Genetic Study of Milk Constituents in Purebred and Crossbred Dairy Cattle1

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Abstract
The lactation production of milk and both yields and percentages for fat, SNF and protein of 560 lactations of 328 cows in the Illinois crossbreeding experiment were analyzed. There were significant differences between breed groups in each generation for all traits studied except protein percentage in generations 2 and 4. Regressions on fraction of Holstein blood were homogeneous for the three generations and were pooled. All pooled linear regressions were significant, but quadratic and cubic regressions were not significant, though the signs of the quadratic coefficients indicated that the average of the crossbreds was larger than that of the purebreds for most of the traits. Season of calving, lactation number and age at calving significantly affected total milk, fat, SNF and protein in generation 3 but not in generations 2 and 4. Season of calving significantly affected SNF and protein percentage in generations 2 and 3 and SNF percentage in generation 4. The effect of weight at calving was nonsignificant except for protein percentage in generation 4. Heritabilities were estimated by regression of daughters on dams, with 285 daughter-dam pairs of purebred Holsteins, Gurnseys and various crossbreds. Regression coefficients were calculated within breed of sire, generation and purebred and crossbred daughters of each sire, and were homogeneous for all traits. Heritabilities were, for yield, for milk, 0.38; fat, 0.36; SNF, 0.35; protein, 0.22; and, for percentage, for fat, 0.35; SNF, 0.09; and protein, 0.09. Genetic correlations were strongly positive between yields, negative between milk yield and constituent percentages and strongly positive between constituent percentages. Copyright © 1969. American Society of Animal Science. Copyright 1969 by American Society of Animal Science

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