Study shows heat stress challenges increasing in Ontario dairy cows

Data suggests Quebec cows have been less affected than those in Ontario

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Ongoing research aimed at identifying genetic determinants for heat stress tolerance in Canadian cows has, as a side effect, turned up interesting comparisons between the long-term impact of rising average temperatures on dairy farms in Quebec versus Ontario.

According to analysis conducted by Masters student Ivan Campos at the University of Guelph, decreases in milk production as a result of heat stress have been less severe in Quebec since 2008, compared to Ontario.

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Campos outlined preliminary details of his work at Lactanet’s annual Open Industry Session, which took place in Guelph in early March.

Why it matters: As average temperatures rise and instances of extreme heat and humidity become more frequent, the ability for dairy farmers to breed for heat tolerance would lessen the impact on their herds.

In a follow-up interview with Farmtario, Campos explained his work – being conducted through University of Guelph’s Centre for Genetic Improvement in Livestock – will “contribute towards the long-term goal of developing novel breeding programs to select naturally better-adapted livestock that can maintain robust fitness and production in the face of shifting environmental conditions.”

Continuous records for milk yield, fat and protein were provided by Lactanet for almost 1,400 herds between 2008-2018. All farms were located within 20 km of one of the 58 weather stations from which continuous heat and humidity data were also collected.

Campos says there’s clear evidence showing that heat tolerance declined over the decades in which production was a focus for dairy cattle genetics. Increased milk yield, he explained, “resulted in increased metabolic heat production and, consequently, greater susceptibility to thermal challenges.”

But while the main goal of his current work is to explore the genetics of cattle that are tolerant to heat and humidity, those 11 years of data told an interesting story about farms on both sides of the provincial boundary.

“Based on our findings, Ontario had greater decline in the three production traits compared to Quebec,” he explained. “This indicates that cows in Quebec are less sensitive to the effect of heat than cows in Ontario.”

Data collection did not include heat mitigation measures, stocking densities or types of barns, so Campos is unable to declare why this is true.

However, he said tie-stall barns predominate in Quebec on the whole, compared to a high proportion of freestall barns in the Ontario industry. In tie-stalls, the potentially damaging effects of overheating are hard to ignore, while in a freestall setting farmers might take a false level of comfort that cattle will move to a cooler part of the barn – which might not necessarily happen. It’s generally thought that freestall barns have better ventilation.

In Quebec, Campos mused, “farmers would have more motivation to implement mitigation practices, which include cooling systems.”

He added it’s unlikely that Quebec cows are simply more heat tolerant, given the interconnectedness of the two provinces when it comes to commonly used dairy sires.

According to Campos, one possible next step towards understanding why there’s a difference is to do a similar study taking into account the microclimate of barns. A project at the University of Guelph is using a different source of climate data to track heat tolerance. All of this work would ultimately go towards identifying genetic markers for heat tolerance.

“Once breeding values are calculated with the best model, the next step will be to calculate the economic value of heat tolerance to properly incorporate (the parameter) into the breeding goal, likely through a selection index.”

About the author


Stew Slater

Stew Slater operates a small dairy farm on 150 acres near St. Marys, Ont., and has been writing about rural and agricultural issues since 1999.



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