USDA scientists at the Miles City, Montana Range Research Laboratory used a large data set from their Line 1 Hereford population to evaluate genetic relationships between preweaning calf gain, milk production, and udder score (1=very pendulous, balloon teats; 9=tight and level, small teats). The study involved the following numbers of observations: preweaning gain, 6,835; milk production, 692; and udder score, 1,686.

Estimates of heritability were as follows: direct preweaning gain, 0.13; maternal preweaning gain, 0.25; milk production, 0.25; and udder score, 0.23. Genetic correlations of milk production with maternal preweaning gain and udder score were 0.80 and -0.36, respectively.

These results indicate that maternal preweaning gain was highly predictive of milk production. Consequently, the authors concluded that direct measurement of milk production for use in genetic improvement may not be justified because it is difficult to measure, and selection for maternal preweaning gain may be nearly as effective in changing milk production as direct selection for the trait. The negative correlation between milk production and udder quality is of concern. However, the authors noted that this correlation (-0.36) is not so strong as to preclude simultaneous improvement of milk production and udder quality using appropriate predictive breeding values for each trait (MacNeil and Mott. 2006. J. Animal Sci. 84: 1639).