In the first part of a two-part article Brian Dougherty, field agricultural engineer for Iowa State University-Extension and Outreach, discusses how heat stress and lack of fresh air can cause bunching behavior in dairy cattle. He also provides solutions.  

Cows in freestall barns sometimes congregate in one part of a barn despite the availability of free space. Youngstock in group housing and cattle in lots or pastures also may exhibit the behavior. It’s often difficult to determine what’s causing cattle to bunch and what can be done to prevent it.

The simple answer is that it's a natural response to stress and that alleviating the stress will help prevent bunching. Stressful conditions can result in a herding instinct causing cattle to bunch even though it may be in a less comfortable area of the barn and result in more stress. Attempts to disperse the group are likely to fail and may cause additional stress. The underlying issue causing the stress must be addressed to prevent bunching.

Bunching is thought to be caused by one or more of four interrelated issues – heat stress, biting flies, lack of fresh air and light avoidance. Management strategies must be implemented to minimize the frequency of cattle bunching. If they favor one area more than another try to determine why. Then extend that benefit to other areas of the barn, lot or pasture.

Watch the cattle and ask yourself a series of questions. ‘When and where does the bunching occur?’ ‘Is there a noticeable timing or location pattern or is it more random?’ Once you have a general sense of the behavior, answering more specific questions can help pinpoint the cause and point toward potential solutions.

Heat stress is a common culprit of cattle bunching. If bunching occurs during hot weather less air will move through the group of animals. That will make them even more susceptible to heat stress. Assess the barn conditions. Do cows bunch more as temperature or the temperature-humidity index increases? Do they disperse once the temperature decreases to a certain point? If the answer to either question is yes, heat stress is likely playing a role in bunching behavior.

Solutions – To improve cow cooling in freestall barns address critical areas where heat stress occurs. At a minimum fans must be located and spaced to provide adequate airspeed to sufficiently cool cows in both their resting area and the holding area.

In the cow resting area aim for a minimum airspeed of 200 feet per minute or 2.25 mph at head height as the cows are laying down. Take measurements in several locations and identify dead spots or areas with poor airflow. They typically occur near crossover alleys, divider walls and other obstructions. Wind speed, temperature, humidity and other measurements can be made with handheld devices, generally starting at a cost of about $150. Check with an Extension service, veterinarian or agribusiness representative to see if they provide devices or assistance.

If recirculation fans are used they should be spaced 24 feet to 30 feet apart over every row of stalls. They should be angled downward to point toward the stall surface underneath the next fan in the row. Additional fans can be located to address areas with low airspeed. Large-diameter recirculation fans – 72 inches – can be spaced as much as 40 feet apart. They should have louvers to help distribute air more evenly across the stall surface.

Dirty fans can lose as much as 40 percent of their airflow capacity. Make sure fans are clean and well maintained and that drive belts are tight to ensure maximum airflow. That’s especially important in mechanically ventilated barns where airflow is provided by fans.

Providing cooling in the holding area is critical for heat-stress abatement. Cows undergoing heat stress in the holding area may have difficulty cooling later on and may exhibit bunching as a result. Limit time spent in the holding area and provide sufficient fan capacity.

Holding-pen ventilation rate should be about 1,000 cubic feet per minute per cow. Holding-pen sprinklers should be capable of providing at least 0.03 gallons per minute per square foot of area. Holding-pen sprinkler timers should be set to provide one minute of wetting followed by six minutes off. Directly wetting the cow is more effective when combined with fans because air velocity increases the rate of evaporation. Sprinklers in the holding area should never be used without mechanical ventilation because the increase in temperature-humidity index can cause severe heat stress.

Fans and sprinklers along the feed bunk can provide additional cooling. Avoid wetting stalls and feed if possible. The cooling effect from sprinklers comes from letting water evaporate from the cow. Continuous wetting is counterproductive because it doesn’t allow time for evaporation to occur. Sprinklers need to be on a timer system with programmable on-off periods.

Feed-alley sprinkler systems are typically designed with nozzles and piping sized to provide about 0.03 gallons per minute per square foot of wetted area per cycle. The recommended cycle is 0.5 to 1.5 minutes on – long enough to soak to the skin – followed by 10 to 15 minutes off to allow for evaporation. The cycles can be shortened to five minutes off as temperatures increase, but the cow still needs to time to dry to receive the cooling benefit.

In both the holding area and feed alley it’s important to use sprinkler nozzles that provide a large droplet size. That helps to ensure cows are wetted through to the skin. Avoid misting systems as they create small droplets that can cause air to lock between hair and skin, which prevents evaporation. Low-pressure systems with regulators to keep water-line pressure less than 20 pounds per square inch work best for producing larger droplets.

Access to plenty of clean fresh water is a must regardless of production system. Water helps remove body heat via cooling the digestive system, respiration and sweating. Cows consume as much as 50 percent of their daily water intake within an hour of milking so providing fresh clean water at the parlor or barn exit is a good strategy.

For pasture-based systems try to provide 40 square feet to 50 square feet of shade per cow on hotter days. Consider portable shade systems with a north-south orientation to prevent wet areas from developing underneath.

Lack of fresh air can cause stress and is directly linked to poor ventilation. There are additional questions one can ask to determine if lack of fresh air may be contributing to bunching. ‘In mechanically ventilated barns, is the fresh-air-intake area adequately sized?’ ‘Do cows bunch near fresh-air inlets or areas with faster moving air?’ ‘In naturally ventilated or hybrid-ventilation barns, do cows bunch more on days with slower wind speeds?’

Solutions – It’s critical that the natural- or mechanical-ventilation system brings fresh air into the barn rather than just recirculating it. Introducing fresh air is the only way to displace hot, humid, stale air from the barn. In mechanically ventilated barns – tunnel or cross-ventilation – measure airspeed entering the inlet area. Inlet airspeed should be 500 feet to 800 feet per minute – 5.7 mph to 9 mph – to ensure good air mixing inside the barn. Adjust the inlet area to achieve proper airspeed and-or add four-fan capacity to introduce more fresh air.

In natural or hybrid-ventilated barns with open sides, airspeed will vary. Make sure that at least half of the sidewall area is open on both sides of the barn. In either case a fogger or smoke stick can be used to determine airflow patterns and to detect dead spots.

In the second half of the article Dougherty will discuss the effect of flies and light avoidance on bunching behavior. He also will offer some solutions.