Heat stress, also known as ‘hyperthermia’ is when a dairy cow's heat load is greater than their capacity to lose heat. It’s a common issue faced by dairy farmers in summer and we have obligations under the Animal Welfare Act 1999 to provide certain things to help protect cows against it. So what are some ways to deal with it? And how do you spot it in the first place?
Heat stress happens when both high air temperatures and humidity are present. The temperature heats them up and the high humidity decreases evaporation, which in turn lowers a cow’s ability to control their body temperature by sweating and breathing.
It’s not just a daytime issue either. During the night the cows radiate heat making their supposedly cool surrounding warm up. Combined with humidity and cloud cover this can exacerbate heat stress.
It’s actually a combo of both air temp and humidity. This is measured with a THI – temperature-humidity index. When it reaches 68 for Friesians and 75 for Jerseys (equivalent to 21 and 25.5°C respectively at 75% relative humidity) the effects of heat stress start to kick in. Use DairyNZ’s calculator to work out THI and help manage heat stress.
When air temperature reaches 23 or greater and the relative humidity is over 80%, cows eat less and therefore produce less milk. On average that drop is around 10g milksolids per day per unit increase in THI. As an example, if THI went up 10 units that would equate to a 100g drop in milksolids. There will also be a change in milk composition such as declines in the fat and protein. Their water intake goes up as their feed intake goes down and you might also notice heavier or faster breathing and more sweating. They may even suffer from a change in blood hormone concentration, e.g. increased prolactin.
If you notice your herd seeking shade, crowding together (especially around or in water sources), eating less and refusing to lie down – they are suffering from heat stress.
The obvious and easiest is to provide them with lots of clean drinking water. A lactating cow needs over 100L a day consumed typically in 2-6 sessions.
Use sprinklers while the cows are in the yard. Be aware though, while this can be a temporary aid cooling the cow down through evaporative cooling for a few hours, it can also increase the humidity around them, especially if they’re close together. Pair sprinklers with a fan to create air movement.
Water solutions are all well and good, however, during summer many regions experience drought or near-drought conditions and water saving measures are crucial. So what else can be done?
The pasture eaten at the peak of summer needs to be of high quality. If it has too much fibre it will increase the heat of fermentation in the rumen, thus increasing the heat load on the cow. The supplementary feed can be dished out when it is cooler at night.
Try to use paddocks that have shade-bearing trees so the cows can get out of direct sunlight. Another option is to have an indoor option like a barn or shed. Dairy Barn System's dairy sheds aren’t just beneficial in winter as an off-paddock solution. They provide shade and a comfortable place to rest for the herd. You’ll also have the benefit of being able to better monitor and manage their feed and water intake and general health and stress levels when they are in the shed.
If possible, shorten the distance the cows must walk to the dairy, or at least let them take it slow. Minimise handling and time spent in holding yards as this adds to the stress.
If you notice certain animals struggling more than others you could manage them separately with later afternoon milking times and more shade.