Response to Selection of Breeding Stock for Milk Solids Production

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Abstract
A trial experiment of young' sire testing for bull breeding was applied to closed pop- ulations of 300 Holstein and 300 Ayrshire cows. The data and selection practiced cov- ered 10 years of year-round stable feeding and standard total digestible nutrient allow- ance. The selection differential for dams of young bulls gave results closest to the ex- pected 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 ef- fects on trends of milk solids yield was used to estimate annual rates of genetic im- provement for 180-day milk solids, 4.46 kg for Holsteins and 4.36 kg for Ayrshires. Compared with the expected increase of 1.30% based on an h 2 of 0.30, these in- creases 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 signif- icant change in percentages of fat, solids- not-fat, or protein in Ayrshires or for per- centage of protein in Holsteins. Milk sol- ids is a practical measure of productivity in dairy cattle, and present selection tech- niques and artificial insemination practices are an economic advantage.
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