It is important to stun an animal prior to slaughter in order to render it insensitive towards any pain or distress. There are a variety of different methods of stunning ranging from a simple blow to the head, a captive bolt into the brain or electrical stunning. In most developed countries captive bolt or electrical stunning devices are most commonly used.
Larger animals, such as adult cattle, are normally stunned using a captive bolt. Smaller animals such as sheep and pigs are often stunned by electrical means. Sometimes animals may be slaughtered without prior stunning on religious grounds.
There are two different methods of electrical stunning; the head only system and head-to-body system. The head only system involves applying two electrodes to the head, either on the forehead or on either side of the head using tongs that look rather like earmuffs.
An electrical current is applied to the brain that instantly induces an epileptic seizure causing the animal to become totally unconscious.The animal will remain in this unconscious state for around 30 seconds, so bleeding must be completed before this to avoid stress to the animal should it regain consciousness.
The head-to-body system applies an additional electrode the body of the animal. Usually it is placed on the back just behind the shoulders, although it can also be applied to the side of the animal or a leg. With this particular system, the electrical current not only passes into the brain, but also crosses the heart causing cardiac arrest as well as unconsciousness.
The advantage of this system is that it produces permanent insensibility; in effect, it kills the animal as it does not regain consciousness. Therefore, the time between stunning and bleeding (the stun-to-stick time) is less critical. Also, this method produces a more relaxed carcase with few involuntary movements such as leg kicks etc making it safer and easier to process. Many researchers consider head-to-back stunning to be the best method for sheep, calves and pigs.1
Damage to leather and prevention
Problems can arise if head-to-back stunning is not carried out properly; electrical arcing can occur between the electrode placed on the back and the skin causing burn marks that cause a permanent blemish on the grain surface of the skin. The damage is usually around 5cm in diameter and has a speckled appearance around the periphery.
There are many factors that can lead to incorrect skin-to-electrode contact:

  • Movement of the animal: Proper restraint is necessary to keep the animal still long enough for the electrodes to be properly positioned. Restraint also ensures that the animal does not fall away from the electrode once the electrical current is applied (it is necessary to apply the current for around three seconds for it to be effective). Usually restraint is achieved by a purpose-designed stunning pen that holds the head still and prevents both forward and backward movement during the stunning process.
  • Wool length: Due to their covering of long, thick wool, it can be more difficult to achieve good electrode-to-skin contact with sheep. Clipping the wool is one solution but better contact can be achieved by using peg-shaped electrodes. Contact can be further enhanced by using a jet of water to wet the skin where the electrode is to be placed.
  • Operator tiredness: A busy abattoir may be processing many hundreds of animals each day. Tiredness or boredom can lead to mistakes. Such difficulties can be overcome by careful design such as ensuring that the operative is positioned well above the stunning pen so that they are not repeatedly lifting the equipment. Automated stunning pens are used in some abattoirs where they help ensure repeated reliable stunning provided they are maintained properly.
  • Dirty electrodes: Dirt from the fleece can reduce good contact as can carbon build up or corrosion. Electrodes should be regularly cleaned with a wire brush.
  • Poor design: the surface area of the electrodes should not be too small.

Further difficulties can occur if there are variations in electrical resistance2:

  • It has been found that animals of different age and species have different electrical resistance, most probably due to differences in the thickness of the skin and sub-cutaneous fat.
  • Dry animals have more resistance than wet animals. Therefore, the effectiveness of electrical stunning can be weather dependant. The application of a jet of water can alleviate this problem.
  • Animals who have had free access to drinking water prior to slaughter are easier to stun as they are better hydrated.
  • If the stunning pen has a wet floor, some of the electrical current can be grounded thus reducing the amount at the electrodes.

The slaughterman needs to take these factors into account and adjust the stunning device accordingly. Stunning equipment should have adjustable voltage, amperage and contact time. Settings of 1 amp, at 300 to 400 volts, 50 Hz for 3 seconds are typical settings.  Proper maintenance, adjustment and application of electrical stunning devices should ensure humane slaughter without compromising skin quality.
1,2. Temple Grandin, Cardiac Arrest Stunning Of Livestock And Poultry, Advances in Animal Welfare Science. 1985/86