Minimum weight for onset of sexual maturity in female chickens: heritability and phenotypic and genetic correlations with early growth rate

No evidence that selection for egg production persistency causes loss of bone quality in laying hens

Background

The physiological adaptations that have evolved for egg laying make hens susceptible to bone fractures and keel bone damage. In modern laying hen breeds, longer periods of egg laying could result in a greater risk of poor bone quality, and selection for increased egg production has frequently been stated to be a cause. However, the existing literature does not support this hypothesis. To test the hypothesis that egg production is associated with quality, breaking strength and density of bone, genetic correlations between these traits were estimated in White Leghorn and Rhode Island Red breeds. Genetic correlations of cortical and medullary bone material chemical properties with bone quality were also estimated, in order to identify methods to improve bone quality with appropriately targeted measurement of key traits.

Results

Estimates of heritability for bone quality traits were moderate (0.19–0.59) for both White Leghorn and Rhode Island Red breeds, except for the keel bone trait, which had a heritability estimate equal to zero. There was no evidence for genetic or phenotypic relationships between post-peak egg production and bone quality. In the White Leghorn breed, the estimate of the genetic correlation between pre-peak production/age at first egg and bone quality was significant and negative (− 0.7 to − 0.4). Estimates of heritability of thermogravimetric measurements of tibial medullary bone mineralisation were significant (0.18–0.41), as were estimates of their genetic correlations with tibia breaking strength and density (0.6–0.9).

Conclusions

The low genetic correlation of post-peak egg production with bone quality suggests that selection for increased persistency of egg production may not adversely affect bone quality. Onset of puberty and mineralisation of the medullary bone, which is a specialised adaptation for egg laying, were identified as important factors associated with the quality of the skeleton later during egg production. These are traits for which genetic, as well as environmental and management factors can positively impact the overall quality of the skeleton of laying hens.

Explanations for keel bone fractures in laying hens: are there explanations in addition to elevated egg production?

The current article served to provide the most up-to-date information regarding the causes of keel bone fracture. Although elevated and sustained egg production is likely a major contributing factor toward fractures, new information resulting from the development of novel methodologies suggests complementary causes that should be investigated. We identified 4 broad areas that could explain variation and increased fractures independent of or complementing elevated and sustained egg production: the age at first egg, late ossification of the keel, predisposing bone diseases, and inactivity leading to poor bone health. We also specified several topics that future research should target, which include continued efforts to link egg production and bone health, examination of noncommercial aves and traditional breeds, manipulating of age at first egg, a detailed histological and structural analysis of the keel, assessment of prefracture bone condition, and the relationship between individual activity patterns and bone health.

Correlated responses to long-term divergent selection for 8-week body weight in female White Plymouth Rock chickens: Sexual maturity

Reported here are correlated responses for reproductive traits to long-term divergent selection (54 generations) for 8-week body weight (BW8). Comparisons involved both selected and relaxed lines. Traits measured were age at first egg (AFE), body weight at first egg (WFE), and ratio of body weight and age at first egg (WAFE). Although sexual maturity was delayed in the selected lines, the effect was more pronounced in the low than high selected and relaxed lines. Selection for low BW resulted in decreases in WFE and WAFE. Correlated responses to selection for high BW were increased WFE and WAFE. Minimum AFE, WFE, and WAFE in relation to sexual maturity were line specific and influenced by selection for BW8. WAFE provided a "yardstick" for target body weights that were optimum for successful attainment of sexual maturity and higher reproductive rates. Such may be line specific. There was opposition between relaxed and artificial selection, resulting in a higher reproductive performance and fitness for the former.

Overlap of quantitative trait loci for early growth rate, and for body weight and age at onset of sexual maturity in chickens

Critical age, weight and body composition have been suggested as necessary correlates of sexual maturity. A genome scan to identify quantitative trait loci (QTL) for age and body weight at first egg (AFE and WFE) was conducted on 912 birds from an F2broiler–layer cross using 106 microsatellite markers. Without a covariate, QTL for body WFE were detected on chromosomes 2, 4, 8, 27 and Z and a single QTL for AFE was detected on chromosome 2. With AFE as a covariate, additional QTL for body WFE were found on chromosomes 1 and 13, with abdominal fat pad as covariate a QTL for body WFE was found on chromosome 1. With body WFE as covariate, additional QTL for AFE were found on chromosomes 1, 3, 4, 13 and 27. The QTL generally acted additively and there was no evidence for epistasis. Consistent with the original line differences, broiler alleles had positive effects on body WFE and negative effects on AFE, whereas the phenotypic correlation between the two traits was positive. The mapped QTL for body WFE cumulatively accounted for almost half the body weight difference between the chicken lines at puberty. Overlapping QTL for body WFE and body weight to 9 weeks of age indicate that most QTL affecting growth rate also affect body WFE. The co-localisation of QTL for body weight, growth and sexual maturity suggests that body weight and growth rate are closely related to the attainment of sexual maturity and that the genetic determination of growth rate has correlated effects on puberty.

Effects of photoperiod on ovarian morphology and carcass traits at sexual maturity in pullets

This paper is concerned with the effects of photoperiod on ovarian morphology and carcass traits at sexual maturity in egg-type hens. Two hundred fifty-six commercial egg-type pullets were initially subjected to a photoperiod of 23L:1D, which was reduced to 22L:2D at 1 wk, to 18L:6D at 2 wk, and to 16L:8D at 3 wk. From 4 to 20 wk, the photoperiod was 8L:16D. At 20 wk, 32 pullets were individually caged in individually lit cages, with 8 cages per unit. Two cage units were placed into 4 photoperiods of 17L:7D, 15L:9D, 13L:11D, and 11L:13D, respectively. Each bird was processed when it reached sexual maturity (SM), and carcass and ovarian morphology were assessed. The results showed that photoperiod had an effect on the timing of SM, and the age at first egg was 5.7 d earlier for hens exposed to the 17L:7D photoperiod than the 11L:13D photoperiod. However, photoperiod had no effect on BW at SM. A photoperiod of 11L:13D limited ovarian follicle formation and increased carcass protein and lipid compared with birds on longer photoperiods, whereas the 17L:7D photoperiod restricted ovary and oviduct full development. These results indicated that excessively long and short photoperiods can restrict reproductive development in egg-type hens.

A Comparison of Growth and Development Patterns in Diverse Genotypes of Broilers. 2. Pullet Growth

Genetic selection within commercial broiler lines continues to generate improvements in BW, feed conversion, and breast meat yield. The objective of the current experiment was to compare carcass and reproductive characteristics of broiler breeder pullets from 2 dam lines that produce heavy broilers with increased breast yield (A and B) with pullets from a commercial line that does not have the extremes in breast yield (C). Restricted-fed BW were similar at all ages of the experiment in the 3 genotypes. All pullets were photostimulated at 23 wk of age, and carcass and reproductive tract measurements were made at 27 wk of age. There were no differences in BW among the lines at 27 wk of age, but the weights of the pectoralis major and minor breast muscles were heavier in lines A and B compared with line C (P < 0.001). The weight of the abdominal fad pad, however, was heavier in line C (P < 0.04). There was no difference in total drum weight or total thigh weight among lines A, B, or C. Tibia length and tibia width were similar in lines A and C, and the measurements were larger in these lines than in line B (P < 0.02). Femur length was longer (P < 0.001) in line C than in lines A and B, whereas femur width was greater in line C than in line A (P < 0.001) but similar to line B (P < 0.001). Oviduct weight was greater in line C compared with lines A and B (P < 0.004), but there were no differences in total ovarian weight, follicle number, or follicle weight. The information gathered in the present experiment suggested that line C may prove to be useful for reproductive comparisons with commercial lines exhibiting significant differences in carcass traits.

Effect of selection for growth on onset of sexual maturity in randombred and growth-selected lines of Japanese quail

An experiment was conducted to determine the effect of selection for growth (BW) on the onset of reproduction (first egg) in Japanese quail hens from growth-selected (HW) and randombred (R1) lines. The HW line had undergone over 40 generations of selection for increased 4-wk BW and was originally developed from the R1 line. Data were collected over two consecutive hatches and summarized by hatch and within lines. Body weight at 35 d of age was positively correlated with BW at sexual maturity (first egg) in both the R1 and HW lines, but these correlations were not significant in both hatches for either line (Hatch 1-R1; Hatch 2-HW; P < or = 0.05). There was a negative correlation between age at sexual maturity (days to first egg) and 35 d BW in both lines with the exception of the HW hens in Hatch 2. Age at sexual maturity and BW at sexual maturity was positively correlated (P < or = 0.001) in the R1 line but did not show the same relationship in the HW line. Age at sexual maturity and weight of the abdominal fat pad were positively correlated in the R1 line (Hatch 1, P < or = 0.01; Hatch 2, P < or = 0.001), but this relationship was not significant in the HW line. Age at sexual maturity and follicle number was negatively correlated in both lines but was not significant in Hatch 2 of the HW line. Follicle size, however, was positively correlated with days to first egg in only Hatch 2 (P < or = 0.001) of the HW line. The data suggest that the relationships between the onset of sexual maturity and both reproductive parameters and carcass traits in hens from consecutive hatches of HW quail are less clear than in the R1 line.

The effects of age at photostimulation and dietary protein intake on reproductive efficiency in three strains of broiler breeders varying in breast yield

The effects of strain, age at photostimulation, and dietary CP intake during the early lay period on sexual maturation and egg production parameters in broiler breeder hens were determined. Three Arbor Acres strains, Classic, Feather Sexable Yield (FSY), and an experimental line (EXP), were reared in floor pens. At 19 wk of age, pullets were moved into individual cages. At 20 wk of age one-half of the birds were photostimulated by increasing the day length from 8L:16D to 15L:9D. The remaining birds were photostimulated at 23 wk of age. The birds were fed 16 or 18% CP diets from 19 to 32 wk of age. The Classic hens achieved sexual maturity 4 d before the EXP hens. Delaying photostimulation to 23 wk of age delayed sexual maturity but the rate of sexual maturation was accelerated after photostimulation occurred. Egg weight at sexual maturity was higher for the later photostimulated birds. The FSY and EXP hens, selected for more white-meat yield than the Classic hens, were heavier in BW but had lower settable egg production than the latter. Feeding 18% CP resulted in a higher BW throughout the trial, as compared to the 16% CP diet. There was a response to dietary CP and photostimulation age on initial egg weight and the number of double-yolked eggs. Early photostimulation was not advantageous because egg production parameters were not affected. Both Classic and FSY strains performed well when fed 18% CP; however, EXP hens benefited from 16% CP intake.

Carcass traits, ovarian morphology and egg laying characteristics in early versus late maturing strains of commercial egg-type hens

Three hundred commercial Single Comb While Leghorn (SCWL) pullets of two strains differing in age at first oviposition (early maturing = EARLY; later maturing = LATE) were reared and photostimulated at 18 wk. Selected birds of each strain were killed at weekly intervals from 17 to 23 wk, at sexual maturity, or at 68 wk of age for the study of carcass and reproductive organ traits. Egg production was recorded for surviving hens to 68 wk of age. The two strains differed in age at sexual maturity (EARLY, 137.5 d; LATE, 142.1 d). The EARLY strain birds appeared to allocate a greater proportion of nutrients to reproductive development (oviduct) from 19 to 22 wk of age than did the LATE group, which showed greater breast muscle development during this same period. The LATE strain hens were significantly heavier than the EARLY hens at sexual maturity, and this difference in BW persisted to the end of the laying period. Although total egg production did not differ between strains, the LATE strain hens had a longer prime egg-laying sequence length (LATE, 70.2 d; EARLY, 52.6 d) as well as a longer mean sequence length (LATE, 12.8 d; EARLY, 8.7 d). Egg weight did not differ between strains. These data suggest that LATE lines of egg-type hens offer equivalent production efficiency due to improved rates of lay, albeit starting later.

Response of broiler breeder females to feed restriction below recommended levels. 3. Characterizing the onset of sexual maturity

The objective of this study was to characterize how the physical attributes associated with sexual maturity would be affected by feed allocation as female broiler breeders passed through the pullet-layer transition period. Treatments consisted of five feeding programs, or body weight goals, during the period of 0 to 20 wk of age. Those were 8% above breeder standard, standard (primary breeder recommended body weights), and 8, 16, and 24% below standard. The characteristics measured included body and fat pad weights, pubic spread, comb development, head score (comb and wattle appearance), shank length, total plasma lipid concentration, ovary weight, oviduct weight, bursa of Fabricius weight, and age at sexual maturity. The generalized effect of increased severity of the feed restriction programs was to delay the birds' development without altering their ultimate physiological values. The exceptions to this were relative body weight and shank length after maturity, which were reduced in the most severely restricted birds. Measurements of pubic spread and comb growth and subjective scores of head appearance would be useful and easily obtained information when estimating the progression of a flock toward sexual maturity.

Performance of early and late feathering broiler breeder females with different feeding regimens

1. Growth, reproductive characteristics and immunocompetence were evaluated in a line of broiler breeder females segregating at the sex-linked feathering locus when maintained on three feeding regimens with two forms of food in a factorial design. 2. Long-term obesity but not short-term weight gain had deleterious consequences for reproductive characteristics, response to sheep red blood cell antigen, resistance to Escherichia coli and livability. 3. Reproductive performance of early feathering females was superior to that of late feathering ones. Differences were attributed to an association between the allele for late feathering and an endogenous viral locus ev21, which encodes for avian leucosis virus. 4. Reproductive performance of breeders fed crumbles was superior to that of breeders fed a combination of crumbles and fines.

Two-way selection for threshold body weight at first egg in broiler strain females. 2. Effect of supplemental light on weight and age at first egg

In order to undergo the processes leading to lay, female chickens must reach a minimum threshold age and body weight, and be exposed to some minimum amount of light stimulation. By two-way selection over six generations lines were developed that differed markedly in body weight at first egg. The effect of supplemental light on age and body weight at first egg of these lines was studied in two experiments. In the first experiment, birds of the two lines were provided ad libitum access to feed from hatch, reaching the weight threshold at an early age. Supplemental light was provided at either 91 or at 126 days of age (before or after the age threshold, respectively). Differences in body weight and age at first egg of the two lines were the same in both light treatments. In the second experiment, birds of the two lines were raised under feed restriction to 154 days of age and then released to ad libitum feeding under supplemental or natural (autumn) light. Differences between the lines in age and weight at first egg were greater in the group maintained under natural light than under supplemental light. The results are interpreted in terms of a model relating age at first egg to weight and age thresholds for onset of competence to respond to light stimulation, and to cumulative light stimulation required from onset of competence to first egg. It is proposed that selection affected the total amount of light stimulation required from onset of competence to onset of lay, but did not affect weight or age thresholds for onset of competence.

Selection for high and low threshold body weight at first egg in broiler strain females. 1. Direct response to selection and correlated effects on juvenile growth rate and age at first egg

A selection program for high and low threshold body weight at first egg was carried out in a broiler line. Selection was on the basis of weight at first egg, following gradual release from feed restriction at a relatively advanced age. After six generations of selection, the lines differed by 862 g in the trait under selection. In addition, 6-wk body weight of high-line (HL) birds was 91 g greater, and age at first egg was 32 days greater than in low-line (LL) birds. When raised under an ad libitum feeding regimen from hatch, HL birds entered lay 20 days later than LL birds. When onset of lay was markedly delayed by maintaining feed restriction until 29 wk of age, body weight at first egg of HL birds was greater by 645 g than that of LL birds. Similarly, when birds of the two lines were subjected to forced molt and brought to a body weight well below that of initial body weight at first egg, and then allowed to gain weight and reenter lay, body weight difference at first egg of HL and LL birds following rehabilitation was similar to that found on original entry into lay. It is proposed that the results may most plausibly be explained as resulting from a primary effect of the selection procedure on the time required to first egg from onset of sexual competence, defined as onset of responsiveness to lay-inducing factors such as light. Alternative explanations involve effects of the selection procedure on threshold weight or threshold age requirements for sexual competence.

Divergent selection for growth in Japanese quail under split and complete nutritional environments. 4. Genetic and correlated responses from generations 12 to 20

Divergent selection during Generations 12 to 20 for high (H) and low (L) 4-wk BW under split-(SD) and complete-(CD) diet environments resulted in continued progress in both upward and downward directions. Following 20 generations of selection, there were 4.8- and 3.6-fold differences between BW of H and L lines in the SD and CD environments, respectively. Regression coefficients of mean 4-wk BW on generation of 1.88 and 1.58 g in the H lines and -1.50 and -1.37 g in the L lines indicated symmetrical selection responses. Realized heritabilities for 4-wk BW ranged from .2 to .4; selection differentials were significantly (P less than .01) larger in the H lines than in the L lines. Directional changes in 2-wk and adult BW and in egg weight across generations were similar to that of the selection trait (4-wk BW). Selection resulted in decreased hatchability and egg production and an increase in the age of sexual maturity. However, these changes were smaller than corresponding changes observed in earlier generations (0 to 11). Egg size of lines selected in the SD environment continued to be larger than egg size of corresponding lines in the CD environment. Considerable genetic variation for BW remains under both SD and CD environments following 20 generations of divergent selection. Smaller gains resulting from lower heritabilities agree with theoretical expectations of decreasing responses accompanying continuous selection.

Restricted feeding in early and late-feathering chickens. 2. Reproductive responses

Reproductive traits of females from a broiler-breeder parent stock segregating for early and late feathering were measured under four feeding regimens. Feeding regimens consisted of ad libitum (AL, feed was always available), ad libitum restricted (ALR, feed restricted daily to control body weight), and skip-one-day and skip-two-days (SOD and STD, given two or three times ALR allowance on Day 1 and not fed on the next 1 or 2 days, respectively). At 160 days of age, pullets on SOD and STD were changed to ALR feeding, and daily feed allowances were increased to 135 g by Day 180 and 138 g by Day 250. Early and late feathering chickens responded similarly, except that hen-day ovulation rates, relative egg shell weights, and incidences of extracalcified and compressed eggs were greater for the former than the later. For most traits measured, differences among restricted feeding regimens were minor, but considerable differences were noted between them and the AL regimen. Females fed AL reached sexual maturity approximately 60 days earlier than those whose feed intake was restricted. Also, body weights at onset of lay were about 1,200 g greater for AL than for feed-restricted females. Percentages of hen-day ovulation, normal egg production, and duration of fertility were lower, whereas incidence of defective eggs was higher for AL than for feed-restricted chickens.

Restricted feeding in early and late-feathering chickens. 1. Growth and physiological responses

Broiler-breeder females from a parent stock segregating for early and late feathering were fed ad libitum (AL, feed was always available), ad libitum restricted (ALR, feed restricted daily to control body weight), skip-one-day and skip-two-days (SOD and STD, given two or three times ALR allowance on Day 1 and not fed on the next 1 or 2 days, respectively). At 160 days of age, pullets on SOD and STD were changed to ALR feeding, and daily feed allowances were increased to 135 g by Day 180 and 138 g by Day 250. Responses of early and late-feathering females were similar for traits measured. Generally, body weights of AL chickens increased to 4,600 g at 130 days of age and then reached a plateau. Body weights of feed-restricted groups were less than half of that of AL chickens by 160 days of age. Controlled release from feed restriction enabled chickens to reach a weight of approximately 3,200 g by Day 210. Daily feed consumption for AL chickens increased to about 220 g by 140 days of age and then decreased to approximately 150 g by Day 250; it eventually decreased to approximately 120 g by Day 350, where it reached a plateau. Mortality, plasma glucose levels, and surface and cloacal temperatures were lower for females whose feed was restricted than for those fed AL. Plasma protein levels were greater for STD than for AL, ALR, and SOD groups whereas plasma lipids were higher for AL and SOD than for ALR and STD groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Divergent selection for growth in Japanese quail under split and complete nutritional environments. 1. Genetic and correlated responses to selection

Japanese quail were divergently selected for 4-wk body weight under two different nutritional environments. In one environment quail were divergently selected for high and low body weight (H and L) under a split diet (SD) with quail having the opportunity to self-select feed from high protein and high energy diets. In the other environment, quail were similarly selected (H and L) under a normal (28% protein) complete diet (CD). After 11 generations of selection, H-SD and H-CD lines were 48.9 and 49.7% higher in 4-wk body weight than controls. Quail from L-SD and L-CD lines were 46.5 and 45.4% lower in 4-wk body weight than controls. Realized heritabilities for 4-wk body weight were .36 +/- .03, .30 +/- .03, .52 +/- .02, and .47 +/- .03 for H-SD, L-SD, H-CD, and L-CD lines, respectively. Selection differentials were significantly higher in the SD than in the CD environment. Changes in 2-wk, adult body, and egg weight across generations paralleled that of the trait under selection (4-wk body weight). Selection also resulted in decreased hatchability and egg production with increased age to sexual maturity in both environments. The SD environment was determined to be a stress environment because of birds' inferior growth there compared with growth in the CD environment. Rate of growth, however, to reach adult body size was superior for L-SD quail once the stress environment was removed. There was also evidence that eggs were larger for quail in the SD than the CD environment.

Effect of diet and early quantitative feed restriction on the minimum weight requirement for onset of sexual maturity in white rock broiler breeders

The effect of diet and feed restriction on minimum weight for onset of sexual maturity was studied in White Rock broiler breeders. In the restriction experiment, three groups of birds were subjected to moderate, severe, and very severe quantitative food restriction during the rearing period (19 week target weights: 1900, 1300, and 700 g, respectively). From 20 to 24 weeks body weight was equalized at 2200 g, and from 24 weeks birds were allowed to gain about 150 g/week until first egg. In the diet experiment, two groups of birds were raised under severe quantitative food restriction during the rearing period and then allowed to gain about 150 g/week on a low metabolizable energy/protein (ME/P) ratio diet (25.4% protein, ME/P = 116) and on a high ME/P ratio (14.7% protein, ME/P = 219) until first egg. All birds were killed at first egg. Birds of the three restriction groups entered lay at the same lean body weight and ash and protein content, but at different age, carcass weight, shank length, dry matter, and fat content. Birds of the two diet groups entered lay at the same lean body weight, shank length, and ash content but at different carcass weight, dry matter, and fat content. Considering both experiments together, birds of the different treatment groups entered lay at similar lean body weight and ash content but differed in age, carcass weight, shank length, dry matter, and fat content.(ABSTRACT TRUNCATED AT 250 WORDS)