Physiological and Behavioral Responses of Stabled Horses (Equus caballus) to Three Types of Environmental Enrichment

Small stalls and regulated feedings restrict horses' natural foraging and locomotion, increasing risks to welfare. Environmental enrichment may promote more naturalistic behavioral time budgets, yet little is known about how enrichment type or timing affects physiology and behavior. This study examined nine stabled Quarter Horses provided with hay feeders, activity balls, or mirrors across randomized trials. Each trial included 30 min observations, four times per day, with enrichment removed between sessions and 5-day washouts between trials. Nightwatch^® Smart Halters™ recorded heart and respiration rates, while behaviors were video-scored using instantaneous scan sampling. Observers were not blind to the treatments. Enrichment effects, item type, time of day, and possible interactions for each variable were tested using a GLMM; Tukey's HSD multiple comparison procedure was used for post hoc comparisons (at <i>p</i> ≤ 0.05). Enrichment significantly increased heart rate compared with the control (<i>p</i> = 0.03), indicating heightened arousal, with hay feeders producing the strongest effects. Respiration rate was unaffected. Mirrors reduced evening heart rates compared with other times (<i>p</i> = 0.02). Across treatments, enrichment increased foraging (<i>p</i> = 0.01) and locomotion (<i>p</i> = 0.03), while reducing frustration behaviors (<i>p</i> = 0.03). Hay feeders produced time budgets most similar to wild horses, suggesting greater effectiveness at meeting behavioral needs. Effects were most pronounced at 12:00 h and 16:00 h, outside routine meals. Overall, enrichment may improve physiological and behavioral outcomes, supporting its role in promoting welfare for stabled horses. Larger studies are needed to assess item-specific and long-term impacts.