Preventing Return to Sensibility after Captive Bolt, Electrical Stunning in CO2 Stunning of Cattle, Pigs, and Sheep

January 2001

Electrical Stunning - Trouble Shooting Guide

Problem - Animal blinks within 5 seconds after stunning


  1. Electrode is placed in the wrong position and the electrical current fails to go through the brain. The animal blinks because the stunner failed to induce a grand mal epileptic seizure which is required to induce instant insensibilty.

  2. The electrical amperage may be too low. Even though the electrode is in the correct position there is not enough current passing through the brain to induce a grand mal epileptic seizure. The amperage must be increased.

  3. High electrical resistance of the animal. This is especially a problem in old sows or dehydrated animals.

Problem - The initial stun appears to be done correctly but the animal blinks or shows other signs of return to sensibility 30 to 90 seconds after stunning.


  1. The stunning to bleed interval is too long. This is especially a problem with head only reversible stunning. The solution is to shorten the interval between stunning and bleeding.

  2. Poor bleeding may be the cause of an animal showing signs of return to sensibility after it has been bled. This can occur in cardiac arrested animals because there are always a few animals where the heart is not stopped. Training of the person doing the bleeding will usually solve this problem.

  3. Poor initial contact results in the animal receiving a stunning time that is too short. A common cause is a fatigued operator.

  4. Interrupted contact - The stunning wand or tongs may bounce or slide during the stun and result in a stunning time that is too short. Poor design of the stunning wand is a likely cause. Another cause is a line speed of 1100 or more pigs per hour on a single restrainer.

  5. Placement of the head electrodes in the wrong position on the head. Reposition the electrodes so that the electrical current will pass through the brain.

Captive Bolt Stunning - Trouble Shooting Guide

Reasons for Poor Stunning

  1. Stunner has not been maintained. A dirty stunner which has not been maintained will lose bolt velocity. High bolt velocity is required for an effective stun.

  2. Damp cartridges. Cartridges for a cartridge fired stunner must be kept in a dry place. Do not store them in the slaughter room.

  3. An overheated cartridge fired stunner will lose bolt velocity. Rotate cartridge fired stunners to prevent overheating.

  4. Worn cylinder bore on a pneumatic stunner. Even when the stunner has been serviced correctly, the machined cylinder bore eventually wears out and the stunner will lose hitting power. At this point the stunner will have to be replaced.

  5. Poor ergonomics of bulky pneumatic stunners. Adding additional handlers will aid positioning. When a pneumatic stunner is used with a conveyor restrainer it is often easier to position it if it is hung from the balancer on a 30 degree angle.

  6. Stunner operator chases the animal's head. The operator should be trained to wait for the animal to stop moving and then position the stunner. Chasing the head will result in poor stunning.

  7. Excited animals. Careful quiet handling and driving of animals into the stun box or restrainer will provide calm animals that are easier to stun correctly.

  8. Air pressure too low to power a pneumatic stunner. Use the air pressure setting recommended by the manufacturer. This usually requires a dedicated compressor which powers only the stunner.

CO2 Stunning - Trouble Shooting Guide

Reasons for return to sensibility

  1. Low CO2 concentration. Increase the gas concentration. 90% is ideal.

  2. Exposure time is too short. Slow down the number of pigs or birds which are moved through the system.

  3. The time between exit from the CO2 chamber and bleeding is too long. To prevent recovery from the anesthesia, bleed the animals more quickly.

  4. Poor bleeding technique. If animals show signs of return to sensibility after bleeding the person doing the bleeding may need more training.

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