EDP-OLOGY MILK EPD
by Darrell Wilkes, Ph.D., International Brangus Breeders Association (IBBA) executive vice president
I get a lot of questions about the Milk EPD. About half of the questions pertain to the units of measurement and the other half are generalized questions about what level of Milk EPD is too low or too high or just right. The first question is easy to answer. The second one is impossible to answer without specific knowledge of a herd’s environment (i.e. nutrition level). Milk EPD is expressed in pounds of weaning weight. It is NOT expressed as an estimate of the actual pounds of milk produced by a cow – a common misperception. Several studies have concluded that the correlation between Milk EPD and actual milk production is fairly high, but few such studies have been done. These kinds of studies are very difficult. Basically, you have to gather up a herd of beef cows with known Milk EPDs, separate from their calves for 12 hours, and then literally milk them by hand and weigh the milk produced. At best, such a milking is a questionable estimate of the milk that a cow actually produces when she is mothered up to a calf who might nurse 6-10 times per day. One can give the cow a shot of the hormone that causes milk let-down, oxytocin, but that may or may not be equal to the natural milk let-down that results when a calf nuzzles a cow. But even with these imperfections, the bulk of the studies that have been done this way have reached the conclusion that Milk EPD is correlated to actual milk production. One of the larger and more sophisticated studies in this area estimated that one pound of Milk EPD equates to 24 lbs of fluid milk produced during a 205-day lactation/suckling period. That’s about 3 gallons of milk, or 384 ounces, or a bit less than 2 oz per day. Stated differently, an 8-lb spread in Milk EPD – which most Brangus breeders would consider extreme -represents enough daily milk to fill a typical water bottle or beer can. One study in particular provides some straightforward assurance about Milk EPDs. Researchers at Oklahoma 16
AU G U ST 2022 B R A N G U S J O U R N A L
State University produced two lines of cattle based on Milk EPD, using Angus and Hereford bulls. Within each breed, they produced High Milk daughters and Low Milk daughters for 3 years using the same set of commercial cows as a base. The difference in sire’s Milk EPD between the High and Low lines was 27 lbs. When the High and Low Milk females were 6, 7 and 8 years old, the researchers measured actual milk production as described above, and also gathered calf weaning records. The observations, summarized below, are highly supportive of the utility of Milk EPD. • Calves from the High Milk lines weaned 30-lbs heavier than calves from the Low Milk lines compared to a 27-lb difference predicted by the Milk EPDs. • High Milk Angus-sired females, at maturity, were significantly lighter than the Low Milk Angussired females. This weight difference was not observed in the Hereford-sired cattle. • Cows in the High Milk line had significantly lower body condition scores than those in the Low Milk line, and tended to have longer calving intervals, later calving dates, and lower calving percentages. The third bullet point above tells an important story. There absolutely is such a thing as too much milk. A cow will sacrifice her next calf (i.e. her next pregnancy) in order to nurture the live calf on the ground. This probably has some evolutionary logic. Namely, a live calf on the ground has higher present value in the biological system compared to a prospective “next calf ” which faces many hurdles – ovulation, fertilization, gestation and parturition. Raising the current live calf takes priority over getting pregnant with the next calf. Here is where I steal a quote from Brad Wright: “A cow that weans the heaviest calf, but calves every OTHER year, is not a profitable cow.” There is no other way to say it, Milk is a difficult EPD. It is affected by what statisticians call “confounding”. I’ll use an example of confounding to help make the point. Assume you have two hay fields, each served
