Pentobarbital is a barbiturate drug that causes sedation, induces anesthesia or causes death depending on dosage administered. In veterinary medicine, the most common uses are to induce anesthesia in pets and to euthanize large and small animals. Pentobarbital is administered by intravenous injection. In cattle, it should rarely, if ever, be used for anything other than euthanasia.  
Pentobarbital has an affinity for fat, so when an animal is euthanized, pentobarbital residues will be found in body fat. For this reason, any animal euthanized by pentobarbital should not be sent for rendering. Such animals should be composted or buried. States have different requirements for required depth of burials; check with your state for details. Incineration may be allowed; again, check with your state for details. Leaving carcasses in open areas where wildlife or other predators have access is not recommended because of pass through toxicity.  
Pentobarbital residues in rendered fat have become more common in recent years. For example, according to Kerry Courchaine, director of technical services for Darling International, Darling’s Blue Earth, Minnesota, rendering plant has become the nation’s hot spot for pentobarbital residues. This plant accepts cattle and swine carcasses from southern Minnesota and Iowa. It does not accept horses, sheep or goats. For the period of November 2020 to December 2021, the positivity rate for pentobarbital in vats of rendered fat was over 23%. Each positive vat costs Darling around $10,000. Contaminated fat can be used for biodiesel production, and Darling ships contaminated fat to its biodiesel plant but has to thoroughly clean rail cars after shipping. Darling spent over 2 million dollars cleaning rail cars in the last 12 months. Other Darling rendering plants have been residue hot spots in previous years. Such high positivity rates threaten to make rendering unsustainable, thus risking the loss of a valuable service for livestock producers.
Based on interviews with producers that have had pentobarbital residues, Darling is convinced that one of the main causes of residues is the use of pentobarbital for sedation of cattle. However, it is very unlikely that pentobarbital is being used by many veterinarians for sedation because it is a controlled drug, meaning it must be kept under lock and key, and every cc used or disposed of must be accounted for. Furthermore, it is not approved for sedation in cattle, and another sedative, xylazine, is approved, effective and inexpensive. Darling is now asking producers who submit carcasses if their animals have had recent surgery and, if so, what drug was used to sedate the animal.  
What can one do to prevent pentobarbital residues? First, if you cannot safely compost or bury carcasses, ask your veterinarian to use an alternative method of euthanasia. For example, all the doctors in our practice carry captive bolts for euthanizing animals. A captive bolt is a device that uses a penetrating bolt and blank cartridges to penetrate the skull and cause sufficient brain damage to induce unconsciousness. At least one secondary step must be used to cause death following stunning via captive bolt. One option, pithing, can be accomplished by inserting a rod into the hole and rotating to destroy brain tissue. Another option, IV potassium chloride, a paralytic, can be given following captive bolt stunning to cause death by stopping the heart.
Second, any time your veterinarian euthanizes an animal, make sure you know what method was used and if the animal is safe for rendering.
Third, even though it is extremely unlikely that a veterinarian would use pentobarbital for sedation, if an animal dies after surgery, it is appropriate to ask your veterinarian what, if anything, was used for sedation and if the animal is safe for rendering. Working together, we can reduce pentobarbital residues in rendered tissues.
    Bennett is one of four dairy veterinarians at Northern Valley Dairy Production Medicine Center in Plainview, Minnesota. He also consults on dairy farms in other states. He and his wife, Pam, have four children. Jim can be reached at bennettnvac@gmail.com with comments or questions.