Heat stress affects cows in many ways. While the visible discomfort can start at moderate temperatures around 22 degrees Celsius, other effects that are not directly visible can start at even lower average temperatures. In this article, we explore the various effects of heat stress with the help of a research report by the University of Wisconsin.
Indicating the effects of heat stress with the Temperature Humidity Index (THI)
According to the report, heat stress occurs when a cow produces more heat than it can passively release. To indicate when different heat stress effects occur in cows, researchers use the temperature humidity index (THI). The index combines ambient temperature and relative humidity (RH) to generate a score. You can then identify different stages of heat stress based on the scores. Both a high temperature and a high relative humidity can make heat dissipation more difficult for dairy cows. With a relative humidity of 100%, no heat can evaporate, allowing heat stress to occur at lower temperatures.
Reduced feed intake (dry matter intake) and milk yield
One commonly studied effect of heat stress in dairy cows is the reduced food intake and milk yield. Cows will sweat and breathe more and stand for longer periods throughout the day. This disrupts rest patterns and leaves less energy to produce milk. In addition, a cow will eat less and reduce the amount of milk it produces to generate less heat. This results in a double impact on the feed efficiency and milk production.
Studies show that the modern threshold for the optimal milk yield of a dairy cow is around a THI score of 68. Above this threshold, milk production will start to decrease. The University of Wisconsin demonstrated this effect using a table with the average loss in milk production per cow per day. On a day with a max THI score of 75, 2.5 pounds (1.1L) of milk production could be lost per cow per day. This calculation takes into account that the THI remained above the 68 threshold for 8 hours. Reducing the exposure to heat stress to only 2 hours in turn would result in a 50% lower loss of milk of around 1 pound per cow per day (0.5L). To see the formula and research results used in this example, view the full paper here.
Besides a direct effect, heat stress will also decrease the milk production of a cow in the days after. “In a 2011 study conducted at the University of Florida, a group of cows subjected to heat stress during their dry period produced an average of 11 pounds (5L) per cow per day less during the subsequent lactation than those that received cooling.” Source: Dairy Cooling: The Benefits and Strategies
Increased time between calving and conception
The fertility of cows is affected during heat stress as well as in the months after, causing the rate of successful inseminations to decrease which then increases the time between calving and conception. This further lowers overall milk production and increases breeding costs for the farmer.
Decreased weight of calves
Heat stress does not just affect the cow itself—it also affect its calves. Researchers have found that calves born to heat-stressed cows weigh less than those whose mothers had received cooling. “The difference in weight continues through the weaning period, after which the calves from heat-stressed mothers average 322.8 pounds (146kg) compared to 340.8 pounds (154kg) for calves whose mothers had received cooling.” Source: Dairy Cooling: The Benefits and Strategies
Sub-acute ruminal acidosis (SARA)
Heat stress can also increase the risk of Sub-acute ruminal acidosis (SARA). In sub-acute ruminal acidosis, bacteria in the cow's rumen produce more lactic acid than normal. This causes problems with the cow's digestion and absorption of nutrients, which can lead to weight loss and dehydration. The increased amount of lactic acid also affects the body in ways that may cause more serious health issues.
Increased risk of lameness
When cows are heat stressed, they stand more as it allows them to breathe more easily and release more heat. This causes additional strain on the feet and legs, which can increase the risk of lameness and laminitis. Laminitis is a condition that affects the hooves of cattle and is more common in cows who suffer from rumen acidosis induced by heat stress.
Conclusion
The additional costs for a dairy farmer due to heat stress can pile up quickly. From a loss of feed efficiency and milk production to increased veterinary, breeding and culling costs. If you would like more information on this, The University of Wisconsin covers this in more depth in their report. Luckily, there are several ways to prevent heat stress from happening in the first place. Two ways to reduce the risk of heat stress are by providing fresh air through ventilation and creating a wind chill effect with circulation fans. We cover these topics in our other articles. If you have any questions concerning fans or ventilation for dairy cows, feel free to contact us.
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