Heat stress has been shown to decrease milk yield by 10 to 25 per cent, feed intake by 10 to 20 per cent, decrease the six-week and 100-day in-calf rates, drop conception rates, and increase the risk of clinical mastitis in high-yielding cows.
The conditions under which a heat stress event occurs can be due to a combination of factors including temperature, humidity, wind speed and sunlight, with an animal’s characteristics of breed, feeding, stage of milk production and health status.
Cattle try to maintain a stable core body temperature through increased water intake, panting, sweating and behavioural changes. If these mechanisms cannot reduce body temperature below acceptable levels, this is when heat stress will occur.
Cow behaviour is a dominant indicator of heat stress progress and a cow goes through the following progression when experiencing heat stress:
- shade seeking;
- increased standing;
- decreased dry matter intake;
- crowding water troughs;
- increased water intake;
- bunching to seek shade from other cattle;
- changes to or increased respiratory rate; and
- immobility or staggering.
To manage the impact of heat stress, the following actions can be taken when a high heat day is expected:
- Use low-stress stock handling techniques, as this helps reduce physical body heat.
- Delay afternoon milking until 5pm.
- Wet the dairy yard an hour before cows arrive.
- Use yard sprinklers, which encourage heat loss and are an effective method of cooling mobs of cows quickly. Aim for a moderate to large water droplet, use on/off cycle, sprinkle cows for one to three minutes every 15 minutes and if you don't have shade on your farm, bring the milking herd back to the dairy yard around noon and use the sprinkler system to cool the cows.
- During hot weather cows can drink 200 to 250 litres per cow per day — install a large water trough on the exit side of the dairy.
- Place fans above sprinklers and tilt them down 20 to 30°, so they blow air between and underneath cows to enhance whole-body cooling.
- Provide cows with the highest quality pasture available to graze overnight when they are cooler.
- Importantly, if affected animals show no signs of improvement, contact your local veterinarian for assistance.
The impact of heat stress also differs between breeds, with Brown Swiss and Jersey breeds less susceptible than Holstein Friesian, as black-coated cows absorb more solar radiation than cows with lighter coloured coats during the day.
For further information, visit: www.coolcows.com.au
Richard Smith,
Dairy industry development officer
