Aug. 10 2020 01:22 PM

A combination of factors, including heat stress and flies, can cause cows to stand in groups.

The author, a former dairy farmer, is now a field ag engineer for Iowa State University Extension and Outreach.

Their herding instinct leads cows to bunch together as a reaction to stress, even in hot weather.

Bunching of animals is a common problem in dairy facilities during summer months. It can be very frustrating as it is often difficult to determine what is causing cattle to bunch and what can be done to prevent it.

The simple answer is that bunching is a natural response to stress, and alleviating that stress will help prevent bunching. Stressful conditions can result in a herding instinct that causes cattle to group together, even though the resulting bunching may be in a less comfortable area of the barn and it often further elevates the level of stress.

Attempts to disperse the group are likely to fail and may cause additional stress to animals and frustration to their caretakers. The underlying cause must be addressed to prevent bunching.

A combination of factors

Bunching is thought to be caused by one or more of these four interrelated issues:

  1. Heat stress
  2. Lack of fresh air
  3. Flies
  4. Light avoidance

Assessing the causes of bunching starts with observing the cattle and asking a series of questions. When and where does the bunching occur? Is there a noticeable timing or location pattern?

Once you have a general sense of the bunching behavior, answering a more specific set of questions can help narrow down the cause and point toward potential solutions.

Heat stress is a common culprit of cattle bunching. Do cows bunch more as temperature humidity index (THI) rises? Do they disperse once the THI drops below a certain point? If yes, heat stress is likely playing a role in the bunching behavior.

Keep the air moving

Lack of fresh air can also cause stress and is directly linked to poor ventilation. Do cows bunch more on days with slower wind speeds or near fresh air inlets?

To reduce bunching, it is essential to bring fresh air into the barn and implement heat abatement strategies. Access to plenty of clean, fresh water in multiple locations is a must.

There should be adequate airspeed to sufficiently cool cows in both the resting area and the holding area. Limit time in the holding area and provide 1,000 cubic feet per minute (cfm) or more airflow per cow. Aim for a minimum airspeed of 2.25 mph in the cow resting area. Take measurements in several locations and identify any areas with poor airflow near crossover alleys, divider walls, and other obstructions.

Add fans as needed to achieve the target airspeed. In mechanically ventilated barns, inlet airspeed should be 5.7 to 9 mph. Dirty fans can lose up to 40% of their airflow capacity, so regular cleaning and maintenance are critical. In barns with open sides, make sure at least half of the sidewall area is open on both sides.

Sprinklers can provide additional cooling in the holding area and feed alleys. Sprinklers are more effective when combined with fans because air velocity speeds up the rate of evaporation. The cooling effect comes from letting water evaporate off the cow, so continuous wetting is counterproductive. Sprinklers need to be on a timer system and should provide a large droplet size to ensure that cows are wetted to the skin. They should never be used in the holding area without mechanical ventilation because the increase in THI can cause severe heat stress.

Airspeed and other measurements can be made with handheld devices starting at $150. A fogger or smoke stick can be used to determine airflow patterns and detect dead spots. Check with your local extension service, veterinarian, or agribusiness professional to see if they provide devices or assistance.

Reduce the flies

Flies are another common culprit of cattle bunching. Do you notice the cows displaying fly-avoidance behaviors?

Observing cattle behavior can help determine which type of flies may be causing bunching and guide strategies for remedying the problem. Stable flies prefer cattle’s legs and bellies and can deliver a painful bite. Foot stomping is a common symptom.

Horse flies and deer flies are much larger than stable flies and also cause a painful bite. They are more common near natural water sources and typically feed on the back, neck, and sides of cattle.

Horn flies spend most of their time on cattle and generally occur as small clouds of flies over the animals’ backs. They often cause cattle to throw their heads over their shoulders.

Face flies congregate around the head but do not bite and only stay on cattle for short periods of time. Cattle typically react by flapping ears and shaking their heads from side to side.

If cows are exhibiting fly avoidance behaviors, take steps to reduce fly population. Eliminating breeding areas is an effective control method. Fresh manure and piles of moist, decaying feed are common areas where flies lay eggs. Remove vegetation and keep wet or spoiled feed away from the barn, fill in areas with water ponding, and maintain general facility cleanliness to reduce breeding areas.

Premises spraying, fly sprays, pour-on insecticides, or in-feed chitin inhibitors may help resolve the situation. Always check the label for restrictions before using any insecticide or other fly control product.

Out of the light

Bright light by itself will not cause cattle to bunch. Rather, light avoidance is a secondary response to other stressors. It is thought that cattle equate “light” with “hot” due to their natural grazing instinct. When stressed, cows will seek faster air movement and/or darker areas of the barn even though it may be a hotter area.

Are some areas of the barn darker than others, and are cows bunching in the darkened areas? Do cows still bunch after sundown? Answering these questions can help determine if cattle are displaying light avoidance behavior.

Implement heat abatement strategies, control flies, and limit the variability in light intensity to prevent bunching in darker areas of the barn. If cattle bunch in darker areas during the day, closing the curtains (starting from the top down) or adding shade cloth blinds on the brighter side of the barn may help. It may be necessary to angle shade cloth away from the sidewall, leave a gap at the bottom, or add additional fans to ensure adequate airspeed in the barn.

Adding lighting to darker areas has also worked in some situations. If cattle still bunch in darker areas after sundown, clean light fixtures and consider adding additional lighting in darker areas to ensure that artificial lighting is evenly distributed.

Careful observation of cattle behavior can point toward solutions to cattle bunching. Implementing heat abatement strategies, providing plenty of fresh air and water, controlling fly populations, and reducing variation in light intensity can help to alleviate the problem.