Reducing impact of heat stress

The recent hot and dry spell is putting pressure on dairy cows and grass growth as many herds are starting to see yields affected.

Fine-tuning buffer feed can help reduce the impact of heat stress, according to nutrition specialists.

Biotal head of ruminant products Brian Doran says: “Heat stress can have a big impact on dairy performance.

“Combinations of changing weather conditions, variable forage quality and environmental conditions mean cows will often be under nutritional and environmental stress.

“In simple terms, cows do not like high temperatures or high humidity and the combination of the two can cause big problems. Cows’ comfort zone is between five and 20degC and the recent hot spell means grazed cows will be suffering from heat stress.”

Biotal has collected data from the UK this summer which clearly shows the combination of temperature and humidity recorded as the temperature humidity index has reached levels where productivity is suffering.


The visible symptoms of heat stress include increased respiration rate and panting, lethargy, lower feed intake and rumination and reduced milk yield, but Mr Doran says there are other invisible consequences.

“Heat stress puts cows under a state known as oxidative stress. This is linked to increases in clinical mastitis, higher cell counts, lower immunity and poorer fertility.”

Mr Doran says new research in the USA has shown feeding live yeast combined with organic selenium can play a significant role on offsetting the effects of heat stress.

He says supplementing diets with live yeast helps to minimise the impact of heat stress by encouraging better rumen health and performance which, in turn, can stimulate intakes. Organic selenium helps reduce the extent of oxidative stress and improves immunity and health.

“This new work shows feed efficiency can be increased in cows suffering environmental stress by up to 7 per cent - equivalent to an extra 120g of milk per kg of dry matter intake.

“For a typical UK herd, this is equivalent to 2.5kg more milk per cow per day from the same feed intake, by helping reduce yield depression caused by heat stress.”

Ensuring a supply of clean, fresh water will help offset the impact of heat stress and according to Martin Smith from UFAC UK, the choice of ingredients in the diet can have a beneficial effect.

“The current heatwave is placing a lot of stress on livestock, but care with nutrition can help alleviate this problem,” he says.

“When cows liberate energy from food, heat is produced. This is called heat increment and is much higher for foods containing starch and sugar than for fat sources.

“With farmers looking to introduce buffer feeds as grass growth rate slows, it pays to incorporate ingredients which help keep livestock as cool as possible by reducing this internally-produced heat.

“Heat stressed cows are prone to acidosis. Reducing ingredients which can cause acidosis will bring additional benefits, including encouraging higher intakes.”

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