Heat stress can have a huge impact on cow health and milk production by reducing feed intake and rumination.
Heat stress is caused not only by temperature, but humidity, and is measured using a temperature humidity index (THI), which calculates the combined effect of maximum daily temperature and humidity.
High-yielding cows start experiencing heat stress when the THI rises above 68 (when ambient temperatures are 22C with 60% humidity). But the impact on fertility starts at lower levels.
Many producers underestimate the impact of heat stress because they fail to acknowledge the risk caused by the higher humidity of the UK climate, says independent nutritionist Hefin Richards of Rumenation Nutrition Consultancy.
“We tend to gauge the risk [of heat stress] on our own feeling of comfort, but quite often we don’t feel hot until it reaches 18-20C whereas, given the choice, a cow would prefer 7-10C,” Mr Richards explains.
Nutrition plays a big part in mitigating the effects of heat stress.
Fermentable carbohydrates such as sugar, starch and fibre generate heat when they are broken down in the rumen, so cows eat less, and also stand more frequently, to cool down.
What are the signs of heat stress?
- Cows panting
- Standing rather than lying down
- Cows may crowd in cooler parts of the shed or around water troughs
- Dry matter intakes will drop
This reduces effective rumination time and increases the incidence and duration of acidosis, further reducing feed utilisation and intake.
This causes rumen pH to drop and puts the cow at risk from acidosis.
Heat stress can also cause secondary issues such as ketosis, increased days open and a decrease in colostrum quality, warns Dave Gilbert of Horizon Vets.
Below, Mr Richards, Mr Gilbert and Dr Richard Kirkland from Volac tell us what feeding strategies can help mitigate the effects of heat stress.
1. Feeding strategies
The starting point is to get the basics of day-to-day feed management right before spending money on supplements.
Understand how big a problem heat stress is on your farm and which groups are at the highest risk.
Poorly ventilated sheds will present the biggest risk, as will time spent in the collecting yard and parlour, where airflow is limited.
Dry cows are a high-risk group and the priority needs to be placed on maintaining a good level of rumen fill to prevent metabolic issues.
- Use temperature and humidity monitors inside buildings to help alert you to risks.
- Consider feeding twice per day, if not already, to keep feed fresher and improve intakes.
- Remove any rejected feed and ensure water supply is clean and meets demand.
- If feeding more frequently isn’t feasible, consider feeding cows in the evening when temperatures are lower and cows will maintain more normal intakes and feeding patterns.
- Ensure feed is pushed up regularly to maximise intakes.
See also: Advice for improving ventilation
To reduce the heating effect caused by fermentation in the rumen, consider reducing the amount of starch in the diet and increasing fat content to maintain energy.
Unlike carbohydrates, fat is a “cool ingredient” because it bypasses the rumen and is absorbed into the blood stream.
Consider the most appropriate type of fat supplement based on the fatty acid profile of the supplement to best meet farm targets.
Traditional calcium salt supplements offer a source of rumen-protected oleic acid (C18:1), which helps improve body condition, the development of eggs and embryos, and total diet fat digestibility.
High-C16:0 (palmitic acid) supplements are effective at stimulating milk fat production and may be of interest under heat stress conditions, where milk fat often declines.
However, palmitic acid can cause partitioning of nutrients away from body fat, resulting in lower body condition, which may not be desirable.
For example, it may be best to feed to early-lactation cows to maintain body condition and promote good fertility, whereas it may be better to feed C16 to the whole herd to lift butterfat.
3. Provide good-quality forage
Good-quality forage should be offered to your highest-yielding cows under the most stress to help improve fibre consumption and maintain good rumen activity.
Poor-quality forage should be avoided because it requires more work by the animal to ferment in the rumen, which generates more heat.
While swapping lower-energy forage for high-fermentable cereals can improve the energy density of the ration, this should be avoided, because starchy supplements will lead to an increase in acid in the rumen.
4. Yeasts and rumen buffers
Yeasts and rumen buffers can help reduce the risk of acidosis by having a positive effect on rumen pH.
If already feeding these products, it may be worth considering temporarily increasing feed rates, or the other option is to target high-risk animals – ie, early lactation where energy balance is likely to be already negative and animals are at risk of losing condition at breeding.
Always check products are proven/backed by good research.
Equaliser/CoolCow is an osmolyte and rumen buffer that helps regulate the core body temperature by hydrating the cow at a cellular level and restoring the electrolyte balance.
The powdered additive, from Cargill, should be added to the lactating cow ration at a rate of 100-150g a head a day when heat stress is a risk. It costs about 9.5p a head a day.
6. Water provision
Cows suffering from heat stress will drink up to 30% more water.
Consequently, you see more traffic at the water trough and some animals can be bullied away from the trough by others.
- Ensure water is clean and palatable.
- Be careful not to put your water supply under pressure by cleaning tanks. Consider if you need to reduce the frequency of cleans or change the time you do it – for example, milking time can see a peak in water demand.
- Provide at least 10cm of space per cow.