Estimates of genetic parameters and genetic trends for body weight, body composition, and reproductive traits in U.S. Suffolk sheep

Authors: ROCHA, ARTUR - Purdue University; SALEEM, ALI - Purdue University; YAZAR GUNES, HILAL - University Of Nebraska; DE CARVALHO, FELIPE - Purdue University; Wilson, Carrie; Freking, Bradley; Murphy Jr, Thomas; Burke, Joan; LEWIS, RONALD - University Of Nebraska; BRITO, LUIZ - Purdue University

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2026
Publication Date: 5/8/2026
Citation: Rocha, A.O., Saleem, A.H., Yazar Gunes, H., de Carvalho, F.E., Wilson, C.S., Freking, B.A., Murphy, Jr., T.W., Burke, J.M., Lewis, R.M., Brito, L.F. 2026. Estimates of genetic parameters and genetic trends for body weight, body composition, and reproductive traits in U.S. Suffolk sheep. Journal of Animal Science. 104. Article skag124. https://doi.org/10.1093/jas/skag124.
DOI: https://doi.org/10.1093/jas/skag124

Interpretive Summary: Up-to-date variance components for traits such as body weight at different ages, body composition (e.g., muscle, fat), and reproductive performance are essential for selecting superior breeding animals. In this study, we re-estimated genetic parameters, genetic trends, and the accuracy of estimated breeding values using over 70,000 pedigree and 48,000 performance records from U.S. Suffolk animals enrolled in the National Sheep Improvement Program since the early 1980s. Growth and body composition traits were found to be moderately heritable while reproduction traits were lowly heritable, with steady genetic improvement observed in body weight and muscling traits over the years. Although Suffolk breeders have made progress in growth traits, signs of declining mothering ability suggest it is important to keep monitoring this area. The updated genetic parameters presented in this study will support more accurate and effective selection decisions for Suffolk sheep.

Technical Abstract: The Suffolk is a meat-type sheep that is the predominant terminal-sire breed in the U.S. Known for its rapid growth, high carcass yield, and efficient feed conversion, Suffolk rams are crucial in commercial crossbreeding systems. Despite its economic importance for the U.S. sheep sector, the genetic parameters used in the National Sheep Improvement Program (NSIP) have not been updated in over two decades. Therefore, we updated genetic parameters and calculated theoretical accuracies and genetic trends for various growth, body composition, and reproductive traits in U.S. Suffolk sheep. The traits evaluated included birth weight (BWT), weaning weight (WWT), post-weaning weight (PWWT), post-weaning eye muscle depth (EMD), post-weaning fat depth (CFAT), number of lambs born (NLB), and number of lambs weaned per ewe lambing (NLW). The dataset consisted of a pedigree comprising 70,271 individuals and up to 48,000 phenotypic records per trait across more than 100 NSIP flocks. Variance components were estimated using the Restricted Maximum Likelihood (REML) method and animal models with maternal and permanent environmental effects fitted when appropriate. Contemporary group (CG; defined as a concatenation of flock, year of birth, lambing season, management group, sex, age class, and recording date for all traits but BWT) was the only fixed effect included in the models for lamb traits. For reproductive traits, ewe age was also included as a categorical fixed effect and the CG defined as a concatenation of flock and year of lambing. Heritability estimates (±SE) ranged from 0.04±0.008 for NLW to 0.24±0.025 for CFAT. Maternal heritabilities ranged from 0.02±0.01 for PWWT to 0.16±0.011 for BWT. Moderate to high genetic correlations were found among growth traits, and positive genetic correlations were observed between PWWT and body composition traits. A strong genetic correlation (0.81±0.033) was observed between NLB and NLW. EBV accuracy was highest for proven animals with progeny records. Genetic trends showed sustained progress in growth and muscling traits, but declining maternal genetic merit, highlighting an antagonistic response resulting from selection emphasis on direct genetic merit. The updated genetic parameters diverge from legacy NSIP estimates, likely due to shifts in population structure, management practices, and trait recording definitions. These results emphasize the need for regular re-estimation of genetic parameters to ensure accurate genetic evaluations and promote balanced genetic progress across various traits.