Heat stress occurs when heat and humidity combine to affect a cow’s behaviour and result in changes in her metabolism in order to respond to these conditions. Combining temperature and humidity values give a Temperature Humidity Index (THI) score, which reflects the severity of the environmental conditions and therefore the likely effects on cows.
THI values 72 or more are indicative of heat stress affecting animals, although some researchers feel that adverse effects occur at THI values of 68 or more. In this more severe assessment, heat stress can occur at temperatures as low as 22°C, at humidity levels we see in UK summers.
Cows respond to heat stress conditions by eating and ruminating less, especially during the day, and trying keep cool by panting, sweating, looking for shade and drinking more water. They may also spend more time standing. All these behavioural changes aim to either generate or absorb less heat, or allow more heat to escape from the body. Cows suffering heat stress suffer both reduced milk production (both yield and constituents) – by more than 4 litres when THI > 80 – and reduced fertility, with both submission rates and conception rates affected. Reduced intakes are not the sole cause of these signs; there is also a direct effect of heat stress on the body systems.
Another consequence of heat stress is an increased risk of subclinical rumen acidosis (or SARA). This arises for several reasons. Firstly, reduced feed intakes, coupled with the likelihood of a smaller number of larger feeds (as cows tend to eat more at night, when it’s cooler) increase the peaks of volatile fatty acid production in the rumen. This situation is exacerbated if the M/D (energy density) of the ration is increased via sugar and starch to compensate for reduced intakes. Secondly, reduced rumination times means less saliva (containing the buffering agent, sodium bicarbonate) is produced. Cows drooling also ‘wastes’ saliva. Thirdly, less bicarbonate is available to buffer low rumen pH. One of the responses of cows to heat stress is to pant, in order to lose heat. However, this also blows off more carbon dioxide, a condition called metabolic alkalosis. In order to keep blood pH stable, more bicarbonate is excreted leading to a lower bicarbonate concentration in the blood; therefore, less bicarbonate is available to combat the low rumen pH. So, altogether, the rumen is under pressure.