GENETIC AND ENVIRONMENTAL ESTIMATIONS FOR TEST-DAY AND STANDARDIZED MILK-YIELD OF DAIRY SHEEP

Three sets of data, standardized 120-day milk yield, corresponding tes t-day yields and the first two test-day yields of Churra ewes were con sidered to estimate genetic and environmental parameters of milk yield . A total of 22599 lactation milk yields and their corresponding 84488 test-day yields for 12785 ewes, collected between 1992 and 1994, were used to estimate environmental parameters employing fixed models. A s ubset including data from the daughters of the rams participating in t he breeding program was used for the estimation of genetic parameters (3379 lactations and their corresponding 12662 test-day yields for 210 2 ewes, the daughters of 121 rams). Heritabilities and repeatablities were estimated by derivative free restricted maximum likelihood and br eeding values of the rams were estimated by a repeatability animal mod el. For test-day milk yield, the environmental effects of the fixed mo del, including flock test-date (FTD), age at lambing, type of birth, a nd linear, quadratic, and cubic coefficients of days in milk and their inverses, were all highly significant (P < 0.001). For 120-day milk y ield, flock-year-season, age at lambing, and type of birth all had hig hly significant effects (P < 0.001). However, interval from lambing to first test had a less significant effect (P < 0.05). The heritabiliti es estimated for all test-day yields, the first two test-day yields an d 120-day milk yields were 0.14, 0.15 and 0.18, respectively. These he ritabilities were all low, mainly because of the higher phenotypic and residual variances obtained. The corresponding repeatability estimate s for the same traits were 0.44, 0.46 and 0.42, respectively. Product moment and rank correlations between breeding values of rams based on 120-day milk yield and those based on all test-day milk yields and the first two test-day yields ranged from 0.77 to 0.88. Product moment an d rank correlations between evaluations based on all test-day yields a nd on the first two test-day yields were 0.94 and 0.92, respectively. FTD models presented a possible alternative to models fitting standard ized yield records. (C) 1998 Elsevier Science B.V.