Heritability of Birth Weight and Its Relationship with Production in Dairy Cattle

Abstract

Summary The data included in this study were taken from the University of Illinois dairy herd and consisted of 1,234 birth weights representing the five major breeds of dairy cattle. A comparison of the confidence intervals demonstrates significant breed differences at the 0.001 level of probability for all possible comparisons. The fixed effects of sex of calf and calving sequence were estimated by the method of least-squares. These combined effects were found to be significant at the 0.01 level of probability in all five breeds. The adjustment factors so obtained were applied to the birth weights before the estimates of heritability and genetic correlations were derived. The differences associated with the sires of the calves were found to be significantly different from zero in all breeds except Guernseys. The pooled within-breed estimates of heritability of birth weight obtained by multiplying, respectively, by 4, 2, and 2, the paternal half-sib correlation, the intra-sire regression of offspring on dam, and the full-sib correlation were 0.38, 0.48, and 0.51. Theoretically, it should be relatively easy to change the average birth weight of a population. The differences between estimates by different methods could be due to sampling errors, epistatic and dominance effects, to additive genetic effects of birth weight as a characteristic of the dam, or to maternal effects of the dam on birth weight of the calf. The small magnitude of the differences between the three estimates indicates that the within-breed maternal effect was small and that birth weight was mostly a characteristic of the calf. The correlations between the birth weight of the calf and the subsequent milk and fat production of the dam were calculated on a within-sire basis for the five breeds and for a within-breed pooled estimate. The absence of correlations significantly different from zero indicated that the birth weight of the calf and the immediately subsequent lactation production of the dam were essentially independent. Intra-service-sire correlations between the birth weight of a calf and its subsequent production were not significantly different from zero in any of the five breeds or for a within-breed pooled estimate. The pooled within-breed estimates of the genetic correlations between birth weight and milk and fat productions were small and not significantly different from zero. It appears that birth weight and production are genetically independent.