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Genetic and Environmental Parameters of Milk Constituents for Five Breeds. II. Some Genetic Parameters ¹

Departments of Veterinary and Animal Sciences and of Statistics, University of Massachusetts, Amherst

ABSTRACT

Estimates were made of some genetic parameters of milk yield and its components and of the breeding values of sires for yields and constituent percentages. The data consisted of 1.449 cows of the Ayrshire, Guernsey, Holstein, Jersey, and Brown Swiss breeds located in 43 herds.

Based on the average of five breeds, the herd effect as a percentage of the total variance in the unadjusted data ranged from 25 to 31% for yields and 7 to 16% for constituents. The repeatability of lactation records by intraclass correlation ranged from 0.31 to 0.48 for yields and 0.56 to 0.79 for constituents. The heritability estimates by paternal half-sibs on a breed average basis were: for yields of milk, 0.35; fat, 0.27; protein, 0.30; SNF, 0.34; TS, 0.31; and for percentages of fat, 0.42; protein, 0.40; SNF, 0.36; and TS, 0.45. The expected breeding values obtained by regressing the least-squares sire constant showed large genetic differences in the transmitting abilities of the sires for the traits studied. Because of the negative relationship between most of constituent percentages and milk yield, an index is needed in a selection program to secure the optimum yield with the desired composition of milk.

¹ A contribution of the Massachusetts Agricultural Experiment Station. This investigation was supported by Hatch 177 and NE-46, The Genetic and Environmental Factors Affecting the Composition of Milk Produced by Dairy Cattle, and from private grants to Dr. S. N. Gaunt, Project Leader. This study is part of a thesis submitted by the senior author to the Graduate School of the University of Massachusetts for the Ph.D. degree.

² Present address: Biometrics Section, Food Research Division, Armour and Company, Oak Brook, Illinois 60521.