GENETIC CONTROL OF TEST-DAY MILK YIELD IN SAHIWAL CATTLE

Heritabilities of test-day (TD) and 305-day milk yield were estimated using performance records of 780 first lactation Sahiwal cows, following two models. In the first model, estimated 305-day lactation milk yields were analyzed through an individual animal model with period-season of calving as fixed effect and additive genetic effect as random factor. The second model was a repeatability model where monthly milk yield records were analyzed using period-season of calving as fixed effect and additive genetic effect and permanent environmental effect as random factors. The age at calving, ratio of days in milk to 305-day (as linear and quadratic components) and their reciprocal logs were used as covariables. The heritability estimates for 305-day and TD milk yields were 0.082 and 0.024, respectively. Heritability estimates of individual TDs ranged from 0 to 0.274. The relative proportion of permanent environment variance to total variance was 0.498. Heritability of individual test-day milk yield was highest in the mid-lactation (TD6-TD8). Lower genetic control of test day and lactation milk yield in the present study needs validation using larger data sets with accurate pedigree recording.