Genetic Covariation between Milk Yield and Fat Percentage in Dairy Cattle

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Abstract
The genetic correlation between milk production and fat per cent was studied in herds with varying levels of fat production. For 5458 Holstein daughters and their dams the genetic correlation was -0.38[plus or minus] 0.06, and for 1825 Guernseys, -0.57 [plus or minus] 0.10. The genetic variance in fat percentage for Holstein was about one-half that for Guernseys, but for milk production it was larger for Holsteins than for Guernseys. There was no consistent trend in the negative genetic correlation between milk production and fat percentage for either breed, when fat production increased from 300 to 480 lb. Selection indices, derived from statistics obtained from this study, indicated that most of the emphasis in selection should be placed on milk production.
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