Genetic parameters for ewe reproductive performance and peri-parturient fecal egg counts and their genetic relationships with lamb body weights and fecal egg counts in Katahdin sheep

ASReml-based estimation of early genetic parameters in hybrid mutton sheep populations

The aim of this study was to analyze the effects of non-genetic factors on the estimation of genetic parameters of early growth traits in hybrid mutton sheep using ASReml software, in order to provide theoretical basis for screening the optimal hybriding combinations and accelerating the breeding process of new breeds of specialized housed-feeding mutton sheep. We selected the wellgrown hybrid Southhu (Southdown × Hu sheep) and Dorhu (Dorset × Hu sheep) sheep as the research objects, constructed weight correction formulae for SH and DH sheep at 60 and 180 days; and used ASReml software to investigate the effects of non-genetic factors on the estimation of genetic parameters of early growth traits in hybrid sheep. The results showed that the birth month and birth type were found significant for all traits (p < 0.001); the heritability of maternal effects ranged from 0.0709 to 0.1859. It was found that both SH and DH sheep emerged the potential for rapid early growth and development, early growth traits are significantly affected by maternal genetic effects, thereby the maternal effect should be taken into consideration for the purpose of improving accuracy in parameter estimations and therefore increasing the success of breeding programs.

Comparison of different animal models for estimating genetic parameters for early growth traits and reproductive traits in Tianmu Sainuo sheep

As the economic level of individuals rises, so too does the demand for mutton. Enhancing the breeds of mutton sheep not only boosts production efficiency and economic benefits but also fosters the sustainable growth of the mutton sheep breeding industry. Thus, this study examines the early growth and reproductive traits of Tianmu Sainuo sheep, analyzing the genetic interactions among these traits to furnish a theoretical foundation for refining breeding strategies and expediting the genetic advancement of this breed. The investigation compiled 29,966 data entries, involving 111 sires for birth weight (BWT) and 113 for other metrics. The data encompassed 10,415 BWT records from 1,633 dams, 12,753 weaning weight (WWT) records from 1,570 dams, 12,793 average daily gain (ADG) records from 1,597 dams, and 13,594 litter size (LS) records from 1,499 dams. Utilizing the GLM procedure in SAS 9.2 software, the study analyzed the non-genetic influences on lamb BWT, WWT, ADG, and LS. Concurrently, DMU software estimated the variance components across various animal models for each trait. Employing the Akaike Information Criterion (AIC) and likelihood ratio test (LRT), six models were tested, incorporating or excluding maternal inheritance and environmental impacts, to identify the optimal model for deriving genetic parameters. The findings reveal that birth year (BY), birth quarter (BQ), birth type (BT), age of mother (AM), and birth sex (BS) exerted significant impacts on BWT, WWT, and ADG (p < 0.01). Additionally, BQ and AM significantly influenced LS (p < 0.01). The most accurate genetic evaluation model determined the heritability of BWT, WWT, ADG, and LS to be 0.0695, 0.0849, 0.0777, and 0.1252, respectively.

Pedigree diversity and implications for genetic selection of Katahdin sheep

The Katahdin hair breed gained popularity in the United States as low input and prolific, with a propensity to exhibit parasite resistance. With the introduction of genomically enhanced estimated breeding values (GEBV) to the Katahdin genetic evaluation, defining the diversity present in the breed is pertinent. Utilizing pedigree records (n = 92,030) from 1984 to 2019 from the National Sheep Improvement Program, our objectives were to (i) estimate the completeness and quality of the pedigree, (ii) calculate diversity statistics for the whole pedigree and relevant reference subpopulations and (iii) assess the impact of current diversity on genomic selection. Reference 1 was Katahdins born from 2017 to 2019 (n = 23,494), while reference 2 was a subset with at least three generations of Katahdin ancestry (n = 9327). The completeness of the whole pedigree, and the pedigrees of reference 1 and reference 2, were above 50% through the fourth, fifth and seventh generation of ancestors, respectively. Effective population size (Ne) averaged 111 animals with a range from 42.2 to 451.0. The average generation interval was 2.9 years for the whole pedigree and reference 1, and 2.8 years for reference 2. The mean individual inbreeding and average relatedness coefficients were 1.62% and 0.91%, 1.74% and 0.90% and 2.94% and 1.46% for the whole pedigree, reference 1, and reference 2, respectively. There were over 300 effective founders in the whole pedigree and reference 1, with 169 in reference 2. Effective number of ancestors were over 150 for the whole pedigree and reference 1, while there were 67 for reference 2. Prediction accuracies increased as the reference population grew from 1k to 7.5k and plateaued at 15k animals. Given the large number of founders and ancestors contributing to the base genetic variation in the breed, the Ne is sufficient to maintain diversity while achieving progress with selection. Stable low rates of inbreeding and relatedness suggest that incorporating genetic conservation in breeding decisions is currently not of high priority. Current Ne suggests that with limited genotyping, high levels of accuracy for genomic prediction can be achieved. However, intense selection on GEBV may cause loss of genetic diversity long term.

Optimum timing for assessing phenotypic resistance against gastrointestinal nematodes in Pelibuey ewes

The objective was to identify the optimal stage of production to evaluate the resistance of Pelibuey ewes against gastrointestinal nematodes (GIN). Faecal egg count (FEC) was used to classify the ewes as resistant, sensible or intermediate against GIN. Forty-seven ewes were mating during 30 d. The gestation was verified by ultrasonography, and the breeding date was used to calculate the productive stages. Faeces were taken weekly to determine the FEC. Blood samples were taken to determine the packed cell volume (PCV), the peripheral eosinophils count (PEC), plasma protein concentration (PP), and Immunoglobulin A (IgA) against Haemonchus contortus. The body condition score (BCS) was recorded at each visit. Six moments during the study (early, mid and late gestation; early, mid and late lactation) were considered. The ewes were classified according to FEC (mean FEC ± three standard errors). The higher FEC occurred during all lactation stages than during early and mid-gestation stages (P<0.05). PCV, PP, and BCS during early gestation stage were higher than shown during the lactation stages (P<0.01). The PEC and IgA were higher during all lactation stages than early and mid-gestation stages (P<0.05). Concerning the type of birth, double births showed higher FEC than single birth (P<0.01). The highest values of accuracy (100 %) and concordance (Youden's J = 1.0) were found during early lactation. Therefore, it is concluded that the optimal stage of production to evaluate phenotypic resistance against GIN infections in Pelibuey ewes was during the early lactation.

Economic values of reproductive and growth traits in Chinese Yiling sheep

This study aimed to calculate the economic value (EV) of reproductive and growth traits for Yiling sheep. A bio-economic model was developed to assess the economic value of litter size (LS), litter size at weaning (LSW), age at first lambing (AFL), lambing interval (LI), birth weight (BW), weaning weight (WW), and 6-month body weight (6MW). The sensitivity of the economic value of traits to changes in market prices was also analyzed. In this study, the trait with the highest EV was LSW (427.97 ¥), followed by LS (419.96 ¥), BW (52.13 ¥), 6MW (14.46 ¥), WW (11.03 ¥), AFL (-0.51 ¥), and LI (-9.09 ¥). LS was the most important trait in the production system with a relative economic weight of 22.81%, followed by 6MW and LSW with relative economic weights of 18.98% and 19.01%, respectively. All traits assessed, except AFL and LI, had positive economic values, indicating that genetic improvement of these traits would have a positive impact on profitability. The results of the sensitivity analysis showed that the economic value of AFL was not sensitive to price changes. All growth traits were unaffected by price changes in labor and medical costs. In addition, the LS, LSW, LI, WW, and 6MW were sensitive to changes in liveweight and feed prices. Generally, as feed prices increased, the economic value of all traits except LI and BW decreased. Except for LI and BW, the economic value of all traits increased due to the rise in liveweight prices. This suggested that liveweight and feed prices significantly affect the profitability of the production system.

EVALUATION OF PELIBUEY LAMBS BORN TO MOTHERS PHENOTYPICALLY SEGREGATED ACCORDING TO RESISTANCE TO GASTROINTESTINAL NEMATODES IN THE HUMID TROPICS OF MEXICO

The objective was to evaluate the effect of the resistance of ewes to gastrointestinal nematodes (GIN) on the parasitologic and productive performance of their lambs during preweaning and postweaning periods. Offspring from 44 Pelibuey ewes classified as resistant or susceptible to GIN were evaluated. During the preweaning period, birth weight (BW), live weight (LW) at 35 and 70 days of age, and live weight changes (LWC) were determined in 73 lambs. During the postweaning period, 20 lambs to susceptible or resistant ewes were evaluated. The lambs were challenged under natural GIN infections and artificial infection with L3 of Haemonchus contortus. The fecal egg count (FEC), hematocrit (HT), peripheral eosinophil count, plasma protein, LW, and LWC were determined. There were no differences among parameters evaluated during preweaning (P > 0.05). The type of lambing (single or multiple) affected the BW, LW, and LWC (P < 0.01). The sex (male) of the lamb had a positive effect on BW at 35 and 70 days of age (P < 0.05). After weaning, lambs of susceptible ewes showed higher FEC than resistant ewes (P < 0.05). The HT of lambs to resistant ewes was higher than those the susceptible group (P < 0.01). In conclusion, offspring of resistant ewes show low FEC compared with those susceptible during the postweaning stage.

The cost of host genetic resistance on body condition: Evidence from divergently selected sheep

Trade-offs between host resistance to parasites and host growth or reproduction can occur due to allocation of limited available resources between competing demands. To predict potential trade-offs arising from genetic selection for host resistance, a better understanding of the associated nutritional costs is required. Here, we studied resistance costs by using sheep from lines divergently selected on their resistance to a common blood-feeding gastro-intestinal parasite (Haemonchus contortus). First, we assessed the effects of selection for high or low host resistance on condition traits (body weight, back fat, and muscle thickness) and infection traits (parasite fecal egg excretion and loss in blood haematocrit) at various life stages, in particular during the periparturient period when resource allocation to immunity may limit host resistance. Second, we analysed the condition-infection relationship to detect a possible trade-off, in particular during the periparturient period. We experimentally infected young females in four stages over their first 2 years of life, including twice around parturition (at 1 year and at 2 years of age). Linear mixed-model analyses revealed a large and consistent between-line difference in infection traits during growth and outside of the periparturient period, whereas this difference was strongly attenuated during the periparturient period. Despite their different responses to infection, lines had similar body condition traits. Using covariance decomposition, we then found that the phenotypic relationship between infection and condition was dominated by direct infection costs arising from parasite development within the host. Accounting for these within-individual effects, a cost of resistance on body weight was detected among ewes during their first reproduction. Although this cost and the reproductive constraint on resistance are unlikely to represent a major concern for animal breeding in nutrient-rich environments, this study provides important new insights regarding the nutritional costs of parasite resistance at different lifestages and how these may affect response to selection.

Evaluation of resistance to gastrointestinal helminth nematodes in two naturally infected indigenous Nigerian goat breeds

Tropical goat breeds often have at least modest resistance to gastrointestinal nematode parasites (GIN), but enhancement of GIN resistance is important for breed improvement. This study compared changes in fecal egg count (FEC), packed cell volume, and body weight in Red Sokoto (RS) and Sahelian (SH) male and female weaner kids and adult goats. The RS is found throughout Nigeria, but the SH is found only in the arid Sahel. Goats were evaluated fortnightly for 20 times (MT) under normal grazing conditions and natural GIN infection over 9.5 months, beginning in the dry season (November) and ending at the end of the subsequent wet season (August). Animals were dewormed at the start of the study and during the rainy season (MT 18). Breed differences in FEC and PCV were not observed in weaners. Weaner females had lower FEC than males but were rapidly re-infected after deworming, perhaps in association with attainment of puberty. Adult SH goats of both sexes had lower FEC than RS goats in MT 8 through 17, suggesting a stronger acquired immune response. The FEC in lactating females of both breeds increased rapidly after deworming, to ≥ 3000 eggs per gram of feces at MT 19 and 20. The optimal time to evaluate GIN resistance in weaners was during the early rainy season, but the decision to focus on the initial high FEC near MT 15 or wait until mobilization of the acquired immune response near MT 17 requires further consideration.

Genetic Selection for Resistance to Gastrointestinal Parasitism in Meat Goats and Hair Sheep through a Performance Test with Artificial Infection of Haemonchus contortus

Simple Summary

Internal parasitism has been an important constraint to small ruminant production and anthelmintic resistance has become a worldwide issue. This study evaluated a 3-year genetic selection program through activities on-farm and a centralized performance test and also provided estimates of genetic parameters of growth and response to artificial infection with Haemonchus contortus by goats and sheep in the southcentral USA. Considerable species as well as breed differences were found in average daily gain and response to parasite infection. Average daily gain was greater for Boer than for Kiko and Spanish goats and slightly greater for Dorper than for St. Croix sheep. Infection level (number of eggs found in feces) of Spanish and St. Croix were relatively low each year, whereas that of Kiko and Dorper was lower after selection. An indicator of anemia (packed cell volume) did not always reflect infection level, which is probably reflective of differences among animals in resilience and susceptibility to haemonchosis. Moderate to high heritabilities were found for growth performance and response to parasite infection for growing meat goat and hair sheep males under a standardized environment that suggests considerable potential for genetic improvement through selection.

Abstract

Internal parasitism has been the leading cause of morbidity and mortality in small ruminants in many areas such as the southcentral USA. Among the different approaches and management practices to cope with internal parasitism, genetic selection for internal parasite resistance is recognized as one with considerable potential long-term impact. A central performance test with artificial infection of Haemonchus contortus for selection of growing meat goats and hair sheep for breeding to increase resistance to internal parasitism and on-farm selection of females was conducted for 3 years. The results varied considerably among breeds of goats and flocks of sheep. Spanish goats and St. Croix sheep maintained relatively low fecal egg count (FEC) each year, whereas for goats categorized as being of high resistance and Dorper sheep FEC decreased with advancing year. Packed call volume (PCV) and total serum immunoglobulin (Ig) levels were not strongly related to FEC. Genetic parameters varied between the two species, which might be related to previous selection pressure exerted for parasite resistance. Heritability of FEC was higher in goats than sheep. The genetic correlation between FEC and IgM and IgG was negative for both species, which suggests possible genetic association. Genetic and phenotypic correlations between ADG and FEC were nonsignificant for both species. In conclusion, different relationships of FEC and PCV between species require careful attention during selection and the lack of relationship between ADG and FEC suggests that selection of growing male meat goats and hair sheep for resistance to internal parasitism will not adversely affect growth performance.

Correlation of subclinical gastrointestinal nematode parasitism with growth and reproductive performance in ewe lambs in Ontario

Infection with gastrointestinal nematode parasites (GINs) is an important cause of productivity loss on sheep farms in Ontario and worldwide. However, efforts to quantify the effect of GIN infection on growth have demonstrated mixed results. Furthermore, there has been limited investigation of their effect on reproductive performance. This study evaluated the effect of subclinical GIN parasitism on growth and reproductive performance of ewe lambs under Ontario grazing conditions. Rideau cross ewe lambs (n = 140) born in spring 2016 on a farm in central Ontario were followed for two years from before weaning through to November 2017, including their first lambing and lactation. These animals grazed from May to November of each year and were sampled every 6-8 weeks during both grazing seasons and once at mid-gestation in March 2017. At each sampling the ewe lambs were weighed, body condition scores assigned, fecal egg counts (FECs) performed, and pasture samples collected to assess number of infective GIN larvae. Study animals with a FEC of 500 eggs per gram or higher were selectively treated with anthelmintics to prevent morbidity and mortality. Fecal samples were cultured to determine infecting GIN species, and climate data were obtained from a weather station 26 km away from the farm. Precipitation levels and numbers of infective larvae on pasture were low during the first grazing season but were more typical of Ontario conditions in the second grazing season. The three most common GIN species were Haemonchus contortus, Teladorsagia circumcincta, and Trichostrongylus spp. General linear mixed models were generated for weight change over time, litter size at lambing, and weaning weights of offspring. Despite moderate peak GIN burdens in both grazing seasons, FEC was not significantly associated with weight change or litter size, apart from periparturient egg rise in study ewe lambs with larger litters (p = 0.05). Significant positive quadratic and negative linear associations were identified between late lactation FECs and offspring weaning weights; the association between FECs and weaning weights changed from negative to positive at a FEC of 361 eggs per gram. These results indicate that when GIN burdens are moderate as evidenced by fecal egg counts and infection is subclinical, there appears to be low to no impact on growth and reproductive performance in ewe lambs in the first 18 months of life. This suggests that when GIN parasitism is regularly monitored and controlled using targeted selective treatment, animal performance is minimally affected.

Genome-wide association study to identify genetic loci associated with gastrointestinal nematode resistance in Katahdin sheep

Resistance to gastrointestinal nematodes has previously been shown to be a moderately heritable trait in some breeds of sheep, but the mechanisms of resistance are not well understood. Selection for resistance currently relies upon faecal egg counts (FEC), blood packed cell volumes and FAMACHA visual indicator scores of anaemia. Identifying genomic markers associated with disease resistance would potentially improve the selection process and provide a more reliable means of classifying and understanding the biology behind resistant and susceptible sheep. A GWAS was conducted to identify possible genetic loci associated with resistance to Haemonchus contortus in Katahdin sheep. Forty animals were selected from the top and bottom 10% of estimated breeding values for FEC from a total pool of 641 sires and ram lambs. Samples were genotyped using Applied Biosystems™ Axiom™ Ovine Genotyping Array (50K) consisting of 51 572 SNPs. Following quality control, 46 268 SNPs were included in subsequent analyses. Analyses were conducted using a linear regression model in plink v1.90 and a single‐locus mixed model in snp and variation suite. Genome‐wide significance was determined by a Bonferroni correction for multiple testing. Using linear regression, loci on chromosomes 2, 3, 16, 23 and 24 were significantly associated at the genome level with FEC estimated breeding values, and we identified a region on chromosome 2 that was significant using both statistical analyses. We suggest a potential role for the gene DIS3L2 for gastrointestinal nematode resistance in Katahdin sheep, although further research is needed to validate these findings.

Lifetime reproductive performance of Chokla sheep at an organized farm in semi-arid conditions

The Chokla sheep, renowned as the finest carpet wool producer of India, plays an important role in supporting the rural economy of the arid and semi-arid regions of its native state of Rajasthan. This study was carried out to assess the lifetime reproductive efficiency of Chokla ewes by analyzing the records of 1,001 ewes bearing 3,541 lambs over a period of 22 years (1994–2016) to assess five traits, namely total lambs born (TLB; 3.38±0.11 lambs), total lambs weaned (TLW; 2.97±0.11 lambs), cumulative lamb weights at birth (TLBW; 9.59±0.37 kg), weaning (TLWW; 39.59±1.68 kg) and 12 months of age (TLAW; 50.15±2.51 kg) produced by an ewe in her lifetime. Period of birth of ewe had a significant effect on all the traits whereas season of birth, inbreeding coefficient and birth weight of ewe had no influence. REML procedures were employed to estimate the genetic parameters through an animal model. Analysis revealed low heritability estimates for the lifetime reproductive traits (TLB; 0.06±0.04, TLW; 0.05±0.04, TLBW; 0.09±0.05, TLWW; 0.05±0.04 and TLAW; 0.02±0.04), suggesting greater role of environmental and managemental factors. Positive correlation among cumulative lamb weights (0.51–0.92) suggested selection for birth weights would result in higher weights at subsequent life stages of lambs. The low heritability estimates and high CV% for all the traits suggest the suitability of a management focussed approach for improvement of lifetime reproductive performance of ewes.

Genetic parameter estimates for fecal egg counts and their relationship with growth in Avikalin and Malpura sheep

Breeding for resistance against nematodes has become the need of the hour due to emergence of anthelmintic resistant strains of major pathogenic nematodes of economic importance and rising demand for chemical residue free food by consumers. ICAR-Central Sheep and Wool Research Institute Avikanagar (Rajasthan) has developed Haemonchus contortus resistant lines of sheep in Avikalin and Malpura breeds by harvesting benefits of over-dispersion in fecal egg counts (FEC) through executing a breeding program since year 2004. Aim of the present study was to assess the genetic parameters for nematode resistance in these lines and also to develop suitable criteria for selection targeting resistance as well as growth improvement in these two lines. The data on 1240 Avikalin and 2172 Malpura sheep generated over 13 years (2004-16) for FEC along with deep pedigree and growth records for live weight at 6 (6WT) and 12 month were used for study. Data were analyzed using Average Information Restricted Maximum Likelihood (AIREML) approach. Results revealed moderate heritability (h2) for pre-drench log transformed fecal egg count (LFEC) in Avikalin (0.21±0.06) and Malpura (0.18±0.04) sheep. The post-drench h2 for LFEC was low in Avikalin (0.04±0.03) and Malpura (0.11±0.03) sheep. Effective selection program can be carried out for further improving the resistance against H. contortus in both the breeds using pre-drench LFEC estimates. The genetic correlation between the pre-drench LFEC and growth traits was not in the desired direction. Existence of substantial genotype × environment (G×E) interaction was seen in Malpura sheep, where major shift in ranks of sheep based on pre-drench LFEC as that of post-drench LFEC was observed owing to genetic correlation of 0.65±0.15. The G×E was absent in Avikalin sheep. Unreliable genetic correlation between growth and LFEC does not warrant a multi trait selection index development and its utilization in breeding program. The independent selection for LFEC followed by corrected 6WT can precisely help in achieving the goal of improving growth in nematode resistant sheep.