Live chickens are hung upside down by the legs in metal shackles and then carried by a moving belt to a water bath. An electric current is passed through their bodies stunning them. However, the current recommended by the Ministry of Agriculture, 105 milliamps, is not adequate for the job, Peter Stevenson, who has written a report on the slaughter of chickens for Compassion in World Farming, said.
'A current of at least 120 milliamps per bird is needed,' Mr Stevenson said. 'This will cause a cardiac arrest in 90 per cent of the birds. A smaller current is not sufficient to guarantee that the birds will not regain consciousness before entering the scalding bath.'
However, stunning with a larger current may cause blemishes to the carcasses, reducing their value. This makes poultrymen reluctant to use higher currents, although the smaller number of birds affected in this way can still be cut up and sold as chicken portions. After being stunned, the birds have their throats cut in a special machine and are left to bleed for about a minute and a half. They then die from loss of blood. But the method of cutting the throat is not efficient enough, Mr Stevenson said.
To be certain that the bird is dead before being scalded - a process which is necessary to loosen the feathers for plucking - the throat should be cut on both sides, severing both branches of the carotid artery, as well as the jugular veins, he said.
Poultrymen are reluctant to cut the throat on both sides because it makes the head too loose. The head may then fall into the feathers during plucking, reducing the value of the feathers.
Some chicken farmers believe that the bird's heart should continue to beat after its neck is cut so that the blood is fully pumped from its body. This has made them reluctant to use a larger current in the stunning machine. However, scientific experiments have shown that just as much blood leaves the body if the heart is not beating when the throat is cut.
New compulsory regulations, rather than a code of practice, are needed to prevent this needless suffering, Compassion in World Farming said. New slaughtering machinery would need to be designed to meet the suggestions of the pressure group. Existing machines use a constant voltage to stun birds but it is the current that should be constant to do the job reliably, Mr Stevenson said.
Compassion in World Farming has tried without success to obtain information through parliamentary questions on practices in UK slaughterhouses. Although slaughterhouses are inspected by Ministry of Agriculture veterinarians, the ministry said that no information was available on the proportion of birds which are stunned in practice.
Peter Clark, British Chicken Association chairman, said: 'We refute all charges of unnecessary suffering being caused at any stage of the production process, or that the welfare of the birds is sacrificed to commercial gain.'