Why Do Dairy Cattle Bunch During the Summer?

If you house cattle in a freestall, bedded pack, or on pasture, you may have seen them bunch to one end or area of the barn or part of the pasture during the summer. You may ask yourself, why are my animals grouped in a hot huddle when spreading out would allow them to dissipate more heat? There are a few theories as to why they do this. Since cattle are prey animals, it has been theorized that they may group closely together on pasture for protection when they feel threatened. If cattle are experiencing bunching behavior due to a predator stressor, they will most likely act stressed, with some of the outer animals pacing back and forth (Grandin, 2008).

Bunching behavior could also occur due to fly pressure (Hart, 1994) both on pasture and in barns. To avoid being bitten by flies, cattle may bunch with their heads facing inward and their tails facing outwards to swat flies away (Schmidtmann and Valla, 1982). Animals who are treated with fly spray are less likely to bunch due to fly pressure (Ralley et al., 1993).

Cattle may also group due to factors that contribute to heat stress. Cows on pasture may group under shade sources or around water tanks when it is hot outside. When constructing shade structures on pasture, there should be 3.5 to 4.5 m2 (40 to 50 ft2) of space per cow (Collier et al., 2006). If shade structures are smaller than this, cows may group too closely together or compete for space. Ideally, shade structures should be movable so that soil erosion does not occur in one place over time.

If structures are not movable, orienting them north to south allows the sun to occasionally shine under the shade structure, preventing an accumulation of mud (Collier et al., 2006). When cows don't have access to shade on pasture, they are more likely to group around water to try and compensate for the added heat load (Muller et al., 1994). It has been theorized that cattle bunch inside barns because of light intensity, ventilation, or ambient air temperature. Depending on the orientation of a barn, the sun may shine intensely into parts throughout various parts of the day.

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