The Temperature-Humidity Index (THI) is a measure that combines the temperature and humidity of an environment. This allows us to assess the stress put on livestock due to the conditions they are living in. THI is a more reliable indicator of heat stress risk than just using temperature alone. It is a useful tool to allow farmers to identify which sheds, or areas of the farm, are not providing an optimum environment. When the THI exceeds 70 the cows’ ability to cope is put under strain and as this index increases, the animal’s production and ability to thrive diminishes. When the THI exceeds 78, cows’ milk production is seriously affected and above 82, very significant losses in milk production are likely.
It is not only dairy cows where this is useful – calves that are in a shed with a high THI will be less likely to cope with a bacterial or viral challenge, and their immunity and health will struggle. This may result in stunted growth and long term health issues.
The best method to improve humidity levels is to provide adequate drainage combined with and maximising air flow, with a large outlet at the highest point of the shed and plenty of inlet space. If this does not produce enough air flow to adequately reduce the THI, then fans could be installed and insulated roofs can help further control temperature.
Humidity meters that measure THI can be purchased from Amazon for £20-30. I have been checking cow cubicle sheds during May with my meter, and several places have had THI over 78, one reached 90. At this level the risk of toxic mastitis is very high as beds will be the perfect breeding ground for pathogens. Cows feed and water intake will be reduced as they stand looking for the coolest place.
Sometimes a fan can appear to be working because it produces a breeze but just make sure this breeze utilises fresh air not recirculating old warm damp air. If fans are used they need to be pushing fresh air into the sheds or forcing old air out. Cows have a natural desire to find a dark place when it’s hot (when outside this is sensible because it’s usually under trees and near streams) but inside these dark places are usually in the middle or a corner of a shed which has the worst air flow.
Don’t forget your in calf heifers and dry cows out at pasture as these can be hot unrelenting places that may not have enough shade for all the stock in the field. Are they being forced to gather under one tree or are they just standing trying to keep cool and not eating?
My advice would be to buy a meter and start checking your sheds and fields!
Written By Jenny Clayton, Veterinary Surgeon
