Effects of grazing muzzles on behavior and physiological stress of individually housed grazing miniature horses

Grazing muzzles are used on obese-prone equids to limit intake of pasture while grazing. However, the length of time grazing muzzles must be worn to prevent weight gain and whether they impede normal grazing behavior to the degree they impact welfare is unknown. The objective of this study was to assess the effects of muzzling for different lengths of time on horse behavior and physiological stress. Six mature miniature horses (initial BW of 230 +/- 19.6 kg; BCS 6 +/- 0.4) were studied using a 3 x 3 Latin Square repeated measure design with horses receiving one of three treatments over three 21 d periods. Treatments were unmuzzled (MO), muzzled for 10 h (M10, 0830-1830 h), or muzzled for 24 h (M24). Horses were housed in adjacent individual 0.11 ha grass paddocks for 24 h/d. Daily at 0800 h, muzzles were removed, horses were groomed and fed 14 g vitamin and mineral supplement. Muzzles were re-applied at 0830 h. Body morphometrics were assessed at the start and end of each period. Each horse's willingness to accept muzzle application was measured daily using a 1 (strongly accepts) to 5 (strongly rejects) scale. Horse behavior was monitored twice weekly with video recordings for 1 h each in the morning and afternoon. Physiological stress was assessed weekly using salivary cortisol concentrations (SC), heart rate (HR), and indices of heart rate variability (HRV). Data were analyzed as repeated measures with treatment and wk as main effects. M24 horses lost BW while MO and M10 horses gained BW over the course of the study (P = 0.01). M24 horses spent more time grazing than M10 horses (P = 0.04). MO horses spent more time walking than M10 or M24 horses (P = 0.02). M10 horses spent more time resting than MO or M24 horses (P < 0.01), and a trend was observed where M10 horses spent more time lying down than M24 horses (P = 0.06). There was no effect of treatment on behaviors associated with frustration including pawing and head shaking. There was no effect of treatment on muzzle acceptability score; however, all horses were less accepting of muzzle application by the third wk of each period (P = 0.01). HR was lower and beat-to-beat interval was higher in M24 horses than in MO horses (P = 0.02, 0.04 respectively). Salivary cortisol concentrations were not different between treatments. Muzzling horses while on pasture for 24 hs a day resulted in a small loss of BW and altered time spent performing behaviors, but no observed impact on our physiological stress measures.