Although the positive effects of turnout have long been known, a new study from Ohio suggests that to fully benefit from being on pasture, horses need room to roam.

Researchers at The Ohio State University Agricultural Technical Institute found a minimum of 342 square meters—the equivalent of about one-tenth of an acre—per horse is needed to reduce stress and minimize aggressive behaviors in the herd.

“My students actually prompted the study,” says Jessica Suagee-Bedore, PhD. “They were working on facility management plans, and someone asked me, ‘How large do I make the dry lot?’ I said, ‘Let’s search the literature and equine extension and see what we find.’ We really didn’t find much at the time, so we designed a study to try to start answering that question.”

She adds that the study focused only on the amount of space needed to reduce stress in a herd rather than to support grazing—two acres per horse is the generally accepted minimum to support healthy pasture growth and adequate nutritional intake.

The research was based on 12 mature, healthy horses who were familiar with each other. In the first stage of the study, each horse’s “flight zone” was determined: While one horse ate his feed ration, a second horse was led toward him until he began to show signs of aggression, such as pinned ears or turning the rump; then the researchers measured the distance between the two horses. On average, the horses each had a flight zone measuring about 10.4 meters (roughly 33 feet).

This figure was then used to calculate the total amount of space a horse would need in any direction to avoid contact with another horse. That area was 342 square meters.

For the second phase of the study, the horses were kept in pens of various sizes and observed for signs of physiological stress. They were divided into three groups of four—two geldings and two mares—and then turned out for an hour into one of three pens: one measuring 342 square meters per horse, another measuring 263 square meters per horse, and one that provided 184 square meters per horse. Researchers drew blood from the horses prior to turnout, then again 15 and 60 minutes after turnout. They also monitored the herds for signs of aggressive behaviors, such as chasing or kicking. When not in the study pens, the horses were kept in individual stalls.

The data showed the horses kept in the pens that allowed 342 square meters per horse had reduced levels of blood cortisol, a hormone associated with stress, 15 and 60 minutes after turnout. In contrast, cortisol levels did not decrease in horses turned out in the smaller pens and they displayed aggressive behaviors more frequently.

The data collected during the one-hour study time may not paint a complete picture of the stresses a horse restricted to a cramped space for longer periods of time may experience, says Suagee-Bedore. “Cortisol is a diurnal hormone and develops a cyclical pattern around sleeping and eating routines,” she says. “In chronically stressed horses, cortisol will remain high and not develop a pattern. We would need to investigate cortisol concentrations over a 24-hour period to know if these findings apply. Also, since horses would have to be fed twice during that 24/7 window, I would assume that aggression scores would go up in the small pen whenever feed would be introduced.”

Suagee-Bedore says the findings may have also been different if the horses had not been familiar with each other. “I can say through personal experience that horses that are very friendly with each other tend to get along better, but my study wasn’t designed to answer that question.”

While larger turnout areas are clearly less stressful for horses, Suagee-Bedore acknowledges that it’s not always possible to provide more space. “It would be nice to study the time it takes for horses to adapt to a smaller paddock size,” she says. “For instance, maybe interactions and stress responses change after a number of days or weeks. If I only had smaller spaces, I would do smaller groups, because that minimizes the number of possible interactions—a timid horse only has to avoid one other horse.”

Reference: “Effect of pen size on stress responses of stall-housed horses receiving one hour of daily turnout,” Journal of Equine Veterinary Science, March 2021

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