Heat Stress in dairy cattle

With an increase in herds than are housed or partially-house all-year-round there has been an increased awareness of the problems associated with heat stress in dairy cattle. High humidity and temperature levels in large buildings combined with poor air flow has been blamed for a 5-15% decrease in milk production and lowered conception rates, due to high cortisol (stress hormone) levels depressing intakes.

As with any chronic forms of stress, cows can experience metabolic changes which can result in stress-related illnesses and depressed immunity. This can lead to a lowering of the cow's defences against mastitis-causing pathogens. This, coupled with conditions in cattle housing that may favour the spread of bacteria can lead to an increase in clinical mastitis and raised Somatic Cell Counts.

The most visible symptoms of heat stress tend to be elevated breathing rates in a significant proportion of the herd. The first step in improving conditions for heat-stressed cows is to identify means of improving air flow and ventilation in buildings replacing doors with gates, or taking out alternate boards on spaced boarding are obvious suggestions. As a last resort, fans can be installed where other improvements cannot be made.

Drinking water supply is another essential area in which conditions can be improved for heat-stressed cows; water intakes can increase by up to 10-20% in hot weather, particularly in high-yielders, which are more prone to suffering the effects of heat stress. Overcrowding should be avoided and cows allowed larger than normal space requirements by reducing stocking densities in buildings, if possible.

It may also be wise to buffer feed cows either earlier in the morning or later in the evening when temperatures are lower. Rations should be balanced so that the ingredients most likely to degrade due to the heat are fed as efficiently as possible or replaced by other more suitable products. The mineral content of rations should also be reviewed so that the minerals lost in sweat in hot weather are replaced.

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