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Selection Indices Based on Milk and Fat Yield, Fat Per Cent, and Type Classification¹

Department of Dairy Science, University of Illinois, Urbana

ABSTRACT

The necessary statistics for deriving several selection indices were obtained from the lactation and type records of 2,810 daughters and mates of 756 sires from 414 Jersey herds. The heritability of differences in single records of milk yield, fat yield, fat per cent, and type were 0.25, 0.20, 0.56, and 0.25, respectively. The intra-sire phenotypic correlations were –0.36 between milk yield and fat per cent, 0.88 between milk yield and fat yield, 0.08 between milk yield and type, 0.15 between fat per cent and fat yield, 0.05 between fat per cent and type, and 0.11 between fat yield and type. The corresponding genetic correlations were –0.50, 0.72, 0.07, 0.20, –0.01, and 0.08, respectively.

The genic value for milk yield can be estimated 10% more accurately by also taking into consideration the cow's fat yield, while fat yield alone is a good criterion of selection for improving the genie value of fat yield. The expected genetic improvement of milk and fat yield was greatest when milk yield alone was the basis of selection. Selection for type along with milk and fat yield resulted in a 15% decrease in the expected genetic gain of milk and fat yield.

¹ The data in this paper are from a thesis presented by the senior author in partial fulfillment of the requirements of the Ph.D. degree, University of Illinois.

² Agent of the Dairy Husbandry Research Branch, USDA.