Cool it – dealing with heat stress in dairy cows

Dr Jan du Preez, managing director of the Institute for Dairy Technology at the Milk Producers’ Organisation and Michael Hutjens from the Department of Animal Sciences at the University of Illinois in the US, recently discussed the impact of heat stress, and ways to deal with this problem in South African dairy herds
Issue Date: 30 March 2007


Dr Jan du Preez, managing director of the Institute for Dairy Technology at the Milk Producers’ Organisation and Michael Hutjens from the Department of Animal Sciences at the University of Illinois in the US, recently discussed the impact of heat stress, and ways to deal with this problem in South African dairy herds, at the Large Herd Conference held in Jeffreys Bay in the Eastern Cape. Glenneis Erasmus reports.

Heat stress annually causes major economic losses in dairy herds, and its impact on reproduction is often much worse than on milk production and milk quality. It can cause conception rates to fall by up to 40%, forcing farmers to use more artificial insemination (AI) than they would have under more favourable climatic conditions. The best way to combat this impact on conception, according to Dr Jan du Preez, managing director of the Institute for Dairy Technology at the Milk Producers’ Organisation of South Africa, is to cool down cows 12 hours before and two days after AI.

Heat stress might make it difficult for farmers to identify exactly when cows are ready for AI, as heat stress can shorten time in heat to only a few hours when it usually is around 16 hours long, and it might lengthen the cycle from 21 days to 28 days. “Farmers should closely observe animals to ensure they accurately identify when cows are in heat,” Dr du Preez advises.

Critical stage
The first four to six days after conception are critical for embryo survival, and it is worth ensuring that animals are not subjected to heat stress during this time. Animals can be fanned or sprayed through evaporation cooling to cool them down. Heat stress can also affect reproduction by causing cows to calve before their due dates. These “early” calves have a lower body weight and weaker immune system than calves carried a full term. As a result, they are also more susceptible to diarrhoea and other diseases. Cows become more susceptible to diseases – such as mastitis and udder infections – when they experience heat stress, and milk production can drop by up to 40%, depending on the severity of stress.

Milk quality is affected, with milk fat dropping by up to 30%. solids and milk protein content can fall by up to 20%. The ability of white blood cells to destroy bacteria in the udder is also affected, resulting in increased somatic cell counts (SCCs). This in turn can result in economic losses, as farmers often receive a premium for milk with SCC under 300 000 per millilitre. Dr du Preez estimates more than 90% of South Africa’s dairy cattle are affected by heat stress during the hot months from November to March. “One of the problems is that our primary cattle breeds, the Friesians and Holsteins, are temperate-climate animals of northern hemisphere origin, and they find it difficult to adapt to South Africa’s climatic conditions,” he says. Farmers should therefore take the climatic conditions of an area as well as the breed into account to ensure it is suitable for production.

Temperature and humidity levels should not be excessive, and there must be sufficient shade. Artificial shade areas can be built if there is not enough natural shade. Jersey cattle are less susceptible to heat stress than Holsteins and Ayrshires, and should rather be used in marginal climatic areas. Farmers can also breed animals that are more resistant to heat stress. Another problem is that cows’ body temperature regulating mechanisms are compromised during heat stress. Unlike humans and horses, they don’t have any sweat glands, but sweat on their muzzles.

Cow management and activities, such as vaccination, dosing and dipping, should for this reason be avoided between 11am and 3pm during peak warm periods. Heat stress can also be reduced with fans to enhance ventilation in the dairies and stalls, and by spraying cows with water once or more often a day when it is very hot. Spraying can be administered through evaporative cooling systems that automatically switch on when air temperatures rise above 23ºC. Cows should be sprayed for half a minute to two minutes, and should afterwards be fanned to cool them down. Sprayers should be located above the cows, and cows should be wetted to their skin for best results. Dr du Preez points out that air pockets can form between the skin and the hair if only the hair is wetted, and this type of wetting will not have the desired cooling effect. The cooling area should have a concrete floor with a slight incline for water run-off. Fans should have a flow capacity of 11 000 cubic feet per minute or more, and air should move at between 400 feet to 600 feet per minute for effective cooling.

Importance of feed

High levels of roughage in feed can aggravate heat stress by increasing body temperature.
Dr du Preez advises farmers to enhance the energy feed level in the rations during heat stress, and to reduce the roughage to as low as possible. Feed should preferably be under shade; stale, and hot feed should be removed regularly to prevent secondary fermentation, which can suppress feed intake. Cows suffering from heat stress tend to eat less, have lower dry matter intake, and accordingly have a higher nutrient requirement than cows which are not subjected to heat stress. “Offering ration dry matter at night when it is cool, multiple feedings and fresh feed after each milking can help to significantly improve dry matter intake,” says Michael Hutjens from the Department of Animal Sciences at the University of Illinois. Adding water might however increase the risk of secondary fermentation.

Adding water to feed can also help to increase dry matter intake, reduce sorting and help to reduce dust that reduces palatability, although care must be taken to avoid secondary fermentation. The increased nutrient demand can be met by increasing forage quality, and shifting from neutral detergent fibre (NDF) sources to by-product NDF sources such as citrus pulp, soy hulls or corn gluten feed, says Hutjens. He adds that effective NDF levels should be maintained at around 20% of the ration to sustain rumination and forage raft in the rumen. The proportion of grain supplied can also be increased, but it should be finely milled and accompanied by an increased amount of physical effective fibre to prevent acidosis.

Fat or oil can also be added to diet to increase energy density. Hutjens warns that unsaturated oils and fats should be avoided as these can cause digestive problems. Electrolyte balance is also affected during heat stress, and farmers should supply animals with more minerals. “The total potassium ration can be increased by 1,4% to 1,6%, while sodium can be increased by around 0,4% and magnesium by 0,35%,” Hutjens advises. Danish researchers and researchers from Utah found that high-protein diets result in milk yield reduction. Crude protein intake should be limited to below 18% of dry matter basis, with 6,5% to 7% as rumen undegraded protein. Water quality and quantity can also manage the impact of heat stress on cows. “Cows should receive between 1,2 to two times more water during times when there is a heat-stress threat than during normal climatic levels. Water should be cool, not cold, and be placed under shade,” Hutjens stresses.

For more information e-mail Dr Jan du Preez at [email protected] or Michael Hutjens at [email protected]. |fw

So how do you know whether cows are suffering from heat stress?
Dr Jan du Preez provides this easy rule of thumb: cows will definitely experience heat stress at temperatures that humans find uncomfortably hot. A cow’s body temperature, normally between 38ºC and 38,5ºC, will rise by 1ºC to 1,5ºC during heat stress. The cow might start to pant, with respiration rate increasing from 40 to 50 to more than 65 breaths per minute. The animal might also start drooling.

Heat-stressed cows spend less time feeding and lying down during the day than non-stressed cows do. They try to avoid the source of heat and actively seek out shade. They might also lie in mud, increasing their chances of getting diseases such as mastitis, metritis and foot rot.

Heat stress can also be determined with Temperature Humidity Indices calculated for South Africa and Namibia. These indices take the impact of temperature, relative humidity, radiation, altitude above sea level and wind into account. Dr du Preez says it can be accepted that cows will experience some heat stress at ambient temperatures above 21ºC and relatively humid conditions. He points out that cows perform best at temperatures between 0ºC and 15ºC.