Evaporative systems for cooling dairy cows
Dairy cows experience heat stress in hot and humid weather because they have trouble getting rid of heat generated by digestion and metabolic processes. Cows experiencing heat stress have reduced milk production and reproductive performance. Heat stressed cows will have increased respiration rates, water intake and sweating and decreased dry matter intake.
Tools for fighting heat stress include providing plenty of fresh clean drinking water, increasing ventilation air exchange, increasing air velocity at cow level, using low-pressure sprinklers, high-pressure misters or evaporative cooling pads. Recently I was asked about the similarities and differences between low-pressure sprinkler and high-pressure misting systems. Both systems and evaporative cooling pads are used to reduce the heat stress dairy cows can experience.
Low-pressure sprinkler systems, high-pressure misting systems and evaporative cooling pads produce cooling by evaporating water. Evaporation converts liquid water to water vapor. It takes energy to evaporate water and as water evaporates the energy source is cooled. The low-pressure sprinkler system gets the energy from a cow's skin. The high-pressure misting system and evaporative cooling pads get the energy from air.
All of these evaporative cooling systems add moisture to the air and raise the relative humidity. Good ventilation air exchange is important to bring in drier air and avoid excessive relative humidity levels.
Low-pressure sprinkler systems wet a cow's back to the skin with a brief 3 minute shower followed by a 12 minute non-sprinkling period. During the non-sprinkling period the cow's body heat is used to evaporate the water from the skin and hair coat. This evaporation provides a cooling effect because it helps the cow get rid of heat from its skin. Sweating produces the same effect. It is the same feeling you get coming out of a shower or swimming pool. It is critical to have the non-sprinkling period for the water to evaporate from the skin surface. The evaporative cooling effect is lost without a non-sprinkling period.
Low-pressure systems operate at water pressures between 20 and 40 pounds per square inch (psi) and use nozzles that have flow rates near 0.5 gallons per minute. The low-pressure nozzles produce large droplets that soak a cow's back to the skin. Avoid excessive sprinkling, it wastes water and adds water to the manure system. Low-pressure systems can be used in the holding area and along feed lines. Sprinklers along the feed lines should be installed to avoid getting feed in the feed manger and bedding in the freestalls wet. Water lines need to be sized to provide adequate water flow for uniform distribution along a feed line or across a holding area based on the number of sprinklers.
High-pressure misting systems are sometimes called fogging systems. They produce very fine water droplets that are shot into the air. As the very fine water droplets evaporate they take energy from the air, which cools the air and lowers the air's temperature. The cooler air temperature helps the cows loose body heat which is the cooling effect you want. High pressure misting systems can run continuously if cooling is needed.
High-pressure systems operate at water pressures 200 psi or higher. Higher water pressures can produce smaller droplets. Avoid letting water pressures decline. At low water pressures droplet sizes increase so that they do not evaporate before reaching the ground where they can wet bedding or feed. High-pressure system nozzles can be attached to fans that direct the mist or they can be located in the inlet side of a cross-ventilated barn or above head locks. High-pressure systems require clean water and in-line filters to minimize plugging of the high-pressure nozzles.
Evaporative cooling pads are common in low-profile cross-ventilated barns. Evaporative cooling pads have been used for years in mechanically ventilated poultry, swine and greenhouse buildings. They work by drawing outdoor air through wet porous pads. Within the pad some of the water evaporates, taking heat from the air and lowering the air's dry-bulb temperature. The amount of ai r temperature drop depends on the amount of moisture in the outdoor air. In dry climates evaporative cooling can reduce incoming air by 10 to 20 F, but 5 to 10 F is more common in more humid climates. Water used to wet the pads can be recirculated. Evaporative cooling systems and pads require regular maintenance to minimize algae growth and accumulation of dirt and minerals on the pads and in the water.
All three systems can be effective if designed, installed and managed well. All three have been used by livestock producers for many years.
Additional information on dairy cow cooling systems can be found at http://www.extension.umn.edu/dairy/management/facilities.htm.