Most of the dairy cattle which provide the commercial milk supply in Asia are imported breeds such as Jersey and Holstein. They originated in temperate countries, and their optimum temperature range is 5-23°C. Temperatures in Asian countries are often much higher than this. In tropical countries, temperatures are generally in the thirties, and for short periods may be even higher than this.
The production of milk is directly related to the level of feed consumption. In hot weather, cattle generally reduce their feed intake. It has been estimated that at 40ºC, feed intake (on a dry matter basis) is only about half that eaten by cows living in their optimum temperature range. As a result, milk production falls.
Heat stress is brought about by any combination of high levels of temperature, humidity and solar radiation with little wind, so that temperatures are higher than the animal’s comfort zone.
In the long term, dairy cattle can be made more tolerant to hot and humid weather conditions by selective breeding. Colored breeds such as Jerseys and Brown Swiss seem to show greater tolerance to heat stress. Jerseys are also better producers of butterfat and protein, while needing a lesser quantity of high-quality feed.
In the short term, cows must be helped to withstand high temperatures by modifying their environment. Careful strategies of milking and feeding also help achieve higher milk production in hot weather. For example, cows may be encouraged to graze at night.
While heat stress causes a decline in dry matter intake, the cow’s energy and protein requirements for maintenance and production increase. It is important to increase the energy and protein content of diets, if dairy cows are to maintain their performance in hot environments.
Profuse sweating by heat-stressed cows results in a considerable loss of potassium. The level of potassium, and also sodium and magnesium in the diet should be increased.
Effect of Heat Stress on Reproductive Performance
Studies have shown that under heat stress, the hormonal profiles of the cows are altered. Studies in Hawaii have demonstrated that the levels of estrogen and luteinizing hormones are lower in cows without shade. compared to those which can take shelter from the sun in the shade. The profiles of these hormones also showed a delayed response.
The result was that cows suffering from heat stress had a lower conception rate. And a cow without a calf is a cow without milk!!
Modifying the Environment
The major objective of a cooling system is to reduce the air temperature inside the barn, so as to keep the cow’s body temperature as close as possible to the normal (38.5 – 39.3°C). Evaporative cooling can be accomplished in two ways.
The first is by direct evaporation from the skin surface of the cows. The second is by indirect evaporation, by cooling the air around the cows with cooling pads and fans in an enclosed barn (see below).
In hot and humid subtropical regions, effective evaporative cooling always requires the use of forced ventilation. Sprinkling without fans, or fans without sprinklers, will not be effective.
While there are many methods of reducing heat stress, selection of the most appropriate technique and its proper application is essential. If one method proves successful in one place, this does not guarantee success elsewhere. There are also limitations related to the local climate, the educational level of farmers, and the amount of money farmers can afford to invest.
A combination of fans, wetting, shade and well-designed housing can help alleviate the negative effect of high temperatures on dairy cows. Careful milking management, feeding strategies and sensitivity to animal behavior are also important in achieving efficient milk production in tropical dairy herds.
source:Food & Fertilizer Technology Center, Taiwan