Wound characteristics after disbudding: Part I-Effects of caustic paste dose and presence of hair

Disbudding is a common painful procedure in dairy calves. Farms vary in method (e.g., hot iron or paste), whether horn bud hair is shaved, or the quantity of paste applied, for example. Yet, little research compares the effects of differing paste applications. Our objective was to assess the effects of dose and shaving horn bud hair on caustic paste disbudding outcomes including disturbance from rubbing, wound size, sensitivity, healing time, horn regrowth, and the need for removal with a second procedure. Female Holstein calves were assigned to 1 of 4 caustic paste treatments (n = 24-25/treatment): 0.2 or 0.3 mL on either shaved or unshaved horn buds, respectively (0.2 Shaved, 0.2 Unshaved, 0.3 Shaved, 0.3 Unshaved). Before disbudding at 3 d of age, calves received a local block and systemic pain relief. Wound tissues and regrowth were scored 1 to 2×/wk until fully healed. Wound depth, diameter, and mechanical nociceptive thresholds (MNT) to evaluate sensitivity were measured ∼4, 14, 28, 42, 56, and 70 d after disbudding. Calves with shaved horn buds were more likely to disturb their paste by rubbing it on themselves or their housing up to 4 h after application. This risk was lowest with 0.2 Unshaved (12%) and highest with 0.2 Shaved and 0.3 Shaved (both 58%). Rubbing, which may result from irritation when the local anesthetic wears off, had minimal effects on wound outcomes. Wound depth initially increased by ∼1.5 mm for all treatments in the 6 wk after paste application. Similar sensitivity was observed for all wounds for 42 d, whereas at 56 and 70 d, 0.2 Shaved calves were less responsive than 0.3 Shaved. Wound diameter was affected by dose and shaving with 0.2 Unshaved wounds being an average of 6 mm smaller than those from 0.3 Shaved. On average, wounds from 4 methods of applying caustic paste took 14.3 to 18.0 wk to re-epithelialize and 15.9 to 19.4 wk to fully heal, with 0.2 Unshaved reaching these stages the fastest and 0.3 Shaved the slowest. Applying 0.2 Unshaved led to smaller, faster healing wounds, but also resulted in the highest rates of horn regrowth (21% regrowth present; 8% required a second procedure), indicating this approach to paste disbudding was the least effective option tested. At the other extreme, 0.3 Shaved caused the most rubbing initially, and long term, created wider, slower healing wounds, likely beyond what is needed for a successful procedure. Two treatments, 0.2 Shaved and 0.3 Unshaved created comparable, long-lasting damage in our population of limit-fed calves. Although these 2 approaches were the best of the 4 compared, they take on average 4.4 mo to heal, raising concerns about the ethical implications of using chemicals to destroy horn growing tissue. Overall, the dose of disbudding paste and presence of hair affected calf responses, the wound profile, and efficacy of the procedure.