Genetical aspects of maximum rate of flow during milking

Abstract

Summary: Rates of flow of milk during milking have been observed on twin and single-born dairy cows with the aid of a device for continuous recording of weight of milk extracted. Each cow has been characterized by a maximum rate arrived at by averaging morning and evening observations on 2 days. Only this maximum rate has been studied.Repeatability was high (0·8–0·9) whether measured from day to day or from lactation to lactation. Maximum rate of flow was 10–16% higher in second than in first lactations. The range extended from under 2 lb/min to over 7 lb/min.Lactation yield of milk varied significantly with rate, rising about 50 gal for an increase of 1 lb/min. This relation is reduced or eliminated by taking into account by partial regression analysis either maximum weekly yield or the increase from the second to the peak week of lactation.Although the higher yielding cows had larger udder areas bounded by the four teats, the regression of 305-day milk yield on rate was not changed much by taking udder area into account. Cows with short teats milked faster and had higher yields than cows with longer teats.Differences in yield among the four quarters widened slightly between 1 and 3 months of lactation regardless of rate of flow, except possibly in the three slowest milking cows. In these the differences increased three times as much as in the other cows. The wider the inter-quarter differences in yield the less the proportion of milk obtained at the maximum rate.Heritability of maximum rate of flow derived from observations on paired singleborn cattle and twins was 85%. Field data would probably yield a value of 60–70%. Appropriate selection of breeding stock would effectively alter rate of flow, but there is no reason yet to expect yield of milk to change except perhaps by reducing the incidence of very slow milking cows.