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

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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 5 breeds, the herd effect as a per cent 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; solids-not-fat (SNF), 0.34; total solids (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 trans -mitting abilities of the sires for the traits studied. Because of the negative relationship between most of constituent percentages and milk yield, a selection index in needed in a selection program to secure the optimum yield with the desired composition of milk.