Sheep & Goats

Genetic analyses of growth, body composition, parasite resistance, and reproductive traits in U.S. Polypay sheep

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Abstract

Polypay is a semi-prolific sheep breed developed in the United States during the 1970s. In this study, we estimated genetic parameters, phenotypic and genetic correlations among trait pairs, estimated breeding value (EBV) accuracies, and genetic trends using data from the National Sheep Improvement Program (NSIP) collected from 1980 to 2023. The evaluated traits included body weight at birth (BWT, n=106,919), weaning (WWT, n=69,811), post-weaning (PWWT, n=51,119), and yearling (YWT, n=551); post-weaning ultrasound-based measurements - carcass fat depth (CFAT, n=1,170) and eye muscle depth (EMD, n=1,168); gastrointestinal parasite resistance assessed by fecal egg count at weaning (WFEC, n=713) and post-weaning (PFEC, n=3,160); and reproductive traits, including number of lambs born (NLB, n=52,563) and weaned (NLW, n=52,563), and post-weaning scrotal circumference (PSC, n=1,081). The pedigree consisted of 127,693 individuals, and animals with phenotypic records were progeny of 2,451 sires and 23,163 dams. Variance components were estimated using the BLUPF90 software suite, fitting linear animal mixed models that included additive genetic, maternal genetic, maternal permanent environment, and animal permanent environmental effects, where applicable. The models included contemporary groups as the concatenation of flock, birth year, management group, sex, age categories, and measurement date. Ewe's age at lambing was included as a fixed effect for reproductive traits. Heritability estimates ranged from low to moderate for weight (0.09 - 0.21), low to high for carcass (0.09 for CFAT; 0.68 for EMD), and moderate for parasite resistance (0.30 for WFEC; 0.36 for PFEC) traits. For reproductive traits, heritability estimates ranged from 0.02±0.01 (NLW) to 0.48±0.10 (PSC), with low repeatability estimates (0.12±0.005 for NLB; 0.06±0.004 for NLW). Phenotypic correlations revealed a strong positive association between weight traits (WWT and PWWT: 0.80). Carcass traits (CFAT and EMD) showed a weak relationship (0.18), while NLB and NLW were strongly correlated (0.65±0.01). Genetic correlations were high among some trait pairs, such as WWT and PWWT (0.89), CFAT and BWT (0.51), and NLB and NLW (0.82±0.03), indicating a substantial shared genetic basis between these traits. Theoretical accuracies varied considerably amongst traits, with the highest values observed for BWT (0.51 ± 0.12) and the lowest for CFAT (0.24 ± 0.10), reflecting differences in trait heritability, sample size, and data structure. Increasing genetic trends were observed for body weight traits (based on direct EBV). Carcass traits showed no significant genetic progress (P > 0.10). Parasite resistance improved steadily, with PFEC declining by 0.088 cube root of count/yr. Limited genetic progress was observed for reproductive traits (NLB: 0.002 lambs/yr; NLW: 0.003 lambs/yr; PSC: 0.108 cm/yr). In conclusion, these results provide a foundation for improving genetic evaluations in Polypay, with future work focusing on integrating genomic data to enhance prediction accuracy for key traits.

Keywords: 2026

How to Cite:

Saleem, A., Rocha, A., Yazar Gunes, H., Wilson, C., Freking, B., Murphy, T., Burke, J., Lewis, R. & Brito, L., (2026) “Genetic analyses of growth, body composition, parasite resistance, and reproductive traits in U.S. Polypay sheep”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2286938. doi: https://doi.org/10.31274/wcgalp.24175

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