Pharmacokinetic-Pharmacodynamic Modeling of Analgesic Drugs in Beef Cattle

Bovine castration is performed on approximately 15 million cattle in the United States annually. Surgical castration with a scalpel followed by testicular removal using manual twisting (cattle <90 kg) or an emasculator (cattle >90 kg) are the most common methods of castration performed in the USA. A survey of US veterinarians found that one in five practitioners report using analgesics or a local anesthetic at the time of castration. However, these drugs have not been formally approved by the FDA for pain relief. Validated methods of pain assessment are required in order to determine if analgesic drugs are efficacious. Potential pain assessment methods include plasma cortisol determination, plasma Substance P determination, accelerometers, thermography, chute exit speed, heart rate determination and electrodermal activity measurements. Studies have shown that a combination of butorphanol (0.01 - 0.025 mg/kg); xylazine (0.02 - 0.05 mg/kg) and ketamine (0.04 - 0.1 mg/kg) can mitigate these responses. Research is underway to further characterize the activity of analgesic drugs administered prior to castration.