BIAS AND ACCURACY OF SINGLE MILKING TESTING SCHEMES TO ESTIMATE DAILYAND LACTATION MILK-YIELD

This study evaluated the adequacy of an alternate a.m.-p.m. testing sc heme for milk yield in comparison with the official test method based on weighing two milkings within 24 h. A total of 88,558 single test da y milking weights from 15,670 Italian Friesian cows in 610 herds in th e province of Milano (Italy) were collected between September 1991 and October 1992. Ratios were computed for daily milk yield to a.m. and p .m. milking weights (direct yield ratios) and ratios of a.m. and p.m. milking weights to daily milk yield (inverse yield ratios). Analysis o f variance indicated that the milking interval is the most important s ource of variation for yield ratios. Factors for estimating daily milk yield from single milking weights adjusted for milking intervals were derived through regression analysis of direct and inverse yield ratio s on the length of the milking interval. Daily milk yield was estimate d more precisely and accurately when adjustment factors were used than when single milking weights were doubled. The doubling of a.m. and p. m. milking weights overestimated and underestimated (5% on average) da ily milk yield, respectively. For 305-d lactation yield, accuracies of these methods were comparable. In conclusion, alternate recording of a.m. and p.m. milking weights led to reliable estimates of milk yield. Effects of the alternate a.m.-p.m. testing scheme on estimates of mil k components and genetic evaluation of dams of sires should be investi gated.