The causes of differences in production between dairy herds

The extent of the genetic differences existing among a sample of the dairy herds of New Zealand was assessed by a direct experimental approach. In part one of the experiment, 240 young calves were taken from 40 herds of markedly different production levels and reared and milked as a uniformly treated herd of cattle at Ruakura. In part two of the experiment 120 sets of identical twin calves were divided amongst these 40 high- and low-producing herds. Results indicated that in general there were no obvious genetic differences between the cattle in the high and low herds in body size, fertility, milk production or days in lactation. There were, however, genetic differences between the high and low herds in milk composition; approximately 50% of the differences between herds in butterfat and protein content and 25% of the SNF content were due to genetic differences between herds. Genetic differences between herds in yield were found within the high group of herds but not in the low. Evidence was obtained indicating that interactions between herd environment and the genotype of individual animals provided an appreciable source of variation. The production level of a herd in which an animal was reared was without effect upon its subsequent production potential for milk yield.