Cow-Calf Commentary for Iowa Cattleman Magazine
By Randie Culbertson, extension cow-calf specialist
Got Milk EPD?
My least favorite EPD to investigate and explain is a milk EPD due to the complexity of its estimation and how it is measured. It's not a trait that is directly measured compared to other traits such as weaning weight. Outside of the wild cow milking event during a ranch rodeo, we don’t directly measure milk of a beef cow. Even during a ranch rodeo, the goal is to get just enough milk in a bottle to win (not much concern for an accurate measurement) and to not be carried out of the ring on a stretcher.
If we are not measuring the milk produced by a cow directly, then how is milk measured? A beef cow’s milk production is expressed through pounds of calf weaned, based on the theory that a cow who produces more milk will have heavier calves. When we measure the weaning weight of a calf, we are seeing the expression of two traits: milk and weaning weight direct. Weaning weight direct is the genes the calf itself possesses for growth. Those are the genes he inherited directly from his sire and dam for growth, as opposed to milk which is being expressed as a trait of the dam but measured indirectly through the calf.
A milk EPD is typically defined as the genetic merit of a sire’s daughters’ ability to produce milk expressed in the pounds of her weaned calves. A milk EPD is considered a maternal trait and is the effect of the cow on her calves that is being measured through the calf’s weaning weight. A maternal effect is the expression of genes in the dam that influence the performance of the calf because of the environment the dam provides. We see maternal influences on traits measured early on in a calf’s life such as calving ease and weaning. As a calf gets older, especially after weaning, the maternal influence on the calf’s performance dissipates. Maternal influence on yearling weight, feedlot performance, and carcass traits is minimal.
The genetic correlation of milk to weaning weight direct is typically zero or a low negative correlation. It is not uncommon to see a bull’s EPD for milk go down but his weaning weight EPD to go up. Most genetic evaluations set this correlation to zero, minimizing the antagonistic changes in bull EPDs for weaning and milk. Milk EPD is a measure of a bull’s daughter's calf performance, which means this is a trait measured later in life for a bull. As a result, the accuracy for a milk EPD on a bull will be low until the bull reaches an age for weaning weight data to be reported on his grand progeny and changes in EPD could be seen at that time. If the calves of daughters perform lower than expected at weaning, this can result in a decrease for a bull’s milk EPD and vice versa.
How a milk EPD is used in the breeding objective will depend on the operation itself. Milk can be a double-edged sword for a cow-calf operation. The first and most obvious reason is simple, more milk means more pounds for calves weaned. If you are selling your calves at weaning, this could be an economically beneficial trait to pay attention to, and increasing milk in your cow herd may make sense. However, nothing in the life is free, and the same can be said about increased milk potential in your herd. A cow’s highest nutritional requirements will occur during peak lactation. Cows who produce more milk will have a higher metabolic requirement during lactation compared to a cow with a lower milk production. The ability to meet those nutritional requirements would have an economic cost. Depending on your feed resources, the increase in cow herd feed requirements could be costly. Matching your annual production calendar to your environment to ensure that the cow herd is at peak lactation when pastures are optimal to meet the nutritional requirements would be ideal. It is also important to match your genetics to your environment. If you lack the resources to feed a high milking herd, you would either need to provide feed (which could get expensive) or risk the chance of cows failing to rebreed. If the cow’s nutritional requirements are not met to meet her metabolic demand for lactation, she could lose body condition score, delay resumption of estrous, and fail to breed back. At the end of the day, a higher milking cow has the potential for providing more pounds of calf at weaning, but the cost of her increased metabolic demand could result in a larger loss through feed costs and open cows.
As you look back at your records for weaning, it is a good time to evaluate your cow herd performance and consider your herd milk level. How were your weaning weights? What condition were your cows at weaning? How many were open? And what were your feeding costs to maintain your cows to weaning? Depending on your operation, selecting for milk as an intermediate optimum may be ideal. Although the milk EPD is my least favorite to explain it is still an important tool for producers and how it is used will depend on the individual operation.