Dairy farmers are advised to watch out for heat stress

Heat stress might be something we worry about only in the summer but higher yielding herds in particular, and units where cows are housed full-time to optimise management, are seeing signs of heat stress all year round.

It only needs to be 24°C at 100% humidity for a cow to go into heat stress – and if it is 14°C outside, a shed with poor airflow soon adds another 10°C to the inside temperature. In hot, dry summers temperatures can reach 30°C with 20% humidity before affecting cows. Different combinations can be checked against a chart on DairyCo’s Dairy Wizard.

Heat stress is the discomfort caused by a combination of temperature and relative humidity – known as the Temperature Humidity Index (THI). A moderate temperature, but high humidity such as on a foggy November day, can cause heat stress. Poor ventilation and the trend towards increasing numbers of roof lights have created warmer buildings with high humidity – even in winter.

Heat stress can lead to an immediate drop in dry matter intake and milk yield, and the effects on fertility, production and lameness can last for months.

When cows become heat stressed, they can no longer regulate their body temperature so one of the most obvious signs is panting. Affected cows spend a lot of time at troughs splashing their tongues in the water to try and cool down. Their coats also become wet and sweaty.

Work at the University of Wisconsin has shown heat-stressed cows also stand for up to 50% longer. This has a knock-on effect on lameness as they can develop sole ulcers and longer contact with slurry increases the risk of digital dermatitis.

Increasing airflow by adding fans, removing sides from buildings or reducing stocking density can make a real difference to cows. Think about replacing boarding and breeze blocks with climate controlled blinds, which use heat and humidity sensors to open up automatically.