Heat stress in dairy cows is one of the foremost causes of decreased production and fertility during summer months. The losses are obvious in the form of decreased milk production, increased days open and increased matings per conception.
Some heat stress is inevitable, but effects can be minimized if certain managemental practices are followed. The exotic cow breeds like Jersey and Frisien are more prone to heat stress than indigenous breeds. The comfort zone for milking Frisien and Jersey cows is between 6 and 18°C. Between -5°C and +5°, apetite will be stimulated. Above 24°, dry matter (DM) intake decreases by about 3% for every rise of 1.2°C; then, at the upper level for discomfort above 27°C apetite is depressed then both biological and economic efficiencies decline.
A cattle have the limited ability to sweat, and have to lose heat mainly through evaporative cooling from respiratory tract. The increase their breathing rate to increase movement of air over the moistened surface of the mouth and the nasal passages. However, if humidity levels are high, this evaporative cooling is less effective, and cattle may be unable to dissipate accumulated body heat.
The sources of heat include environment, fermentation of rumen contents, body heat from other cows. The severity of heat stress in influenced by many factors including temperature and humidity, length of heat stress period, degree of night cooling available, ventilation and air flow, cow breed and size, level of milk production and dry matter intake prior to heat stress, housing type, overcrowding, water availability, coat colour, if exoposed to sun, hair coat depth.
Normally degree of heat stress is related to both ambient temperature and humidity although fermentation of feed in the rumen also generate heat. Stressed cows are not likely to want to eat as much because it further contributes to their discomfort. More heat can come from the body heat from other cows when they are all clustered to gather in yards as occurs before milking. The movement of cows into a holding yard before milking can therefore exacerbate heat stress if there is no cooling system in place.
Heat stressed cows eat less, produce less milk, and are more difficult to get pregnant. Humidity also plays a signiﬁcant part in heat stress. There are three temperature-humidity ranges of concern. A temperature of 100 degrees Fahrenheit and 20 percent humidity is the range in which you begin serious measures to ease the stress on the cattle. Some type of cooling should be started. The danger occurs as the temperature nears 100 degrees Fahrenheit and 50 percent humidity. The lethal range for cattle is 100 degrees Fahrenheit and 80 percent humidity.
Because cattle sweat at only 10 percent of the human rate, they are more susceptible to heat stress. This is why dairy cattle need mechanical means to reduce heat, such as body sprinkling to aid in evaporation and effective air movement systems to aid in cooling. Stale, stagnant air can reach dangerous or lethal areas in a short time. Therefore, it is essential to have rapid movement of air in any conﬁned area.
During times of heat stress feed intake is reduced by 8 to 12 percent or more. This reduction in feed intake reduces volatile fatty acid production in the rumen that results in decreased milk production.
Signs of Heat Stress
Increased body temperature, respiration rate, pulse rate, eyes and muzzle becomes dry, increased water intake, seeking shade, refusal to lie down, reduced feed intake and/or eating smaller amount more often, crowding over water troughs, body splashing, agitation and restlessness, reduced or halted rumination, grouping to seek shade from other animals, open mouthed and labored breathing, excessive salivation.
Some signs of heat stress in lactating cows are obvious, especially the reduced milk production and the lethargic behavior of the cows. Moderate signs of heat stress may occur when the temperature is between 80° and 90°F with the humidity ranging from 50 to 90 percent. As temperatures rise to 90° to 100°F and humidity remains in the 50 to 90 percent range, the cow will show severe depression in milk yield, usually greater than 25 percent, and in feed intake as her body temperature elevates. She will begin exhibiting more significant signs of heat stress, such as open mouth breathing with panting and her tongue hanging out. Usually, the combined temperature and humidity that results in a temperature/humidity index of greater than 90 will result in severe signs of heat stress in the high producing cow and moderate signs in lower producing cows.
In severe cases, cows may die from extreme heat, especially when complicated with other stresses such as illness or calving. High producing cows exhibit more signs of heat stress than lower producing cows because higher producing cows generate more heat as they eat more feed for higher production.
In general, the decrease in milk production results from less feed intake by the cow. Two pounds of milk production is lost for every pound of decreased dry matter intake when temperature and humidity levels are high. If the dry matter intake of the feed and milk production has decreased by 10 percent or more, your cows are also exhibiting heat stress. During severe heat stress, intake and milk production may decrease by more than 25 percent and weak cattle may die, especially older or sick animals. Prevalence of insects creating diseases may be increased due to increase in environmental temperature.
Measures to combat Heat Stress
Milk yield can be increased by up to 3-5 kg/day through effective cooling strategies. The first step is designing shed according to the climate. In a hot climate, maximum use should be made of ventilation by using open-sided sheds. Good orientation of shed allows early morning and late afternoon sunlight to reach and dry out the floor while protecting cattle from the hot mid-day-sun.
The ideal orientation for ventilation would allow the prevailing winds to hit the shed perpendicular to the side. The shed should be sited so that the prevailing winds in not blocked by obstacles or other buildings. Ideally, the shed should be on the highest ground possible.
A second step to reduce heat stress is to provide cool water and shade for all milking and dry cows plus heifers. Water should be available for cows near their loafing area, either in the shade of native trees or artificial shade. The second step to alleviate heat stress in lactating cows is to provide a more comfortable environment in the holding pen.
A third step to reducing heat stress on your cows is to provide shade and a cooling device around the feed manger. A fourth step to decrease heat stress is to increase the density of the ration.
Keeping lactating cows cool can provide a good return on your investment as it makes cows more comfortable, thereby making them more productive. Shade and cool water should be available to cows and heifers at all times. Cooling devices like fans and showers should also be installed in the holding pen and feeding area if possible. The ration should be properly balanced, and generally the energy density should be increased in the summer to help compensate for decreased dry matter intake of the cow.