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Prediction of Daughter's Performance from Dam's Cow Index Adjusted for Within-Herd Variance

Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24060

ABSTRACT

Lactation records of 1,032,438 Jersey and 1,490,909 Holstein cows were used to compute Modified Contemporary Comparison Cow Indexes for fat percent, fat yield, and milk yield. The analysis of herd-year variables indicated heterogeneity of within herd-year standard deviations of 16-fold and 11-fold differences for fat percent, 10-fold and 12-fold differences for fat yield, and 9-fold and 14-fold differences for milk yield for Holsteins and Jerseys, respectively. Correlations between the mean and within herd-year standard deviation were smaller for fat percent than milk and fat yield with .112 and .115 for fat percent, .509 and .424 for fat yield, and .490 and .450 for milk yield in Holsteins and Jerseys, respectively. Five adjustments to the modified contemporary deviation portion of the Cow Index were compared with the current USDA Cow Index. The six modified contemporary deviations calculated were: the current deviation, natural log-adjusted deviation, and the deviation standardized to a constant variance in combination with either the current correction for contemporaries genetic merit or an adjusted correction. The six different Cow Indexes were compared for the accuracy with which they predicted the daughter's average modified contemporary deviation. The standardized deviation with an adjusted genetic merit of contemporaries was best for predicting daughter's milk and fat yield average modified contemporary deviations in both breeds. However, little differences were observed between deviation methods for fat percent.

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