When you think about it, dairy producers impose a lot of stress on their cattle. Cows are bred to calve once a year or so; they are pregnant for 9 months and lactating for 10 or more months; they are machine milked 2 to 4 times daily; and we continue to feed and genetically select them to produce even MORE milk. Consequently, as the demands for increasing milk yield and efficiency continue, more and more stress is placed on the cow’s productive capacity. Thus, it makes sense to keep them as comfortable as possible.
For example, to enhance cow comfort, we provide housing with freestalls of the proper dimensions filled with bedding that is soft, clean, and dry. Our pastures contain shade structures and are maintained in a clean and dry condition. Water is available ad libitum and provided fresh, clean, and cool. Lastly, we manage our herds to counter heat stress in the summer season, and, because we expect higher production from our cows, controlling heat stress becomes extremely important.
The stresses associated with hot and humid environments have huge adverse effects on a dairy cow’s metabolism by greatly increasing her body maintenance requirements. It must be kept in mind that dairy cattle are of northern European origin and are generally intolerant of high environmental temperature above 77°F, especially when the relative humidity is greater than 80%. Moreover, older, heavier, high-producing cows are more susceptible than smaller or younger animals.
Heat Stress Has a Deleterious Effect on Production
The negative effects of heat stress include:
- Depressed feed intake
- Decreased milk yield, fat, and protein
- Increased weight loss
- Elevated SCC
- Increased clinical mastitis
- Elevated rectal temperatures
- Impaired or compromised immune system
- Increased metabolic problems
- Increased morbidity and mortality
- Impaired reproduction
- Increased respiration rates, panting, and salivation
Several factors influence heat stress such as air temperature, air flow, wind velocity, ventilation, relative humidity, animal health, solar radiation, animal crowding, insect pests, location, and breed. The cow herself can control her body temperature to some degree by:
- Convection: seeking air currents or wind
- Conduction: lying down on cool earth or mud
- Radiation: dilating her blood vessels
- Evaporation: panting, salivation, and sweating
But, under conditions of excessive heat and humidity, we need to help her keep more comfortable. Strategies to control heat stress are aimed at maintaining feed consumption, preventing milk production losses, and minimizing mastitis and other disorders such as acidosis, ketosis, and dystocia.
Heat Stress Must Be Properly Managed
Methods to minimize stress include:
- Providing adequate shade to reduce solar radiation
- Cooling cows with sprinklers and fans, commercial coolers, tunnel ventilation, or cooling ponds
- Providing fresh, cool (50°F) drinking water (cows drink 50% more water at temperatures of 80°F and above compared to 40°F)
- Feeding a high-energy ration with high-quality forages, and doing so in the early morning or late afternoon to encourage consumption
- Increasing dietary potassium, sodium, and magnesium and providing free-choice salt
- Supplementing the diet with vitamin A, ß-carotene, vitamin E, and selenium
- Feeding a fat supplement to increase the energy density of the diet
- Avoiding the handling of cows (e.g., pregnancy checks) during the hottest part of the day
Helping the cow to cool herself is probably most important, and shade is probably the easiest and least expensive way to do this. However, it is important to keep shade structures away from feed bunks because cows tend to defecate and urinate where they eat. If there is shade over the bunk, they will lie down in feces and urine, which is a prime environment for mastitis-causing bacteria, which grow even better when it’s hot and humid. Under these conditions, factors such as rain, mud, manure, and bedding become important as they influence the numbers and types of microorganisms present on udders and teats. Environmental mastitis pathogens such as E. coli and Strep. uberis grow where it is warm and moist, so it is imperative to keep bedding materials and calving areas as clean and dry as possible and to avoid wet, muddy areas where cows lie down.
Cooling with water and/or fans includes:
- Sprinkler systems over the feed bunk in front of freestalls to wet down the cow’s head and shoulder areas in combination with fans to move away the warm, humid air from the cow's body.
- Commercial coolers that combine air turbulence and high-pressure water pressure injectors to lower the ambient temperature under shades. One study showed increased milk production and body weight and reduced death rate.
- Corral manger misters with fans to carry away the humid air. Studies show decreases in milk loss due to heat stress and reduced death loss due to mastitis.
- Mechanical refrigeration with evaporatively cooled shades. Expensive and limited to areas with low relative humidity.
- Shower and fanning station in milking parlor holding pen. Studies show a reduction in cows’ body temperature and an increase in yield. But cows definitely need an adequate drip-dry time before udder preparation prior to milking.
- Shower and fanning station in milking parlor exit lane. Spraying should cover only the top and sides of the cow so that the post-dip germicide is not washed off. In this way, they are temporarily relieved from the sun, and instead of returning immediately to the shade, they follow their normal cool weather practice of eating and drinking after milking, which keeps them on their feet and allows time for teat duct closure before contact with soil and manure.
- Tunnel ventilation provides air movement and air exchange through fans placed in one gable endwall of the barn. Fans create a negative pressure in the barn causing air to be drawn into the opposite endwall opening. Fresh air flows longitudinally through the barn and is exhausted by the tunnel fans.
- Cooling ponds. Must not be stagnant; fresh water must be pumped in continuously and the overflow carried into contained settling areas.
Reproductive Performance Is Also Affected by Heat Stress
In addition to decreasing milk yield and quality and increasing mastitis and SCC, heat stress impairs animal reproduction. Factors include:
- Poor estrus expression
- Reduced egg viability
- Increased embryo mortality
- Retained placenta
- Compromised uterine environment
- Early calving
- Increased dystocia and morbidity
- Reduced calf weight
- Increased calf mortality
Bottom line: Properly cooled cows are more productive because they are comfortable. Cooled cows produce more milk (up to a 10-pound increase/day) and have less mastitis and lower SCC. Likewise, they experience fewer metabolic disorders such as acidosis and fewer feet and leg problems. Reproduction is also improved such as higher conception rates, fewer embryonic deaths, less dystocia, and having larger calves.
Stephen C. Nickerson, University of Georgia