THE MODERN dairy cow is highly susceptible to the effects of heat and in the Australian climate, the effects of heat stress cause millions of dollars in lost production every year.
Temperature and humidity that exceed certain thresholds lead to significant declines in milk production, feed intake and fertility.
These problems can be exacerbated in grazing cows because the opportunity for providing shade, sprinklers and fans is often limited to when the cows are at the dairy.
As a consequence, Agriculture Victoria researchers have been working to identify and breed heat-tolerant dairy cattle.
The Genomic Estimated Breeding Value (GEBV) for heat tolerance in Australian dairy cattle was developed by a DairyBio research team led by Thuy Nguyen at the AgriBio Centre for AgriBioscience, validated by an Agriculture Victoria research team at Ellinbank led by Bill Wales, and implemented by DataGene.
The validation involved selecting heifers that were predicted to be either heat-tolerant or heat-susceptible, and exposing them to a mild-moderate heat challenge in a controlled-climate environment.
“We wanted to mimic the daily fluctuation in temperature and humidity that would occur during a mild to moderate heat event in the temperate climate,” Josie Garner, a member of the Agriculture Victoria research team at Ellinbank, said.
“Therefore we set the temperature to fluctuate between 25°C at night and 33°C during the day, with 50 to 60 per cent relative humidity,” she said.
“We also wanted to simulate the fact that heat events in the temperate environment are commonly sporadic, sudden and short-term, resulting in large temperature increases from generally mild summer conditions.
“The duration of these heat events is commonly four consecutive days, so that is how long our imposed heat challenge lasted.”
The consecutive days are important because they minimise the opportunity for the cows to dissipate body heat and, as a result, the increases in body temperature can accumulate from day to day.
During the 32-day experiment, 24 heifers that had been identified as heat-tolerant and 24 heifers that had been identified as heat-susceptible were exposed to the controlled heat challenge and their production, physiology and metabolism responses were measured.
“What we found was that the genomically-selected heat-tolerant cows maintained higher milk production and feed intake during the heat challenge,” Ms Garner said.
“They also maintained a core body temperature that was 0.6°C lower than heat-susceptible cows — a remarkable finding.
“A proportion of the milk yield difference was directly related to the reduced feed intake, but the experiment also demonstrated thermoregulatory differences between heat-tolerant and heat-susceptible cows.
“The heat-tolerant cows were able to dissipate heat more efficiently via evaporative cooling. For example, they had a skin surface temperature that was 1.7 °C higher, which indicates greater heat dissipation through the skin surface.”
Overall this experiment demonstrated that genomic selection for heat tolerance is a promising option for increasing the resilience of the Australian dairy herd in the face of the increasing threat of heat stress.
This research was funded by the Federal Agriculture Department, Dairy Australia and the Victorian Economic Development, Jobs, Transport and Resources Department.