Horses are highly social animals who want to be around other horses. However, in many situations today, horses are kept by themselves for management reasons and ease of monitoring, feeding, and exercise regimens. Isolating horses, especially young horses, is not a good idea and can lead to problems. One study in the Journal of Applied Animals Behavior Science showed that depriving 2-year-old horses of social contact during a 6-month period resulted in increased aggression and less submissive behavior when mixed with other horses.
A main concern about mixing horses is injuries that could occur during aggressive encounters, and this is avalid concern. When new horses are mixed, a stressful situation develops until the horses get acquainted and during this period, injuries can occur due to fighting. To determine if allowing limited exposure to horses would help prevent fighting when the horses were mixed, a study was performed that allowed 2-year-old horses to be placed beside each other in stalls before being released into a paddock. Results indicated that contact aggression was decreased if horses were introduced in side by side stalls where they could see each other but not touch each other. And biting between horses was reduced if the horses were only in these stalls for 5 minutes. Also, horses that did bite at each other were more likely to be aggressive toward each other when turned out in the pasture. For this reason, consider placing unfamiliar horses in stalls next to each other to monitor them before turning them out together and if biting is occurring, consider not turning them out together.