Heat Stress in Cattle

Author: Emma Holmes BVM&S MRCVS

Many of you will have your first cut of silage firmly under wraps and be looking forward to the balmy summer evenings. With the current heat wave set to continue, spare a thought to how that increased heat is affecting your cows. Some might laugh, however, heat stress is a real problem here in the UK, particularly in housed dairy cattle.

Causes of heat stress

 Cattle are homeothermic animals, needing to maintain a constant body temperature of 38.8°C +/- 0.5°C. They are very sensitive to environmental changes affecting their temperature regulation.

Factors include:

  • Air temperature
  • Radiant heat
  • Air velocity
  • Relative humidity

A cow's temperature comfort zone is 5-25°C.

  • < 5°C she will increase her dry matter intake (DMI) to maintain her body temperature rather than produce milk. 
  • >25°C she has 2 main ways to regulate her body temperature:
  1. Increase heat loss - evaporation (increased subcutaneous blood flow, panting, drooling) → redirects production energy. 
  2. Limiting heat production - reduce all activity including eating (rumen fermentation is responsible for the majority of a dairy cow's heat production). 

As a cow becomes heat stressed her DMI will reduce, consequentially lowering her milk yield. Temperatures >25°C can reduce yields by up to 20%, fertility is adversely affected coupled with an increased risk of infections such as mastitis.

A high yielding dairy cow will generate more heat than a dry cow.

E.g. A cow producing 18L and 31L milk/day will generate 18% and 48% respectively more heat than a dry cow.

Air movement encourages evaporation assisting heat loss. As humidity increases, the temperature at which a cow shows signs of heat stress reduce.

Symptoms of heat stress

Affected cattle will be lethargic and inactive. Panting and raised respiratory rates are seen in an attempt to increase heat dispersal. Oddly cattle tend to congregate in tightly packed groups when suffering from heat stress, further compounding the problem.

Management of heat stress

DMI reduces when temperatures rise, hence strategies for control involve dietary management.

  • Increase nutrient content - higher quality forages → faster digestion → less heat produced.
  • Increase energy density - greater concentrate amounts.

All dietary changes should be carefully balanced to avoid digestive upsets predisposing to acidosis and displaced abomasum. 

  • Change feeding times - higher consumptions between 8pm-8am when cooler.
  • Adequate water provision is essential - easily accessible.

Practical ways to prevent heat stress include:

  • Shade
  • Ventilation
  • Cooling methods 

Simple building alterations, e.g. side or open ridge outlets can significantly improve ventilation. Installation of fans in buildings or sprinkler systems in collecting yards involves capital investment but improvements in airflow and cooling ability will considerably help evaporative heat loss.

A study in the USA suggested that when ambient temperatures reached 27°C (a cattle shed can be 10°C higher than outside), the addition of fans and sprinklers in a collecting yard reduced the cow body temperatures by 1.7°C. This increased milk yields by 0.79kg/day over cows with no fans or sprinklers.

Other considerations could be:

  • Roof insulation to reduce solar penetration - ?costly.
  • Consider lighting position and requirement as an extra source of heat - more on the north rather than south facing side of a building. 

Conclusion

 To conclude and to spark some thought as you drive around in your air conditioned tractors, one of my Edinburgh University professors was renowned for a very simple, yet poignant quote. "A comfortable cow is a happy cow, is a productive cow".