Davis, K. M., Iwaniuk, M. E., Dennis, R. L. et al. 2020. Effects of grazing muzzles on behavior, voluntary exercise, and physiological stress of miniature horses housed in a herd. Applied Animal Behaviour Science 232, 105108.

Grazing muzzles are a popular and effective management tool used to help prevent weight gain in obese-prone equids. Concerns have been raised over their possible negative impact on horse well-being as muzzles may interfere with normal grazing and social behaviors. The objective of this study was to assess the effects of grazing muzzles used for different lengths of time on behavior, voluntary exercise, and physiological stress of grazing horses housed in a herd. Six mature miniature horses (initial BW of 114.9 ± 11.4 kg; BCS of 6.0 ± 0.8) were studied using a 3 × 3 Latin Square repeated measure design with horses receiving one of three treatments daily for a period of 3 weeks. Treatments were unmuzzled (M0), muzzled for 10 h/d (M10, 0830-1830 h), or muzzled for nearly 24 h/d (M24). Horses were housed as a herd on a 0.6 ha grass pasture. Body weight (BW) was assessed weekly. Reaction to muzzle application was scored daily. Behavior was monitored with video twice weekly for two hours a day. An aggressiveness rank was assigned to each horse each period. Physiological stress was assessed weekly using changes in heart rate (HR), indices of heart rate variability, and salivary cortisol concentrations (SC). Voluntary exercise was measured for 24 h twice during each period. Data were analyzed as repeated measures with treatment and wk as main effects. Horses muzzled for 24 h/d lost BW while horses muzzled 0 or 10 h/d gained BW (P = 0.01). Horses muzzled for 24 h/d spent more time grazing (P = 0.04) and less time resting (P = 0.03) than M10 horses. Unmuzzled horses spent more time walking than M24 horses (P < 0.01), more time cantering than M10 (P = 0.02), and more time trotting than M10 and M24 (P = 0.04 and 0.01 respectively). Both muzzled treatments spent less time autogrooming with their mouths than unmuzzled horses (P < 0.01). Horses muzzled for 24 h/d had lower HR than M0 horses (P = 0.01) and higher beat-to-beat intervals than M0 and M10 horses (P = 0.01 and 0.03 respectively) while grazing. There were no effects of treatment on muzzle acceptability score, salivary cortisol, voluntary exercise, or change in aggressiveness rank. In conclusion, muzzling for 24 h/d prevented weight gain and did not cause apparent physiological stress. The lower HR and higher beat-to-beat interval in horses muzzled 24 h may be a potential health benefit and needs further exploration.