Heat Stress

28 June 2018

HEAT STRESS

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.

 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 plus the reduced VFA absorption across the rumen wall related to increased competition from chloride ions. 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:

The solutions are both management and nutrition and indeed closely aligned to the actions required under current grazing conditions.

  1. Ensure that water is easily and freely available at all times and whenever possible maximise access to shade. Remember that cattle sheds can be more humid and warmer than paddocks unless properly managed.                                                                         
  2. On a nutritional basis it is important to maximise energy density to correct for the reduced appetite, replace salts lost due to sweating and manage rumen health.              
  3. Buffer feeding is a key solution. Reducing the quantity of silage in the mixture and increase the concentrate portion, typically by an extra 2 to 4kg per head per day of a high energy, high NDF concentrate in buffer feeds will be an important action during the hot weather. A daily intake of around 7kg NDF is needed for rumen health and butter fat production so pay careful attention to this parameter in ration formulation.                                    
  4. The feeding of mineral supplements to ensure dietary mineral balance is correct can be helpful..