Heat stress negatively affects animals’ performance, including their milk production, growth rate, and fertility. Producers often select their breeding livestock for productivity gains. As many Western regions experience hotter and longer heat waves, both genetic and behavioral traits of heat tolerance increasingly are become culling criteria.

Behavioral responses assist livestock in their adaptation to high heat load. Animals stressed by heat often seek shade, increase their standing time, and lessen their lying down time. They reduce feed intake and defecation. Heat-stressed livestock also increase water intake and drinking frequency, which is coupled with decreased urination frequency.

Heat waves both directly and indirectly affect stock performance. Hot temperatures and drought often decrease the quantity and quality of both forage and drinking water, which leads to reproductive failure, poor growth, and higher disease susceptibility. Producers must consider a whole farm approach to surmount challenges of drought and heat waves. Is there a need for heat adaptation (both behavioral and genetic), improved stock management, or both?

Targeted genetic selection provides animal productivity gains. In some cases though, selection of livestock focused on increased production has resulted in sensitivity to environmental stressors. For instance, heat stresses high producing dairy cows more than low producing dairy cows. While the latter cows produce less milk, they also aren’t as stressed by hot temperatures and carry lower input costs.

“There is constant debate in New Mexico on the subject,” says Dr. Marcy Ward, New Mexico State University Extension Livestock Specialist. “There’s a group of ranchers that really believes in truly a low input animal: low birth weights and small structure. The cattle are not super productive, but they breed every year. Then, there’s the group that stretch more production out of their cows and are successful at doing it.  Angus remain the predominant breed in the state. Producers that prefer Angus often select herd replacements with these two different trains of thought.”

When cattle are managed with adequate feed, heat mitigation, and in an environment with controlled parasite and pathogens, selection for heat tolerance within highly productive breeds may out weigh improving low input breeds. Though, as the New Mexico cattle debate shows, selecting low input cattle for productivity and heat tolerance can work well when cattle are grazed on arid rangelands with variable forage, water and weather. For either approach, there are several methods to evaluate an animal’s performance under heat stress. 

Selecting for heat tolerance

Breeds originated in warm climates often carry adaptive advantages to heat stress. Yet, over time, any breed can adapt to high temperatures through natural selection. Producers selecting breeding stock for heat tolerant behavior and genetics can accelerate this process. Heat tolerant traits include light-colored short and thin hair (called the slick hair gene), lightly pigmented skin, higher density of sweat glands, slender legs, and less subcutaneous fat. The American Angus Association now includes a hair shed score in its EPDs (Expected Progeny Differences) to show the genetic potential of cattle to shed their hair earlier to increase environmental adaptability in heat-stressed regions. 

It is also possible to conduct an on-farm evaluation for heat tolerance. Weather conditions, including temperature and humidity, recorded daily can be merged with animal performance records to quantify livestock’s reaction to heat loads. Other metrics to determine productivity include feed and water intake.

Through a Western Sustainable Agriculture Research and Education ( Western S.A.R.E) grant, New Mexico State University currently researches how water consumption in ranging beef cattle may be linked to genetics. Ward says that feed efficiency is 35% heritable. Her project tests the hypothesis that heritability exists for water intake, too. Ward and her team invented a water system that sets on a scale. Cattle drink individually, and the system records the amount of water drank and the weight of the animal.

 “In a trial with research cattle of various ages – from mature cows to yearlings – on the NMSU campus,” Ward explains, “we found no difference in their water consumption. This surprised me, because I assumed the larger animals would drink more water. Though, preliminary data appears to show that heavier muscled cattle drink more.

“Additionally, we see that cattle – regardless of breeding or age – drink a gallon less per day per animal than conventional estimates. This may be tied to incorrect data on how much water evaporates from troughs.”

Ward’s study is in its first year. Preliminary data shows that water temperature, air temperature and moisture content of forage influences cattle’s water consumption. The project’s final results will be combined with previous research, such as a study that placed tracking collars on cows grazing arid range to record the distances and frequency the cattle travelled to water. That research pinpointed the genetics of the cows that would travel up to two miles to access water every other day.

“The definition of efficiency in a production setting is broad,” says Ward. “It is fairly clear in a feed lot. The cattle eat x pounds of feed to achieve one pound of gain. That is feed efficiency. In production, though, efficiency is often tied to pregnancy rates. Here in New Mexico, we’re in a drought more often than not. A New Mexico cow may not have the largest calf on the market, but she breeds every year under harsh conditions. If she doesn’t eat and drink as much, she’s cheaper to keep as well. Through our study, we want to identify the bloodlines that are water efficient. If a ranch were to isolate and breed for water efficiency, it can lead into reproductive efficiency and feed efficiency.”

In the study’s next step, the New Mexico State water system will be set up on cow/calf operations across the state, to record cows water intake in a range production setting.