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. Cows may bunch to the part in more direct shade when it is hot outside. If using a shade cloth to prevent this, remember to take it down after the sun leaves that area so that ventilation does not become a problem. Nighttime cooling is especially important for animals exhibiting bunching behavior. Because radiant heat moves from the highest temperature body to the coolest, heat will move from the cow to the dark sky at night; the opposite happens during the day. So, if shade cloths remain up at night, heat from the cows will not be able to be expelled, and the barn will stay hot.

A recent study published in the Journal of Dairy Science showed that cows are likelier to bunch in a freestall when the air temperature is above 68°F (Chopra et al., 2024). It has still not been scientifically proven why dairy cattle bunch at high temperatures. However, Chopra et al. (2024) theorized that cows may seek a more favorable microclimate within the barn. Heat stress in dairy cattle has many adverse effects, including increased body temperature and respiration rate, decreased milk production, impaired immune system, and decreased conception rates. It has been widely accepted that dairy cattle begin to experience heat stress at a Temperature-Humidity Index (THI) value above 68. When cows bunch together when it is hot outside, it creates a confounding effect. They cannot dissipate heat as well as they would otherwise be able to if standing alone. The added body heat of the other animals contributes to heat stress.

Bunching behavior has other negative impacts on animal health and well-being. When cows bunch together, they spend less time eating and lying down and, in turn, produce less milk. Because cows spend more time standing when they bunch, lameness increases. When cows bunch on pasture, it often creates muddy areas that contribute to soil erosion and could contribute to mastitis if cows lie in it.

Stray voltage is sometimes suggested as a cause for bunching behavior, but this has not been proven or disproven.

To prevent bunching behavior, ensure enough access to water and shade during the summer on pasture. Take preventative measures to control fly pressure in barns and on pasture. Ensure proper ventilation, focusing on both air exchange and speed, throughout freestall barns and other loose housing. Use shade clothes to block sun exposure during daylight hours and add feedline sprinklers if possible.

References

Chopra, K., H. R. Hodges, Z. E. Barker, J. A. Vazquez Diosdado, J. R. Amory, T. C. Cameron, D. P. Croft, N. J. Bell, A. Thurman, D. Bartlett, and E. A. Codling. 2024. Bunching behavior in housed dairy cows at higher ambient temperatures. J. Dairy Sci. 107: 2406-2425.

Collier, R. J., G. E. Dahl, and M. J. VanBaale. 2006. Major advances associated with environmental effects on dairy cattle. J. Dairy Sci. 89:1244-1253.

Grandin, T and M. Deesing. 2008. Humane Livestock Handling. Storey Publishing. North Adams, MA.

Hart, B. L. 1994. Behavioural defense against parasites: interaction with parasite invasiveness. Parisitiol. 109: S139-151.

Muller, C. J. C., J. A. Botha, W. A. Coetzer, and W. A. Smith. 1994. Effect of shade on various parameters of Friesian cows in a Mediterranean climate in South Africa. S. Afr. J. Anim. Scie. 24:56-60.

Ralley, W. E., T. D. Galloway, and G. H. Crow. 1993. Individual and group behaviour of pastured cattle in response to attack by biting flies. Can. J. Zoo. 71:4.

Schmidtmann, E. T., and M. E. Valla. 1982. Face-fly pest intensity, fly-avoidance behavior (bunching) and grazing time in Holstein heifers. Appl. Anim. Etho. 8:429-438.