On-farm slaughter and killing

What is the legal framework for making end-of-life decisions, and what are the main principles of killing?

01 June 2020, at 8:15am

Encountering animals on-farm when euthanasia or slaughter is the only practicable solution to avoid unnecessary suffering is not uncommon in the life of a veterinarian. Knowing how best to approach such cases from a practical perspective and understanding the principles which underpin the methods available will assist decision making, best manage animal welfare and reassure the owner or keeper in sometimes challenging situations.

This first article outlines the legal framework for making end-of-life decisions and the principles of killing for commonly farmed large animal species. A following article (September 2020) will describe practical options available for a veterinarian needing to administer euthanasia or oversee arrangements for slaughter.

The law

The Animal Welfare Act of 2006 requires all persons to not cause, or allow through failure of action, an animal to suffer. It states that it is the duty of the person responsible for the animal to ensure welfare by meeting its needs, including protection from pain, suffering, injury and disease. Where it is no longer possible to protect an animal’s welfare, a person (responsible or otherwise) must ensure that further, unnecessary suffering is prevented. In circumstances where the prognosis, situation or economic pressures rule out treatment, emergency on-farm slaughter or euthanasia is justified as a means to eliminate further unnecessary suffering. The administration of such interventions is not specifically covered by the Animal Welfare Act and may – exceptionally – be actioned or arranged by a constable or inspector, following certification by a veterinarian. However, in most circumstances, the decision to euthanise or slaughter can be made independently by the person responsible – usually the owner or keeper, or following examination by a veterinarian.

On-farm slaughter or euthanasia of farmed large animals can be administered by one of three methods: death by lethal injection, stunning by captive bolt followed by death caused by bleeding or pithing, and killing by free projectile.

Lethal injection using controlled drugs is authorised for veterinary use only and likely to be least preferred. The practicalities of administration, cost, route of disposal and intended end use of the animal are all considerations. Animals euthanised by lethal injection are unsuitable for human consumption and cannot be used at hunt kennels.

Depending on route of disposal and intended end use, captive bolt stunning or free-projectile killing may be the best options. If sufficiently competent and authorised (in the case of firearms), any individual may euthanise an animal where it is justified by unnecessary suffering.

Animals intended for human consumption through emergency on-farm slaughter must comply with relevant hygiene (853/2004 and 2017/625) and EC (1099/2009; PATOK) and Welfare at the Time of Killing (WATOK, 2015) regulations, requiring antemortem inspection by a veterinarian and slaughter by qualified personnel. Transmissible Spongiform Encephalopathy (TSE) Regulations 2018 compliance must also be met in relevant species, effectively prohibiting pithing. When killing or slaughtering for human consumption, the operator must also hold either a WATOK licence for on-farm slaughter, or a Certificate of Competence (COC), in the relevant species and method used. Further conditions of WATOK must also be met when performing slaughter, including conditions for restraint, stunning and killing. Practical advice for handling and secondary procedures will be explored in the next article.


Humane euthanasia or slaughter requires the operator to effect irreversible unconsciousness and subsequent brain death to the animal in a painless manner, thereby avoiding further unnecessary suffering. This can be achieved using free-projectile killing devices, or by using a captive bolt stunning device to render the animal immediately insensible, followed by a process which causes cerebral death, including bilateral severance of carotid arteries (bleeding) or manual destruction of the brain (pithing).

A free-projectile device is identified as either a rifle, pistol or shotgun, where the projectile (a bullet or mass of lead shot) enters the brain separate from the firearm. Captive bolt stunners are an alternative design of firearm, where the bolt – delivering the percussive blow to the skull – is projected from the barrel upon firing, retracting back once completed.

FIGURE (1A) Penetrating captive bolt stunners
FIGURE (1A) Penetrating captive bolt stunners

Captive bolt stunners fit into two basic types: penetrating and non-penetrating (Figure 1). Non-penetrating devices use a flat or mushroom-ended bolt which distributes the percussive force of impact over a larger area and therefore do not penetrate the brain. Conversely, a penetrating captive bolt device houses a bolt with straight bolt; the impact force is therefore imparted over a much smaller area, resulting in the bolt punching through the skull to penetrate the midbrain.

FIGURE (1B) Non-penetrating captive bolt stunners
FIGURE (1B) Non-penetrating captive bolt stunners

When correctly applied, both methods are highly effective and reliable stunning methods; however, under PATOK regulations, non-penetrating captive bolt devices must not be used for ruminants over 10kg, or pigs. In practice, this means that they are prohibited for on-farm slaughter of ruminant species, if they are destined for human consumption.

The critical element in achieving a painless stun or kill is speed of induction to irreversible unconsciousness, achieved by producing brain dysfunction. Brain dysfunction must therefore occur faster than the animal is able to perceive the application of the method. The measured speed of afferent nerve impulses following sensory input is 100 to 150ms; therefore, any method which can achieve brain dysfunction in less time will render the animal immediately insensible.

Captive bolt devices and free projectiles cause brain dysfunction by energy imparted on the skull causing differential acceleration of the brain inside the cranial cavity. This is equivalent to a profound concussion. Brain movement inside the skull also produces contrecoup haemorrhages and shearing forces within its matter, leading to a deep state of unconsciousness which is often irreversible. The energy imparted on the skull on impact is related to the velocity of the bolt or projectile (Figure 2).

FIGURE (2) The energy imparted on the skull on impact is related to the velocity of the bolt or projectile; here KE is the kinetic energy, m is the mass of the bolt projectile and v is its velocity
FIGURE (2) The energy imparted on the skull on impact is related to the velocity of the bolt or projectile; here KE is the kinetic energy, m is the mass of the bolt projectile and v is its velocity
FIGURE (3) There are optimal stunning and killing positions for each species
FIGURE (3) There are optimal stunning and killing positions for each species

For the most effective transfer of energy upon impact with the skull, the bolt or projectile must impact trabecular bone directly overlying the brain surface. Optimal stunning and killing positions for each species exist (Figure 3) and will be discussed in the second article. Animals with thicker skulls require greater generation of kinetic energy upon impact to achieve the same effect on the brain. Therefore, the exit velocity from the gun must be higher, requiring use of a more powerful cartridge. Manufacturers of both captive bolt and free-projectile cartridges offer a range of cartridge powers, allowing effective stunning or killing of all species, age ranges and breed differences. When using captive bolt devices, following the manufacturer’s recommendations for cartridge power selection is essential. A wide range of free-projectile cartridges are available so some simple power calculations are needed to estimate muzzle energy and velocity to select the combination which will achieve the required muzzle energy of over 200J.

What makes both captive bolt and free-projectile devices so effective at delivering an immediate stun or kill is their speed. When using a free-projectile killing device, the shot or bullet will be travelling at 200 to 450m/s (model and cartridge dependent). Immediate irreversible unconsciousness will follow on impact and brain death from the subsequent destruction of brain matter by the travel and expansion of the projectile upon entry will occur in a fraction of the time required for sensory perception.

Equally, a captive bolt travelling at roughly 50m/s will cause brain dysfunction and unconsciousness from the force of impact in 1.5ms – 100 times faster than the animal is able to perceive its application. When using a penetrating device, the bolt trajectory into the midbrain will cause further mechanical destruction, lessening likelihood of recovery. Nevertheless, this method does not reliably result in brain death and must be followed immediately by a process which does: either bleeding or pithing.

Effective stunning or killing is recognised by immediate loss of posture, absence of rhythmic breathing and fixed glazed expression, followed by a degree of uncontrolled kicking and paddling. Eye movement, corneal reflex, incorrect body posture and presence (or return) of rhythmic breathing indicate a failed stun or kill, or recovery. The animal must be immediately re-stunned or -shot, in a position just above and to the side of the first shot, and signs of unconsciousness reassessed.

Following assessment of an effective stun, secondary processes must be performed to elicit brain death: bleeding or “sticking” interrupts blood supply to the brain, which varies between species due to anatomical differences in the vertebral blood supply to the circle of Willis. In the case of bleeding, brain death will occur after 14 seconds in sheep (Wotton and Gregory, 1986), 18 seconds in pigs (Gregory and Wotton, 1984a) and 55 seconds in cattle (Gregory and Wotton, 1984b). Animals must be monitored for signs of recovery during the process. Pithing, using a reusable or disposable “cane”, requires insertion of the rod through the captive bolt hole, rotating it inside the cranial cavity to disrupt brain and spinal cord architecture, resulting in immediate brain death.

Following brain death by whatever means, the animal must be promptly transferred to either an abattoir or fallen stock handling establishment, dependent on its intended use. The next article (September 2020) will cover practical applications of the principles of welfare at slaughter and euthanasia, ensuring – via careful preparation and management – that unnecessary suffering is avoided and human safety is protected.

Author Year Title
Gregory, N. and Wotton, S. 1984a Sheep slaughtering procedures. II. Time to loss of brain responsiveness after exsanguination or cardiac arrest. British Veterinary Journal, 140, 354-360
Gregory, N. and Wotton, S. 1984b Time to loss of brain responsiveness following exsanguination in calves. Research in Veterinary Science, 37, 141-143
Wotton, S. and Gregory, N. 1986 Pig slaughtering procedures: time to loss of brain responsiveness after exsanguination or cardiac arrest. Research in Veterinary Science, 40, 148-151

Claire White, BVSc, MSc, PgC, MRCVS, occupies a varied role at Bristol Vet School, operating their teaching and commercial abattoir, and lecturing in veterinary public health and state veterinary medicine. She also delivers industry-facing courses and makes technical contributions to national policy change.

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