Good practice in the poultry lairage and during shackling requires continual monitoring by personnel responsible for bird welfare. Lairage conditions should take into account ambient temperature, humidity and the general condition of birds arriving at the processing plant. Bird activity within and outside the transport containers should be kept to a minimum.
Flapping, for example on the shackle line, will increase downgrading and reduce the effectiveness of electrical waterbath stunning. The shackling procedure itself combined with bird inversion has been shown to be painful to birds, therefore the time birds are shackled before stunning should be kept to a minimum (12 s for chicken and 25 s for turkeys). By minimizing this period, the welfare of the birds can be more easily maintained in the event of a line breakdown, in that there will be less birds to remove or, preferably, stun/kill with a back-up device. Research has demonstrated that there are doubts as to the welfare aspects of decapitation and neck dislocation, which has led to the development of an alternative killing system for use in the casualty slaughter of poultry (Hewitt, 2000). A pneumatically powered percussive device has been developed for use either as a back-up to the killer or for the dispatch of shackled birds in the event of a line breakdown. It is hoped that both neck dislocation and decapitation will be phased out.
Electrical stunning
Pre-stun shocks
The turkey can be used as a prime example of the welfare problem of pre- stun shocks, as the average incidence of pre-stun shocks in a survey of turkey plants was found to be 45% (range 0–87%). The prevalence of shocks in turkeys is exacerbated by the anatomy of the bird. Turkeys have wings that hang lower than their heads when the bird is inverted and suspended on a shackle line. This means that their wings will enter the
‘live’ water first and the bird will receive a pre-stun shock. It is also important that the water does not overflow at the entrance ramp, creating a wet route through which live contact can be made, otherwise birds will receive a painful pre-stun shock on the way into the bath. This is
particularly a problem at slow line speeds and with badly designed waterbath entrances. Contact with a ‘live’ ramp will induce painful muscle contractions, which may result in birds flying the stunner or making and breaking contact throughout the stunner length. Waterbath entry ramp design and manipulation can be the solution for plants by holding back the bird at the top of the ramp for sufficient time to ensure that they swing down into the ‘live’ water in a fast, clean entry.
Electrical stunning equipment
Electrical stunning is the most commonly applied stunning method by the poultry industry. Birds are electrically stunned in waterbath stunners where the water is ‘live’ and the stunning current flows through the head (brain) and body of the bird to ground through an earthed shackle.
Sufficient electrical current must penetrate the brain to induce a stunned state that will enable the bird to remain unconscious until it is dead either through cardiac arrest, induced at the point of stun, or by exsanguination.
Commonly, electrical stunners apply mains frequency (50 Hz) alternating current (AC) of sinusoidal waveform. A 50 Hz sinewave is one of the optimum frequencies and waveforms for inducing cardiac arrest though ventricular fibrillation. In addition to the induction of brain dysfunction, the applied voltage will also stimulate muscles to contract.
This muscle stimulation is brought about in three ways: first, through direct muscle stimulation; second, through stimulation of the motor cortex in the brain; and third, through the stimulation of motor nerves in the periphery. The direct muscle stimulation can result in muscle haemorrhages and broken bones and this has led the industry to apply higher frequencies, which have a reduced effect on muscle stimulation.
Methods for assessing effective electrical stunning Laboratory methods:
1. The method of EEG assessment is not without certain limitations; for example, it has failed to provide unequivocal indication of the state of unconsciousness as produced by sleep or anaesthesia.
2. Somatosensory-evoked responses (SERs) represent a basic level of response, which can be used to investigate the patency of a nervous pathway. The presence of an evoked response does not necessarily indicate consciousness, as they occur in conscious and anaesthetised animals (Gregory and Wotton, 1983). However, the abolition of SERs does indicate a profound loss of consciousness in poultry.
3. The return of rhythmic breathing has been used extensively in red meat species and poultry to indicate the start of the recovery process. The presence of rhythmic breathing indicates that the brain stem and spinal cord are still functioning. It is not a proof of consciousness, but indicates the need for further tests to establish whether the birds are conscious.
In the processing plant:
1. The use of rhythmic breathing assessment is a basic method that can be used in the processing plant to determine the effectiveness of a stunning system. If the bird has been stunned effectively, rhythmic breathing will not resume for about 8 s or more from the bird’s exit from the waterbath.
Looking for signs of rhythmic breathing is not a valid test of consciousness and/or death if the spinal cord has been broken or severed by neck cutting.
Effect of stunning current on efficacy of stun
The use of SERs has allowed researchers to measure the effect of increasing current amplitude on the effectiveness of electrical waterbath stunning. The abolition of the SER suggested that 120 mA should be the minimum recommended current level per bird. Measurement of whether a treatment can abolish a multisynaptic response is an objective method;
however, it can be argued that it gives a very conservative answer, whereas the return of neck tension more closely follows the bird’s recovery. These results, taken together with the subjective results using the return of neck tension, produced a recommendation for a minimum current of 105 mA per bird for chickens when an AC voltage is applied. Recent research combining EEG analysis by Fast Fourier Analysis and SER abolition has suggested a minimum rms AC of 100 mA per bird for 100 and 200 Hz, and that the current should be increased for frequencies above 400 Hz.
The poultry industry has adopted waterbath stunning using a high- frequency pulsed DC waveform with a 25–30% duty cycle, because of the improvements in carcass and meat quality that they can achieve. In addition, the use of low-frequency AC stunners at the minimum current to stun (105 mA per bird) will result in some birds receiving less than the minimum current, due to variation in impedance between birds whereas, with high-frequency pulsed DC stunners, three or four times the minimum current can be applied i.e. 40–50 mA per bird, and all birds should receive more than the minimum recommended current.
Slaughter
It is important that the correct blood vessels are severed and that the cut is made as quickly as possible following electrical waterbath stunning. When 105 mA is applied per bird at 50 Hz, about 90% of birds will be killed in the stunner (ventricular fibrillation). However, it is still important that the neck-cutting procedure is accurate. When higher frequencies are applied, the majority of the birds will survive the stunning treatment. The death process starts from the severance of major blood vessels when insufficient oxygenated blood reaches the brain. It is essential that both carotid arteries are severed at neck cutting to ensure the birds do not recover. An additional advantage can be achieved if heads can be removed by the killer
for immediate maceration, which is a solution to the welfare concern over:
(i) neck cutting procedures; (ii) the duration of unconsciousness produced by the electrical stun; and (iii) misinterpretation of normal post-kill bird movement.
References
Anil, M.H. (1991) Studies on the return of physical reflexes in pigs following electrical stun- ning.Meat Science30, 13–21.
Anil, M.H., McKinstry, J.L., Wotton, S.B. and Gregory, N.G. (1995a) Welfare of calves – 1.
Investigations into some aspects of calf slaughter. Meat Science41, 101–112.
Anil, M.H., McKinstry, J.L., Gregory, N.G., Wotton, S.B. and Symonds, H. (1995b) Welfare of Calves – 2. Increase in vertebral artery blood flow following exsanguination by neck sticking and evaluation of chest sticking as an alternative slaughter method. Meat Science 41(2), 113–123.
Anil, M.H., McKinstry, J.L. and Wotton, S.B. (1997) Electrical stunning and slaughter of pigs.Fleischwirtschaft77(5), 473–476.
Cook, C.J. (1993) A guide to better electrical stunning. Meat Focus International (March), 128–131.
Daly, C.C. (1990) Stunning and slaughter an overview – meat quality from gate to plate.
Proceedings of a two-day course organized by the Meat Technology Service, Division of Meat Animal Science, University of Bristol, Langford, UK.
Daly, C.C., Kalweit, E. and Ellendorf, F. (1988) Cortical function in cattle during slaughter:
conventional captive bolt stunning followed by exsanguination compared with shechita slaughter. Veterinary Record122, 325–329.
DEFRA (1995) The Welfare of Animals (Slaughter or Killing) Regulations. Statutory Instruments No. 731, HMSO, London.
Gregory, N.G. and Wotton, S.B. (1983) Studies on the central nervous system: visually evoked cortical responses in sheep. Research in Veterinary Science34, 315–319.
Gregory, N.G. and Wotton, S.B. (1984a) Sheep slaughtering procedures. 2. Time to loss of brain responsiveness after exsanguination or cardiac arrest. British Veterinary Journal 140, 354–360.
Gregory, N.G. and Wotton, S.B. (1984b) Time to loss of brain responsiveness following exsanguination in calves. Research in Veterinary Science37, 141–143.
Hewitt, L. (2000) The development of a novel device for humanely dispatching casualty poultry. PhD thesis, University of Bristol, UK.
Raj, A.B.M. and Gregory, N.G. (1996) Welfare implications of the gas stunning of pigs 2.
Stress of induction of anaesthesia. Animal Welfare5, 71–78.
Wotton, S.B. and Gregory, N.G. (1986) Pig slaughtering procedures: time to loss of brain responsiveness after exsanguination or cardiac arrest. Research in Veterinary Science40, 148–151.
5.2 Hygiene of Slaughter – Cattle
The general flow of operations during cattle slaughter and dressing is illustrated in Fig. 5.1.