Feeding Heat Stressed Dairy Cows

In Manitoba, cattle undergo both cold and heat stress during the course of a year. During the winter months, cold stress is the result of both temperature and windspeed. In the summer months, the degree of heat stress is determined by the temperature-humidity index (THI) - a combination of temperature and humidity For example, the THI of a day at 24°C and a relative humidity of 85% is 74. A 29°C day with only 15% relative humidity has a THI of 72. When the THI exceeds 72, cattle begin to experience heat stress. It has been estimated that dairy cattle in Western Canada experience some degree of heat stress on 40% of summer days.
The effects of heat stress are costly. Cows lower their feed intake during hot weather by 10-12% or more. This leads to reduced milk yield (15-40%), poor reproductive performance and a greater incidence of metabolic health problems. Understandably, the bulk of research relating to the feeding and management of heat-stressed dairy cows has been undertaken in the southern U.S. where cows experience heat stress for long periods of time. In Manitoba, heat stress tends to occur periodically rather than continuously. Research at the University of Manitoba by Dr. Kim Ominski shows, however, that the effects of even short-term heat stress can have long-lasting effects. Dairy cows were exposed to a 3-day thermoneutral phase, a 5-day heat stress phase and a 7-day recovery phase. The response to heat stress, both in terms of feed intake and milk production, was not immediate. Feed intake began to drop with the start of heat exposure but the lowest intake was not achieved until day 4 of the heat stress phase when intake was 9% lower than during the thermoneutral phase. Intake remained depressed until day 4 of the recovery phase. Maximum milk loss (3.5 kg/day) did not occur until day 1 of the recovery phase. Milk yield was still depressed on day 7 of the recovery phase, resulting in an overall loss in yield of 8.3%. Dr. Ominski calculated the economic value of lost production to be $0.45/cow/day and $1.78/cow/day during the heat stress and recovery phases, respectively.
There are a number of strategies that dairy producers can undertake to reduce the effects of heat stress during our Manitoba summers.
    1. Compensate for the drop in feed intake by increasing the nutrient density of the diet.
      • For example, a diet with 16% crude protein at an intake of 22kg/day will need to be reformulated to 17.6% crude protein if intake falls to 20 kg. Avoid feeding excess protein as the excess is excreted and this process creates heat and also utilizes energy which should go towards milk production.
      • Supplemental fat, high energy grain (eg. corn), and/or increasing % concentrate may be considered as ways to increase energy density. Be sure to continue following ration guidelines for fibre when adjusting diets.
      • Feed high quality forages during times of heat stress. High quality forages are more palatable, more nutrient dense and produce less heat during digestion.
    2. Provide unlimited fresh, clean water. Water intake increases significantly during times of heat stress. Locate water bowls and/or troughs to encourage consumption and keep them clean. Shading of water troughs helps to increase water intake. Research shows that chilled drinking water helps to cool the cow and improve feed intake.
    3. Increase the number of feedings each day. Frequent feeding provides fresh feed, stimulates cow curiosity and encourages more frequent feeding.
    4. Keep feedbunks clean. Remove left-over feed daily. Clean away any moldy and/or heating feed from the corners and edges. This improves palatability and decreases the fly population.
    5. Feeding at night is a popular management practice in heat-stressed areas such as the southern United States. Night feeding allows the heat of digestion to be produced during the cooler hours of evening when it is more easily dissipated. Feeding 60 to 70% of the ration between 8:00 p.m. and 8:00 a.m. has successfully increased milk production during hot weather. However, the University of Manitoba found that this practice was not effective in alleviating production losses associated with short-term, episodic heat stress.
    6. Requirements of potassium, sodium and magnesium are increased during heat stress. Diets for heat-stressed cows should contain 1.5% potassium, 0.5% sodium and 0.35% magnesium.
    7. Keep cows as cool as possible - provide shade and ensure barns are well ventilated with good airflow (ie. fans). Sprinklers are also an effective means of cooling cattle.
    8. Feed additives, such as yeast and Aspergillus oryzae, have variable results in preventing the drop in intake and production associated with heat stress. Research at the University of Manitoba showed that supplementation with the fungal culture Aspergillus oryzae did not reduce body temperature or prevent a drop in milk production.