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Looking at size, Wyoming studies leaning toward smaller to moderate size cows

by Wyoming Livestock Roundup

Torrington – Despite a sharp jump in cow size, rangeland in Wyoming has still stayed in consistent condition, according to University of Wyoming (UW) Extension Rangeland Specialist Derek Scasta. Scasta spoke about how cow size impacts Wyoming rangeland during the recent Southeast Wyoming Beef Production convention in Torrington. 

In 1975, the average beef cow in the U.S. weighed 1,000 pounds, which has become the range management standard for calculating animal unit months (AUM). 

Increasing size

However, recent data suggests the average beef cow now weighs 1,400 pounds. 

“In 2010, 16 percent of the U.S. beef cows were more than 1,500 pounds,” Scasta said. “That’s millions of beef cows that weigh more than 1,500 pounds on range and pasture in the U.S.”

Despite a more than 400-pound increase in cow size in the last 40 years, Scasta says no evidence exists to suggest that increase has resulted in weaning larger calves.

“We have enhanced the production and performance potential of cows, but we may not be realizing that in terms of calf weaning weight,” he said.

The expected progeny difference (EPD) for yearling weight has increased 100 pounds in the Angus breed, which basically shows ranchers have been selecting for growth in cattle. In 1985, the average carcass weight was 725 pounds, and in 2015, it was 892 pounds, which is 165 pounds larger. 

“Cattle are basically 20 percent heavier than 35 years ago and 10 percent heavier than 15 years ago,” he explained. 


With that amount of growth has come some negatives in relation to animal welfare. Cattle pots were originally designed to haul smaller cattle.

“With these bigger cattle, a lot of them will bump their back going into that lower deck, which leaves a bruise on their back, leading to a cut out. It costs the industry $35 million a year because the cattle are bigger today than what the trailers were originally designed for,” Scasta said.

Milk production

It is not just a matter of muscle growth. Ranchers have also selected for milk production. 

“As we have enhanced the performance of our cattle, what has been happening to rangeland? Actually, rangeland has stayed pretty flat despite the production potential of cattle increasing. We have managed to optimize what we get from the range, and it has stayed pretty consistent over time,” he said. “Ranchers have done a good job of matching their cattle genetics with range productivity.”

Optimal size

Scasta said there is a lot of disagreement over optimum cow size. 

Some studies suggest smaller cows are better because of live weight production and income, while others find larger cows to be more efficient because they have a larger rumen, which could be an advantage for the efficiency of processing low quality forages. 

A lot of the data available comes from feeding trials, where they did a lot of modeling, Scasta said. 

“What I found was a lot of mixed studies and a lack of information in Wyoming,” he said.


One study he shared that was published in the Journal of Animal Science studied how cow size impacts calf weaning weights relative to precipitation extremes. The four-year study involved 80 cows grazing rangeland northwest of Laramie. 

The study showed that during the driest years, the larger cows had an advantage, and the smaller cows weaned lighter calves. However, the results were opposite during wet years and variable during average years. 

“Taking the average of all four years into account, they found no significant difference in terms of cow size class,” Scasta explained. “Smaller cows weaned calves statistically similar to those weaned from the bigger cows, riding the roller coaster of wet-dry-wet-dry.”

Calculating the input-output ratio, which is the pounds of grass consumed relative to the pounds of calf weaned, the smaller cows were weaning similar size calves across all wet-dry cycles, Scasta said, noting smaller cows also eat less because their nutritional requirements were lower. 

A 1,000-pound cow consumed 7.5 pounds of grass per pound of weaned calf, according to the study. For a 1,200-pound cow, that number jumped to 8.5 pounds, and for 1,400-pound cow, it was 9.5 pounds. 

“Basically, the larger cows had to eat more per pound of calf weaned,” he said. “Most ranchers have an efficiency target for the cow weaning a calf that is at least 50 percent of the cow’s body weight.” 

“So, a 1,000-pound cow should wean at least a 500-pound calf. In this study, the smaller cows were the only ones to reach that target,” Scasta said.

More data

In another study, Scasta worked with a Wyoming ranch to analyze 8,000 cow/calf records with 13 years of data to determine which cow size is most efficient. The cow size on this ranch varied from 800 to 1,600 pounds, but the majority of the cows weighed 1,100 to 1,300 pounds, Scasta noted.

From this data, Scasta found that the smaller to moderate size cows were closer to hitting the 50 percent cow size to weaning weight target, compared to their larger counterparts. 

“The 1,600-pound cows were actually pretty inefficient for the amount of grass they eat,” he said. “I think the data indicates managing for moderate size cows and to not let them get bigger over time.”

For some ranchers, it is easier said than done, Scasta admitted. He shared a story about his grandfather, who struggled to keep his cow size steady. 

“What we have is a lot of information to go through. When my grandfather would go to a bull sale, he was looking for EPDs for low birthweight and higher weaning weight, but he may have ignored the maternal traits and then kept the higher end of the heifer calves for replacements,” he explained. 

Looking at the bull’s maternal EPDs will indicate how the heifer calves will look. The bull may have had a positive EPD for milk and mature size, producing larger daughters. 

“That is why you really need to sort through the bull catalog and look at those EPDs,” he adds. 

Gayle Smith is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to

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