Relationship of tissue dimensions and stun placement on cadaver heads from 2- to 3-yr-old male bison (Bison bison).

The bison industry in the United States has been growing rapidly. In 2023, over 75,000 bison were commercially slaughtered, but stunning methods for bison did not appear to be scientifically evaluated at the time of our study. Our primary objective was to evaluate tissue thicknesses at the frontal (FRONTAL: 2.5 cm above a line connecting the bottom of the horns) and poll (POLL: below the crest of the head at the midline and angled toward the ventral aspect of the nose) placements in bison. The secondary objectives were to assess the potential for bolt-thalamus contact and determine an optimal stunning placement based on thalamic location. Fifteen cadaver heads from 2- to 3-yr-old bulls (hot carcass weights: 300.50 ± 27.42 kg; mean ± SD) with hide-on were obtained from a commercial slaughter establishment after being stunned with a 0.410-bore shotgun at a frontal location. All heads were shaved, and external head features were measured. A 0.25R-caliber Jarvis free-flight penetrating captive bolt (PCB) was applied at the POLL placement. Heads were split at the midline and soft tissue and cranial thicknesses were measured with a digital caliper. Digital images were obtained of the split head to evaluate the brain and thalamic contact plane. From these images, an IDEAL placement was determined to ensure bolt-thalamic contact. The IDEAL placement is in a significantly different (P < 0.0001) location than the FRONTAL placement and is located 8.33 ± 0.36 cm above the base of the horns at the midline with the shot placed perpendicular to the surface of the head. For the IDEAL placement, the bolt path was in the plane of the thalamus for 14/14 heads (100.00%, 95% CI: 76.8% to 100.0%) while only in the plane of the thalamus at the FRONTAL placement for 11/14 heads (78.57%, 95% CI: 49.2% to 95.3%) and at the POLL placement for 10/14 heads (71.43%, 95% CI: 42.0% to 91.6%). Total tissue thickness was significantly greater (P < 0.0008) at the FRONTAL placement (75.00 ± 1.84 mm; upper reference limit (URL): 89.31 mm) than the IDEAL (60.92 ± 1.90 mm; URL: 70.64 mm) and POLL (64.60 ± 1.84 mm; URL: 85.52 mm) placements. There was no evidence to support a difference (P = 0.5166) in total tissue thicknesses between the IDEAL and POLL placements. Thalamic depth at the IDEAL placement was 115.27 ± 8.36 mm (URL: 133.96 mm). Therefore, to reach the thalamus consistently and cause insensibility, it is recommended to stun at the IDEAL placement and have a penetration depth of at least 133.96 mm for PCB use in young male bison.