SoutheastDairyExtension.org: Tip of the Month

Heat stress has definitely arrived, and with a vengeance. The average temperature and humidity in Knoxville, TN recently would cause cows to suffer from mild to severe heat stress for nearly 20 hours of a single day. Typically, the heat stress season in Tennessee lasts from early March through October.

Heat stress is caused by a combination of temperature and humidity. Cows begin suffering from heat stress when the temperature is 72 degrees Fahrenheit with a relative humidity of 95 to 100%. When temperatures climb into the 80's, relative humidity can be quite low and still negatively affect dairy cattle by decreasing feed intake, decreasing conception rate, decreasing milk production and increasing metabolic problems of heat stressed dry cows at calving. The following tips may help your cows survive this long, hot season.

Provide abundant fresh water. Water intake may increase by 50% during times of heat stress. Encourage water consumption by:
1) Ensure enough water bunk space (at least 2 waterer locations per group, prefer 1 station per 20 cows)
2) Provide water in parlor exit lanes
3) Have water supply of at least 3-5 gallons per minute
4) Put waterers in the shade
5) Keep water tanks clean
6) Keep temperature of water between 70 and 86 degrees F

Remember, you can never offer too much water!

Cows will seek shade during daylight hours. Providing shade structures will reduce direct effects of solar radiation. Structures can be permanent, such as a barn, or structures could be temporary or mobile using shade cloth. In order to be effective, the shade cloths must block at least 80% of the solar rays. Remember to provide feed and water under shade structures because cows will not leave the shade to eat or drink.

Using some type of cooling system will provide the most relief from heat stress. However, if your chosen cooling system is not maintained properly, it could make the situation worse. Most cooling systems in Tennessee use a combination of water and air movement.

Air movement is vital in freestall barns and holding pens. Fans should be cleaned at least once per year, and in some situations up to 4 times during the summer. Dirty fans do not function at full capacity and do little to move air in conventional barns. If your fans are on a belt system, be sure to replace any worn belts to maximize air movement. Fans should provide an airflow of about 11,000 cfm and should be tilted downward at 20 to 30 degree angles. You will need at least one 36 inch fan for every 30 feet, or a 48 inch fan for every 40 feet.

Sprinkler systems are designed to wet the hair and skin of the cow using a large water droplet, allowing evaporation to cool the cow (with the aid of fans). Use either 180-degree (along feed bunks) or 360-degree (in holding areas), low pressure (20 to 25 psi) sprinkler nozzles that produce a shower of large droplets that readily wets the cows' skin, not a fine mist. Irrigation nozzles and solid-cone coarse droplet spray nozzles with flow rates between 0.2 and 0.5 gallons per minute work very well. Be sure to check older nozzles for clogs and proper flow rates.

Misting systems have a smaller droplet size than sprinkler systems and cool primarily by cooling inspired air. They do not work well in windy conditions or with fans. Too often, the high humidity in our area causes the mist droplets to expand, and they will not fully evaporate before settling to the ground, bedding or feed. These systems are not intended to wet the animals or the environment they are in. A fine mist that wets the hair of the cow will create an insulating layer around the cow which will prevent her from releasing body heat. A misting system is probably not advisable for most dairy operations in Tennessee, especially for freestalls bedded with sawdust or shavings.

Feed Modifications:

Forage digestion increase the amount of heat a cow must release (typically called the heat of fermentation), especially poor quality forage that is high in ADF. During times of heat stress, it is important to maximize digestibility in the diet without causing problems such as rumen acidosis. The most popular solution is increasing the concentrate in the ration and decreasing the forage. However, many herds are already feeding the maximum amount of concentrate, but there are other options to ration management during heat stress.
1) increase feeding frequency (smaller quantities at more intervals)
2) feed during the cooler times of the day (early morning and late evening)
3) provide adequate bunk space to prevent crowding (2.5 to 3 linear feet per cow)
4) increase ration buffering capacity (such as adding sodium bicarbonate)
5) feed supplemental fats to increase energy intake (whole cottonseeds, whole soybeans, etc.) but do not exceed 7 to 8% fat in total ration dry matter
6) minimize overfeeding of protein (it takes energy to excrete extra protein)
7) feed higher quality, more digestible forages
8) avoid slug feeding

The most important tip on heat stress is recognizing that just because you think it feels comfortable outside, does not mean your cows are comfortable.