Prof. Linda Little
November 7 2013
The Effects of Heat Stress on Dairy Cattle
The major effects caused by heat stress on dairy cattle are decreased feed intake, increased respiration rate, increased body temperature, reduced activity, enlarged water intake and increased health problems (Kendall 2007). These effects can cause decreased production in dairy cattle meaning less milk is produced making a smaller profit for the farmer. Farmers want to manage the amount of heat stress on dairy cattle as well as possible to get the best production rate and the biggest profit. Strategies to prevent heat stress on dairy cattle would be to provide them with natural shade or a man-made shade structure and have unlimited supply of water available at all times. Some use sprinkler systems to keep body temperatures down during the hot months (Legrand 2011). The upper limit body temperature of dairy cattle is between 25-26 degrees Celsius (West 2003). If the temperature rises above this, practices should be used to try and reduce it. High humidity levels, combined with high temperatures, is a big problem for farmers in the Southeastern United States. Holstein, Jersey, and Brown Swiss cattle milk yields were shown to be 97, 93, and 98% respectively, of the normal milk yield at a temperature of 29 degree Celsius and 40 % humidity (West 2003). When the humidity was increased to 90%, the normal milk yields dropped to 69, 75, and 83% respectively, of the normal milk yields (West 2003). The best way to cool cows is through an evaporative chilling process. It is when a device is used to lower the air temperature through the evaporation of water (West 2003). With high humidity levels which make the evaporation less effective, it is not a reliable method of cooling to use in the southeastern United States (West 2003). Thermal Gradient consists of these processes of conduction, convection, and radiation; as the air temperature gets warmer and becomes greater, then the critical point on the thermal gradient gets smaller, causing the evaporation cooling system to be less effective.
Nutritional Management for lactating dairy cows in hotter climates is much different than cows in cooler climates. The crops that are able to grow in these different climates make it harder for some farmers to manage nutrition for their herd. There many problems to think of when managing nutrition of dairy cattle in warmer climates and they are: reduced dry matter intake, larger nutrient requirements and providing enough water (West 2003). The most important nutrition for a lactating dairy cow is water. The milk yield and dry matter intake are linked closely to the amount of water a cow drinks in a day. A cow’s intake of water rises by 1.2Kg/ degrees Celsius as the temperature rises (West 2003). No matter the increase in temperature there should always be lots of water available for the dairy cattle in hotter weather. In these conditions nutrient density of the diet rises but in the warmer weather, the dry matter intake decreases (West 2003). To increase the nutrient density of the diet excess protein is added (West 2003). There is a cost with feeding excess protein because it decreases the milk yield and increases feed cost making the farmer’s profit smaller (West 2003).
A cow’s performance can be reduced by a combination of air temperature, solar radiation, relative humidity (Kadzere 2006). Research conducted in the United States studied the effects of heat stress on a cow’s milk production and health. A hundred different counties in the United States were used, with 4.3 million cows in the research. The data determined that the dairy cow spends an average of 47 days a year over upper limit body temperature that is between 25-26 degrees Celcius. High environmental temperatures are linked to a reduction of milk and fat production in dairy cattle. This is caused because heat stress has negative effects on the function of the udder and