The expected progeny difference (EPD) for maternal weaning wt. (sometimes referred to as “milk” EPD) is calculated from the preweaning gain of calves. This EPD can then be used by producers as an aid in selection to alter milk production levels of cows. In spite of over 20 years of using maternal weaning wt. in this manner, there is still a degree of distrust of this statistical prediction among some producers. There is also some concern that selection for increased milk production may result in a deterioration of udder and teat quality, thereby reducing longevity in the herd.

USDA scientists at the Miles City, Montana Research Station used 10 years of data from their Line 1 Hereford herd with the objectives of: 1) confirming the usefulness of the EPD for maternal weaning wt. as a predictor of genetic merit for milk production; and 2) evaluating the relationship between udder structure and milk production. Milk production was measured four times per year on each cow. Within 24 hours of calving, the udder of each cow was scored subjectively on a 1 to 9 scale (1=Very pendulous udder and balloon teats; 9=An ideal mammary system).

Heritability estimates for preweaning gain, milk production, and udder score were 0.25, 0.25, and 0.23, respectively, indicating that reasonable progress can be made to improve these traits through selection. The genetic correlation between maternal weaning wt. and milk production was 0.80, which indicated that the EPD for maternal weaning wt. is an effective predictor of genetic differences in milk production. Genetic correlations of maternal weaning wt. and milk production with udder score were -0.26 and -0.36, respectively, indicating that a moderate genetic antagonism exists between selection for increased milk production and udder quality. These results indicate that selection pressure on maternal weaning wt. EPD will change milk production, but some caution is advised in selecting for extremely high milk production due to a potential erosion of udder quality (SOURCE: Fort Keogh Research Newsletter, Dec., 2005).