Idaho Study Looks At Why Cattle Graze Where They Graze

A new study led by the University of Idaho sheds light on why some cattle graze hillsides on during hot summer days while others stick close to shade and water.

Researchers found cattle that convert feed into body mass more efficiently are better able to handle heat while grazing rangeland.  Less efficient cattle are more likely to stay in cooler lowland areas, where they may trample sensitive streamside habitats.

“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.

During cooler June conditions, all the cattle grazed hillsides. But in August heat, efficient cattle continued grazing higher terrain, while inefficient animals stayed near shade and water.

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 study was published in the Journal of Animal Science in December.

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