Summer temperatures mean heat stress for dairy cattle. Heat stress has negative impacts on both lactating and dry dairy cows. In addition to decreasing milk production in lactating dairy cows, heat stress causes decreased feed intake, reproductive performance and immune function in cows.
The amount of heat stress experienced by a dairy cow depends upon the air temperature and the humidity. Research has shown that high-producing dairy cows (more than 77 pounds of milk/day) start to decrease milk production when the temperature-humidity index (THI) exceeds 68. As an example, a temperature of 72° F with 45 percent relative humidity or a temperature of 80° F with no humidity both produce a THI of 68. Research indicates dairy cows can experience a negative impact on fertility factors such as estrus expression, conception rate and embryo survivability at even lower THIs in the 55-60 range.
In order to minimize the detrimental effects of heat stress, the dairy manager needs to make sure they have an effective heat abatement program in place. Cattle sweat at only 10 percent of human rate so any heat abatement program must include the use of fans and sprinklers to provide evaporative cooling. A June 2016 article on the eXtension Dairy site, "Dairy Feeding and Management Considerations during Heat Stress" listed the following key points regarding the use of fans and sprinklers:
■ Fans over freestalls, in the housing area, and over feed bunks should be automatically programmed to turn on when the temperature and humidity reach a THI of 68.
■ In more humid climates, fans should be used in combination with sprinklers (nozzles need to deliver 0.5 gallon/minute of water, 20-40 pounds/square inch of pressure which will wet the hair coat of cows. Sprinklers should generally be on for 1-3 minutes, then off for the remainder of a 15-minute cycle. The length of time sprinklers run increases with increasing temperature. Fans should run continuously. (Janni, University of Minnesota Engineer, Evaporative systems for cooling dairy cows)
■ Fans and sprinklers (in humid environments) should be used in the holding pen to cool cows waiting to be milked, and time in the holding pen should be kept to a minimum.
■ Adequate number of fans should be spaced at about 12 feet high along the length of the freestall barn. The recommended distance between fans is 30 feet for 36-inch fans and 40 feet for 48-inch fans (Gay, Virginia Tech Extension Engineer, Pub 442-763).
■ Check fans to make sure they are angled correctly (20-degree angle) and are operating properly. Fans should be cleaned regularly.
Good ventilation is necessary for sprinklers to be an effective cooling option. Water added to a poorly ventilated area will produce a more humid environment and make the heat stress worse.
Drinking water is a critical component of heat abatement. A dairy cow's water consumption will increase by 29 percent as air temperature increases from 64 to 86 degrees F. Cows will drink about 50 percent of their total daily water intake immediately after milking, so having access to plenty of cool, clean water at this time is very important. Clean waterers on a routine basis to encourage water consumption.
Dry matter intake decreases when a cow is heat stressed, a contributing factor to reduced milk production. In addition to the energy requirement for lactation, there is an additional energy requirement due to increased respiration rates and panting associated with heat stress. There may be a 30 percent increase in energy needs during periods of high heat stress. In order to meet those needs when feed intake is declining, dairy managers need to pay attention to diets. According to the eXtension article mentioned previously, consider the following when formulating dairy rations during periods of heat stress:
■ Maintain effective fiber intake to insure rumination and rumen buffering. Decreasing fiber content and increasing the amount of starch in the diet in an attempt to increase the energy content could result in ruminal acidosis. High quality forages are essential during periods of heat stress.
■ Add yeast cultures to the diet. Yeast culture can help improve fiber digestion and stabilize the rumen environment. Several studies have even shown yeast supplementation to increase in milk production of heat stressed cows.
■ Modify the mineral content of the diet. Potassium and sodium needs increase as cows sweat. Increasing those minerals while maintaining an adequate cation-anion difference (DCAD) in the diet will require additions of sodium bicarbonate, potassium bicarbonate, or both to the diet.
Finally, when implementing a heat abatement program, do not neglect dry cows. An article in the April 2017 University of Kentucky Dairy Notes, "Heat Abatement for Dry Dairy Cows," included the following list of detrimental effects:
■ Cows that were heat stressed during the dry period gave birth to calves 13 pounds lighter.
■ Heifers born to heat-stressed dams had lower milk production compared to heifers born from dams not heat stressed.
■ Cows heat stressed during the dry period had lower milk production in the next lactation.
Heat abatement is a necessary management practice to maintain cow comfort and keep dairy cows healthy and productive.
Rory Lewandowski is an OSU Extension Agriculture & Natural Resources educator and may be reached at 330-264-8722.