Understanding Heat Stress
Heat stress strikes dairy cattle whenever the heat load placed upon an animal exceeds her ability to lose heat. The consequences are reduced dry matter intake, decreased feed efficiency, lower milk production and quality, and diminished reproductive performance. Sources of heat load include environmental heat, exercise, metabolism (body fat production), and heat of fermentation (rumen). Heat stress is most severe when daily low temperatures exceed 70 degrees Fahrenheit.

The Cow's Response
1. Increased blood flow to the skin.
2. Increased sweating.
3. Increased respiratory rate.
4. Feed efficiency plummets when animals expend energy to cool themselves by panting.
5. Sweating and increased rumination contribute to the loss of potassium and sodium.
6. Have a tendency to stand rather than lie down.

The Economic Results
1. Milk volume is reduced as maintenance energy requirements increase by 7-25%, water intake increases up to 33% and feed intake declines.
2. Increased blood flow to the skin results in reduced blood flow through the udder, thus less milk will be produced.
3. In addition to reduced milk volume, butterfat percent typically drops, and somatic cell count commonly increases by 10-15%.
4. Heat stress can impair reproduction by altering hormone (estrogen, progesterone) production or by directly damaging the ova, sperm, or embryos.

Nutritional Strategies To Help Minimize Heat Stress
1. Dietary Adaptations
  a. Choose highly digestible forages.
  b. Add dietary fat or rumen inert fat.
  c. Increase sugar content.
  d. Increase buffer and electrolyte content
2. Proper Feed Bunk Management
  a. Keep feed fresh.
  b. Keep bunk clean.
  c. Increase number of feedings per day.
  d. Never allow feed bunk to go empty.
  e. Feed during cooler times of the day or during the night and feed a greater percentage of dry matter.

Forages
Higher quality forages should be fed because they are more digestible, contain more energy and reduce heat load when compared to poorer quality forages. Rations formulated with high quality forages are more energy dense and help maintain caloric intake even when dry matter intake is reduced.

Sugars
1. Sugars can be added to the diet since sugar has a lower heat of fermentation than grains.
2. Bacteria and yeast additives may also prove beneficial during periods of heat stress by increasing rumen fermentation efficiency.

Protein
1. Maintain protein intake (total pounds) when dry matter intake has been reduced by heat stress.
2. Increase dietary protein density.
3. Undegradeable intake protein should be increased.

Fats
In times of heat stress, fats provide two major benefits. Fats increase the energy density of the diet, which helps maintain constant caloric intake despite reduced dry matter intake. Secondly, fats are not fermented in the rumen, and thus do not add to the heat load.

Fat Feeding Guidelines:
1. Oilseeds should provide no more than 25% added dietary fat (1 pound of roasted soybeans or whole cottonseed per 8 pounds of dietary dry matter).
2. Feeding rates of animal fats (tallow) should be limited to no more than 0.75 pounds per cow per day.
3. Rumen inert fat can be added at the rate of 0.5 to 1 pound per cow per day depending on the amount of other fat products included in the diet.
4. Total fat content of the diet should not exceed 6.5% or the diet should contain no more than 4% added fat.

Water
Water is perhaps the most critical of all nutrients, especially during heat stressed times. Provide a readily accessible source of clean fresh water as intake may exceed 40 gallons per cow per day. Placing water in the shade as water consumption may increase over 54%. Along with water, provide plenty of shade, which can reduce body temperature by 3 degrees and increase milk production by 2 pounds per cow per day.