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

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Abstract
The lactation and type records of 2810 daughters and mates of 756 sires from 414 Jersey herds comprised the data from which the necessary statistics for several selection indices were obtained. The heritability of differences in single records of milk yields, fat percent, 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 percent, 0.88 between milk yield and fat yield, 0.08 between milk yield and type, 0.15 between fat percent and fat yield, 0.05 between fat percent 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.