Usefulness of subjective ovine milk scores: II. Genetic parameter estimates

Genetic parameters for a subjective milk score given to ewes within 24 h of parturition were estimated to determine the usefulness of milk score as a selection trait to improve milk production, which influences total litter w eight weaned. Heritability of milk score and the genetic correlation of mil k score with litter weight weaned were estimated by REML separately for fou r sheep breeds, Rambouillet (n 1,731), Targhee (n = 1,638), Columbia (n = 1 ,731), and Polypay (n = 1,129). Litter weight weaned was the total weight o f lambs weaned at approximately 120 d of age under a western range producti on system. Observed heritability estimates for milk score at first parity w ere moderate and similar among breeds, ranging from 0.18 to 0.32. Heritabil ity estimates adjusted for a binomial distribution of milk scores at first parity were high (Columbia, 0.43; Polypay, 0.35; Rambouillet, 0.50; Targhee , 0.84). Estimates of observed heritability for second-parity milk score we re moderate to high, ranging from 0.23 to 0.46. Milk score at first or seco nd parity was genetically correlated with milk score records at maturity (t hird parity and greater), with estimates ranging from 0.69 to 1.00. Milk sc ore and litter weight weaned were genetically correlated at first or second parity in Rambouillet (r(g) = 1.00) and Targhee breeds (r(g) = 1.00 and 0. 61, respectively), but not in the Columbia and Polypay breeds. Estimates of heritability for lifetime records for milk score ranged from 0.16 to 0.26 across breeds. Estimates of genetic correlations of annual lifetime milk sc ore records with litter weight weaned were high (Columbia, 1.00; Polypay, 0 .81; Rambouillet, 1.00; and Targhee, 0.77). Repeatability estimates for mil k score were similar across breeds, 0.23 for Columbia, Rambouillet, and Tar ghee ewes and 0.28 for Polypay ewes. Milk score measured at first or second parity may be a good predictor of future potential milking ability. Furthe r, milk score can be used as a selection trait to improve maternal ability for increasing litter weight weaned. The need for increasing ewe milking pe rformance and lamb growth rate at first parity in commercial range sheep pr oduction systems may be addressed by selection for milk score at first pari ty.