Response to Selection of Breeding Stock for Milk Solids Production

Abstract

A trial experiment of young sire testing for bull breeding was applied to closed populations of 300 Holstein and 300 Ayrshire cows. The data and selection practiced covered 10 years of year-round stable feeding and standard total digestible nutrient allowance. The selection differential for dams of young bulls gave results closest to the expected value; less success was achieved from selection by cow culling and progeny-testing of bulls. The statistical method that had the most stringent control of environmental effects on trends of milk solids yield was used to estimate annual rates of genetic improvement for 180-day milk solids, 4.46kg for Holsteins and 4.36kg for Ayrshires. Compared with the expected increase of 1.30% based on an h2 of 0.30, these increases were 1.06 and 1.30% of the most recent annual average milk solids yield. Correlated genetic responses for percentage of fat and solids-not-fat in Holsteins were significant but small. There was no significant change in percentages of fat, solids-not-fat, or protein in Ayrshires or for percentage of protein in Holsteins. Milk solids is a practical measure of productivity in dairy cattle, and present selection techniques and artificial insemination practices are an economic advantage.