A Selection Index for Fat Production in Dairy Cattle Utilizing the Fat Yields of the Cow and Her Close Relatives

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Abstract
Improvement of traits with low heritability can often be enhanced if, in the selections, appropriate attention is paid to the phenotypes of close relatives in addition to the animal's own phenotype. When selection is directed toward improving a single trait, the importance of the individual's o~1 phenotype, as compared with the phenotype of a particular relative, for predicting an animal's breeding value is determined by the heritability of the trait and the closeness of the relative's genetic relationship. The individual's phenotype receives nearly all the enlphasis when heritability is high, but there is room for more attention to the relative's phenotype when heritability is low. Lush (3) explored the relative worth of the individual's performance alone and the performance of progeny alone for predicting individual breeding values. Information on at least five progeny was needed to equal the information provided by the individual's phenotype, mfless some selection had already been practiced and only the differences among the individuals surviving were being considered. Lush (5) also investigated the results anticipated from selecting on individuality alone, solely on the family average, or on the optimum combination of the two. Only if the ph~notypic correlation between family members and the genetic correlation between family members are distinctly unequal, does the family provide information which makes selection much more accurate than if selection were based on the individual's own performance alone. The present study was undertaken to derive an index for more accurate intraherd selection for fat production by utilizing properly all of the information on the individual cow and her close relatives. Fat yields of the cow, her dam, her daughters, her maternal sisters, and her paternal sisters are used to predict the cow's breeding value. The proposed index combines these items of information about the cow and these relatives in such a way that the index value for the cow will be correlated more closely with her breeding value than if the information were combined in any other linear manner. EXPERIMENTAL Data for this study were obtained from the American Jersey Cattle Club. The data included fat production records of all herds on Herd hnprovement Registry Test for at least 4 years during the period from 1943 to 19~7. In all,