Effect of genetics on handling and CO2 stunning of pigs

Meat Focus International (July 1992) pp.124-126
(with July 2008 updates)

Temple Grandin
Assistant Professor
Department of Animal Sciences
Colarado State University
Fort Collins, Colorado 80523, USA

There are increasing problems in the U.S. with very excitable pigs which are difficult to handle at the slaughter plant. These pigs have been bred for rapid weight gain and leanness, but they often have very high levels of PSE. Quiet, gentle handling in a high speed pork slaughter plant is almost impossible. Observations in fourteen slaughter plants in the US, Canada, Australia, and Germany indicate that these pigs have behavioral traits which make handling difficult, such as:

  1. hyper-reactivity to touch
  2. extreme flocking instinct (one pig will not leave the bunch and start up the single file race) and
  3. constant backing out of race (Grandin, 1991).
If this trend continues, the use of single file races to direct pigs into stunning equipment will cause both animal welfare and meat quality problems because of excessive electric prodding. During the summer months, PSE levels rose to 60% in some groups of excitable pigs. These high PSE levels were due to agitation during handling.

The incidence of very excitable pigs in the U.S. varies from 5% in the midwest to 30% on the east coast. High numbers of excitable pigs in the U.S. are on large farms which fatten hybrid pigs which have been genetically selected for rapid growth. However, some of the worst groups of excitable pigs were observed in Canada and Australia. Observations in a Danish slaughter plant indicated that most pigs were calm and excitability was seldom a problem. Excessive excitability is caused by a combination of environmental and genetic factors. A barren environment in the fattening pens will make pigs more difficult to load onto a transport vehicle (Warris et al.). Providing pigs with rubber hose toys to play with will reduce excitability (Grandin et al.). Research with other species has shown that a barren environment which restricts sensory input makes animals more excitable and easier to startle. Puppies kept in barren kennels became more reactive to stimulation (Melzack 1969) Trimming the whiskers of baby rats causes the areas of the brain that receive sensory input to become more excitable (Simons and Land, 1987). The nervous system needs a certain amount of stimulation to function normally. Playing a radio in the fattening barn will reduce excessive startle reactions to strange sounds.

Providing increased environmental stimulation will reduce excitability, but genetics is a major cause of excessive excitability.

Hybrid pigs from commercial breeding companies in the US are often bred from a Hampshire type boar line and a Yorkshire x Landrace-type sow line. The offspring are often very excitable. The exact composition of the pigs is unknown because it is a trade secret. Observations on both farms and in slaughter plants indicate that pigs which have some Duroc genetics were calmer. When one large farm replaced Hampshire boars with Duroc boars, both excitability and PSE decreased. The Hampshire breed can have a high incidence of poor quality meat (Hampshire effect) even when they are Halothane negative (Monin and Sellier, 1985). The significant effect of genetics could be seen by observing different breeds of pigs on the same farm. On one farm, Yorkshire x Landrace sows startled more easily than white hybrid sows that had some Duroc blood. One must be careful not be become too critical of a particular breed. Breed characteristics can vary greatly, depending on genetic selection. In a German plant, there was a definite difference in temperament between German Landrace and Dutch Landrace. The heavily muscled German Landrace were more excitable. Observations in all the plants indicated that there is a tendency for selection for leanness to increase excitability.

There are two possible solutions to this increasing meat quality and welfare problem:

  1. Select pigs for a calm temperament, or
  2. Develop CO2 stunning equipment where the use of a single file race could be eliminated.

Ninety percent of the handling problems associated with excitable pigs occur in the single file race and in the crowd pen which leads up to the single file race. Handling would be greatly improved if a group of five to ten pigs could be driven into a small pen that then descended into the gas.

There has been controversy about the humaneness of CO2 stunning. Dutch research indicated that the excitation phase which occurs during CO2 stunning starts prior to the onset of unconsciousness (Hoenderken et al., 1979)

This research raised the question that pigs may be distressed during the induction of CO2 anesthesia. More recent research (Forslid, 1987) indicated that unconsciousness occurred prior to the onset of the excitation phase, therefore, CO2 is definitely humane. The differences in the results between these two studies may be due to genetic factors. All of the research conducted by Anders Forslid at the Swedish Meat Research Institute has been on Yorkshire pigs (Forslid personal communication, 1991). Other studies have shown that there is large variation in pig reaction to CO2 (Dodman, 1977, Grandin, 1988). Observations by the author in a commercial slaughter plant indicated that white crossbred pigs with Yorkshire confirmation had a much milder reaction to CO2 than crossbred pigs with Hampshire breed coloration. Many of the Hampshire-type pigs started to react in the first few seconds after they contacted the gas. The animals were stunned in a Wernberg Compact plant. Crossbred pigs with Hampshire coloration rode quietly in the gondola until they contacted the gas. They attempted to rear up to avoid the gas while they were fully conscious.

Experiments with Pietrain x German Landrace pigs indicated that Halothane positive pigs had a more vigorous reaction to CO2 than Halothane negative pigs (Troeger and Woltersdorf, 1991). The reaction started about 20 seconds after the animals contacted the gas. These pigs had little or no reaction during initial contact with the gas. Seventy percent of the Halothane positive pigs had strong motoric reactions and only 29% of the Halothane negative pigs reacted in this manner. The researchers are concerned that reactions in Halothane positive animals may possibly be of animal welfare concern. The use of high CO2 concentrations of 80% or greater reduced the incidence of vigorous reactions.

An earlier German study with unspecified pig genetics indicated that the animals were anesthetized before the excitation phase (Ring, 1988). It is likely that some Halothane positive pigs were tested. Further studies with both Halothane positive and negative Hampshire pigs are still needed.

Human beings also vary in their reaction to CO2. People who have panic attacks which have a strong genetic basis will react very badly to CO2. The gas may induce panic attacks in these people (Griez, 1990). Neville Gregory from the Meat Research Institute in England reviewed a number of studies, (Lambooy, 1990) which indicated that most people find the smell of the gas to be pungent when it is breathed at a concentration of 50%. Years ago, CO2 was used in psychiatric therapy. Patients breathed a mixture of 30% CO2 and 70% oxygen until they became unconscious (Clark, 1954). One patient loved the treatment and some others disliked it. Twenty to fifty breaths were required to induce unconsciousness.

There is a need for more research and measurement of recordings from the brain. Forslid has proved that CO2 is a humane method of stunning for the Yorkshire breed, but the same kind of careful measurements need to be done on different breeds of pigs. It is especially important to test the Hampshire breed. Both Halothane positive and Halothane negative animals should be tested. Until further research is conducted, one can conclude that CO2 may be humane for certain genetic types of pigs and stressful to others. In order for CO2 to be completely acceptable from a humane standpoint, pigs may need to be genetically selected for a good reaction to CO2.

The pork industry needs to address problems caused by genetic over selection for a single trait. Both PSE meat and excitability have increased when pigs are selected for leanness and rapid growth. Breeders also need to select pigs for low levels of PSE and a calm temperament. I hypothesize that the genetic lines of pigs which become highly excited during handling may possibly be the same genetic types which will react badly to CO2. The worst problems with excitable pigs and high PSE occur in countries where the marketing system allows the pork producers to pass PSE losses on to the slaughter plant.

May 2000 update on genetics handling and CO2

More recent research on the aversiveness of CO2 has not fully answered the question about discomfort during CO2 inhalation and genetics. Raj et al, 1997, reported that CO2 inhalation is aversive to pigs, but Jorgman et al, 2000, reported that a shock with an electric prod is more aversive than inhaling 60% or 90% CO2. They used Landrace X Large White pigs. Pigs which had inhaled 60% CO2 were willing to re-enter the CO2 crate. There is still a need to study CO2 with Halothane stress positive and stress carrier pigs. The Hampshire breed also needs to be tested. Fortunately, some large companies that have switched to CO2 are eliminating the stress gene.

Research done with people still supports the fact that the reaction of people to CO2 inhalation is highly variable. It causes anxiety in some people and it has little effect on others (Perna et al, 1994, 1996, and Biber and Alkin, 1999). Research on people supports my hypothesis that genetic factors will affect a pig's reaction to CO2.

In the U.S., vertically integrated pork production companies have reduced or eliminated the halothane gene which has resulted in calmer pigs which are less likely to die during transport. The use of the halothane stress gene still occurs in marketing systems where producers are paid on the basis of the largest loin eye and the thinnest backfat. This type of payment system encourages the producers to select pigs for maximum quantity instead of quality pork which has low PSE. Fortunately some breeders are selecting calm pigs which are less excitable. The vertically integrated companies were the first ones to select for calmer pigs which would produce higher quality meat. Pigs which have been selected for large overdeveloped hams have larger muscle fibers and the meat is often tougher. Monin et al (1999) reports that heavy weight pigs with the stress gene were judged to be tougher by a taste panel.

July 2008 update on Pig Handling

Pig breeders have now started to correct some of the handling problems caused by poor leg conformation. They have learned that selecting a pig strictly for a huge loin and low back fat is often counter productive because the lean muscle may be dry and tough. Pig excitability problems caused by genetics will probably be reduced when a little more fat is bread back into lean genetic lines of pigs.

Feed additives such as Ractopamine (Paylean) can increase the incidence of fatigued non-ambulatory pigs. This is especially likely to occur at higher doses. If these products are misused, there may be some pigs that will have difficulty walking from the truck up to the stunner.

Research now shows that the handling of the pigs when they are brought from the lairage to the stunner is very critical. Excitement and agitation shortly before stunning increases lactic acid in the pig's blood. This will cause more PSE. Pigs that jam in the single file race to the stunner or get shocked with electric prods five minutes before stunning will have higher lactate levels. Careful, quiet handling shortly before slaughter is essential to maintain pork quality and reduce PSE.

Group handling systems that eliminate the single file race on the new CO2 systems greatly reduce handling stress. In all types of systems, steps should be taken to make pigs easier to handle before they come to the slaughter plant. This will make them enter a single file race more easily. People should walk through the finishing pens on the farm several times a week to train the pigs to quietly get up and move away. Walking pigs in the aisle will also train them and make them easier to drive. Pigs that are raised in an Auto-Sort system have to go through a short single file race to be weighed before they can enter the feeding area. These pigs will easily move into a single file race at the plant because they think it leads to the feeders.


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Dodman, 1977, British Vet Journal 133: 71; and Grandin, 1988, International Congress of Meat Science and Technology, pp.96-97

Forslid, 1987, Acta Physiol. Scand. 130:1-10

Grandin et al., 1986, Journal of Animal Science (Suppl. 1) 64:161

Grandin, 1991, International Congress of Meat Science and Technology, Paper 2:8, pp.249-251

Griez, 1990, American Journal of Psychiatry 147:796-797

Hoenderken et al., 1979, Fleischwirtschaft 59:1572-1578

Lambooy, 1990, Fleischwirtschaft 4:10-18

Melzack, 1969, Annals New York Academy of Science 159:721-730

Monin and Sellier, 1985, Meat Science 13:49-63

Ring, 1988, International Congress of Meat Science and Technology 98-100

Simons and Land, 1987, Nature 326:694-697

Troeger and Woltersdorf, 1991, Fleischwirtschaft 4:43-49

Warris et al, 1983, Meat Science 9:271-279

May 2000 References

Biber, B. and Alkin T., 1999. American Journal of Psychiatry. 156: 739-744.

Jong, E.C., Barnett, J.L., and Hemsworth, P.H., 2000. Applied Animal Behaviour Science. 67:67-76.

Monin, G. et al., 1999. Journal of Animal Science. 77:408-415.

Perna, G. et al., 1994. Psychiatry Research. 52:159-171.

Perna. G., Bertani, A., Caldirola, D., and Bellodi, L., 1996. American Journal of Psychiatry. 153:1060-1064.

Raj, A.B., Johnson, S.P., Wotton, S.B., and McInstry, J.L., 1997. Veterinary Journal. 153:329-339.