Heat stress in the lactating dairy cow

Physicological strategies the dairy cow uses include: 

Dissipation of body heat

• Increase in panting and sweating resulting in loss of water
  • Sweating results in a loss of potassium, an important osmo-regulator
  • Exhaling more carbon dioxide
  • Drooling more, increasing the loss of bicarbonate
  • Decreased rumen pH, mimicking acidosis
• Vasodilation of skin and lungs
• Increase in water intake

Reduce heat producing activities

• Vasoconstriction in digestive tract, reproductive tract and mammary gland
  • Lower appetite
  • Slower rate of passage of digesta
  • Less ruminal contractions
  • Decrease in nutrient absorption
  • Decrease in performance (growth, milk production, reproduction etc.)

However, recent research from Dr. Lance Baumgard at Iowa State University has also noticed an increase in glucose utilization and breakdown of protein stores as well. Dr. Baumgard’s team has conducted studies that compared heat-stressed animals with feed restricted (pair-fed) animals to mimic the 30% lower intake of the heat-stressed animals, which saw the heat stressed animals produce less milk than their pair-fed counterparts. This showed that the decreased dry matter intake (DMI) could only explain 50% of the reduced milk yield. Heat-stressed cows also lost more body weight in the form of muscle rather than body fat. Although lower milk yield was caused by lower lactose secretion, the heat stress increased insulin secretion and stimulated glucose utilization, demonstrating a much higher need for glucose during heat stress. Therefore, even though they consumed the same amount of feed, heat-stressed animals produced less milk and lost more weight.

Dr. Baumgard has also presented studies demonstrating a rapid increase in endotoxins following heat stress, which will result in gut inflammation. ‘Leaky gut’ is a term used to describe increased intestinal permeability resulting in unwanted molecules (e.g. toxins, bacteria) to ‘leak’ from the gut into the blood stream stimulating an immune response. There are several on-farm circumstances which can impact gut health, including heat stress, acidosis, weaning, feed restriction, psychological stress and calving.

Leaky gut could be the explanation not only for a large percentage of drop in milk production, but also the DMI decrease and the loss of body weight a result of glucose energy being directed to the immune system during heat stress.