Ken Olson: Relationship between cow size and nutrient requirements
March 4, 2011
It is commonly understood that cows are getting bigger. When one considers that the cow population in the U.S. has shrunk since 1974, while total pounds of beef produced annually has been maintained at nearly 50 billion pounds over the same time period, it is obvious that cattle have had to steadily increase growth potential and size.
Perhaps the more important question for a cow-calf producer is how big is the modern cow? More particularly, each producer might wonder how big their cows are. If everyone had a scale on the ranch, this would be known. The best alternative on a ranch-specific basis is to look at sale weights of cull cows and then try to adjust for any differences between the culls and the cows that remain in the herd.
In place of thinking specifically of each ranch, let’s look at indicators of cow size in general. One indicator is the shift in expected progeny differences (EPD) genetic trends for cattle weights. For example, the genetic trend through time for the Angus breed has displayed a steady increase in weaning, yearling and mature cow weights. In particular, yearling weight, which is considered a reliable indicator of mature weight, has increased by 96 lbs. since the mid-1970s. We can expect that an increasing trend has occurred in other breeds as well.
Another source of information about mature cow weights is the USDA Germplasm Evaluation Program being conducted at the Meat Animal Research Center (MARC) near Clay Center, NE. Based on direct head-to-head comparison of cow weights from several sire breeds, they reported the following mature weights using 2009 data (Table 1).
These weights suggest two things. First, the average cow weight across all breeds shown here is 1,390 lbs. Second, there is difference among breeds. Our perception of which are the “big” breeds may not be supported by these data. That may be a function of genetic trends that are changing the size of cows in each breed at different rates.
Finally, a third way to estimate mature cow size is to evaluate weight of finished offspring. The general rule of thumb is that mature cow weight, and live weight at slaughter of their progeny, should be the same. Again, USDA MARC data supports this relationship. In 1990, average market weight for slaughter cattle was 1,180 lbs.; and it had increased to 1,343 lbs. in 2009. This suggests that mature cow weight has increased by 163 lbs. over the last 19 years.
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It is a fact that nutrient requirements increase as the size of an animal increases, but is it a straight-line, direct relationship? For mammals in general, nutrient requirements per pound of body weight increase at a decreasing rate. Said another way, the nutrient requirements per pound are lower for a large animal than for a small animal. This is particularly true for the requirement for energy. The energy required by the 1,400-lb. cow is about 11 percent higher than that required by the 1,200-lb. cow, despite the fact that she is about 16 percent heavier.
The question about nutrient requirements is how do we get additional nutrients into the larger cow using similar resources (i.e. the rangeland on your ranch) to those that were used for smaller cows in the past? The nutrient densities of the forages are about the same, so forage intake needs to increase. Continuing with the same example using 1,200- vs. 1,400-lb. cows, annual dry matter intake will be 9,353- and 10,406 lbs. for the 1,200- and 1,400-lb. cows, respectively. This is a difference of 1,053 lbs., or about an 11 percent increase for the larger cow.
The bottom line that can be drawn from this discussion includes: Cattle have changed dramatically over the last two or more decades; they are probably 200 lbs. bigger than two decades ago. The primary reason cows have gotten bigger is genetic selection for increased rates of growth, which leads to larger size. Bigger cows take more feed to meet their nutrient requirements. Management practices, such as grazing management, need to allow forage intake to increase to match the cow’s nutrient requirements. If that is not possible, then more harvested feeds will need to be fed. One needs to evaluate the economics of the additional feed costs versus the additional productive capacity of the larger cows.