Research in VeterinaryScience1991, 51, 200-202
Effect of depth of immersion in the waterbath on the effectiveness of electrical stunning in chickens N. G. GREGORY, S. B. WOTTON, Department o f Meat Animal Science, University o f Bristol, School o f Veterinary Science, Langford, Bristol BS18 7D Y
Broiler chickens were electrically stunned either by immersing their heads, necks and upper breast in a waterbath stunner or by immersing their heads only. The time to recovery of muscular activity was assessed, and it was found that the deeper immersion was associated with a slightly (10 second) shorter time to resumption of head righting. When hens which had previously been implanted with electroencephalogram electrodes were electrically stunned it was found that the incidence of somatosensory evoked responses during the first 60 seconds after applying the current was not influenced by the depth of immersion. It was concluded that depth of immersion had little influence on the effectiveness of electrical stunning. THE depth of immersion used in waterbath stunners depends on the size of the bird relative to the distance between the water surface and the overhead line. Since bird size varies within a batch delivered to a processing plant the height of the stunner is usually adjusted so that the majority of the birds will make contact with the water. The larger birds are inevitably immersed up to the base of their wings while the smaller birds may only have their heads in contact with the water. It has been suggested that the immersion depth could help to determine whether a bird is likely to be properly stunned. For example, in those birds that are immersed to the base of their wings, it is possible that the current bypasses the head and instead flows between the base of the wings and the legs. In this situation the stun could be inadequate, whereas if only the head were immersed in the water, current is more likely to flow through the brain. This study set out to determine whether the depth of immersion influenced the effectiveness of the stun by comparing the effect of shallow and deep immersion on somatosensory evoked responses in the brain and the time to recovery of muscular activity. Both techniques have been used in earlier studies to assess the level of brain disturbance achieved with different electrical stunning procedures (Gregory and Wotton 1990a).
Materials and methods
Somatosensory evoked responses Sixty hens (mean bodyweight 1-8 kg ± 0.2 SD) were implanted with electroencephalogram (EEG) electrodes while under pentobarbitone anaesthesia using the method of Gregory and Wotton (1986, 1989). Stainless steel electrodes for evoking somatosensory potentials were implanted in the wing on the contralateral side to the recording electrodes in the positions described by Gregory and Wotton (1989). After at least four hours recovery from the anaesthesia, each bird was electrically stunned using a glass waterbath stunner while suspended by the feet from a stainless steel shackle. The shackle and bird were lowered into the waterbath using a rope and pulley and the birds were stunned with 60 to 103 mA per bird for four seconds. Thirty birds were immersed in the water so that only their heads were submerged, and 30 birds were immersed so that their heads and necks were totally submerged and the cranial portion of the body up to the base of the wings (that is, crop and cervical girdle regions). Shallow immersion was achieved by holding the neck in the water with a plastic noose and tube which was similar in design to a dog catcher. Before, during and after the stunning current application the wing was stimulated with 3.4 V pulses delivered at 2 pulses s - l and a 2 ms pulse duration. The EEG and evoked potentials were recorded and analysed by the method of Gregory and Wotton (1989). Potentials in the averaged EEG traces were recognised as somatosensory evoked responses if their polarity and latency were repeatable.
Recovery o f muscular activity Four hundred and sixty-three broilers (mean bodyweight 1.7 kg ± 0-4 SD) were subjected to electrical stunning using a shackle suspended from a rope and pulley and a waterbath. One hundred and ninety-nine broilers were immersed up to the base of their wings in
200
Effect of immersion on stunning the water and 264 broilers had only their heads immersed in the water using the restraining device described in the previous subsection. The birds received a 50 Hz sinusoidal alternating current of 65 mA ( ± 1 SD) for four seconds, delivered from a constant current stunner unit. The time from the start of stunning to the onset of breathing movements was determined with a stop watch. Those birds which became flaccid and did not resume breathing were assumed to have experienced a ventricular fibrillation. When breathing occurred the bird was immediately removed from the shackle and placed on its side on a table. While in that position the fingers of one hand were placed under the head and upper neck of the bird, and its head was repeatedly raised (once every one or two seconds). This was continued until the maintenance of neck posture returned and the bird could hold its head away from the operator's hand. The time from the onset of stunning to the return of this function was measured with a stopwatch. As soon as the neck tension test was performed, the bird was placed on its legs and pushed gently to test whether it could balance on its shanks and feet.
Results
Somatosensory evoked responses The average current delivered to the birds in the shallow immersion treatment was almost the same as that in the deep immersion treatment (89 ± 2 versus 90 ± 2 mA ± SE ). There was no difference in the prevalence of a ventricular fibrillation between the shallow and deep immersion treatments. Forty-seven and 53 per cent of the birds in the shallow and deep immersion treatments, respectively, developed ventricular fibrillation at stunning. Similarly, there was no difference between the two treatments in the prevalence of
TABLE 1 : Effect of depth of immersion in a waterbath stunner on the prevalence of ventricular fibrillation and the time to recovery of muscular activity Depth of immersion Shallow Deep Significance Number of broilers % fibrillated Non-fibrillated birds Number of broilers Time to resumption of breathing s ( ± SE) Time to return of neck tension s ( ± SE) % birds which balanced
264 78
199 70
59
59
23 ± 1
23 ± 1
65 ± 3 78
55 ± 3 68
NS Not significant at P = O - 0 5 * Fisher protected least significant difference t X2 test on absolute values
P
Effect of depth of immersion in the waterbath on the effectiveness of electrical stunning in chickens N. G. GREGORY, S. B. WOTTON, Department o f Meat Animal Science, University o f Bristol, School o f Veterinary Science, Langford, Bristol BS18 7D Y
Broiler chickens were electrically stunned either by immersing their heads, necks and upper breast in a waterbath stunner or by immersing their heads only. The time to recovery of muscular activity was assessed, and it was found that the deeper immersion was associated with a slightly (10 second) shorter time to resumption of head righting. When hens which had previously been implanted with electroencephalogram electrodes were electrically stunned it was found that the incidence of somatosensory evoked responses during the first 60 seconds after applying the current was not influenced by the depth of immersion. It was concluded that depth of immersion had little influence on the effectiveness of electrical stunning. THE depth of immersion used in waterbath stunners depends on the size of the bird relative to the distance between the water surface and the overhead line. Since bird size varies within a batch delivered to a processing plant the height of the stunner is usually adjusted so that the majority of the birds will make contact with the water. The larger birds are inevitably immersed up to the base of their wings while the smaller birds may only have their heads in contact with the water. It has been suggested that the immersion depth could help to determine whether a bird is likely to be properly stunned. For example, in those birds that are immersed to the base of their wings, it is possible that the current bypasses the head and instead flows between the base of the wings and the legs. In this situation the stun could be inadequate, whereas if only the head were immersed in the water, current is more likely to flow through the brain. This study set out to determine whether the depth of immersion influenced the effectiveness of the stun by comparing the effect of shallow and deep immersion on somatosensory evoked responses in the brain and the time to recovery of muscular activity. Both techniques have been used in earlier studies to assess the level of brain disturbance achieved with different electrical stunning procedures (Gregory and Wotton 1990a).
Materials and methods
Somatosensory evoked responses Sixty hens (mean bodyweight 1-8 kg ± 0.2 SD) were implanted with electroencephalogram (EEG) electrodes while under pentobarbitone anaesthesia using the method of Gregory and Wotton (1986, 1989). Stainless steel electrodes for evoking somatosensory potentials were implanted in the wing on the contralateral side to the recording electrodes in the positions described by Gregory and Wotton (1989). After at least four hours recovery from the anaesthesia, each bird was electrically stunned using a glass waterbath stunner while suspended by the feet from a stainless steel shackle. The shackle and bird were lowered into the waterbath using a rope and pulley and the birds were stunned with 60 to 103 mA per bird for four seconds. Thirty birds were immersed in the water so that only their heads were submerged, and 30 birds were immersed so that their heads and necks were totally submerged and the cranial portion of the body up to the base of the wings (that is, crop and cervical girdle regions). Shallow immersion was achieved by holding the neck in the water with a plastic noose and tube which was similar in design to a dog catcher. Before, during and after the stunning current application the wing was stimulated with 3.4 V pulses delivered at 2 pulses s - l and a 2 ms pulse duration. The EEG and evoked potentials were recorded and analysed by the method of Gregory and Wotton (1989). Potentials in the averaged EEG traces were recognised as somatosensory evoked responses if their polarity and latency were repeatable.
Recovery o f muscular activity Four hundred and sixty-three broilers (mean bodyweight 1.7 kg ± 0-4 SD) were subjected to electrical stunning using a shackle suspended from a rope and pulley and a waterbath. One hundred and ninety-nine broilers were immersed up to the base of their wings in
200
Effect of immersion on stunning the water and 264 broilers had only their heads immersed in the water using the restraining device described in the previous subsection. The birds received a 50 Hz sinusoidal alternating current of 65 mA ( ± 1 SD) for four seconds, delivered from a constant current stunner unit. The time from the start of stunning to the onset of breathing movements was determined with a stop watch. Those birds which became flaccid and did not resume breathing were assumed to have experienced a ventricular fibrillation. When breathing occurred the bird was immediately removed from the shackle and placed on its side on a table. While in that position the fingers of one hand were placed under the head and upper neck of the bird, and its head was repeatedly raised (once every one or two seconds). This was continued until the maintenance of neck posture returned and the bird could hold its head away from the operator's hand. The time from the onset of stunning to the return of this function was measured with a stopwatch. As soon as the neck tension test was performed, the bird was placed on its legs and pushed gently to test whether it could balance on its shanks and feet.
Results
Somatosensory evoked responses The average current delivered to the birds in the shallow immersion treatment was almost the same as that in the deep immersion treatment (89 ± 2 versus 90 ± 2 mA ± SE ). There was no difference in the prevalence of a ventricular fibrillation between the shallow and deep immersion treatments. Forty-seven and 53 per cent of the birds in the shallow and deep immersion treatments, respectively, developed ventricular fibrillation at stunning. Similarly, there was no difference between the two treatments in the prevalence of
TABLE 1 : Effect of depth of immersion in a waterbath stunner on the prevalence of ventricular fibrillation and the time to recovery of muscular activity Depth of immersion Shallow Deep Significance Number of broilers % fibrillated Non-fibrillated birds Number of broilers Time to resumption of breathing s ( ± SE) Time to return of neck tension s ( ± SE) % birds which balanced
264 78
199 70
59
59
23 ± 1
23 ± 1
65 ± 3 78
55 ± 3 68
NS Not significant at P = O - 0 5 * Fisher protected least significant difference t X2 test on absolute values
P
