Hot and Bothered?

Posted by rebecca on Jul 16, 2013, 11:19 AM

As we all enjoy a bit of hot weather and fantastic conditions for getting silage in and hay made, spare a moment to consider the impact on your cows.

Heat stress is defined as when the Temperature Humidity Index (THI) is at or above 72. Severe heat stress occurs when this is above 90. The chart below gives temperatures and humidity levels and the associated THI. Foe example, 26 degrees celcius and 72% relative humidity results in a THI of 75.6.


Data taken from showed that this week, Preston had these conditions, with the humidity having been above 80 a few days before. So, we do achieve heat stress conditions quite easily.

In lactating cows, heat stress has been very well researched. It will decrease dry matter intakes (by up to 35%), lower milk yields 1-6 litres (more than the drop in intake would predict), increase disease rates and impair fertility (eqq quality greatly lowered).

For late lactation/dry cows there is also a knock impact to the calf, with depressed growth of the calf pre-calving and a reduction in growth through to weaning. The calves will also have impaired immune function and greater disease rates. The mammary gland of the cow will also be slower to generate under heat stress conditions prior to calving.

What can you do?

Lots of shade! and plenty of fresh, clean drinking water (cows will drink up to 40% more in heat stress conditions).

Lower the fibre intake, as higher fibre rations increase the dry matter intake effect. Consider using Yea-Sacc yeast as this will help rumen function.

Feed FiMLAC diets, especially to dry cows, to ensure the best mineral package to support immune function.

Consider allowing access to buffer feed (if you have it) at cooler times of the day

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