WELFARE IMPLICATIONS OF GAS STUNNING PIGS .3. THE TIME TO LOSS OF SOMATOSENSORY-EVOKED POTENTIALS AND SPONTANEOUS ELECTROCORTICOGRAM OF PIGS DURING EXPOSURE TO GASES

Changes in the spontaneous electrocorticogram (ECoG) and somatosensory evoked potentials (SEPs) were recorded in 12 pigs in each of three ga s killing treatments. The treatments were 90% argon in air with 2% res idual oxygen; a mixture of 30% carbon dioxide and 60% argon in air wit h 2% residual oxygen; or 80-90% carbon dioxide in air. The mean times to loss of SEPs were 15, 17 and 21 s, respectively. The mean time to l oss of SEPs recorded during killing with a high concentration of carbo n dioxide was significantly longer than those recorded for the other t wo gas killing treatments (P<0.05). Slow wales (high amplitude and low frequency) appeared on average 15 s after exposure to argon. In some pigs killed with the carbon dioxide-argon mixture, a decrease in the f requency of electrical activity was apparent, although slow waves did not appear during killing with a higher concentration of carbon dioxid e. A suppressed ECoG (reduction in amplitude of signals) was recorded at 22 and 20 s respectively, during exposure to the carbon dioxide-arg on mixture and 80-90% carbon dioxide in air, but the onset of ECoG sup pression could not be determined exactly during exposure to 90% argon in air. The time to onset of an isoelectric ECoG was 54, 39 and 32 s a fter exposure to argon, carbon dioxide-argon mixture and a high concen tration of carbon dioxide, respectively. The mean time to the onset of an isoelectric ECoG during exposure to argon was significantly longer than that recorded for the other two gas killing treatments (P<0.05). Based on the time to loss of SEPs, it is concluded that during killin g with a high concentration of carbon dioxide, pigs would have to endu re a moderate to severe respiratory; distress induced with this gas fo r a considerable period of time prior to the loss of brain responsiven ess. Argon-induced anoxia appears to be the first choice from a welfar e point of view for killing pigs, based on its lack of aversive proper ties and its effectiveness in rapidly abolishing brain responsiveness. A mixture of 30% carbon dioxide and 60% argon in air is considered to be more humane than using a high concentration of carbon dioxide, as the time to loss of brain responsiveness is similar to that using 90% argon in air.