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Justin Sexten

On Target

Finding the ideal mature cow size.

Few topics today are cussed and discussed as widely as ideal mature size for the average beef cow. This is not the first time I have joined in, and it likely won’t be the last, but two recent articles in the Journal of Animal Science provide an interesting platform on the roles of mature size, weaning efficiency and environment.

Paul Beck’s team at the University of Arkansas evaluated the role of mature size on two groups of cows that weighed on average 1,020 pounds (lb.) and 1,258 lb., at variable stocking rates locally. Across the Plains to the northwest, Derek Scasta led efforts at the University of Wyoming to examine five groups varying in average weight 100 lb. from 1,000 lb. to 1,400 lb. on semi-arid rangelands.

As you can assume, these settings stand out for their contrast. Arkansas cattle grazed fertilized, warm-season pastures interseeded with ryegrass, while the Wyoming cattle grazed native range. Stocking rates were markedly different as well, from 140 lb. per acre on the range to 1,273 lb. per acre in Arkansas. Such differences limit our ability to directly compare results, but they reinforce the importance of environmental context to cow size.

With respect to stocking rate anywhere, we have to consider two factors: mature cow size in weight — not frame, because requirements are based on mass, not height — and grazing acres not to include land reserved for haying. While the forage environments were very different in this case, there were several similarities between the trials.

Fortunately for the researchers, though not for ranchers, both experiments spanned the drought years of 2011 and 2012, so widespread as to affect both regions. Drought provided a natural limit for studies of cow size in the context of limited resources. That was not a key focus of the Arkansas work, but the Wyoming group suggested planning the herd’s genetic potential around the possibility of sustained drought from global climate change.

I’ll note that other research has shown virtually no correlation between selection to include superior beef marbling, and any other economically important traits, across the wide range of environments where cattle are raised.

In these trials, the Angus-based herds were evaluated in October for weaning efficiency based on pounds of calf weaned divided by mature cow weight. In both herds, the smallest cows had the greatest weaning efficiency; that is, they weaned more pounds of calf per unit of mature weight, which was no surprise having been reported in other studies.

The different stocking rates evaluated in Arkansas showed increased weaning efficiency per acre as stocking rate increased, regardless of cow size. That may point to short-term opportunities that don’t need to wait for genetic change, but stocking rates taken to extremes require caution to ensure sustainability over time.

In Wyoming, researchers further determined the efficiency of weaned-calf weight relative to forage intake using metabolic animal-unit-equivalent calculations. “Equivalent” is the operative word in that formula, assuming the forage-intake relationship for all cattle is dependent only on mature weight. While mathematically correct to assume a consistent relationship, we know there are genetics that stand out for efficiency and allow ranchers to select bulls with smaller mature size and exceptional pre- and postweaning growth. More and more bulls are also being tested for metabolic efficiency, where residual feed intake evaluates their ability to consume less forage than contemporaries with comparable performance.

The take-home? There is no universal ideal. Wyoming data suggest smaller cows are more efficient in restricted-resource environments. Arkansas data agreed in part, but the greater efficiency in smaller cows there provided no profit advantage over large cows because greater forage resources were available.

This debate on cow size will continue because the cow is responsible for transmitting half of the genetic potential to her calf, and moving beyond the universally ill-defined ideal may increase her nutrient demand beyond what the environment can fulfill.

These experiments evaluated efficiency from the perspectives of weaning weight and cow weight, among many possible “endpoints.” While these traits are easily measured with a scale and recognized by ranchers, they do not represent the true endpoint for any calf in the beef production system, nor do they address genetic opportunities to reduce nutrient demand.

The abundance of genetic knowledge and diversity available within the leading breeds offer commercial ranchers the chance to select for larger or smaller cows that match each ranch environment while ensuring calves carry genetics to supply the increasing demand for premium quality beef beyond the ranch gate.

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Editor’s Note: Justin Sexten is the supply and development director for Certified Angus Beef LLC.



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