A new University of Idaho-led study explores the scientific reason why some cows spend hot summer days lazing along a shady streambank while others brave the heat and graze the hillsides. 

The study suggests cattle that inefficiently convert feed into body mass are more apt to struggle with heat stress while grazing rangeland in hot weather than their efficient counterparts. Consequently, efficient cattle tracked for the study took fuller advantage of the forage throughout their range, including on steep slopes, during hot days while their inefficient counterparts hunkered in lowlands and were more apt to trample riparian areas.  

“There are adaptations that cows take to adapt to their environment, and it’s different between the efficient and inefficient cattle,” said Jim Sprinkle, a UI Extension beef specialist and principal author of the study. “They adapted their grazing differently, and it’s influenced by their physiology.” 

The researchers conducted the experiment in 2016, closely tracking 12 efficient and 12 inefficient 2-year-old Hereford-Angus cross cows on rangeland at U of I’s Rinker Rock Creek Ranch in the Wood River Valley of central Idaho. They followed the cattle over four days in June and for four more days in August. 

Unlike in a feedlot, where ration sizes can be controlled, it’s extremely difficult to estimate how much forage cattle consume on rangeland. The team devised a creative method to quantify rangeland grazing — adding a small amount of a marker molecule to the rumen before starting the trial. The researchers followed each cow and collected its manure throughout the experiment to calculate forage consumption based on the dissipation of the marker in the manure over time. 

To validate estimates from their marker-based approach, they conducted the same analysis with cattle given the same marker and fed a known amount of forage in a feed intake monitoring system known as a GrowSafe unit. Those calculations enabled the researchers to create a table for adjusting forage-consumption estimates obtained from the rangeland trials. 

Cattle in the study were classified as efficient or inefficient as yearlings, based on the amount of feed they consumed in a GrowSafe unit. 

As expected, inefficient cattle spent more time grazing to meet their nutritional requirements. 

“The efficient cows are more purposeful about how they graze. They get in and get it done,” Sprinkle said. 

In June, when forage was lush and easily digestible, efficient and inefficient cows grazed rugged terrain and slopes. In August, however, efficient cattle continued grazing the hillsides, but inefficient cattle stuck to the lowlands, near shade and water. Sprinkle and his colleagues discovered inefficient cows had significantly more undigested forage in their rumen than their efficient counterparts during August grazing, when vegetation was dry and full of less digestible fiber. The inefficient cows were driven to keep eating to gain adequate nutrition and became overwhelmed by the combination of the summer heat and the heat generated inside of their rumen as forage accumulated. 

Based on the study results, Sprinkle advises ranchers who graze on rugged terrain in the summer to consider selecting their replacement heifers with efficiency in mind. 

“The majority of ranchers are not going to test their cows. However, you can buy bulls that have been tested for feed efficiency,” Sprinkle said. “You could do some selection over time to try to get some of those efficient cattle that presumably would use more of the rugged terrain.” 

The paper was published in the Dec. 13, 2025, edition of Journal of Animal Science. 

Source: University of Idaho