Heat stress can be an issue when temperatures exceed the comfort zone (thermo neutral zone) of 25oC, known as the upper critical temperature (UCT).
At temperatures above the UCT, cows become highly stressed and use two main control strategies to maintain thermal balance:
1. Increasing heat dispersion, in particular through evaporation, by increasing subcutaneous blood flow and exhibiting the classic signs of heat stress of panting and drooling. These activities increase the maintenance energy needs of the animal by an estimated 20% so reducing that available for milk production. Sodium and potassium requirements increase, the latter being the primary osmotic regulator in sweat glands of cows.
2. Limiting heat production – by reducing all activity and changing the feeding pattern. As the majority of heat production in dairy cows is essentially due to rumen fermentation, the cow will reduce her DM intake by 10-30% and be selective in what she eats, namely less roughage. Roughages increase rumen activity and therefore heat production.
3. Also, rumination, which produces heat, decreases dramatically. Rumen acidosis becomes a concern, further exacerbated by the reduced saliva flow into the rumen associated with slobbering. Varying manure consistency is the classic sign.
The impacts of heat stress are seen on dairy units as decreased milk production, with lower milk fat and protein content, loss in body condition, fertility issues, higher incidence of mastitis and hoof claw issues leading to more lameness.
The solutions are both management and nutritional as follows:
1. Ensure that water is easily and freely available at all times and whenever possible maximise access to shade. Spray water over the cows at milking time in the collecting yard especially in the evening time when the sun is on the wane. Remember that cattle sheds can be more humid and warmer than paddocks unless properly managed and ventilated.
2. Maximise the energy density of diet to counter reduced Dry Matter intakes.
3. House cows by day and turnout out to grass at night.
4. Buffer feeding is key. Reducing the quantity of silage in diet and increasing the concentrate. A daily intake of around 7kg NDF is needed for rumen health and butter fat production.
5. The feeding of mineral supplements to ensure correct dietary mineral balance especially lost Potash & sodium salts through sweating can be helpful.
For more information on heat stress, please contact your local FCG office or Andrew at firstname.lastname@example.org