This can result in reduced milk production as well as shade-seeking behaviour, increased breathing rates and crowding around water troughs. Daily grazing patterns are also likely to change.
New Zealand dairy cows start to suffer from heat stress once temperatures get to around 24deg C with moderate relative humidity of about 75%. Parts of Northland, the greater Waikato, the Bay of Plenty and some inland South Island areas regularly have several weeks of weather each summer where conditions are likely to cause heat stress for milking cows.
The extent to which a cow is affected will depend on a number of factors. High-producing cows show signs of heat stress before their low-producing herd mates because high-producing cows generate more metabolic heat. Friesian cows and cows with a dark coat are more susceptible to heat stress, as are older and larger framed cows, especially those that may have some underlying health issue such as lung disease.
Farm infrastructure and management practices can also influence the degree of heat stress the cows may experience. The amount of shade available to cows in paddocks, the distance walked each day and the time of day that cows walk all affect the risk of heat stress. Like people, cows seek shade, like long cool drinks and limit physical activity wherever possible to manage the effects of heat.
Keeping cool
So what can farmers do over the summer months to reduce the heat load for the cows and maintain milk production?
* Assess each individual paddock for heat load risk. Paddocks with shade and close to the dairy are lowest risk, and those further from the dairy, with limited or no shade, are highest risk.
* Reserve the low risk paddocks for use during the day on days when the risk of heat stress is high. These paddocks could also be used for the most susceptible cows – the high producers or older, larger cows.
* Higher risk paddocks would be better used for night grazing or lower producing cows such as once-a-day (OAD) milkers.
Cows will drink a lot of water when it’s hot and humid and the water supply should be able to supply at least 120 litres of water/cow/day. The peak demand on drinking water is likely to be after afternoon milking so there should be an adequate flow of good quality water to meet this demand.
Water palatability is important as cows will not drink unpalatable water. Water should also be kept cool by burying water reticulation pipes and providing water in large volume troughs.
Remember to provide adequate trough space so all cows have access to water. In hot conditions more dominant cows will crowd around water troughs making it difficult for the more submissive cows to get access.
Use of sprinklers in the dairy is a very effective means of reducing the body temperature of hot cows, even though research shows that cows prefer shade over sprinklers on hotter days. This cooling process is most effective when medium to large droplets are used and the hair is wet to skin level. Cows should be able to spread out in the holding yard so that air can circulate between them and allow evaporation of the water. There is also benefit in using water to cool concrete surfaces prior to cows standing in these areas.
During hot periods cows are likely to increase the amount of grazing they do during cooler parts of the day and at night. Providing the main part of the daily ration at these times will allow cows to maximise the time spent in the shade during the hotter parts of the day.
More summer management tips are available at dairynz.co.nz/seasonaldiary. Early warning about temperature and humidity is available in MetService forecasts.
Nita Harding is DairyNZ’s team leader for animal husbandry and welfare.
