%. The proportion of fat in the fresh yolk of the 30-wk-old group was 23.8% compared with 27.4% in the 50-wk-old group, whereas the proportion of protein was 17.9% compared with 15.6%, respectively. During incubation, results indicated that water and protein infiltrated from other egg compartments to the YS. Accordingly, the calculated change in the content of water and protein between fresh yolk and sampled YS does not represent the true uptake of these components from the YS to the embryo, and only fat uptake from the YS can be accurately estimated. By embryonic d 15, fat uptake relative to embryo weight was lower in the 30-wk-old group than in the 50-wk-old group. However, by embryonic d 21, embryos of both groups reached similar relative fat uptake, suggesting that to hatch, embryos must attain a certain amount of fat as a source of energy for the hatching process. The amount of COH in the YS increased similarly during incubation in eggs from hens of both ages, reaching a peak at embryonic d 19, suggesting COH synthesis in the YS. At hatch, the amount of protein, water, and COH in the residual YS, relative to the weight of the yolk-free chick, was similar in eggs from young and old hens. However, chicks from the younger hens had less fat in the YS for their immediate posthatch nutrition compared with those from the older hens.

The effects of feed restriction and ambient temperature on growth and ascites mortality of broilers reared at high altitude

The development of ascites was investigated in broilers at low versus high altitudes, cold versus normal ambient temperatures (AT), and 3 feeding regimens. One-day-old chicks obtained at sea level were reared at high altitude (highA; 1,720 m; n = 576) with 2 AT treatments, low AT from 3 wk onward at highA (highA/cold) and normal AT from 3 wk onward at highA (highA/norm), or at sea level (normal AT from 3 wk onward at low altitude, lowA/norm; n = 540). Under highA/cold, AT ranged between 16 to 17 degrees C in the fourth week, 17 to 19 degrees C in the fifth week, and 19 to 21 degrees C thereafter. Under highA/norm and lowA/norm, AT was 24 degrees C in the fourth week and ranged between 22 to 24 degrees C thereafter. Broilers in each condition were divided into 3 groups: feed restriction (FR) from 7 to 14 d, FR from 7 to 21 d, and ad libitum (AL). Ascites mortality and related parameters were recorded. Low mortality (0.4%) occurred under lowA/norm conditions. Under highA/norm, mortality was lower in females (8.6%) than in males (13.8%) and was not affected by the feeding regimen. The highA/cold treatment resulted in higher mortality but only in males; it was 44.2% among highA/cold AL-fed males and only about 26% under the FR regimens, suggesting that FR helped some males to better acclimatize to the highA/cold environment and avoid ascites. However, mortality was only 13.3% in AL-fed males at highA/norm and FR did not further reduce the incidence of ascites under these conditions. Thus, avoiding low AT in the poultry house by slight heating was more effective than FR in reducing ascites mortality at highA. Compared with FR from 7 to 14 d, FR from 7 to 21 d did not further reduce mortality and reduced growth. At 47 d, the majority of surviving broilers at highA had high levels of hematocrit and right ventricle:total ventricle weight ratio (>0.29), but they were healthy and reached approximately the same BW as their counterparts at low altitude. This finding may suggest that in broilers reared at highA from day of hatch, the elevation in hematocrit and in right ventricle:total ventricle weight ratio happens gradually and therefore is not necessarily indicative of ascites development.

Physiological parameters in broiler lines divergently selected for the incidence of ascites

Ascites syndrome (AS) is manifested in flocks of contemporary broilers that are allowed to fully manifest their genetic potential for rapid growth. After successful selection, a pair of divergent lines was established, AS-susceptible (AS-S) and AS-resistant (AS-R). These lines facilitate comparisons between genetically resistant and susceptible healthy young broilers when reared under standard brooding conditions (SBC). The aim of the present study was to look for predictive indicators for AS susceptibility by comparing relevant physiological parameters in the AS-S and AS-R lines under SBC and after exposure to extreme ascites-inducing conditions (AIC). In this design, a trait differing significantly between the 2 lines under SBC is expected to be a reliable indicator for selection against AS susceptibility in breeding stocks when reared under noninducing conditions. Males from the AS-S and AS-R lines were reared together under SBC to 19 d of age, then under the AIC protocol. Cumulative incidence of AS mortality was 93.2% in the AS-S line and only 9% in the AS-R line, confirming the genetic divergence between the lines. Exposure to AIC enhanced the imbalance between oxygen demands and supply in the AS-S birds and induced differences in blood parameter level between the 2 lines. The AS-S birds exhibited elevated hematocrit and red blood cell counts and a decline in oxygen saturation in the arterial blood. No difference in hemoglobin concentration was found, but calculation of hemoglobin content per 1,000 red blood cells revealed a significant reduction in hemoglobin content in the AS-S birds. Under SBC, there were no significant differences between the lines for hematocrit, red blood cell count, hemoglobin concentration, hemoglobin count per 1,000 red cells, and blood oxygen saturation. However, heart rate during the first week of life was significantly higher in the AS-S birds than in the AS-R birds on d 1 and 7, suggesting that high heart rate may potentially serve as an early criterion for selection against AS susceptibility.

DNA fingerprints of poultry

Human minisatellite probes cross-hybridize to DNA of several species of poultry (chicken, duck, turkey and goose), and detect high levels of polymorphism. The resulting DNA fingerprints are individual specific, and allow the discrimination even between closely related birds. The pattern of poultry DNA fingerprints is different from that of humans and other animals, having a higher average proportion of large DNA fragments. Pedigree analysis revealed a low number of allelic pairs of variable DNA fragments, indicating that most of the alleles are unresolved in the DNA fingerprint or too small to be detected. The total number of detectable loci in broilers, using probe 33.6, was estimated as 62, of which 13 loci are on average scoreable and available for use. Poultry DNA fingerprints can be used for individual identification, linkage studies and as an aid in breeding programmes.

DNA fingerprints of farm animals generated by microsatellite and minisatellite DNA probes

A multi-locus DNA probe, R18.1, derived from a bovine genomic library, detected DNA fingerprints of highly polymorphic loci in hybridization to genomic DNA from poultry and sheep, and of moderate polymorphic loci in cattle and human DNA. The average numbers of detected bands in chickens and sheep were 27.8 and 21.4, and the average band sharing levels were 0.25 and 0.33, respectively. In hybridization to cattle and human DNA, the results were less polymorphic; nevertheless, individual identification is feasible using probe R18.1. The results obtained by R18.1 were compared to results obtained by Jeffreys minisatellite probe 33.6 and two microsatellite oligonucleotides, (GT)12 and (GTG)5. The total number of detected loci using probes R18.1 and 33.6 were estimated in chickens through family analysis of broilers and the maximal number of detectable loci was calculated.

Restriction fragment length polymorphism analysis of major histocompatibility complex class IV (B-G) genotypes in meat-type chickens

Restriction fragment length polymorphism (RFLP) was used as a molecular genotyping approach to characterize differences in major histocompatibility complex class IV genes in meat-type chickens. A high level of polymorphism was observed following digestion with each of the two restriction endonucleases PvuII and BglII. Examination of DNA from 54 chickens revealed 23 polymorphic fragments. Application of RFLP techniques in the analysis of family groups should make possible the determination of B-G genotypes in the meat type chickens.

Effects of the genetically reduced feather coverage in naked neck and featherless broilers on their performance under hot conditions

Under hot conditions, contemporary commercial broilers do not reach their full genetic potential for growth rate, body weight (BW), or breast meat yield because dissipation of their excessively produced internal (metabolic) heat is hindered by the feathers. Therefore, it was hypothesized that heat stress can be alleviated by using the naked-neck gene (Na) or the featherless gene (sc). The study consisted of 4 experimental genetic groups (fully feathered, heterozygous naked neck, homozygous naked neck, featherless), progeny of the same double-heterozygous parents (Na/na +/sc), and commercial broilers. Birds from all 5 groups were brooded together until d 21 when one-half of the birds from each group were moved to hot conditions (constant 35 degrees C), and the others remained under comfortable conditions (constant 25 degrees C). Individual BW was recorded from hatch to slaughter at d 45 and 52 at 25 and 35 degrees C, respectively, when breast meat, rear part, heart, and spleen weights were recorded. Body temperature was recorded weekly from d 14 to 42. Feather coverage significantly affected the thermoregulatory capacity of the broilers under hot conditions. With reduced feather coverage (naked-neck), and more so without any feathers (featherless), the birds at 35 degrees C were able to minimize the elevation in body temperature. Consequently, only the featherless birds exhibited similar growth and BW under the 2 temperature treatments. The naked-neck birds at 35 degrees C showed only a marginal advantage over their fully feathered counterparts, indicating that 20 to 40% reduction in feather coverage provided only limited tolerance to the heat stress imposed by hot conditions. Breast meat yield of the featherless birds was much greater (3.5% of BW, approximately 25% advantage) than that of their partly feathered and fully feathered counterparts and the commercial birds under hot conditions. The high breast meat yield (at both 25 and 35 degrees C) of the featherless broilers suggests that the saved feather-building nutrients and greater oxygen-carrying capacity contribute to their greater breast meat yield. Because of these results, further research on genetically heat-tolerant broilers should focus on the featherless phenotype.

Development of Ascites-Resistant and Ascites-Susceptible Broiler Lines

The rapid growth of modern broilers is associated with enhanced appetite and high metabolic rate and, consequently, high O(2) demand. Ascites syndrome (AS) develops in individuals that fail to fully supply the increasing demand for O(2) in their bodies under ascites-inducing conditions (AIC) such as high altitude or low temperatures. The tendency of broilers to develop AS is heritable, but efficacious selection against AS susceptibility (without affecting the normal expression of other important traits) requires identification of indirect selection criteria. In the present study, divergent AS-susceptible (AS-S) and AS-resistant (AS-R) lines were developed to confirm the heritability of AS and to facilitate future detection of criteria for indirect selection against AS susceptibility. The base population consisted of 85 sire families with a mean of 73 progeny per sire, reared in a commercial broiler house under low-challenge AIC (cold environment and pelleted feed). Chicks dying with AS manifestations were designated AS-susceptible, whereas the surviving birds were designated AS-resistant. By the end of the trial (d 48), AS mortality had accumulated to 17.2%, but AS incidence per family (%ASF) ranged from 0 to 49%, with a high heritability (0.57). Parents of 7 families with very high %ASF produced the first generation (S(1)) of the AS-S line, and parents of 7 families with very low %ASF produced the S(1) of the AS-R line. The S(1) males and females reproduced generation S(2) of the selected lines, whereas additional S(1) males were tested under high-challenge AIC (individual cages, cool wind, and pelleted feed). Progeny testing under this high-challenge AIC, followed by sib selection, was repeated in generations S(2) and S(3), resulting in a divergence of 86.6% in the incidence of AS between the AS-S (91.3%) and AS-R (4.7%) lines. The rapid genetic divergence, and family analysis of %ASF suggested that a single or few major genes are responsible for the difference between the 2 selected lines. These lines may facilitate more sensitive and effective genomic research aimed at detecting these genes or identifying the primary physiological cause of AS.

Evaluation of Growth Rate, Body Weight, Heart Rate, and Blood Parameters as Potential Indicators for Selection Against Susceptibility to the Ascites Syndrome in Young Broilers

The continuous selection for rapid growth has been accompanied by an increasing occurrence of ascites syndrome (AS), which develops in broilers failing to supply the increasing demand for O(2) in their bodies. Moderate heritability has been reported for AS in broiler populations, suggesting that selection against AS is feasible. However, direct selection based on AS mortality requires exposure of candidate birds to AS-inducing conditions (AIC), which hinder selection for performance traits. Noninvasive indicators of AS, expressed under standard husbandry, may facilitate the integration of selection against AS into breeding programs. This study was designed to look for differences in heart rate, hematocrit, O(2) saturation of hemoglobin in arterial blood (SaO(2)), BW, and weight gain, all measured at early ages under standard brooding conditions, between birds that later developed AS and those that remained healthy under AIC, and to estimate the heritability of these AS-related parameters and their genetic correlation with the tendency of broilers to develop AS. The experimental population was derived from a broiler dam line. Male progeny of 34 half-sib sire families were reared under standard brooding conditions to 19 d of age, then under an AIC protocol consisting of housing in individual cages, cool air high-speed ventilation, and growth enhancement using high-energy pelleted feed and 23 h/d of light. Birds were necropsied upon mortality or at the end of the trials and were recorded as being susceptible, with manifestations of AS (SUS), or resistant and healthy (RES). About 44% developed AS, confirming the efficacy of the novel AIC protocol. The SUS and RES chicks did not differ in BW and weight gain up to 19 d of age, suggesting that there was no association between AS susceptibility and rapid early growth. The SUS chicks exhibited lower SaO(2) and heart rate than the RES chicks. Moderate heritability was estimated for all traits, but only SaO(2) exhibited consistently significant genetic correlation (-0.5) with AS, suggesting that it may serve as an early indicator for selection against AS, albeit with a limited efficacy.

Marker-assisted selection based on a multi-trait economic index in chicken: experimental results and simulation

A method proposed herein allows simultaneous selection for several production traits, taking into consideration their marginal economic values (i.e. the economic value of a trait's additional unit). This economic index-marker assisted selection (EI-MAS) method is based on the calculation of the predicted economic breeding value (BV), using information on DNA markers that have previously been found to be associated with relevant quantitative trait loci. Based on the proposed method, results with real birds showed that sire progeny performance was significantly correlated with expected performance (r = 0.61-0.76; P = 0.03-0.01). Simulation analysis using a computer program written specifically for this purpose suggested that the relative advantage of EI-MAS would be large for traits with low heritability values. As expected, the response to EI-MAS was higher when the map distance between the marker and the quantitative trait gene was small, and vice versa. A large number of distantly located markers, spread 10 cM apart, yielded higher response to selection than a small number of closely located markers spread 3 cM apart. Additionally, the response to EI-MAS was higher when a large number (ca.150) of progeny was used for the prediction equation.

Genetic variation in major histocompatibility complex class I alpha2 gene among broilers divergently selected for high or low early antibody response to Escherichia coli

The MHC genes have a profound effect on animal abilities to respond to specific antigens because they play a role in presenting foreign antigens to T cells during the course of the humoral or cellular immune response. In the current study, polymorphism in the MHC class I alpha2 domain was compared in 2 lines divergently selected for high (HH) or low (LL) antibody response to Escherichia coli vaccine. These lines also differ markedly in their antibody response to natural E. coli exposure and to vaccination with Newcastle disease virus, infectious bronchitis virus, and infectious bursa disease virus. Recent trials have shown that the LL chicks exhibit a significantly higher percentage of CD8+ T lymphocytes in their peripheral blood lymphocytes and spleen than HH chicks. Despite symmetrical selection intensity in both lines, polymorphism of the alpha2-domain gene was higher in the LL line than in the HH line. Among 29 single-nucleotide polymorphism positions found, 3 were unique to the HH line, 15 were unique to the LL line, and 11 were polymorphic in both lines. These single nucleotide polymorphism positions were not 100% line specific and were in agreement with the genetic variation in antibody level or cellular response still found within the selection lines. Five amino acid positions showed significant differences in polymorphism between the selection lines. These were located within the antigen-binding cleft, suggesting that these positions might influence the ability of MHC class I to bind foreign antigens and leading to differences in immunocompetence between the lines.

Microsatellite markers associated with quantitative trait loci controlling antibody response to Escherichia coli and Salmonella enteritidis in young broilers

A unique resource population was produced to facilitate detection of microsatellite markers associated with quantitative trait loci controlling antibody (Ab) response in broiler chickens. Three F1 males were produced by mating two lines divergently selected on Ab response to Escherichia coli vaccination. Each F1 male was mated with females from four genetic backgrounds: F1, high-Ab line (HH), low-Ab line and commercial line, producing three resource families, each with four progeny types. About 1700 chicks were immunized with E. coli and Salmonella enteritidis vaccines. Selective genotyping was conducted on the individuals with highest or lowest average Ab to E. coli and S. enteritidis within each progeny type in each sire family. Twelve markers were significantly associated with Ab to E. coli and six of them were also associated with Ab to S. enteritidis, mostly exhibiting a similar low effect (approximately 0.35 phenotypic SD) in all progeny types. Four markers exhibited a highly significant and much larger effect (approximately 1.7 SD), but only in progeny of females from the HH, suggesting that a backcross to the high parental line should be preferred over the commonly used F2 population. Results from two markers suggested a quantitative trait locus on chromosome 2 around 400 cM. The marker MCW0083, significant in two sire families, is closely linked to the bone morphogenetic protein 2 (BMP2) gene, known to be associated with the control of T-cell transformation in humans.

Genotype-by-environment interaction with broiler genotypes differing in growth rate. 4. Association between responses to heat stress and to cold-induced ascites

Males and females, selected from a commercial line to represent its phenotypic variation for BW, were mated with similarly ranked mates to produce sire families representing a wide genetic variation in potential growth rate (GR). Following 5 wk of rearing at normal ambient temperatures, birds representing all sire families were exposed to cold (Days 37 to 47, Trial 1) or hot (Days 43 to 48, Trial 2) environments. Birds exhibiting ascites syndrome (AS) in the cold environment (Trial 1) were counted, and the incidence of AS (%AS) per family was calculated. Sire families' least-square means of BW at 37 d of age in Trial 1 and BW at 43 d of age in Trial 2 represented the families' potential GR (i.e., GR under normal conditions). A significant positive correlation was found between potential GR and %AS (r = 0.479, Trial 1), indicating that families with higher potential GR under normal conditions are more likely to suffer from AS under cold stress, compared to families with lower GR. Heat stress markedly reduced weight gain in all families (Trial 2); however, the genetic potential GR was negatively correlated with actual GR under heat stress (r = -0.411, Trial 2). Since offspring of the same sire families were exposed to the two stressful environmental conditions, correlations between sire families' means under the two environments could be calculated. A negative correlation was found between growth under heat stress (Trial 2) and %AS (Trial 1) (r = -0.439), indicating that families whose GR is more depressed under heat stress are more likely to suffer from AS under cold stress. These results suggest that the two stress responses may share similar control of the genetic variation in each trait and their negative genetic correlation with potential GR.

DNA markers and crossbreeding scheme as means to select sires for heterosis in egg production of chickens

Genotypes for 24 microsatellite markers, dispersed across the chicken genome, were used to predict progeny performance and heterosis for egg production (number and mass) in 'layers' (egg-type chickens). These markers were used to evaluate genetic distance between each of 39 sires sampled from two-layer male-lines; Rhode Island Red (RIR) and White egg Leghorn (Leghorn), and a DNA pool of 30 randomly sampled females from a Brown-egg female line (Silver). Each sire was analysed for egg production across months in the laying period and cumulatively in each of three subperiods; onset (2 month), mid (9 month) and late (1 month). The average Reynolds' genetic distance between Leghorn sires and the Silver female line (theta;=0.6) was significantly higher than that between RIR sires and the Silver female line (theta;=0.5). Neither performance nor heterosis values in the RIR sire's daughters were associated with genetic distance values between sires and the Silver female line. On the other hand, performance as well as heterosis values of Leghorn's daughters were positively associated with genetic distance. This association was particularly evident in the mid-subperiod. If 25% of the most genetically distant Leghorn sires from the Silver female line had been selected in a single generation on the basis of DNA markers information only, average egg production of the crossbred daughters would have been improved by about nine eggs (3%). In principle, further improvement is possible if selection to increase genetic distance between the parental lines is carried on.

Genotype-by-environment interaction with broiler genotypes differing in growth rate. 3. Growth rate and water consumption of broiler progeny from weight-selected versus nonselected parents under normal and high ambient temperatures

One cycle of high-intensity selection on BW was conducted to study correlated effects on performance under high ambient temperature (AT). From a large flock of a commercial sire-line, 3 males and 15 females with the highest BW at 35 d of age were mated and produced a group of 120 BW-selected chicks. Three average-BW males and 15 average-BW females from the same flock were mated to produce a control group of 120 chicks. On Day 17, the two groups were equally divided between two temperature-controlled chambers and housed in individual cages. One chamber was set to a normal AT (NAT; constant 22 C) and the second chamber to high AT (HAT; constant 32 C). Under NAT, the relative advantage of the selected broilers over the controls did not change from 17 to 42 d of age, averaging about 15% for BW gain and 9.7% for feed consumption. These differences were halved under HAT from Days 17 to 28 and were reversed from 28 to 42 d of age, when the selected broilers consumed significantly less feed and gained less BW than the controls. Water-to-feed ratio was measured in each AT treatment. From 28 to 42 d of age, averaged over the two groups, birds under HAT consumed 2.5 g water/g of feed compared to only 1.5 g water/g feed under NAT. The diminished superiority of the selected broilers under HAT led to a substantial genotype-by-environment interaction involving high AT and within-stock genetic differences in growth rate. It appears that broilers selected for rapid growth under optimal conditions do not achieve their genetic potential under high AT. Thus, specific indicators of adaptation to heat, possibly water consumption or body temperature, should be added to commercial selection for rapid growth to improve broiler performance in hot climates.

Genetic and phenotypic correlations between antibody responses to Escherichia coli, infectious bursa disease virus (IBDV), and Newcastle disease virus (NDV), in broiler lines selected on antibody response to Escherichia coli

The genetic control of antibody (Ab) response to Escherichia coli (EC), infectious bursa disease virus, and Newcastle disease virus and the genetic and phenotypic correlation between these Ab responses, were evaluated under farm conditions in which chicks were simultaneously exposed to these antigens. The experimental population comprised five groups: two lines divergently selected for high (HH) or low (LL) Ab response to EC vaccination; a commercial broiler dam-line (CC), from which HH and LL had been derived; and the HH x CC and LL x CC hybrid groups (HC and LC, respectively). Lines LL and HH expressed similar symmetric divergence to all three antigens. The ranking of the LL, LC, CC, HC, and HH genetic groups according to their mean Ab responses and their very high linear correlation with the LL vs. HH genomic scale clearly indicate the additive nature of the genetic divergence between these lines. Several estimates of correlation were calculated between Ab responses of each pair of antigens and between BW and Ab to each antigen. The high correlation between group means, the near-zero within-group correlation, and the low phenotypic correlation indicate the strongly positive genetic correlation between Ab responses and no correlation with BW. The results of this study suggest that overall immunocompetence of commercial broilers can be improved by selection for high Ab response of young chicks to controlled immunization with a single antigen, without counteracting further selection for high BW.

Antibody responses and morbidity following infection with infectious bronchitis virus and challenge with Escherichia coli, in lines divergently selected on antibody response

We evaluated the association between antibody (Ab) production and disease resistance. A controlled-challenge protocol was developed to mimic natural infection and to yield a higher rate of mortality following Escherichia coli (EC) challenge. Chicks were first infected with infectious bronchitis virus (IBV) by injecting a high dose of vaccine (attenuated virus) into their air sacs and then were infected with pathogenic EC introduced intratracheally. The experimental population consisted of lines divergently selected for high (HH) or low (LL) Ab response to EC vaccination, an HH x LL cross (HL), and commercial broilers (CC). When chicks were vaccinated with EC vaccine, mean Ab titer 15 d post-EC challenge was threefold higher in HH than LL lines, but both lines exhibited very low mortality (approximately 2%). When chicks were not vaccinated prior to EC challenge, high mortality (8 to 20%) occurred in the slow-growing HH, LL, and HL lines, and much higher mortality (approximately 40%) occurred among the CC broilers that were 38% heavier than the HH, LL, and HL lines. Mean level of Ab to EC, 7 d after EC challenge, was about twofold higher in HH vs. LL chicks and intermediate in HL and CC chicks. Within each line, Ab levels were higher in chicks exhibiting colibacillosis than in healthy ones, suggesting that these Ab were produced as a result of ongoing infection but were too late to fully prevent morbidity and mortality. These results indicate that rapid growth rate substantially reduces broiler viability, whereas Ab levels produced in response to acute pathogenic challenge without prior vaccination do not contribute to disease resistance. Among the relatively slow-growing lines, mortality was about twofold higher in HH than in LL lines. This finding may confirm previous reports that without prior vaccination, high Ab response to acute challenge increases consequent mortality; alternatively, the LL line may be superior in nonspecific defense mechanisms.

Immune response and resistance to infectious bursal disease virus of chicken lines selected for high or low antibody response to Escherichia coli

Two experimental broiler lines were developed by divergent selection for high (HH) and low (LL) antibody response to Escherichia coli. Antibody response of these lines to immunization with a commercial vaccine (whole inactivated virus, WIV) against infectious bursal disease virus (IBDV) or with proteins VP2 and VP3 of that virus, and their resistance to challenge with a virulent IBDV, were tested. The study was performed with 213 male and female chicks from the tenth generation of the HH and LL lines. At 15 d of age, after disappearance of maternal antibodies, chicks from each line were randomly divided into four groups and injected with WIV, VP2, VP3, or adjuvant alone as a negative control. Chicks were bled 18 d postinjection, and antibody titers were determined by ELISA. Ten days later, the chicks were challenged with a virulent strain of the virus and killed after 10 d; the ratio of bursa of Fabricius to 100 g BW was determined for each bird. Significant differences in antibody titers were found among immunized and control chicks. Chicks from the HH line exhibited significantly higher antibody titers than LL chicks in response to WIV and VP2 vaccines but not to VP3 vaccine. Following challenge, bursa weight (relative to BW) of HH and LL chicks vaccinated with WIV and VP2 was significantly higher (P < 0.01) than that of chicks vaccinated with VP3 or the challenged unvaccinated control. No difference was found in this parameter between the latter two groups. Possible explanations for the differences in the line response to VP2 and VP3 are discussed.

Genotype-by-environment interaction with broiler genotypes differing in growth rate. 1. The effects of high ambient temperature and naked-neck genotype on lines differing in genetic background

High ambient temperature (AT) significantly depresses growth rate and meat yield of commercial broilers, thus making it a major factor hindering poultry meat production, especially in hot climates. The effects of high AT were variably moderated when feather coverage was reduced by the naked-neck gene (Na). In this study, the effects of high AT and Na were investigated in broiler progeny of hens from a sire line and two dam lines, differing in growth rate and meat yield due to different breeding histories. Heterozygous naked-neck (Na/na) males were mated with normally feathered (na/na) hens from the three lines. The 500 progeny were segregated for Na/na and na/na genotypes. After brooding, chicks of each maternal background, Na genotype, and sex were equally divided to two similar chambers and were reared on litter to 53 d of age. One chamber was set to normal AT, averaging 25 C; the second chamber was set to high AT, averaging 30 C. The high AT treatment reduced growth and meat yield in the progeny of all three groups. This reduction increased with age and was highest in the broilers produced by hens from a sire line bred for high growth rate and breast meat yield. The two other groups, produced by hens from selected and relaxed dam lines, differed in growth rate but were similarly affected by the high AT. It is suggested that the magnitude of the high AT effect depends not only on differences in potential growth rate but also on differences in overall genetic background. It was also shown that broiler performance in the final weeks could be improved by introducing the Na gene into commercial flocks. The advantage of the Na/na genotype was much more pronounced at high AT and in broilers with genetically higher growth rate and breast meat yield.

Genotype-by-Environment Interaction with Broiler Genotypes Differing in Growth Rate: 2. The Effects of High Ambient Temperature on Dwarf Versus Normal Broilers

High ambient temperatures (AT) reduce feed consumption (FC) and BW in broilers, thereby leading to lower efficiency and profitability of poultry meat production in hot climates. These negative effects have been found to be more pronounced in chicken lines with high BW. The effects of high AT were investigated in a broiler population segregating for the Dw gene and, thus, consisting of normal-sized and dwarf broilers, which differed markedly in BW but had the same genetic background. All chicks were reared under normal AT (constant 22 C) up to Day 44, when AT was gradually raised, over 24 h, to 32 C and then held constant to Day 49. The dwarf chicks had 23% lower BW and BW gain (WG) at all ages until Day 44. During the first 24 h at 32 C, FC of the dwarf broilers was reduced by 35% and their average WG was 7.6 g/d, whereas FC of their normal-sized counterparts was reduced by 46% and they lost BW (average WG: -42.1 g/d). Thereafter, however, the dwarf and normal broilers adapted similarly to the chronic heat stress, with FC and WG of both phenotypes averaging 72 and 35%, respectively, of the corresponding means at normal AT. The two phenotypes had similar body temperatures at normal AT, but following its increase, body temperature in the normal-sized broilers rose by 1.14 C, whereas in the dwarf ones it rose by only 0.47 C. This finding suggested better thermoregulation during acute heat stress, apparently due to the latter's smaller body size. Under chronic heat stress, however, FC and WG were similarly reduced in the dwarf and normal broilers. We concluded that the dwarf gene has no value with regards to broiler tolerance to chronic heat stress, either for production or as a model.

Minimal number of chicken daily growth velocities for artificial neural network detection of pulmonary hypertension syndrome (PHS)

Previously, evaluation of the first 2 wk of daily growth velocity with an artificial neural network (ANN) provided an effective noninvasive approach for predicting the susceptibility of broilers to pulmonary hypertension syndrome (PHS). This study was conducted to define the minimum number of days of growth data and the type of ANN required for the best prediction of PHS susceptibility. Four experiments were conducted in which broilers were weighed daily at 0800 h. In Experiment 1, Hubbard male broilers were reared to 50 d of age, with 13 developing PHS and 33 remaining normal (N), for a PHS:N ratio of 13:33. In Experiment 2, ANAK broilers were exposed to cool temperatures (16 to 17 C) from 17 to 42 d of age, resulting in a PHS:N ratio of 16:46 for males. In Experiments 3 and 4, Hubbard male and female chicks from a base population and a PHS-resistant line were exposed to cool temperatures from 17 to 42 d (Experiment 3) or 49 d of age (Experiment 4). The PHS:N ratios were 40:68 for males and 6:96 for females in Experiment 3 and 26:91 for males and 10:58 for females in Experiment 4. Four ANN, back propagation (BP3), Ward back propagation (WardBP), probabilistic (PNN), and general regression (GRNN), were evaluated for their ability to predict PHS in the shortest number of days based on daily growth velocities (BWd+1-BWd). A 100% prediction of PHS and N birds was considered the criterion of success. Starting with 14 d of data, each ANN was trained on daily growth velocity, and the number of predictive days was reduced with each run of the ANN. The best ANN was a GRNN, which correctly diagnosed PHS and N male broilers on 4 and 6 d of growth velocity data for Experiments 1 and 2, respectively. The results were poorer with the BP3, WardBP, and PNN. The diagnostic ability of the neural network was not consistent over all four experiments. In Experiment 2, a minimum of 6 d was required for 100% PHS detection for males. In Experiment 3, the best diagnostic value for males was 93% PHS detection and 100% N detection at 15 d. For females, the 100% PHS detection occurred at a minimum of 8 d. In Experiment 4, males had 100% PHS and N detection at a minimum of 11 d. Females had a 100% PHS and N detection at a minimum of 10 d. An attempt to build a single neural network that would detect PHS susceptibility in Hubbard (Experiment 1) and ANAK (Experiment 2) broilers was unsuccessful. The application (validation) of neural networks between experiments also was not successful (data not presented). However, these studies demonstrate that within a breed or line reared under similar selection pressures for ascites, a GRNN based on the first 14 d of growth velocity can detect, with at least 93% accuracy, broilers susceptible to PHS.

DNA microsatellites linked to quantitative trait loci affecting antibody response and survival rate in meat-type chickens

Selection for immune response parameters may lead to improved general disease resistance. Because disease resistance and immune response are hard-to-measure quantitative traits with low to moderate heritability, they may respond more efficiently to marker-assisted selection (MAS) than to phenotypic selection. To detect DNA markers linked to quantitative trait loci (QTL) associated with immune response, a resource half-sib family of 160 backcross (BC1) and intercross (F2) birds was derived from a cross between two meat-type lines divergently selected for high or low antibody (Ab) response to Escherichia coli. By using 25 microsatellite DNA markers covering approximately 25% of the chicken genome, initial genotyping of 40% of the resource family was followed by complete genotyping of the entire family with four suggestive markers. Three of these markers exhibited significant association with immune response: (1) ADL0146 on Chromosome 2 associated with Ab to SRBC and Newcastle disease virus (NDV), (2) ADL0290 on linkage group 31 affecting Ab to NDV, and (3) ADL0298 on linkage group 34 associated with Ab to E. coli and survival. The family was also genotyped with five linked markers from two of the suggested regions, and interval mapping was applied. The results confirmed the significant effects, suggested the location of the QTL, and confirmed the genetic association between immune responses and disease resistance. These findings support the idea of improving poultry immunocompetence by MAS.

The dynamics of antibody response to Escherichia coli vaccination in meat-type chicks

The dynamics of serum antibody (Ab) response in young broilers were studied in lines divergently selected for high (HC) or low (LC) Ab response to Escherichia coli vaccination at an early age, and their cross (HL). Chicks were divided into three vaccination-age (VA) groups: 8, 10, and 12 d of age (VA8, VA10, and VA12, respectively). Antibody response was determined five times for each chick, at 6, 8, 10, 12, and 14 d postvaccination (dPV). The effects of line, VA, and dPV on Ab titers were highly significant. The HC and LC chicks exhibited the highest and lowest mean titers, respectively, in all VA groups. The HL chicks exhibited midparent Ab values for all VA and dPV combinations, indicating additive inheritance of early Ab production. In LC, the highest mean Ab titer was obtained on Day 26 (14 dPV of the VA12 group), whereas in HC, the same titer had already been obtained on Day 18 (VA8-10 dPV and VA10-8 dPV combinations). The VA8 and VA12 chicks differed markedly in their Ab titer dynamics curves, and the VA10 chicks exhibited an intermediate curve. The three VA groups exhibited a similar change in Ab level from 6 to 10 dPV, but they differed in Ab change from 10 to 14 dPV. This significant dPV x VA interaction suggests that the VA12 and VA10, but not VA8, chicks maintained the capability to produce persisting Ab.

Immunocompetence and viability under commercial conditions of broiler groups differing in growth rate and in antibody response to Escherichia coil vaccine

Mortality and morbidity due to infectious diseases are an increasing source of losses to the broiler industry. Breeding chickens for improved disease resistance may reduce these losses. A study was designed to evaluate the contribution of selection for immune response to viability of broilers under farm conditions. The experimental populations consisted of six groups: two lines divergently selected for high (HH) or low (LL) antibody (Ab) response to Escherichia coli vaccination; commercial broilers (CC); and the HH x CC, LL x CC, and HH x LL crosses. Chicks were tested under standard vaccination program and management on commercial farms in two years (1997 and 1998). Mortality was recorded in the whole groups, each consisting of several hundred or thousand of chicks, whereas BW and Ab to natural exposure to E. coli and to vaccination with Newcastle disease virus (NDV) were determined in samples of 50 to 120 chicks/group per yr. Groups were clustered into three levels of BW: CC representing contemporary fast-growing broilers; HH, LL, and HL representing broilers 10 yr earlier; and HC and LC with intermediate BW. The HH and LL groups exhibited the highest and lowest E. coli Ab titers, respectively. Mean Ab of the CC group equaled the average of the selected lines, and all crosses exhibited mid-parent Ab titers, indicating additive genetic control. Group means for Ab to NDV were highly correlated with those of E. coli, suggesting a common genetic control for the immune response to these two antigens. In both years, the highest mortality was found in the fast-growing group (CC), and the lowest mortality was in the slow-growing HH, LL, and HL groups. In the crosses, despite their similar mean BW, mortality was one-third higher among LC vs. HC birds. These results suggest that Ab response and potential growth rate interact in their effect on mortality due to infectious diseases.

Microsatellite polymorphism between and within broiler populations

Two independent broiler chicken populations were genotyped with microsatellite markers to determine genetic polymorphisms within and among broiler populations. Birds were genotyped with primers from the US Poultry Genome Mapping Kits 1 and 2. The 59 primer sets selected for this study provided wide genomic coverage. All 59 primer sets amplified a polymerase chain reaction product in Population L, whereas 57 primer sets produced a product in Population C. The average allele number per line per microsatellite was 2.8 and 2.9 for Populations L and C, respectively. Considering the 57 primer pairs generating product in both lines, 72.3% of the total alleles were unique to one or the other population. This study illustrates the high polymorphism level in broiler populations of microsatellites amplified from primers developed from Red Jungle Fowl or White Leghorn sequences.

Detection of RFLP markers associated with antibody response in meat-type chickens: haplotype/genotype, single-band, and multiband analyses of RFLP in the major histocompatibility complex

Improving disease resistance in poultry by direct selection or by selecting for immune response is hardly feasible due to the quantitative nature of these traits, their low heritability, and the difficulties associated with reliable measurements. In this situation, marker-assisted selection (MAS) is expected to be a more effective breeding approach. The major histocompatibility complex (MHC), known to affect immune response and disease resistance, was examined as a set of candidate genes for association between DNA markers and antibody response. Backcross (BC1) and F2 families were generated from a cross between lines divergently selected for high or low antibody response to Escherichia coli vaccination. Restriction fragment length polymorphism (RFLP) analysis of the highly polymorphic MHC class IV (B-G) region suggested an association with antibody response to several antigens (E. coli, SRBC, NDV). The multiband data generated with the class IV probe were used to compare the efficacies of three alternative analyses: "single-band" (carriers versus noncarriers of each RFLP band separately), "multiband" (multiple regression on all RFLP bands), and "genotype" (determined from family analysis of RFLP patterns/haplotypes). Groups of birds identified by the "multiband" analysis were identical to the haplotype-based genotypes, suggesting that the laborious step of haplotype determination can be omitted without unduly sacrificing power of analysis.

The effects of the naked neck (Na) and frizzle (F) genes on growth and meat yield of broilers and their interactions with ambient temperatures and potential growth rate

High ambient temperatures (AT) decrease the growth of broilers because of difficulty in dissipating heat through the feather coverage. Broilers selected for higher growth rate eat more and generate more heat per unit of time; hence, they may become more sensitive to high AT. Reduced feather coverage, either by decreased number or by modified shape, may help birds to dissipate internal heat more efficiently. Two broiler stocks were studied; each was segregated for four genotypes with regard to the genes for naked neck (Na) and frizzled feathers (F): heterozygous naked neck (Na/na f/f), heterozygous frizzle (na/na F/f), double heterozygous (Na/na F/f), and normally feathered (na/na f/f). One stock had a high growth rate (GR) similar to current commercial broilers, whereas the second stock had a lower GR. Birds of each stock, genotype, and sex were reared under constant standard AT (24 C) or high AT (32 C). Body weight at 4 and 7 wk, weight gain (WG) from 4 to 7 wk, breast meat yield, and feather weight were recorded. Reduction in WG from 4 to 7 wk because of high AT was greater in high-GR birds than in low-GR birds, but, in both stocks, the high AT effect was greater on normally feathered birds than on the other three genotypes. AT 32 C, in low- and high-GR stocks, the F allele increased WG from 4 to 7 wk and increased the BW at 7 wk of fully feathered (na/na) broilers but had no effect on meat yield. The effects of the Na allele were similar to or greater than those of the F allele. The Na allele did not affect breast meat yield of low-GR broilers but increased it significantly in high-GR broilers. Combining the two allles resulted in an additive effect, which was more pronounced in the high-GR stock.

The effects of naked neck genotypes, ambient temperature, and feeding status and their interactions on body temperature and performance of broilers

The effect of ambient temperature (AT) and feeding status on body temperature (BT) were investigated in broilers of the three naked neck genotypes (Na/Na, Na, na, and na/na). From 29 to 49 d of age, chicks were reared in a temperature-controlled chamber, where AT alternated daily between 24 and 32 C. At Day 47, all birds were deprived of feed for 12 h at 32 C, followed by 12 h of ad libitum intake at 24 C, then 12 h of ad libitum intake at 32 C, and finally feed deprivation for 12 h at 24 C. Body temperature was measured at the end of each of these 12-h periods. Body weight, feed consumption, feather coverage, and breast yield were determined. The Na/na and Na/Na birds had 20 and 40% less feather mass than the na/na birds. The three genotypes had similar BW at Day 49, but the naked neck birds had a higher breast yield. At high AT, BT was positively associated with feather mass of the three naked neck genotypes. The highest BT was exhibited by the fully feathered birds, and the lowest by the homozygous naked neck birds. The feeding status also affected BT of all birds, but to a larger extent in the normally feathered than in the naked neck birds. It appears that the lower negative effects of high AT on growth rate and meat yield in naked neck broilers can be attributed to their lower BT. Thus, it is suggested that measuring BT of broilers can be used as an indicator of the level of stress imposed on them by high AT.

Major histocompatibility complex (MHC) related cDNA probes associated with antibody response in meat-type chickens

The major histocompatibility complex (MHC) region was examined as a set of candidate genes for association between DNA markers and antibody response. Intercross F2 families of chickens were generated from a cross between high (HC) and low (LC) Escherichia coli(i) antibody lines. Restriction fragment length polymorphism (RFLP) analysis was conducted by using three MHC-related cDNA probes: chicken MHC class IV (B-G), chicken MHC class I (B-F), and human MHC-linked Tap2. Association between RFLP bands and three antibody response traits (E. coli, sheep red blood cells and Newcastle disease virus) were determined by two methods: by statistically analyzing each band separately and also by analyzing all bands obtained from the three probes by using multiple regression analysis to account for the multiple comparisons. The MHC class IV probe was the highest in polymorphisms but had the lowest number of bands associated with antibody response. The MHC class I probe yielded 15 polymorphic bands of which four exhibited association with antibody response traits. The Tap2 probe yielded 20 different RFLP bands of which five were associated with antibody production. Some Tap2 bands were associated with multiple antibody response traits. The multiband analysis of the three probes' bands revealed more significant effects than the analysis of each band separately. This study illustrates the efficacy of using multiple MHC region probes as candidate markers for quantitative trait loci (QTLs) controlling antibody response in chickens.

Comparative effects of added sodium chloride, ammonium chloride, or potassium bicarbonate in the drinking water of broilers, and feed restriction, on the development of the ascites syndrome

A hypothesis that the ionic composition of drinking water might affect development of the ascites syndrome in broilers was investigated in two trials. The first trial comprised four groups of 650 male chicks. A control treatment was normal tap water and the other three treatments comprised the addition to the tap water of 1,000 mg/L sodium as NaCl, 5,000 mg/L NH4Cl, or 5,000 mg/L KHCO3, supplied from age 2 to 47 d. At Day 28, equally sized subsets of these groups were moved to individual cages, where they received a severe exposure to ambient cold. The development of the ascites syndrome was monitored by measurements of hematocrit and arterial blood oxygen saturation (PaO2) by oximetry, body weight, and examination of dead birds for cause of death. Mortality from ascites in cold-exposed birds from Days 28 to 47 was 28, 48, 40, and 16% in the tap water, NaCl, NH4Cl, and KHCO3 groups, respectively; only the NaCl mortality was significantly different from the tap water mortality. The KHCO3 treatment increased PaO2 (compared with tap water treatment) at Day 28 by 5.5% and at Day 35 by 10.5%, but not at Day 42. The KHCO3 caused a reduction in body weight, which was 13% less than the tap water group at Day 42, probably due to a chronic toxicity. The second trial specifically examined the same parameters with lower water levels of KHCO3 (3,000 and 1,000 mg/L), in comparison to a 10% feed restriction protocol, in order to clarify whether the increased PaO2 was due to a specific effect of the KHCO3 or was a metabolic manifestation of a reduced growth rate. The 3,000 mg/L KHCO3 treatment had no effect on PaO2, but the 1,000 mg/L treatment augmented PaO2 by 5.3% at Day 35 (but not at Days 28 or 42), without reducing the final body weight. The feed restriction group showed an elevated PaO2 of 5.4% at Day 35 (but not at Days 28 or 42), with no reduction in the final body weight. The inclusion of 1,000 mg/L of KHCO3 into the drinking water of broilers or a temporary 10% feed restriction may be means to augment PaO2.

Relationship between broiler chicken haematocrit-selected parents and their progeny, with regard to haematocrit, mortality from ascites and bodyweight

A previous work of this group demonstrated that the relative haematocrit value of broilers is inherited and may serve as an indicator to susceptibility to the ascites syndrome in cold-stressed broilers. In this study, a full-pedigreed population was produced from male and female grandparent breeding stock that was selected by haematocrit and by normal selection parameters. Matings were made between low (L), medium (M) and high (H) haematocrit parents: L x L, M x M, and H x H. In their progeny, both before and after cold exposure, there was a statistically linear relationship between actual haematocrit and their H, M and L grouping (P<0.0001); heritability of the haematocrit was high (0.46-0.81). Both the low haematocrit parent and progeny groups showed an increased bodyweight. Exposure of the progeny from all the parental groups to an ascites-predisposing cold environment caused similar losses from ascites in the progeny of all three groups. Although this finding was not the same as in the previous trial where the H haematocrit group was associated with high ascites mortality, it is hypothesized that other factors, such as arterial blood saturation with oxygen, interacted in these birds at genetic or environmental levels.

Thermoregulation in naked neck chickens subjected to different ambient temperatures

1. Heterozygous (Na/na) naked neck chickens and their normally feathered (na/na) sibs, were exposed to constant ambient temperatures (Ta) ranging between 15 and 35 degrees C and 12h: 12h diurnal high:low temperatures of 15 degrees C:35 degrees C. 2. No significant effect of genotype was obtained in weight gain and food intake. However, the naked neck birds tended to gain somewhat more weight at high Ta and consume more food at low Ta. 3. At 35 degrees C Na birds showed better regulation of body temperature (Tb) and demonstrated considerably higher radiation from the neck. 4. The greater food intake of the naked neck chickens at 15 degrees C was associated with significantly higher packed cell volume, haemoglobin concentration, heart and liver size. These appear to involve both higher heat production and haemodynamic changes to accommodate the higher oxygen demands of the naked neck chickens at low Ta. 5. The results indicate the ability of the naked neck chickens, on the one hand to thermoregulate at low Tas and, on the other their slightly better capacity to maintain Tb at high Tas. However, no genotype advantage was obtained under diurnal cyclic temperature conditions.

Performance of naked neck and normal broilers in hot, warm, and temperate climates

Chickens suffer at high ambient temperatures because their feather coverage hinders internal heat dissipation. Naked neck broilers (Na/na) and their normally feathered sibs (na/na) were evaluated in three natural climates. Three experiments were conducted in Turkey, during the summer in the extremely hot region of Adana (Ad-Sm), and in the moderate region of Izmir during the spring (Iz-Sp) and summer (Iz-Sm), always following the same experimental protocol. Ambient temperatures averaged 19, 28, and 32 C in Iz-Sp, Iz-Sm, and Ad-Sm, respectively. About 300 birds per genotype were included in each experiment. Feather weight was lower by about 20% in Na/na broilers than in na/na ones, independent of climate, sex, and age (6 or 7 wk). The Na/na broilers exhibited higher breast weight in all cases, from 2.5 to 10.9% higher than their na/na counterparts. Body weight gain from 4 to 7 wk (BWG4-7) clearly reflected the differences in ambient temperature among climates. The effect of the Na/na genotype on BWG4-7 interacted with climate and sex. In the hottest climate (Ad-Sm), both male and female Na/na broilers exhibited a highly significant advantage over their na/na counterparts. In the more moderate climate (Iz-Sm), the Na/na genotype exhibited superior growth only among males, and the magnitude of this advantage was lower than in Ad-Sm. In the cool temperate climate (Iz-Sp), BWG4-7 and BW7 (BW at 7 wk) means were similar for both genotypes. In Iz-Sp, feed efficiency (FE) of the Na/na birds was lower by about 4%, but in the two summer climates (Iz-Sm and Ad-Sm), FE of the Na/na birds was about 9% higher than that of their na/na counterparts. Body temperature was lower in the Na/na broilers than in their na/na counterparts; in all cases, the difference increasing with ambient temperature. The results indicate that the reduction in feather coverage provided relative heat tolerance, and therefore, under hot climates the Na/na broiler were superior to their normally feathered counterparts. It is concluded that naked neck broilers should be preferred in hot climates.

Comparative evaluation of three commercial broiler stocks in hot versus temperate climates

Hot climate is a major limiting factor of broiler production in tropical and subtropical regions. The use of standard stocks in hot climates may result in large economic losses because genotypes selected in temperate climates may respond differently to the high ambient temperatures in hot regions or seasons. The summer and fall in Izmir, Turkey, provided the natural hot and temperate climates, respectively, for this study. Broiler chicks were obtained from three commercial stocks, all bred in temperate climates. Male and female chicks, 60 per pen, were housed in four pens per stock per season. Individual BW was determined at hatch, and at 4 and 7 wk of age. Feed consumption and efficiency were determined per pen. Feathering was scored at 4, 5, and 6 wk of age. Body temperature was measured twice on three birds per sex per pen, 16 h and immediately before slaughter, and feather weight was determined for each of these birds. The two seasons clearly differed in ambient temperature at the broiler house, and consequently, BW at 7 wk was significantly lower in the summer than in the fall in all stocks, with an average reduction of 23%. The season effect was largest (33.5%) on BW gain from 4 to 7 wk, along with 23 and 15% reductions in feed consumption and efficiency, respectively, during these 3 wk. A significant season by stock interaction was detected for BW gain from 0 to 4 wk and 4 to 7 wk. The three stocks exhibited similar 4- to 7-wk BW gains under the temperate fall climatic conditions, but differed significantly in the summer. These differences were not related to normal differences in feather coverage or body temperature, suggesting that standard broiler stocks must be tested in hot climates in order to find the one most suited to these conditions.

Associations of individual genomic heterozygosity, estimated by molecular fingerprinting, and of dam major histocompatibility complex with growth and egg production traits in layer chickens

Growth and egg production data from F2 layer chickens produced from a mating of MHC-heterozygous B15B44 sires to B15B44 and B15B43 dams were analyzed for associations with the level of the individual's genomic heterozygosity and dam MHC. The F1 sires and dams were produced from mating of birds from two genetically distinct, highly inbred lines. Genomic heterozygosity (expressed as composite band frequency, CBF) was estimated based on the average proportion of shared bands of individual birds compared to a composite pool of Hinf I-digested DNA samples from the two inbred lines. Birds from dams of genotype B15B43 had significantly greater BW than progeny from B15B44 dams at 6 wk and thereafter. Dam MHC genotype differences were also significant for number of eggs laid from 20 to 28 wk of age, with B15B43 progeny being superior. Negative regression coefficients at hatch, 32 and 44 wk were observed for the effect of CBF on BW of progeny from B15B43 dams and at hatch and 32 wk from progeny of pooled dam genotypes. Similarly, negative regression coefficients were observed for the association of CBF with number of eggs within progeny from the B15B44 dam MHC genotypes at 20 to 28 wk and total period recorded, and at 28 to 36 wk for progeny from B15B43 dams. The CBF regression coefficients and the differences among progeny based on dam MHC genotypes suggest that individual heterozygosity and MHC genotype may be associated with BW and egg production in chickens.

Genetic differences and heritability of antibody response to Escherichia coli vaccination in young broiler chicks

Broiler chicken lines, selected divergently for high (HC) or low (LC) antibody titer to Escherichia coli vaccination at an early age, were evaluated for antibody response at the S5 and S9 generations of selection. The full-pedigreed populations consisted of about 300 and 400 chicks per line in S5 and S9, respectively. At S5, all chicks were vaccinated at 10 d of age (VA10) and antibody titer was determined twice for each chick, at 8 and 12 d postvaccination (dPV). At S9, each line was divided into two equal groups; in the HC line, one group was vaccinated at 8 d of age (VA8), and the other at 10 d of age (VA10), whereas in the LC line, one group was VA10 and the other was VA12. Antibody titers were determined twice for each chick, 8 and 10 dPV. The effects of line, age at vaccination (VA), and days for antibody development (dPV) were tested, and the heritability of antibody titer was estimated for each line-VA-dPV set of data. The HC and LC lines differed significantly in the maturation process of their immune systems. The percentage of chicks with detectable antibody at 18 d of age (VA10-8 dPV) among HC chicks was significantly higher than among LC chicks (85 vs 48% in S5 and 96 vs 63% in S9). In S9, 90% of the HC chicks had already responded at 16 d of age and 100% at 18 d of age, whereas among the LC chicks, only 62% were positive at 18 d of age, increasing to no more than 98% at 22 d of age. The results demonstrates that selection of antibody titer to E. coli vaccination at 20 d of age actually affects the earliest age of immune response, as the immune system of the HC chicks matures earlier than that of the LC chicks. The HC S9 chicks at 8 dPV exhibited a fourfold higher antibody titer than their LC 8 counterparts. This difference further increased at 10 dPV, indicating that the lines differed not only in the level of antibody at a specific age, but also in their rate of antibody titer development. The highest estimate of heritability was very similar in both lines (0.44 and 0.42 in HC and LC, respectively). However, in the HC line this heritability was exhibited at 18 d of age, and only at 22 d in the LC line. Thus, both lines have a similar amount of genetic variation for early immune response, but in the HC line this variation is fully expressed 4 d earlier than in the LC line. These results suggest that selection for high or low antibody response in young chicks results in early or late antibody production, respectively. To maximize the efficiency of selection for early immune response, one must determine the best vaccination age and timing of antibody evaluation in any given population, and these values must be revalidated and updated as selection proceeds.

Hematocrit values and mortality from ascites in cold-stressed broilers from parents selected by hematocrit

A hypothesis that the relative hematocrit value of broilers is inherited and can serve as an indicator of partial resistance to the ascites syndrome in cold-stressed broilers was shown to be valid in a field trial. Hematocrits were determined for male and female grandparent breeding stocks. Matings were then made between low (LL), low-medium (LM), medium-high (MH), and high (HH) hematocrit parents: LL x LL, LM x LM, MH x MH, and HH x HH. The progeny of HH parents had higher hematocrit values than the progeny of lower hematocrit parents (P < 0.0001). Exposure of the progeny from all the parental groups to an ascites-predisposing cold environment caused higher losses from ascites in the progeny of the HH parents (P < 0.0001). The progeny of LH parents had an increased mortality from causes other than ascites (P < 0.0001). This work suggests that elimination of birds with HH in broiler breeding programs may be desirable where cold-induced ascites is an important problem.

Effects of dietary protein under high ambient temperature on body weight, breast meat yield, and abdominal fat deposition of broiler stocks differing in growth rate and fatness

The effect of dietary protein on growth, feed intake and efficiency, abdominal fat deposition, and breast meat yield was investigated in broiler males from a commercial stock (WI) and from experimental stocks selected for low (LF) or high (HF) abdominal fat. All birds were kept at constant high ambient temperature (32 C) and were provided with low- (LP) or high-protein (HP) diets from hatch until 8 wk of age (Experiment 1) or from 4 to 8 wk of age (Experiment 2). In both experiments, HP diet significantly increased 4- to 8-wk BW gain in the LF and HF stocks but reduced it in the WI stock as compared with the LP diet. Abdominal fat, as percentage of BW, was almost twofold higher in the HF birds than in the LF ones, with WI mean being intermediate. In contrast to the HF and WI birds, in which abdominal fat decreased with increased protein intake, abdominal fat was not affected by dietary protein in the LF stock. The HP diet substantially increased breast meat yield in LF birds but not in the WI birds, with HF birds exhibiting intermediate increase in breast meat weight. It was concluded that birds of varied inherent growth rate and tendencies toward protein and fat deposition respond differently to dietary protein level under heat stress.

Parental effect on the humoral immune response to Escherichia coli and Newcastle disease virus in young broiler chicks

Genetic and environmental variables influence animal resistance to disease infection. In addition, maternal effects were also found in studies with egg-type chicken lines. In our laboratory, meat-type chicken lines were divergently selected for either early or late maturation of the immune system, based on family and individual antibody responsiveness at 10 d of age. The high-antibody (HC) and low-antibody (LC) lines differed significantly in the early immune response to Escherichia coli, to Newcastle disease virus (NDV) vaccination, and to several other immune functions. Reciprocal crosses between the HC and LC lines were performed over 2 yr at three different locations. Immune responses to E. coli and NDV vaccination provided separate estimates of maternal and paternal effects. Dam effect on immune response to E. coli vaccine was significantly larger than sire effect; the antibody titer in both reciprocal crosses was intermediate between the parental lines, but the mean titer of the HC x LC cross was significantly lower than that of the LC x HC cross. Similar, but not significant, ranking of crosses was observed for the response to NDV. Evidently, the level of the offspring humoral immune response was more a dam than a sire effect.

Direct and correlated responses to multitrait, divergent selection for immunocompetence

Leghorn lines had been selected for an immunocompetence index based on four traits: antibody production to Mycoplasma gallisepticum (MG) and Pasteurella multocida (PM) vaccines, reticuloendothelial clearance of colloidal carbon (CCA), and cell-mediated, wing web response to phytohemagglutinin (PHA). The purpose of this study was to produce replicated lines of chickens with divergent levels of multitrait immunocompetence by index selection. The objectives of analyses of Generations 5 to 7 of this study was to characterize these lines with respect to immune-response traits, correlations among these traits, and correlated responses in other important production traits. Differences (P < .05) existed between the lines selected for high or low immune response and between the two replicates in mean breeding values and in individual immune-response traits. Averages of heritability estimates, weighted by number of offspring and pooled across three generations (two cycles of selection), estimated by using sire variance components and parent-offspring correlations were, respectively, .16 and .09 for the index, .31 and .08 for MG, .21 and -.02 for PM, .06 and .05 for CCA, and .08 and .12 for PHA. Realized heritabilities (response divided by effective selection differential) pooled across the two selection cycles, were .19 and .11 for the index, .06 and -.01 for MG, .44 and .32 for PM, 1.52 and -1.21 for CCA, and .48 and .15 for PHA, for Replicates 1 and 2, respectively. Phenotypic correlations among traits were generally small, and several estimates were negative. Estimates of genetic correlation varied widely. Juvenile and adult body weights, age of first egg, 32-wk egg weight, and rate of egg production were analyzed to evaluate effects of selection on these traits of direct economic importance. Very few differences were noted.