The summer is an interesting time for dairy farmers. We, as people, look forward to the long days, the warmth, the sun. But every dairy farmer will tell you that cows, especially high yielding cows, don’t like the heat. Many outside the industry might think that heat stress shouldn’t be a problem in the UK but in reality, our cows can be suffering heat stress when the temperature is as low as 16-18°C.
Cows continually generate heat through gut fermentation and through the chemical reactions of metabolism. And because milk production requires an increased
metabolic rate, higher milk yields lead to higher heat production by the cow.
Heat stress occurs when heat production by the cow outstrips her ability to offload heat. Cows offload heat by diverting blood flow to the skin where superficial blood vessels can lose heat. They also lose heat through increasing breathing rate. Breathing is cooling because cool air is breathed in and warmed in the lung and breathed out. The effect is enhanced when dry air is breathed in and moist air is breathed out as water in the lungs evaporates, further cooling the animal.
The efficiency of these two processes is affected dramatically by environmental temperature and the relative humidity. High humidity means less evaporation and high temperature means a lower gradient between the cow’s body temperature and the environment. Therefore, we can measure the expected risk of heat stress by measuring the temperature and humidity combined. The Temperature Humidity Index (THI) is increased by increasing temperature and increasing humidity.
Because humidity is so high in the UK, heat stress can begin – particularly in high yielding cows – at quite low temperatures.
(image from NADIS website)The above temperature and humidity readings were recorded in 2009 by The Dairy Group. What is interesting about this
data is how often the humidity in the shed was 100%!
The above study in Belgium showed that heifer yields were negatively affected when the THI was as low as 60. When humidity is 100%, the THI is 62 at around 15°C. When THI is above 72, there will be a significant number of cows in heat stress.
When cows are heat stressed, they eat less, they stand more, they bunch and they get acidosis. The drop in yield is about double what can be accounted for by their drop in feed intake. This is thought to be due to inflammation that is triggered by ‘leaky gut syndrome’ which is a result of heat stress.
How do I know if my cows are heat stressed?
It is suggested that a herd is heat stressed, and would benefit from some form of heat abatement, when > 25% of the cows have breathing rates over 60 Breaths Per Minute (BPM).
How Much Does it cost?
As well as affecting yield, heat stress causes reduced fertility and can even kill cows. Researchers estimate that the real cost is likely to be about double what can be accounted for through lost milk production.
What can Farmers do?
Heat stress affects grazing cows and housed cows. It is essential in hot weather to provide shade for cows. Ideally, cows need 4-6 M2 per head. This will be unachievable on most farms and letting cows rest in the same area for extended periods is a considerable mastitis risk. Therefore, for grazing cows, in extreme weather with high UV index, it is important to give the cows ‘Siestas’. That is, bring them in from about 10 am until after evening milking to rest in the shade of the dairy sheds.
The collecting yard is a high-risk area for heat stress. The best heat abatement for cows in the collecting yard is a combination of soaking cows with large water droplets (low pressure sprinklers) combined with fans that produce wind speeds of > 1m/second throughout the collecting yard. This high air speed should provide relief provided the collecting yard is not so enclosed that humidity rockets.
Reducing heat stress in the cubicle house is best achieved by encouraging cows to lie down. Multiple high-speed fans need to be aimed at the cubicles. Studies have shown that creating air speeds of >1m/s in every cubicle is effective at preventing increases in rectal temperature, preventing standing behaviour
and maintaining milk yield.
Synergy has an anemometer, a tool used for measuring wind speed. If, after investing heavily in fans you wish to test their
efficacy, give us a call and we will come and measure the windspeed in your cubicles.
Written By Graeme McPherson, Veterinary Surgeon
