Genetic analysis of ewe stayability and its association with lamb growth and adult production.

Records from 2,525 adult Targhee ewes and 10,099 lambs were used to estimate genetic parameters in an animal model for ewe stayability (STAY), productive life (PL), adult BW (AW) and BCS, fleece characteristics, and number of lambs born (NLB) and for lamb BW at birth and approximately 45 d, 120 d (120W; weaning), 12 mo, and 18 mo of age. Stayabilities were analyzed as overall stayability (STAYn|2), which indicated presence or absence of a ewe at n yr of age, given that she was present at 2 yr of age, or marginal stayability (STAYn|n-1) recording the presence of a ewe at n yr of age, given that she was present in the previous year. Productive life was measured as the age of ewe in years at her last lambing. Lamb BW were corrected for age at weighing; BW taken at or before weaning were adjusted for effects of type of birth and rearing, age of dam, and sex using National Sheep Improvement Program methods. The NLB was likewise adjusted for effects of ewe age before analysis. Birth year was included as a fixed effect for all traits. Year of record and, for AW and CS, the number of lambs born or born and reared were also included in models for adult traits. Models for all traits included random additive genetic and residual effects. Random maternal genetic and dam permanent environmental effects were also included in the model for lamb (pre)weaning BW, and random permanent environmental effects of the ewe were included in models for adult traits. The heritability estimate for PL was 0.05 (P < 0.10). Additive variance in STAY was only present after 5 yr of age (P < 0.05), with estimated heritabilities ranging from 0.04 to 0.10. Phenotypic correlations between STAY and other traits were near zero, ranging from -0.07 to 0.14. Estimates of genetic correlations of STAY5|4 and STAY6|2 with maternal genetic effects on 120W were positive (both 0.46; P < 0.05), suggesting that STAY and maternal effects on 120W both reflect genetic variation in ewe fitness characteristics. Genetic correlations between STAY5|4 and 120W, AW, and NLB were not different from zero (0.06, 0.13, and -0.06, respectively; P > 0.10). However, genetic correlations between STAY6|2 and 120W, AW, and NLB were all negative, with values of -0.17, -0.32 (P < 0.05), and -0.03, respectively, indicating a possible small antagonism between STAY and genetic effects on body size.