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Genetic Covariation between Milk Yield and Fat Percentage in Dairy Cattle¹

Department of Animal Industry, North Carolina State College, Raleigh

ABSTRACT

Market preference for milk with approximately four per cent fat has stimulated Holstein breeders to select for higher fat percentage, and Guernsey and Jersey breeders have been stimulated to emphasize raising the level of milk production. The level of milk production in Guernsey and Jersey herds needs to be increased, even with some sacrifice to the present high fat percentage. However, the level of milk production in Holstein herds should also be raised, or at least maintained, as the average fat percentage is increased. This goal of simultaneously increasing milk yield and fat percentage in the Holstein breed may be difficult to attain, since studies to date (4, 12, 13) have indicated that milk yield and fat percentage are correlated negatively genetically.

In a random-breeding population of long standing, the coupling and repulsion phases of linked genes would be near equilibrium, and any genetic correlation between two traits such as milk yield and fat percentage would result largely from the pleiotropic effects of genes.

¹ Contribution from the Department of Animal Industry, North Carolina Agricultural Experiment Station, Raleigh, North Carolina. Published with the approval of the Director of Research as Paper No. 754 of the Journal Series.