Dairy cow heat stress affects almost 40 per cent of days

Glacier FarmMedia – New research continues to qualify the cost of heat stress on dairy cattle.

Impacts, from a decline in milk and components production to effects on reproduction, mean that heat stress costs farmers money.

If temperature trends continue, heat stress could cost farmers about $9 per hectolitre by 2049, says Veronique Ouilet, an assistant professor at Laval University.

That’s double today’s costs per hectolitre for heat stress.

Heat stress is not just something for arid or tropical climates, Ouilet said through a translator at a dairy research symposium put on by Agriculture and Agri-food Canada’s Sherbrooke, Que. research station. Research over the past decade on dairy farms in Ontario, Quebec, Wisconsin and Germany shows that heat stress is problematic for temperate climates as well.

Studies have shown that heat stress increases inflammation in cows, affecting blood movement through the cardiovascular system. In an attempt to dissipate heat, blood is moved closer to the skin. This has a negative effect on the cow’s deep organs, including damaging the epithelial cells of the intestine.

Heat stress also leads to a change in metabolism, driven by an increase in insulin.

Changes in the body of cows from heat stress results in an extended response, with the effect of heat stress showing up into September and October in decreased butterfat production, long after the hottest part of the season, says Ouilet.

The weather extremes in eastern Canada drive the challenge with heat stress, especially humidity.

Tie stalls, with lower ceilings keeping the cows from moving, continue to be the housing on the majority of Quebec dairy farms.

A survey of herds found that some had extra fans and a few had water-based cooling, but most farms weren’t taking significant steps to cool their cows.

A study Ouilet worked on looked at the Temperature Humidity Index (THI) and compared the THI from weather stations near farms with what they found in barns.

“There’s a significant risk that we underestimate heat stress, since the average THI in the barns was significantly higher than what we obtained from the weather station,” she said through a translator.

Her research confirmed that a THI of 65 resulted in a drop in production – and in southwestern Quebec the THI was over 65 on 136 days in a year and on 100 days in a year in eastern Quebec. It doesn’t take long to have an effect as the research showed that one day over 65 THI resulted in a decline in milk production.

The next step in Ouilet’s research is looking at milk constituents and heat stress which is of interest to processors.

They’ve separated the research barn into two areas, one with cows who are exposed to heat stress and another where cows are cooled by fans.

They are monitoring temperature, speed and humidity of the air and the animal’s physiological response, including temperature, respiration rate and lying behaviour. They’re also taking blood samples and milk samples, and are evaluating the fatty acid, milk protein and mineral profile of the milk.

To manage heat stress farmers should install fans and misters, but they can also concentrate on helping cows manage their heat stress overnight. If they can recover normal levels from 8 p.m. to 6 p.m., cows handle the next day better if it is hot.

The biggest effect of heat stress can be found earlier in the season, says Ouilet. Her research showed that early-season heat events mean cows will take longer to return to normal behaviour.

– This article was originally published in the July 7, 2025 issue of Farmtario.