Pain assessment based on facial expression of bulls during castration

Abstract

Bovine castration can be performed surgically, chemically, or immunologically and the method used can influence animal welfare. The use of facial expressions is an efficient tool to quantify pain, but there has been limited study of the effects of bovine castration methods on various pain stimuli. The objective of this study was to investigate the differences in pain intensity expressed by Nelore bulls subjected to surgical, chemical, or immunological castration methods, as well as to evaluate if facial action units (AUs) can be used to detect distinct pain intensities from the different castration methods. Observation of 5 AUs (reactivity, vocal, muzzle, mouth, eye and above the eye), was evaluated through photographs taken during the procedure, and an overall score was obtained after the evaluation of the images in degree of intensity 1–5. Blood was sampled for evaluation of serum cortisol levels. Surgical castration (3–5) presented the greatest median (p < .001) for the sum of AUs when compared to the chemical (1–5) and immunological groups (0–3). Howerever, serum cortisol concentrations were not significantly different between groups. We concluded that immunological castration resulted in the least pain and reduction in animal welfare. Moreover, there was a clear association between exhibition of facial AUs and pain intensity in cattle, demonstrating that this tool can be used in the field, due its ease of application and practicality.