By Lorna MacPherson, dairy consultant SAC Consulting

The recent warm spell of weather may have been welcomed by farmers but perhaps not so much by cows housed throughout the summer months.

Cows prefer cooler temperatures, with their thermal comfort zone ranging from -15C to 25C. Once temperatures exceed 25C, they will adapt their behaviour to minimise heat production and increase heat loss.

It is not just high temperatures that are the problem. There is a close relationship between temperature and relative humidity: the higher the relative humidity at a given temperature, the greater the risk of heat stress. This relationship is described in the Temperature Humidity Index (THI) table below.

Cows will experience moderate heat stress when the THI reaches 72. This can occur at temperatures as low as 22C, if the relative humidity is at 90%. In the UK, relative humidity can easily climb above 80% during the summer months, meaning that heat stress is a real threat to cow welfare and productivity.

Heat stress causes an increase in the stress hormone, cortisol, which can depress feed intake and reduce milk yield in the region of 5% to 15%. Furthermore, cortisol is an immune suppressant, meaning that cows are more susceptible to disease. Warmer temperatures and high humidity are the perfect combination for rapid growth of pathogens, greatly increasing the risk of udder infections and clinical mastitis.

Feed intake drops within one day after the initiation of heat stress and the maximum reduction in milk yield tends to occur 48 hours after the onset of heat stress. Fertility is also likely to be affected, with cows exhibiting oestrus for shorter periods and weaker signs, along with increased embryo loss.

Cows will tend to sort their ration more, favouring concentrates over forages, as forages produce more heat from fermentation in the rumen. This can increase the risk of acidosis, which is further compounded by a reduction in both saliva and sodium bicarbonate production which buffers the rumen.

Consequently, a fall in butterfat may also be seen. Research from SRUC has shown that butterfats can drop from 3.85% to 3.48% and milk protein from 3.27% to 3.19% when the THI rises by just 10 points (from 50 to 60 on the temperature humidity index).

Cows respond to heat stress by increasing heat loss through evaporation by panting, drooling and increasing subcutaneous blood flow. If respiration rates are greater than 80 breaths per minute on at least seven out of ten cows, they are likely experiencing significant heat stress.

Whether under grazing conditions or housed, cows will stand longer and spend less time lying down as temperature increases. If heat stress is prolonged, this increase in standing time will increase the risk of lameness.

Several steps can be taken to alleviate the impact of heat stress:

1. Increase nutrient concentration in the ration to compensate for a reduction in dry matter intake but ensure sufficient NDF to reduce acidosis risk and displaced abomasums.

2. Avoid feeding long, stemmy forages as these will be sorted in favour of smaller particle sized feeds. Reduce sorting by using liquid feeds or even water in high dry matter rations.

3. Cows will eat more when it is cooler so alter feeding times, i.e. provide 60% of the ration between 8pm and 8am when temperatures are cooler.

4. Water is critical and intakes may increase by 10 to 20%. Ensure water provision is sufficient so at least 10% of the herd can drink at any one time and provide a minimum of 10cm available drinking space per cow. A cow is unlikely to walk more than 250m for water so make sure that buildings and fields are well supplied.

5. Cows lose electrolytes through sweat, so review mineral requirements. At the very least provide free access rock salt to increase sodium intakes, which will further encourage water intakes.

6. Reduce stocking density in buildings.

7. Improve ventilation/air flow by removing some wooden boarding panels, open up side inlet ventilation and/or ridge outlet ventilation. High humidity levels will be more common in older buildings with lower roofs and poorer ventilation.

8. If possible, have grazing cows in fields that provide some shade and increase water trough access. Consider grazing cows at night and house them through the day.

Invest in a humidity meter. Some versions include temperature and light measurements as well. This will give a good indication of relative humidity levels within buildings and the risk of heat stress can be assessed and acted upon if necessary.