Mechanical Nociceptive Threshold, Tissue Alterations and Horn Growth in Calves after Injection of Isoeugenol or Clove Oil under the Horn Bud

SIMPLE SUMMARY: Hot-iron disbudding of calves is a common but painful practice. We investigated the injection of clove oil or isoeugenol under the horn bud as a potential alternative. Compared to hot-iron disbudding (with local anesthesia), pain sensitivity increased much less and for a shorter time in isoeugenol-injected calves. After injection of clove oil, the differences to hot-iron disbudding were smaller, and there was a high occurrence of swellings of the eyelids. Neither injection was as effective as the use of a hot iron in preventing horn growth. In sum, the injection of clove oil is not recommendable, and regarding the use of isoeugenol further research is needed, in particular on possibilities to improve its effectiveness. ABSTRACT: Disbudding of calves is a common, painful intervention. Due to cytotoxic and anesthetic properties, the injection of clove oil or its component isoeugenol may be less detrimental to animal welfare. We investigated mechanical nociceptive threshold (MNT), possible tissue alterations and horn growth for up to 12 weeks after injection of 1.5 mL clove oil (CLOV), isoeugenol (ISO) or saline (CON) or after hot-iron disbudding (BURN; with local anesthesia and sedation, n = 10/treatment). MNT was measured using von Frey filaments and a pressure algometer at four locations around the horn bud. There was a treatment*time point interaction (linear mixed model, p < 0.05). MNT decreased most strongly and for the longest time for BURN in most calves at least for 3 weeks. For ISO, the decrease was less distinct and most calves’ values returned to baseline after 1–2 weeks. MNT in CLOV was intermediate, with decreased values up to 3 weeks in some animals. 12 weeks after the treatment, horn growth was prevented in about 50% of the horns in CLOV and ISO. Tissue alterations such as swellings of the eyelids often occurred in CLOV, but less so in ISO. Our results suggest that injection of isoeugenol causes less pain and thus seems to be beneficial compared to hot-iron disbudding, while clove oil was not advantageous. Regarding the effectiveness of isoeugenol to prevent horn growth, more studies are needed.