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Hot weather can bring a long list of problems for dairy producers. When cows are heat stressed, they eat less, produce less milk, have reduced immune function and higher SCC, and show reduced fertility. A spike in lameness often follows the hot season. In severe heat waves, cows can even die. In addition to the economic burden, the discomfort from heat stress also reduces animal welfare.
This fact sheet focuses primarily on cooling cows housed in barns, where they are shaded from gaining heat from direct sun exposure. For information specific to pasture and grazing systems, see Dealing with Hot Weather in Pasture/Grazing Systems.
Do Your Cows Need Help Coping?
Consequences of heat stress, such as drops in feed intake and milk production, occur at a delay relative to when cows begin to experience discomfort. By the time you notice a change in milk yield, the problem may have been ongoing for at least a day or more.
To stay ahead of severe problems, we can look for other signs to tell us whether cows are coping well with heat stress, or if facilities need to be modified to provide better heat abatement. Environmental factors, such as air temperature, relative humidity, or Temperature Humidity Index (THI), can only tell us so much. The best strategy for troubleshooting heat stress on each unique farm is to observe how the cows are responding.
The following signs are some natural coping mechanisms cows use to try to dissipate heat and maintain a normal core body temperature. Often, these natural mechanisms are insufficient and they need a hand from additional heat abatement in their facilities.
A clear indicator of severe heat stress is panting. Look for any cows breathing with their mouths wide open, tongues out, stringy drool hanging down, or a combination of these signs (Figure 1).
Before severe panting occurs, cows will show increased respiration (breathing) rates. A rule of thumb is 60 breaths per minute or 1 breath per second. When several cows in a pen are breathing that quickly or faster, they are struggling to cope adequately with the heat and additional cooling would be beneficial.
When cows breathe faster, pant, and sweat, they lose moisture. This needs to be replenished by increasing water intake. A sign that cows are hot is not only drinking more, but also bunching around the water trough without drinking. Cows may also bunch away from the side and end walls of the barn. This behavior reflects shade seeking, as cows associate light intensity with heat.
In addition, cows will eat less, particularly during hot times of the day. If soakers are mounted over the feed bunk, cows will seek out the cooling and will spend more time there both eating and standing without eating. The soakers lower their core body temperature and improve feed intake and milk yield, but the increased standing time is also a risk factor for lameness.
Regardless of whether soakers are provided, cows spend more time standing up and less time lying down when heat stressed because while cows are lying down, their core body temperature rises. When they stand up, they can better dissipate heat, so body temperature falls. Based on this behavior, a higher proportion of cows standing is yet another indicator of heat stress.
This is why it is important to make sure there is sufficient, consistent, fast-moving air over the stalls or bedded pack. It is critical for the air to reach the cows and help them dissipate heat in the place where they should be spending over half their day lying down.
Air Exchange and Air Movement
Barn ventilation systems serve 2 important functions for heat abatement: air exchange and fast air movement.
Ventilation technically refers to air exchange, meaning bringing fresh air into the barn and removing warm, humid air, including noxious gases such as ammonia. This exchange of stale and fresh air is important for both heat abatement and promoting respiratory health in cattle and people working in the barns. In the summer, the target air exchange rate is 40-60 air changes per hour, whereas in the winter, 4-8 air changes per hour can suffice.
In addition, ventilation systems serve an important function for summer heat abatement by providing fast-moving air on the cows, helping them dissipate heat. This fast-moving air is beneficial for cows in many locations, including the holding pen, the parlor, at the feed bunk, and in the lying area.
An important outcome for cow comfort is sufficient resting time. Cows need to spend at least half of their day lying down, so high airspeeds are essential over the stalls or bedded pack. A target of no less than 200 feet per minute (2.3 mph), but ideally 400 feet per minute (4.6 mph) should reach down to a height of 20-30” above the stall base. Airspeeds can be measured using a hand-held device called an anemometer, such as those sold by Kestrel (https://kestrelinstruments.com/agriculture).
To get a good “bang” for your cooling “buck,” the holding area represents an opportunity to turn an area that could cause heat stress into a cost-efficient location to cool cows. Holding areas often have inadequate natural ventilation. Although many are equipped with recirculation fans to direct high velocity air onto the cows, the air can become increasingly hot and humid when it gets recycled within the space. Fast-moving air should be combined with soaker systems (read more below) to best take advantage of a cow’s time in the holding area to cool her. To promote proper heat loss, including the body surface area available for cooling, tight bunching of cows in the holding pen should be minimized. Groups of cows may need to be brought up for milking in a staggered manner to provide the necessary space, especially during extreme heat events.
Naturally vs. Mechanically Ventilated Barns
For some dairy operations, barns may have site challenges such as multiple buildings close together, blocking natural wind. For this and other situations, mechanically ventilated barns can promote air exchange by forcing air to travel through the barn using exhaust fans. These mechanical barns include tunnel barns, where air enters and exits from the short sides of the barns and flows parallel to the feed bunk, or cross-ventilated barns, where air travels perpendicular to the feed bunks.
To ensure proper fast-moving air at the cows’ standing and lying heights, these barns may use various additional features such as baffles to passively push air downward at specific locations (Figure 3), low flat ceilings to keep the moving air at cow height throughout the barn, or fans above the stalls to force a jet of air over the cows.
For details on barn ventilation design recommendations, see The Dairyland Initiative: https://thedairylandinitiative.vetmed.wisc.edu/home/housing-module/adult-cow-housing/ventilation-and-heat-abatement/.
Water-based cooling can take 2 main forms: misting vs. soaking. High-pressure misting or fogging injects the air with very fine droplets to lower the temperature surrounding the cows. At the same time, this results in an increase in relative humidity. Therefore, this strategy works best in drier climates like the southwestern U.S.
In contrast, low-pressure soakers, sprinklers, or showers deliver coarser droplets to wet cows directly. Energy from the cows’ body heat evaporates the water, cooling them. The water also draws heat away from the cows’ skin directly. These processes do not depend on relative humidity, so soaking works across a range of climates and regions. The cooling effects of soakers are enhanced when combined with fast-moving air from fans. In addition, the evaporating droplets also cool the air, just like with misters or foggers.
Soaking has been shown to be very effective for reducing respiration rate and body temperature, and for increasing feeding time, feed intake, and milk yield. Similar principles apply whether soaking cows at the feed bunk in their home pen, in the holding pen, in the parlor itself, or at the exit lane.
Nozzles should deliver approximately 1 gallon per spray application over the course of 3 minutes or less. At the feed bunk, each nozzle can cool at least 2-3 adjacent cows. The spray should activate every 15 minutes or more frequently, especially in hotter weather when evaporation is quicker.
All Age Groups Benefit from Cooling
Lactating cows produce twice the amount of metabolic heat that non-lactating animals do, which makes them particularly sensitive to the heat. Nonetheless, heat stress affects cattle of all ages.
When dry cows are heat stressed, not only is their welfare and future productivity affected, but also the developing fetus they carry. Calves born from heat-stressed dams have reduced health, survival, and performance. Calves and growing heifers benefit from heat abatement, similar to adult dry and lactating cows.
Providing shade or shelter to all age groups, including dry cows and youngstock, is a basic expectation for animal welfare. Checking that all animals have access to shelter is part of many welfare audits or evaluations. Shade can be in the form of either permanent or temporary structures. When positioning a shade structure, its orientation should be north-to-south on the long side. The shadow will move throughout the day as the sun moves across the sky, and cattle will follow the shadow. This will result in the area directly under the shade structure remaining a drier and cleaner place for cattle to rest. The distance the shadow moves is proportional to the structure’s height, so a taller structure will promote more frequent animal movement.
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