When summer arrives, so does heat stress

Dairy farmers need to be alert to the signs of heat stress which could knock performance in the coming months according to Adam Clay, Ruminant Manager with Trouw Nutrition GB.

Heat stress is caused by a combination of higher temperatures and raised humidity and can have a significant impact on performance, reducing milk yields largely due to the depression of feed intakes.

“Last year we saw depressions in milk yield, milk quality and pregnancy rates when climatic conditions induced heat stress, reducing margins and potentially profits,” Mr Clay explains.

“Temperatures do not need to be very high before heat stress becomes an issue. Cows start to be affected by heat stress when the temperature reaches just 20°C (68°F) so it is easy to see how grazing cows are at risk of heat stress.”

The diagram shows the relative heat stress risk at different combinations of temperature and humidity. Mr Clay says that in the week ending 20th June the peak temperature of 21°C combined with 83% humidity gave a stress risk of 70%.

“Visible symptoms of heat stress include more rapid respiration rate and panting, lethargy, reduced feed intake and rumination and lower yields. When stressed, respiration rate can increase to 35-60 breaths per minute, roughly twice the normal rate.”

Mr Clay says heat stressed cows also exhibit changes in rumen function with less frequent cud chewing and reduced saliva production. He says these factors increase the risk of acidosis by disrupting the usual rumen buffering effect of rumination.

He advises farmers to act promptly at the first signs of heat stress as the rumen can take a long time to adjust meaning effects can be seen long after weather conditions have improved.

Mr Clay advises added rumen buffers and yeasts to the diet to help offset the acidosis risk and to promote better intakes. Against usual advice he says cows suffering heat stress should also be fed less forage in the diet.

“Forages are actually a source of considerable heat production in the rumen which can exaggerate the impact of heat stress. Their fermentation produces seven times more heat than concentrates, so under heat stress conditions it will pay to reduce forages and increase concentrates, particularly high fibre concentrates. This also has the effect of increasing energy density which will help offset reduced energy intakes due to lower dry matter intakes.

“Also make sure cows have access to plenty of clean, fresh water as when thirsty water intakes will increase markedly. This problem will be worse when forages are reduced.”

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