Background The ideal field anaesthetic protocol for castration will offer an adequate duration and depth of anaesthesia using a single injection without compromising safety or recovery quality. Objectives The objective of this study was to compare intravenous (IV) xylazine and detomidine as sedatives in combination with midazolam and ketamine for induction of anaesthesia in horses undergoing field castration. Study design Randomised, blinded, clinical study. Methods Fifty‐seven male Quarter Horses were randomly assigned to receive xylazine (group X) (1.1 mg/kg bwt) or detomidine (group D) (0.03 mg/kg bwt) as premedication with midazolam (0.05 mg/kg bwt) and ketamine (2.2 mg/kg bwt) anaesthesia. Using simple descriptive scales, quality of sedation, induction, surgical conditions and recovery were scored by blinded observers. Induction, surgery, recovery time and time from induction to standing were recorded. Results There were a greater number of horses with lower sedation score in group D when compared with horses in group X at 5 minutes after the first dose of sedation (11.1%, 66.7% and 2.2% of horses from group D with sedation scores of 1, 2 and 3, vs 0%, 52% and 48% from group X, respectively, P = .05). A significant (P < .01) difference was found between surgical condition scores for group D (63%, 14.8%, 11.1% and 11.1% with surgery scores of 1, 2, 3 and 4, respectively) and group X (28%, 4%, 28% and 40%). Horses in Group D were less likely to require maintenance doses of ketamine (P = .05) with only 26% (7 of 27 horses) in this group requiring additional doses compared to 68% (17 of 25 horses) in group X. Recovery time in minutes (Group D: 24.74 ± 14.80, Group X: 13.08 ± 8.00; P < .01) and induction to standing time in minutes (Group D: 42.2 ± 13.8, Group X: 29.8 ± 8.2; P < .01) were significantly longer for horses in group D compared with group X. Main limitations Multiple surgeons and multiple blinded observers. Conclusions Detomidine as compared with xylazine as premedication results in good quality sedation and surgical conditions and prolonged surgical plane of anaesthesia, without significant differences in induction or recovery quality.