By Maile Matsimela
In a groundbreaking review of heat stress in dairy cattle, researchers L.M. Erasmus and E. van Marle-Köster from the Department of Animal Science at the University of Pretoria, have highlighted critical challenges facing dairy producers in subtropical climates. Their comprehensive analysis reveals that as global temperatures continue to rise, dairy farmers must adopt more sophisticated approaches to managing heat stress in their herds.
The review emphasises that dairy cows are particularly vulnerable to thermal changes compared to other livestock species. This sensitivity has profound implications for South African dairy farmers, as the region is experiencing temperature increases at rates faster than the global average.
“Dairy cows today are showing signs of heat stress at lower temperature thresholds than previously thought,” explains Erasmus. “This challenges our traditional understanding of when to implement heat management strategies.”
Heat stress manifests in various ways, including reduced milk production, decreased fertility, altered behaviour, and compromised welfare. The researchers found that these effects are particularly pronounced in high-producing cows, which generate substantial metabolic heat as a byproduct of milk synthesis.
One of the most significant findings from the Erasmus and van Marle-Köster review is the limitation of the widely used temperature-humidity index (THI) as a sole predictor of heat stress. While the THI has been the industry standard for decades, the researchers argue that it fails to account for critical factors such as solar radiation, wind speed, and individual cow characteristics.
“The THI was developed in the 1950s and doesn’t reflect the reality of modern dairy farming,” notes van Marle-Köster. “Today’s high-producing cows begin experiencing heat stress at a THI of 68, much lower than previously established thresholds.”
The researchers recommend the adoption of more comprehensive thermal indices that incorporate additional environmental variables and cow-specific factors. These improved metrics would allow farmers to more accurately predict heat stress events and implement timely interventions.
The review also highlights the importance of biotic factors in determining heat stress susceptibility. Breed differences play a crucial role, with Holstein cows generally showing less heat tolerance than Jersey cows or crossbreeds. Hair coat characteristics, body size, stage of lactation, parity, and previous heat exposure all influence how individual animals respond to thermal challenges.
Of particular interest is the identification of the SLICK1 gene mutation, which confers enhanced heat tolerance. Cows carrying this genetic trait demonstrate superior thermoregulatory capacity, maintaining productivity under conditions that would cause significant stress in non-SLICK animals.
“The genetic component of heat tolerance offers exciting possibilities for breeding programmes,” explains Erasmus. “By selecting for naturally heat-resistant traits or incorporating beneficial mutations like SLICK1, we can develop herds that are better adapted to warmer climates.”
South African dairy production relies heavily on pasture-based systems, which present unique heat stress challenges. Cows on pasture are exposed to direct solar radiation and must walk considerable distances to graze, generating additional heat through physical activity.
The researchers note that shaded areas are essential in pasture systems but are often insufficient. They recommend strategic placement of shade structures, water sources, and feeding areas to minimise walking distances during the hottest parts of the day.
“Pasture-based systems require special attention to landscape design,” says van Marle-Köster. “Simple modifications, such as providing adequate shade near water troughs, can significantly reduce heat load.”
The review outlines several practical strategies that dairy farmers can implement to mitigate heat stress:
- In the short term, physical interventions such as shade structures, fans, and sprinkler systems can provide immediate relief. Proper ventilation in housing areas is crucial, as is the use of reflective materials to reduce heat absorption.
- Nutritional adjustments also play a vital role. Feeding during cooler parts of the day, ensuring adequate water availability, and formulating diets with lower heat increments can help cows maintain energy balance during hot periods. The researchers specifically note the importance of phosphorus supplementation, as deficiencies can exacerbate metabolic stress.
- Long-term strategies focus on genetic selection and breeding programmes that enhance heat tolerance without sacrificing productivity. Crossbreeding with heat-tolerant breeds and selecting for beneficial traits like the SLICK1 gene mutation offer promising approaches for developing more resilient dairy herds.
As climate change continues to impact global agriculture, the researchers emphasise the need for continued research and innovation in heat stress management. They call for closer collaboration between scientists, industry stakeholders, and policymakers to develop sustainable solutions that balance productivity with animal welfare.
“We need to think beyond traditional approaches,” concludes Erasmus. “The future of dairy farming in subtropical regions like South Africa depends on our ability to adapt to changing environmental conditions while maintaining efficient production.”
