Should we stress about heat stress?

November 30, 2021

Should we stress about heat stress?

A large US study found that severe heat stress (above 25°C) can cause significant financial losses across all species (St Pierre et al., 2003). An estimated 80% of these losses were associated with loss of productivity and 20% with health issues. In ruminants many of those losses were due to rumen acidosis problems and related depression of feed digestibility, feeding behaviour, milk hygiene (somatic cell count), and fertility.

While cows naturally take on heat from the environment and generate metabolic heat from eating and digesting feed, they are generally good at maintaining their core body temperature between 38.6°C and 39.3°C. However, cows become uncomfortable when temperatures rise above 21°C and start to actively regulate their body temperature at 25°C resulting in energy being diverted away from production and into the dissipation of heat.

While New Zealand is not known as a very hot country (if compared
to Australia), it has become more evident that over the last few years that hot and humid weather events are more common. Recent data produced by DairyNZ suggests even in our temperate climate, average heat and humidity is having an adverse impact on production. See Table 1, published by DairyNZ.

Table 1

*For a 500-cow herd in Hamilton this represents $27,000 in lost production alone at an $8.00 payout.

Act early to reduce the effects of Heat Stress on your herd.

It is essential to recognise the early signs of excessive heat load and allow for early intervention with effective management and mitigation strategies. Common behavioural and physiological changes that can alert you to heat stress are:

  • Cows looking for areas with greater air movement or standing to increase exposure to air.
  • Seeking water and shade.
  • Changing their orientation to the sun.
  • Panting or sweating.
  • Stopping or reducing feed intake, especially fibrous feeds such as summer pasture, decreases rumen heat production – leaving cows at greater risk of rumen acidosis.


*Twice a day feeding with a large amount of fermentable carbohydrates and lower levels of forage can cause an increase in rumen fermentation acids (lowering rumen pH) and a decrease in microbial protein production

The following simple management steps can be taken to minimise the effects of heat stress on your herd.

Access to cool drinking water

Allow 200 to 250 litres per cow per day of drinking water in hot weather – double what cows usually need each day. Make sure cows have access to plenty of clean, cool drinking water wherever they are during the day. A large water trough on the exit side of the dairy allows cows to consume water at their leisure. Water troughs in every paddock will keep cows grazing longer in hot weather. Shorter distances to water reduce the chance of cows stopping grazing due to the heat. High flow rates are essential. Water pipes should be 75mm in diameter. There needs to be sufficient pressure to provide 20 litres per cow per hour. Avoid running black polyethylene pipe along the ground as water will become very hot.

Milking times

Walking cows to the dairy during the hottest part of the day (about 3pm) increases their heat loads. Delaying afternoon milking until 5pm may increase milk yield by up to 1.5 litres per day, regardless of whether the cows are sprinkled with water while in the dairy. Be sure to milk and feed cows before 10am on hot days. Have it done by 9am on heatwave days. Look for ways to offer feed to cows as soon as they exit the dairy. Have the paddock or feed-out area ready and ensure that every cow has adequate access.

Paddock rotation, it is helpful to have a rating for all paddocks based on distance from the dairy and the amount of shade available. Adjust paddock rotation to minimise walking and maximise available shade when high heat and humidity periods are forecast.

During summer, if there is minimal paddock feed available, consideration needs to be given to whether the cows need to go to the paddock during the day. The use of sacrifice paddocks and feed pads will help to minimise heat exposure by reducing walking distance. Care needs to be taken to avoid excess paddock contamination in sacrifice paddocks. This will increase the risk of mastitis and milk quality issues. Ensure any stand-off areas have plenty of access to water with high flow rates into the trough.

Rumen Modifiers

Rumen modifiers such as Rumensin™ can assist during heat stress conditions by altering the balance between the different populations of microbes in the rumen and the proportions of volatile fatty acids (VFAs) they produce. This helps reduce the risk of rumen acidosis and maximises the conversion efficiency of feeds consumed during heat stress periods.

Levucell SC® a rumen specific live yeast has been proven to improve not only the VFA profiles, but also:

  • Improved eating behaviour under heat stress conditions (chewing activity and rumination behaviour)
  • Increased rumen efficiency • A reduction in inflammation

For a complete description of the trial and results click here.

Fats and proteins for heat stress

Higher producing cows and those under greater metabolic stress respond well to supplementary bypass fat sources. Dietary supplementation with extra bypass fat is an excellent way to help increase the energy density of the cow’s diet and maintain the daily energy intake without generating extra heat from ruminal metabolism. Higher producing cows and those under greater metabolic stress respond well to this nutritional strategy. High-quality bypass fats such as GoldenFlake® help to provide energy in the diet without causing acidosis.

How much protein during warmer periods?</h3?

During hot conditions, cows still need enough protein in their diet to maintain rumen microbial function and supply good flows of amino acids to the intestine. They are faced with three challenges:

  • Their daily feed intake is reduced
  • Their rumen microbial function is compromised
  • Summer pastures are lower in protein.

Feed higher quality protein sources in the diet during the hot season. Higher ‘bypass’ or ‘escape’ protein sources (like soya, canola or dried distillers grain (DDG)) that are readily digested in the cows’ small intestine can help offset lower yields of microbial protein from the rumen during hot weather.

Beef, Calf, Dairy

Sign up for a FREE

on-farm consultation