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Relationships of Predicted Differences of Dairy Bulls and the Performance of Their Crossbred Progeny^1,2,

North Carolina State University, Raleigh 27650

Abstract

Actual milk and milk fat yields of 328 crossbred cows in first lactation in four experimental herds and Predicted Differences for the same traits on their 165 sires were the basic data used. Predicted Differences (PD) for milk and fat on a 1974 base were the measures of transmitting value. They were computed from all straightbred progeny of these bulls in milk recorded herds in the United States. Standard deviations of PD for the 95 Holstein and 70 Brown Swiss bulls were similar to those of all bulls of their breeds in use in artificial insemination at the same time. Regressions of milk yields of crossbred progeny on PD for milk were .77 ± .44 for Holstein-sired crosses and 1.42 ± .58 for crosses sired by Brown Swiss bulls. Corresponding values for milk fat were .55 ± .41 and 1.25 ± .60. The four regression coefficients averaged 1.00. Correlations corresponding to the four regressions were .22, .35, .17 and .31, respectively. These averaged .26, slightly higher than their expected value based on heritabilities of .25 for milk and fat yields. Regressions based on 405 straightbred Holstein and Brown Swiss progeny in the same four experimental herds averaged .99, while respective correlations averaged .21. These results suggest that selection of dairy bulls based on their progeny tests from straightbred progeny would be effective in improving lactation yields of crossbred progeny.

¹ Paper no. 8283 of the Journal Series of the North Carolina Agr. Res. Service, Raleigh.

² This research was part of the Southern Regional Dairy Cattle Breeding Project, S-49 (Genetic Methods of Improving Dairy Cattle for the South).

³ Present address: SR-CRSP, P.O. Box 58137, Nairobi, Kenya, E. Africa.