It is desirable to render an animal unconscious before it is slaughtered in order to eliminate pain, discomfort and stress from the procedure. Most developed and many developing countries have legislation that requires pre-slaughter stunning, with the exception of authorised ritual slaughter like Kosher or Halal. In some circumstances, traditional slaughter may be exempt from pre-slaughter stunning. Whatever the stunning method, the animal should be rendered unconscious for long enough so that bleeding results in enough loss of blood to cause death from lack of oxygen to the brain (cerebral anoxia). In other words, death should occur before the animal would have regained consciousness after stunning, had bleeding not taken place. There are three main technologies used to effect stunning-Percussion, Electrical and Gas. Only the first two are commonly used in developing countries.
Percussion stunning
Captive bolt
This method works on the principle of a gun and fires a blank cartridge and it propels a short bolt (metal rod) from the barrel. The bolt penetrates the skull bone and produces concussion by damaging the brain or increasing intracranial pressure, causing bruising of the brain.The captive bolt is perhaps the most versatile stunning instrument as it is suitable for use on cattle, pigs, sheep and goats as well as horses and camels, and can be used anywhere in the world. (Although electrical stunning is preferable to captive bolt pistols for stunning pigs and sheep.) There are several different manufacturers of captive bolt pistols, and after the initial expense, running costs are minimal. Users must ensure sufficient supply of cartridges, which may be different in caliber for stunning guns from the different manufacturers. These features make the captive bolt the stunning instrument of choice, particularly in developing countries.
There are two variations of the gun. One has a handle and trigger. The other comprises hand-held barrel, which is tapped against the skull, which sets of the cartridge explosion.
Another type of bolt has a flat, mushroom end. Unconsciousness is achieved through percussion by strong blow to the skull. The brain is not penetrated, and as the animal is not killed, it is a method that is acceptable in many countries for Halal slaughter. When in use, the captive bolt is positioned on the correct spot on the animal’s head Poor maintenance is a major cause of poor stunning and the guns must be cleaned and serviced regularly, according to the manufacturer’s instructions.
Electrical stunning
This method of stunning is well suited for pigs, sheep or goats, poultry and ostriches. (Use in cattle or other large species is in development, but if not properly applied it may result in excessive haemorrhage in the muscles or spinal fractures.) Electrical stunning induces electroplectic shock or epileptic state in the brain. This state should last for long enough for bleeding to be carried out so that the animal dies from cerebral anoxia. A low voltage alternating electric current is applied by means of two electrodes, which are placed on either side of the brain using tongs. Since the brain of animals is small, the electrodes should be accurately and firmly placed high up on the sides of the head in sheep, goats, pigs and ostriches.
Carbon dioxide gas stunning
The use of carbon dioxide (CO
2) gas is a relatively new method of stunning suitable for pigs and poultry. However, it is applicable only at large industrial plants, as the sophisticated technical equipment is relatively costly to install. Basically, animals are stunned using various concentrations of CO
2 in air. Concentrations of CO
2 for the stunning of pigs are at least 80% in air for 45 seconds and poultry of 65% for 15 seconds. The acceptability of this method on welfare grounds has been questioned however. For some genetic types of pigs, it may be satisfactory, and for others may be stressful.
Currently Argon gas is being tested for stunning purposes. It is assumed that Argon gas may have some advantages over CO
2, but the costs may be higher.