Components of the Relationship Between Level of Production and Rate of Maturity in Dairy Cattle

Summary Evidence is presented by the use of a variance and covariance analyses technique applied to production records of artificially sired cows to support the use of first lactation records on daughters of young dairy cattle sires being evaluated. The heritability of rate of maturity reflected in change of production with age was found to be one-third to one-fourth that of level of production. It is shown that level of herd production has far more influence on the relationship between level of production for successive lactations than does level of production among progeny groups of sires, the latter association being essentially zero. An increase in level of production among herds is accompanied by a decrease in the increase from first to second lactation production. Herd variation was found to be the largest single source of variation in production traits. The fraction of herd variation which is genetic was not estimated, but it was found that sires may be expected to be ranked in the same order according to daughter production in different herds.