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

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 importance, the parameters used by the National Sheep Improvement Program (NSIP) in its genetic evaluation have not been updated in over two decades. Therefore, we re-estimated genetic and phenotypic parameters and calculated theoretical accuracies and genetic trends for various growth, body composition, and reproductive traits in U.S. Suffolk sheep. The traits evaluated were birth weight (BWT), weaning weight (WWT), post-weaning weight (PWWT), post-weaning eye muscle depth (EMD), post-weaning fat depth (CFAT), and number of lambs born (NLB) and weaned (NLW) per ewe lambing. The dataset consisted of a pedigree comprising 70,271 individuals and up to 48,000 phenotypic records per trait across more than 100 flocks. Variance components were estimated using the Restricted Maximum Likelihood method and animal models with maternal and permanent environmental effects fitted when appropriate. Records were pre-adjusted for age at recording, birth type, rearing type, and dam age as relevant. For lamb traits, contemporary group (CG; concatenation of flock, birth year, management group, sex, lambing interval, and recording date, except for BWT) was the only fixed effect included in the models. For reproductive traits, CG combined flock and lambing year; ewe age was added as a categorical fixed effect. 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. Genetic correlations among growth traits ranged from 0.47 ± 0.07 to 0.67 ± 0.04, and genetic correlations between PWWT and body composition traits were positive (0.30 ± 0.09 with EMD and 0.55 ± 0.08 with CFAT). A strong genetic correlation (0.81 ± 0.03) was observed between NLB and NLW. Theoretical accuracy was highest for proven animals with progeny records. Genetic trends showed progress in growth and muscling traits but declining maternal breeding values for growth traits. Several updated genetic parameters diverged from legacy NSIP estimates, likely due to shifts in management, and trait definitions. These results emphasize the need for regular re-estimation of genetic parameters to ensure accurate genetic evaluations and promote balanced genetic progress across traits.