Heat stress occurs when a cow’s heat gain is greater than her ability to lose it. Balancing heat gain and loss over a 24 hour period is the goal.
Productive dairy cows may experience heat stress when the Temperature Humidity Index (THI) is 68 or greater. In ‘more humid’ climates this can occur at temperatures as low as 72oF. Heat stress can be reduced by slowing heat gain to the cow, and improving heat transfer rate from the cow. Basic heat stress abatement techniques include Shade, Air, and Water – or SAW.
Protecting cows from direct solar radiation helps lower their body temperature and respiration rate. Shade can be provided by trees, buildings, or shade structures. Roofs and shade structures should be at least 12 feet high and oriented properly. Buildings and covered outside feeding areas are typically placed east –west to minimize sunlight intrusion throughout the day. Placing pasture or dry lot shade structures north-south allows shade to move from west to east, helping to keep the resting area drier.
An air exchange every minute or less during the summer months is essential to remove moisture, gases, heat, and other pollutants from the animal space. Without a proper air exchange other heat stress abatement techniques will not work effectively.
Mechanically ventilated dairy buildings use exhaust fans and properly sized and placed inlets throughout the animal space. Tunnel ventilation can provide a rapid air exchange – typically less than 45 seconds – in tie stall barns.
Naturally ventilated buildings depend primarily on wind speed and direction to drive the air exchange. Buildings with high, side and end walls fully open to resting cow level create a preferred ‘pavilion-like’ design during the summer.
When the warm weather exchange rate in naturally ventilated buildings is challenged by topography, up wind obstacles, or building limitations, well designed tunnel or cross ventilation systems can be used provide the necessary air exchange.
Turbulent air movement around cows increases convective heat transfer, enhances evaporation, and minimizes ‘hot spots’. Air speeds of 3.5 to 5 miles per hour (mph) are preferred in resting, feeding, and holding areas.
Thirty-six to 52 inch diameter axial circulation fans can provide excellent animal space air movement. To be effective fans placed in-line must be no further than 10 times their diameter apart. For side-by-side applications, place fans two to three times their diameter apart.
Large high volume, low speed (HVLS) fans can also provide air movement at cow level, but they must be placed over the animals, and usually no more than twice their diameter apart.
Increased respiration and urination during hot weather may increase drinking water intake by 20 percent or more. Watering stations need to be located conveniently, allow multiple cows access, and keep up with water demand.
Evaporative cooling uses water to increase heat transfer from cows. The evaporation of a pound (or pint) of water requires about 1,000 British thermal units (Btu) of energy, approximately the heat produced by 1,000 four inch wooden matches.
Direct evaporative cooling (DEC) systems intermittently apply and evaporate water from the cow’s skin, drawing heat directly from her body. Indirect evaporative cooling (IEC) lowers the temperature of air surrounding the cow, increasing her heat transfer rate.
Spray cooling systems are low pressure DEC systems installed in feeding and holding areas that use a five to 15 minute wet-dry cycle. Spray nozzles emit a coarse droplet that penetrates the cow’s hair coat soaking her skin for one to three minutes. Fans provide air movement for the remainder of the cycle to speed evaporation and draw heat away from her body. Studies show the respiration rate of a heat stressed cow decreases with the first wet-dry cycle. DEC seems to be the most effective evaporative cooling method for cows in more humid climates like Pennsylvania. However, it can require a significant water supply and good drainage.
Indirect evaporative cooling (IEC) uses heat in the air to evaporate water, lowering the dry bulb air temperature. The heat transfer rate increases when the difference in temperature between the cow body and surrounding air is greater. Heat transfer from within the body also improves as cows inhale cooler air.
Fogging and misting are examples of IEC systems that use pressure to force water through nozzles emitting very small droplets. High pressure systems emit finer droplets that have a better chance of evaporating before settling on the cows hair coat, resting surface, and floor. Nozzles for lower pressure systems emit larger droplets and typically installed on circulation fans so air movement can aid in evaporation. Pressurized IEC systems are popular in arid climates where the droplets are more likely to evaporate suspended in the air. These systems are prone to ‘drift’ in naturally ventilated buildings.
Evaporative pads are another method of IEC. Thick, water-soaked corrugated pads are installed at inlet opening(s) used with tunnel and cross ventilation systems. Outside air drawn through the pad evaporates as much moisture as the air conditions allow, lowering the dry bulb temperature. All air drawn through inlet is cooled and can only pick up as much moisture as the air conditions allow.
Since evaporative cooling systems incorporate adding water to the animal space air, ventilation systems that provide a good air exchange to remove moisture laden air, and circulation fans to enhance evaporation are essential.
The techniques for combating dairy cow heat stress currently available includes shade, an adequate air exchange, good air movement, drinking water, and evaporative cooling. Used properly these tools can help balance the daily heat gain and loss of dairy cows, minimizing heat stress effects, and improving cow health, production, and well-being during the summer season.
Prepared by Dan F. McFarland, Agricultural Engineering Educator, Penn State Extension