New research shows how cattle are coping with searing summertime heat while grazing on rangeland, which could help prairie ranchers better understand how to manage their herds in a warming climate.
looked at their activity during the , when temperatures repeatedly rose to as high as 35 C in the province. The researchers found that during heat waves, cattle changed how much they walked and when they were active, focusing their movement during specific times of the day.
The discovery points to “marked behavioural shifts in grazing cattle when there is heat stress,” says , a professor of rangeland ecology and management in the Faculty of Agricultural, Life & Environmental Sciences (ALES), and a co-author on the study.
“That’s important to understand as extreme temperatures in Western Canada become more common under global warming.”
Heat stress can alter a cow’s physiology so they burn more energy trying to keep cool, and they may stop eating, lose weight, and produce less milk for their calves.
To measure how extreme weather conditions could affect animals in a northern rangeland like Alberta, the study monitored heifers and young cows with their calves as they grazed on the . Cattle were fitted with leg-mounted pedometers, and their movements, including their steps and time spent lying down, were monitored 24 hours a day while they were on pasture from June to August, and again from August to November.
The researchers assessed the levels of thermal stress the cattle experienced using an integrated measure of heat and humidity known as the Temperature Humidity Index (THI), which has been shown to affect cattle health when it exceeds 68. Calculations from the Kinsella ranch showed that the cattle were subject to average daily values above 68 on at least 16 days during summer, from 8 a.m. to 11 p.m.
When the index scores were broken down hourly, cattle were exposed to that threshold almost around the clock, from 7 a.m. to 2 a.m. over the summer. And though weather conditions in the fall typically lead to much lower risk of heat stress, there were still short periods within the day, especially in September, when the THI threshold exceeded 68.
Cattle showed two distinct changes in activity during periods of high THI: They unexpectedly increased their overall movement, indicated by step counts, during times of heat stress. They also changed when they moved, markedly increasing their step counts at night and midday, and spending more time lying down in the morning and afternoon.
“Taken together, these results indicate it was too hot during much of the day to graze, so cattle instead fed extensively at night to achieve gut fill,” Bork notes. Step counts jumped again at noon — despite being one of the hottest parts of the day — likely because the cattle had rested, digested their food and wanted to eat again. The researchers theorize that cattle also had to move to drinking water to avoid dehydration. Shortly after drinking water, cattle sharply decreased their activity throughout the afternoon — the hottest part of the day — before returning to high activity overnight.
Along with the impact on their health, the energy cattle expended to stay cool could make them less efficient in terms of production, Bork notes.
“If cattle are in a large pasture and they have a long way to walk to shade or water, it’s going to be more physically demanding for them. That comes with a cost,” he says, adding that producers should provide reliable access to forests for protective cover along with ample drinking water to ease the effects of extreme heat.
“These findings highlight the impact of heat stress in altering cattle behaviour and the importance of mitigating the risks of heat stress, even this far north in Alberta,” Bork says. “Aligning cattle needs with resource availability is a key step in making sure sustainable beef production isn’t compromised by climate warming.”
The study’s co-authors include 海角社区 researchers , , , graduate student Temitope Oloyede, postdoctoral fellow Francisco Novais, and John Church from Thompson Rivers University.
