GENETIC TRENDS AND SELECTION IN A DAIRY CATTLE HERD

Abstract

Lactation records completed by 104 Holstein and 51 Ayrshire cows in the University of Guelph herd, in 1955 to 1965 inclusive, revealed a steady improvement in each breed for milk and fat yield. Maximum likelihood constants for environmental change indicated an annual increase of 3.9 and 4.0 breed age-class average (BCA) points for milk, and 2.2 and 2.3 BCA points for fat yield for the Holstein and Ayrshire breeds, respectively. In contrast, fat percent showed an equally strong negative environmental trend of −0.05 and −0.07% annually for the Holstein and Ayrshire breeds. The annual genetic trend in the Holstein breed was insignificant, in contrast to significant positive estimates of 1.3 BCA milk, 1.7 BCA fat, and 0.03% fat for the Ayrshire breed. The weighted average within-year difference in breeding value between dams that left female offspring and all cows making records in a given year was generally small, averaging +0.77 and +0.08 BCA points, respectively, for the Holstein and Ayrshire breeds. The weighted average superiority in breeding value for milk of sires of females in the Holstein and Ayrshire breeds was +0.04 and +1.56 BCA points, respectively. The significant positive rate of genetic improvement for milk yield in the Ayrshire herd was attributed to more extensive use of AI sires with high breeding values for milk production.