Effects of Burdizzo castration on CO2 laser induced thermal nociception of Holstein–Friesian calves of different ages

The objective was to investigate the effects of Burdizzo castration on the thermal nociception (stress-induced hypoalgesia) of Holstein–Friesian bull calves of different ages. Calves castrated at 5.5 mo of age were compared with either intact calves of the same age, or calves castrated at 1.5, 2.5, 3.5, and 4.5 mo of age (n = 10 calves/treatment). Treatments were conducted on the same day for all calves. The baseline surface skin temperatures on the caudal part of metatarsi, and the latency of the calves to perform hind leg withdrawal (i.e., thermal nociception threshold) in response to a CO2 laser beam applied on the same area were measured 72 h before, and 12, 24 and 48 h after treatment. The thermal nociception threshold varied inversely with the baseline skin temperature (pooled correlations, r = −0.45, −0.31 and −0.48 at 12, 24 and 48 h, respectively; P < 0.01). There were no differences (P > 0.05) in the skin temperatures between castrated and intact calves at 5.5 mo of age. Calves castrated at 1.5 mo-old had consistently lower skin temperatures than all other castrated calves throughout the study. These calves had markedly increased skin temperatures following castration, while the opposite trend was observed in older castrated calves, and no change was observed in intact calves. At −72 h, the 1.5 mo-old calves had higher thermal nociception thresholds than older calves. In all calves, the thermal nociception threshold increased after treatment. Calves castrated at 5.5 mo of age tended to display higher thermal nociception threshold than intact calves. However, variations in the initial skin temperature accounted for these differences between treatments or interactions between time and treatment. In conclusion, the laser-based thermal nociception assay can be influenced by the surface skin temperature of the hind legs and the age of animals, particularly in calves less than 2 mo of age which have lower skin temperatures and longer latency to respond to the laser. Burdizzo castration increased the skin temperature of 1.5 mo-old calves, but had the opposite effects on older calves. Within the temporal limits of this study, no conclusive evidence was found to support the presence of acute stress-induced hypoalgesia following castration.