A Model for Quantifying Sources of Variation in Test-day Milk Yield of Holstein Cows
AbstractA cow's test-day milk yield is influenced by several systematic environmental effects, which have to be removed when estimating the genetic potential of an animal. The present study quantified the variation due to test date and month of test in test-day lactation yield records using full and reduced models. The data consisted of 54 237 edited daily teat-day milk records from 372 Holstein cows from Glenara Estates in Zimbabwe that were milked between 1 May and 31 December, 1996. Three models were used for analysis. The reduced model contained the fixed effects of parity, days in milk, the month and year of calving and random cow effects. The full models included either test-date or month of test in addition to the afore- mentioned variables. Analysis was done using the Henderson's method III in SAS (1994). Parity, days in milk, month and year of calving, test date and month of test all had significant effects on test-day milk yield (p<0.001). Both test date and month of test (SS = 12 287.40). The R-square values were 76.3%, 76.8% and 77.8% for the reduced, full model with test date and full model with month of test respectively. The results indicate that there is scope for genetic improvement from use of test-day lactation records.
Transactions of the Zimbabwe Scientific Association Vol. 74 2000: 22-24