by Allan Chestnut, Ph.D.,PAS Vigortone Technical Service Nutritionist

The negative effect of heat stress on dairy cattle performance is well known. Cows experience heat stress when the Temperature-Humidity Index (THI) rises above 72. Table 1 shows ambient temperature and relative humidity combinations that produce mild heat stress (THI 72 to 79), moderate heat stress (THI 79 to 89) and severe heat stress (THI > 89).

Dr. Joe West (Animal & Dairy Science Dept., University of GA), in a recent presentation to the Tri-State Dairy Conference in Ft Wayne Indiana, states that a cow experiences heat stress when she looses the ability to dissipate sufficient heat to maintain thermal balance and her body temperature rises. This has a negative effect on many factors such as dry-matter intake, reproduction, and milk production. The normal body temperature of a dairy cow is 101.5F.

Dr. West goes on to say that signs of heat stress for a cow include:

• Increased body temp. (>102.6F)
• Panting >80 breaths per minute (35-45 normal)
• Reduced activity
• Reduced feed intake (>10-15% reduction)
• Reduced Milk Yield (10-20% or more)

Dr. West states that the cow has several mechanisms to help dissipate body heat. These include:

• Conduction, where the cow conducts heat to a cooler surface.
• Convection, where thermal currents leave the cow’s body
• Radiation, where the cow radiates heat to a cooler environment, such as the cool night air.
• Evaporation, where moisture is evaporated from the surface of her body (sweating) and from her lungs (panting).

The cow will also reduced feed intake to produce less metabolic heat, which is a protective mechanism.

Heat stressed cows are less likely to exhibit standing estrus and often only exhibit signs of estrus at night when temperatures are cooler but when they are less likely to be observed. In addition, duration of estrus is shorter for cows subjected to heat stress (Wolfe and Monty, 1974).

Although we cannot change the weather, we can modify the cow’s environment to minimize heat stress and we can change feeding practices.

Shade - Shading is one of the most important and cheapest ways to modify the cow’s environment during hot weather. Even for dry cows, providing shade can be a big help. It is reported that cows shaded during the dry period gave birth to larger calves and had greater 100-day and 305-day milk yields than un-shaded dry cows. For further information on this topic please refer to "Heat Stress Also Affects Dry Cows" by Richard Goings, PhD.

Fans and Sprinklers - Several options are available. However, if you are going to put water on cows, air movement with fans is a necessity. Sprinklers should wet the cow but not the udder. Fans should move enough air to evaporate the water. Sprinklers can be controlled with timers to cycle the water on and off (i.e. 5 minutes on and 10 minutes off in a 15-minute cycle). Fans and sprinklers are usually placed near the feed bunk. The coolest place in the barn should be near the feed bunk to encourage eating. Fans, but not sprinklers, may be placed over free stalls. Fans and sprinklers should also be used in the holding area where temperatures increase rapidly when cattle are concentrated before milking.

Water - The most critical nutrient needed by the cow is water. Water intake will increase by 30% or more during heat stress. Clean water should be provided to cows where they congregate during the day and while in the holding pen and return alley from the milking parlor. Using wet feeds in the ration or adding water to the ration can also help.

Ration changes - Rations need to be formulated to compensate for reductions in dry-matter. Lower fiber diets produce less metabolic heat, though care must be taken to insure adequate fiber is still provided. Additional fat is an option when needing to increase energy while maintaining necessary fiber.

Minerals may need adjustments as well. Cows sweat just like other mammals, but unlike humans, who sweat more sodium, cow’s sweat contains a large amount of potassium.

Mineral recommendations during heat stress include:

• Potassium: > 1.4% of DM
• Sodium: 0.35 to .45% of DM
• Magnesium: 0.35 to 0.40% of DM
• Chlorine: < 0.40 % of DM

Mineral adjustments should be made several weeks before the onset of high temperatures so minerals are present in the body when needed. Several feed additives, such as Vigortone’s KAT-ION Buffer and yeast culture, have shown benefits when addressing heat stress.

Bunk management - Wet feeds spoil faster in the bunk during hot weather. Keep feed fresh by feeding during early morning hours and in the evening when cows have better appetites. Consider feeding only a third of the ration during the day and two thirds in the evening when temperatures are cooler. Adding a TMR preservative, like Mold Zap, will retard feed heating in the bunk. Keep bunks free of spoiled feed to maximize the cow’s appetite.

Table 1. Temperature Humdity Index (THI)¹ for dairy cows. Modified from Dr. Frank Wiersma (1990), Department of Agricultural Engineering, University of Arizona, Tuscon.

1 THI=(Dry-Bulb Temp. C) + (0.36 dew point Temp. C) + 41.2

For more details about ration formulations for cows during hot weather, please call us at 1-800-553-1712 or email us at contactus@vigortone.com.