Understanding Heat Stress in Dairy Cattle
Glenn A. Shirk, Extension Dairy Agent, Lancaster, Pa.
May 10, 2002
Think of cows as furnaces. The more you stoke them (feed them) the more heat they produce, and as we strive to maximize DMI and milk output, we sure do stoke our high producing cows!
The most comfortable range for lactating cows is around 41 to 77 degrees Fahrenheit, and that will vary with humidity. When humidity levels are under 30% cows can become moderately stressed at temperatures around 90 degrees Fahrenheit, but when humidity levels increase to 80% or higher, moderate stress can occur at temperatures of 75-80 degrees. Roughly, this is when the temperature-humidity index is in the 80's. Severe stress can occur when this index gets into the 90's; that's when temperatures rise to around 95 degrees or higher and humidity levels reach 80% or higher. The problem magnifies when temperatures remain high at night and cows don't have a chance to cool down. If it gets bad enough, heat stress can become life threatening to cows.
What contributes to increased body temperatures? Digestion (stoking the cow's rumen with feed, especially with hard-to-digest fibrous feeds) and other normal metabolic functions performed by the cow's body, cow activity, increased humidity and still air which reduce the ability of cows to evaporate moisture and dissipate heat from their body, high air temperatures, solar radiation, and dark colors (dark-colored cows absorb about twice as much heat as light-colored cows).
How does heat stress affect cows? High producing cows are more sensitive to heat stress than low-producing and non-lactating cows. The first thing you may notice is a drop in feed intake, perhaps as much as 33% or more, which then translates into a similar drop in milk production. But, heat stress also reduces the dry matter intake (DMI) of transition cows (that includes springing heifers), and that's the last thing you want to have happen to cows prior to and after calving! Heat stress in the last trimester of pregnancy contributes to lighter birth weights, reduced vigor in newborn calves, increased calving difficulties - - and all of its related problems, plus a significant loss of milk production in the upcoming lactation. It can also lower cows immune response, thereby reducing their ability to fight off mastitis and other infections. Heat stress increases cows respiration rates and panting, and that increases their nutrient needs. At 95 degrees, cow's nutrient requirements may be 15-20% greater than they were at 70 degrees. You can imagine what will happen to bulk tank weights when DMI becomes depressed at a time when cows nutrient needs become greater. In fact, you probably don't have to guess because chances are, you probably have already experienced these production declines in times past! Cows also lose more body fluids due to increased sweating, slobbering and urination. With these fluid losses, water intakes may increase by about 33%. Potassium losses increase with sweating and sodium losses increase with urination. Feed passage rates and gut motility slow down which in turn, reduces feed intake. The rumen may become more acid, and fat tests
can be depressed.
Body temperatures increase, as indicated by rectal temperatures. Blood flow to the skin is increased in an effort to help dissipate excess body heat. This reduces the flow of blood to internal organs, decreasing the absorption and transport of feed nutrients, thus reducing milk secretion. Excess body heat can damage the ovum, the embryo and sperm. High uterine
temperatures can interfere with implantation of the embryo and trigger early embryonic death (repeat breeders). Cows that lose a lot of flesh and are in negative energy balance, due to reduced DMI, may become anestrus and fail to conceive. Once a severe heat spell ends, it may take several weeks for cows and bulls to recover and produce normal eggs and sperm. Heat stressed cows are more lethargic. They don't feel like walking to the feed bunk or to the watering trough, they may "give up" earlier in times of calving difficulties, and they may show less interest in displaying heats. They will seek out breezes and places of shade, and cool wet (sometime sloppy) places to rest. So, listen to what your cows are telling you by the actions they display. In an upcoming article, I will be discussing some ideas for reducing the impact of heat stress on cows.
