Just how big are our beef cows? Answering this question requires a journey back to what I like to refer to as the beginning of the “genetic revolution” in the U.S. cattle business.

In 1975, a weaned 400-lb. calf was considered a good calf. Today, it wouldn't generate a positive net return in most operations. With today's increased production costs, it takes calves nearer to 600 lbs. to generate a positive net return. Technology has led the way to improved production.

  • The first is genetics. Since 1975, the influence of European breeds has substantially impacted calf weaning weight. Crossbreeding and the exploitation of complementarity and heterosis have contributed, as well. And the development of EPD technology as a selection tool for growth facilitated further advancements in weaning weights.

  • Improvements in our health programs — vaccinations, parasite control and antibiotics — are other contributors. Meanwhile, the management of breeding systems and more controlled calving seasons helped producers wean more uniform and heavier calves.

  • Nutrition programs in general have improved. Despite declines in species diversity, quantity and overall quality in our native range country, we've had some success in improving forage varieties and incorporating legumes into tame pastures. In addition, supplementation strategies have improved based on a better understanding of the nutrient gaps that exist between requirements and what is supplied in the forage. This is especially true for the cow as she progresses through the four phases of production.

The 200-lb. increase in weaning weight over the last 30 years has coincided with substantially increased carcass weights of fed steers and heifers, as well as slaughter bulls and cows.

That's because we selected for growth — more specifically average daily gain and yearling weight — and it worked, to the point that carcasses that yield cuts too large to “fit the box” are now a concern. Animal breeders warned against focusing too intently on direct growth traits, which would lead to an increase in mature size of our cattle due to the high genetic correlation between growth and mature size.

As shown in Figure 1, cow carcass weights have increased by nearly an identical amount as steers; bull carcass weights increased even more, simply because we focused our selection efforts on the sires. The additional 50-lb. increase in the heifers at first seems questionable, until one considers the improvement in implant programs for heifers and the increasing use of MGA. Our efforts to get feeder heifers to perform more like steers appear effective.

Figure 2 states the average carcass weight for slaughter cows in 1975 was 475 lbs.; by 2005, it was 621 lbs. If cow carcass weights have increased 146 lbs., just how much has the mature weight of our cows increased?

To calculate the increase in live weight at slaughter, we must know the average dressing percent for cows at slaughter. One of the major factors impacting dressing percent is the amount of fatness at the time of slaughter. Body condition scoring (BCS) can give us a range estimate of dressing percent. Given the average BCS of our cows at harvest time, we can approximate their average dressing percent, especially when the sample size is large.

According to the “National Market Cow and Bull Beef Quality Audit — 2007,” a survey of 23 packing plants, the average BCS for 2,800 head of cows was 4.53. Based on a BCS of 4.53, the expected range in dressing percent is 45-49%, with the average at 47%. Using .47 as the average dressing percent, we can calculate the live-weight change at slaughter of those cows by dividing the carcass-weight difference by the dressing percent (146 lbs./0.47 = 311 lbs.). Based on these assumptions, the increase in cow live weights at slaughter from 1975 to 2005 has a range of 298-324 lbs., with an expected average of 311 lbs.

So if we've increased cow slaughter weights by 311 lbs., what is the average live slaughter weight of those cows? By dividing the cows' average carcass weight — 475 lbs. in 1975 and 621 lbs. in 2005 — by 47%, we approximate the live weights at slaughter, which is 1,010 lbs. and 1,321 lbs., respectively.

A 1,321-lb. cow at slaughter is a big cow, but that's not her mature weight. The mature weight for cows is established at a constant BCS of 5.0. Because the BCS was estimated at 4.5, the average slaughter weight must be adjusted to a BCS of 5.0 to determine her mature weight. According to National Research Council guidelines, the factor we use to make that calculation is .965. Therefore, the cows slaughtered in 2005 with a BCS 5, on average, would have a mature weight of 1,369 lbs. (1,321 lbs. / .965 = 1,369). Assuming the BCS and dressing percent averages were the same for the 1975 cows, that's a mature weight of 1,047 lbs.

Thus, over the course of 30 years, we've increased mature size of our cows by more than 300 lbs. While some of our assumptions can be challenged, and reasonable challenges may impact the outcome by up to 30 lbs., it's possible that the average mature weight of cows in the U.S. today is about 1,350 lbs.

This is important because a 1,300-lb. cow can be a productive cow in some environments, but not in all environments. The more important issue isn't so much the average itself but the distribution around that average.

If the mature weight of our cows is a normal distribution around the average, then by definition half weigh more than 1,350 lbs. That's where the problem lies in terms of production efficiency.

That leads us to an even more important question. How has this increase in mature weight impacted production efficiency for cow-calf producers?

See Associated Figure 3

Bryan McMurry, Ph.D., is beef brand manager for Cargill Animal Nutrition, Minnetonka, MN.