The effect of disbudding age on healing and pain sensitivity in dairy calves

Hot-iron disbudding, a husbandry procedure performed in dairy calves in which horn bud growth is prevented through tissue cauterization, produces severe burns. Little is known about how long pain persists following disbudding or factors, such as age at the time of disbudding, that may influence animal welfare outcomes. Our objectives were (1) to evaluate whether disbudding age mediates healing, pain sensitivity, and weight gain in calves disbudded near birth (3 d) compared with at a later age (35 d); and (2) to examine how long wounds are sensitive during healing. Forty-eight calves were disbudded or sham-disbudded at 3 or 35 d of age (n = 12/treatment). We assessed pain sensitivity with a pressure algometer on d 3 and 7 after disbudding and then weekly thereafter until the wound had re-epithelialized. Sensitivity was measured at the lateral and caudal edges of each wound, as well as on the rump to test for systemic changes in sensory function. We evaluated healing by scoring the presence of 7 tissue types in the wound bed: attached necrotic tissue, detached necrotic tissue, burns outside the necrotic ring, exudate, granulation, crust, and epithelium. Tissue scores, surface temperature, and girth circumference as an estimate of weight gain were measured twice weekly until the wound had re-epithelialized. Disbudded tissue was more sensitive than nondisbudded tissue for at least 3 wk, after which time the sham calves were removed from the study to be disbudded. Wounds took 62 ± 10 d (mean ± SD) to re-epithelialize and were more painful than new epithelium throughout this time, regardless of whether the procedure occurred at 3 or 35 d of age. The surface temperature of disbudding wounds did not differ from that of non-disbudded tissue nor did disbudding affect weight gain at either age. Age at the time of disbudding did not affect latency to re-epithelialize or wound sensitivity. However, rump sensitivity was greater in calves disbudded at 3 versus 35 d of age on d 28, 42, and 56 after disbudding, in agreement with other studies showing increased pain sensitivity when painful procedures are performed at an early age. In conclusion, wounds remain painful throughout healing, raising animal welfare concerns about hot-iron disbudding. Performing the procedure near birth does not improve welfare outcomes; rather, some evidence suggests it may produce a generalized long-term increase in pain sensitivity.