When it comes to temperature stresses on calves we tend to think about cold weather and the impact of low temperatures on the calf’s ability to keep warm and grow at the target rate. However, for summer born calves, heat stress can also impact on calf health and growth performance.
When Does Heat Stress Occur?
Heat stress is likely to affect calves at temperatures over 25˚C, with the first sign of stress being an increase in respiratory rate. The calf will use extra energy to divert heat away from its core to try and maintain body temperatures. This means less energy is available for growth and immune function and a higher risk of mortality. Energy requirements for maintenance and hydration needs can increase by as much as 20-30%.
Signs Of Heat Stress
Signs of heat stress include:
- Drinking more water
- Reduced milk and starter feed intake
- Higher respiratory rate/panting (for calves less than 1 month old, 24-26 breaths/minute is normal. For older calves the normal range is 15-30 breaths/minute).
- Spending more time standing and less time lying down.
Any sick calves which are scouring will dehydrate more quickly in warmer weather, making electrolyte treatment even more important. If calves are scouring and little water is available, they will be much more susceptible to the effects of heat stress.
Water Requirements In Higher Temperatures
Water requirements greatly increase with temperature, with an extra 1 litre being required at a temperature of 20˚C compared to 30˚C. While prolonged temperatures over 25˚C are uncommon on Scotland, lower temperatures can still cause heat stress effects when humidity is high.
Consider Increasing Milk Allowance
Starter feed intake tends to reduce during periods of heat stress, meaning calves have less energy to meet their maintenance requirement and have less energy available for growth, resulting in lower weaning weights. This will be especially true in calves over 3 weeks of age where start feed intake starts to become significant. As healthy calves are most likely to drink more milk if it is offered to them, increasing the milk allowance provides a means to help increase energy intake during times of heat stress.
Steps to Reduce Heat Stress:
- Provide shade to calves in hutches and increase spacing between hutches to allow more air flow.
- Situate hutches to have the opening facing the prevailing wind.
- Alter bedding material from straw to materials like shavings, sand (although can heat up in direct sunlight) or oat husks which retain less heat.
- Lift hutches 6-8 inches off the ground to increase airflow.
- Increase frequency of bedding to reduce disease built up (warmth will increase growth of pathogens).
- Increase milk allowance.
- Replace water with fresh water regularly.
- Improve ventilation in buildings either natural or provide mechanical ventilation but avoid drafts at calf level.
- Adjust feeding times to take place during the cooler times of the day. Calves are likely to eat more and feed is less likely to spoil.
Heat stress in the dam and subsequently the calf in utero can also affect performance of that calf further down the line. An article in Hoard’s Dairyman last month indicated that calves from dams that had experienced heat stressed before birth showed reducing milk production over their lifetime, lower body weight, poorer immunity, lower antibody absorption from colostrum and a decrease in herd survival for two generations.
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