Interrelationships Among Age, Body Weight, and Production Traits During First Lactations of Dairy Cattle

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Abstract
Objectives of the study were to determine what role body size should play in breeding programs. Based on 22,767 Holstein, 2174 Guernsey, and 1036 Jersey A.I., 1st-lactation, DHIA records, the estimated intraherd heritabilities of Holstein, Guernsey, and Jersey ME milk production were 0.37, 0.28, and 0.00, respectively. The corresponding heritability estimates for age-adjusted taped body weight were 0.16, 0.40, and 0.24. The estimate of the genetic correlation between ME milk and age-adjusted weight was 0.45 for the Holsteins and 0.40 for the Guernseys. Estimates of the intraherd phenotypic correlation between these 2 traits ranged from 0.08 to 0.17. Based on an intraherd multiple regression analysis, body weight was found to be a slightly more important source of the intraherd variation in actual 1st-lactation Holstein production than was age, but just the opposite was true for the Guernseys and the Jerseys. When milk or milk fat production records are available, use of body weight measurements in selection indices would increase the accuracy of such indices only slightly. Body weight could, be used to select for production when no other information is at hand.
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