Estimation of genetic parameters and genetic trends for ewe longevity indicators in U.S. Katahdin sheep

Authors: PINTO, LUIS - Federal University Of Bahia (UFBA); LEWIS, RONALD - University Of Nebraska; ROCHA, ARTUR - Purdue University; Freking, Bradley; Wilson, Carrie; Murphy Jr, Thomas; NILSON, SARA - University Of Nebraska; Burke, Joan; BRITO, LUIZ - Purdue University

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/11/2025
Publication Date: 4/18/2025
Citation: Pinto, L.F.B., Lewis, R.M., Rocha, A.O., Freking, B.A., Wilson, C.S., Murphy, T.W., Nilson, S.M., Burke, J.M., Brito, L.F. 2025. Estimation of genetic parameters and genetic trends for ewe longevity indicators in U.S. Katahdin sheep. Journal of Animal Science. Article skaf125. https://doi.org/10.1093/jas/skaf125.
DOI: https://doi.org/10.1093/jas/skaf125

Interpretive Summary: Ewe longevity is a key economic driver and constraint within most sheep production systems. Increasing genetic potential for longevity of ewes would spread the direct cost of developing replacement ewes over a longer period of time and also generate increased levels of total ewe lifetime productivity. However, direct genetic improvement in this sex-limited trait is limited by the phenotype being only measured and expressed late in the life of an individual ewe making it an ideal trait for assistance through genomic information. Genetic parameters were estimated for ewe longevity and eight longevity indicator traits from datasets provided by the National Sheep Improvement Program (NSIP) and recorded by U.S. Katahdin producers. This is the first study to report genetic parameters for ewe longevity indicator traits in U.S. Katahdin sheep. The heritability estimates were low but indicated that all longevity indicators have a genetic component that can be used in breeding schemes. Also, the high and positive genetic correlations across all eight longevity indicator traits suggests only one longevity indicator trait is needed to apply selection. The genetic trends observed indicate that the selection practices currently carried out by NSIP members based on other traits, such as growth, prolificacy, and parasite tolerance related traits, have not yet indirectly affected ewe longevity. In general, our results suggest that in order to improve longevity there must be an indicator trait included as part of the selection objective in the U.S. Katahdin sheep population. Total number of lambs weaned over a ewe lifetime may be the most reasonable indicator trait for Katahdin breeders to improve both ewe longevity and productivity at weaning, with a consequent increase in both flock profitability and female selection intensity.

Technical Abstract: Direct genetic selection for increased ewe longevity can improve flock profitability and animal welfare. However, longevity indicator traits are not presently evaluated in the National Sheep Improvement Program (NSIP). The primary objective of this study was, therefore, to estimate genetic parameters for 8 longevity indicator traits using data collected in NSIP Katahdin flocks. Ewes (n = 12,734) were born between 1989 and 2020 in 58 flocks across the U.S. and were daughters of 1,245 sires and 6,325 dams. Traits evaluated were age at last lambing (ALL), length of productive life (PL; number of days between the first and last lambing), total number of litters (TNL), total number of lambs born (TNB) and weaned (TNW) over ewe lifetime, total lamb birth weight (TLB) and total lamb weight at weaning (TLW) over ewe lifetime, and TLW divided by the ewe’s 120 d adjusted body weight (TLWadj). Variance components were estimated using the AIREML method based fitting an animal model using either a pedigree (A) or blended pedigree and genomic (H) relationship matrix. Genomic information on 10,032 animals genotyped with a 50K SNP chip was included in the analyses based on H matrix. Age at first lambing and birth-rearing type of the ewe were fitted as fixed effect, while the contemporary group (CG: flock-year-season of ewe’s birth) was fitted as either a fixed (CGF) or random (CGR) effect. Breeding values and their accuracies were obtained for 127,535 animals in the pedigree using either best linear unbiased prediction (BLUP) or single-step genomic BLUP. Genetic trends were evaluated based on all combinations of CG type and method for predictin breeding values. The averages of ALL, PL, TNL, TNB, TNW, TLB, TLW, and TLWadj were 1100 d, 890 d, 2.7 litters, 4.6 lambs, 4.3 lambs, 18 kg, 70 kg, and 2.8 kg/kg of ewe weight, respectively. The H matrix performed better than the A matrix, based on AIC and estimates of breeding value accuracy. Higher average accuracy values were observed when fitting CG as a random effect. The heritability estimates ranged from 0.06 ± 0.02 (TLWadj) to 0.15 ± 0.02 (TLB). All genetic and phenotypic correlations between longevity traits were greater than 0.80. Genetic trends were significative and positive for all traits, but no substantial genetic gains were observed. Considering the observed average values and the estimated genetic parameters, we recommend that longevity becomes part of the selection objectives for U.S. Katahdin sheep.