Gas Killing of Chickens and Turkeys
SUMMARY
Although the majority of birds slaughtered in UK processing plants arestunned using electrical waterbath systems, an increasing number of plants
are killing poultry using gas. This process has the potential to eliminate
some of the welfare concerns associated with electrical waterbath stunning,
including such issues as uncrating the birds which can cause handlingstress, live shackling, pre-stun electric shocks and ineffective stunning.
Any person involved with the stunning, slaughtering or killing of poultry has
a legal obligation to ensure they do not cause 'avoidable pain, excitement
or suffering to any animal’ under The Welfare of Animals (Slaughter orKilling) Regulations 1995 (WASK '95)(as amended). All staff involved withthese procedures must have the knowledge and skills necessary tounderstand fully the operation of the equipment they are using and how todeal with any problems that might arise.
This leaflet focuses on the principles of gas killing and the advantages anddisadvantages of the different types of gas mixture used to kill chickens
and turkeys in the UK. It aims to provide constructive, practical advice tomaximise bird welfare.
Operators must be trained to identify signs of an ineffective killprocess and must know the appropriate action to be taken to prevent
birds from suffering unnecessarily.
BackgroundGas systems are increasingly being used for the killing of poultry. Controlled Atmosphere
Stunning (CAS) systems kill birds by exposure to an anoxic gas mixture (gas mixture that
does not contain oxygen), which rapidly renders them insensible to pain or distress.
Although the name ‘CAS system’ suggests that birds are only stunned whilst in the gas,
UK legislation demands that birds must be killed whilst they are still in the gas mixture andbefore they are shackled.
Extensive research has been carried out since the 1980s into the most appropriate gas
mixtures for use in a commercial environment. Based on this research, the first systembecame available in the UK in 1996.
CAS systems eliminate a number of bird welfare issues associated with the useof electrical waterbath systems including:zzzzz

the stress of uncrating birds
the stress of shackling and inverting live birds
the problem of pre-stun electric shocks
the possibility of variation in current within a multiple-bird electrical waterbath
the possibility of birds missing the electrical waterbath stunner

Comparisons of different stunning techniques also show that gas mixtures give a lower
incidence of broken bones at killing compared to electrical stunning, therefore improving
carcase quality.
Humane Slaughter AssociationNo.12

Legislation
WASK '95 controls the killing of birds by exposure to gas mixtures. Legal requirements
include:zzzzzzzzz

birds must be killed by exposure to an anoxic gas mixture which rapidly renders
them insensible to pain or distressgas mixtures can be either:(i)any mixture of argon, nitrogen or other inert gases with a maximum of 2%
total oxygen by volume (2% total oxygen by volume is the proportion of oxygenin a 90% inert gas, 10% air mixture, as the amount of oxygen in air is
20.9%) OR(ii)
carbon dioxide with any mixture of argon, nitrogen or other inert gases witha maximum of 2% oxygen by volume, provided that the carbon dioxide does
not exceed 30%
devices must be fitted so that carbon dioxide or oxygen (as appropriate) can bemeasured by volume at the point of maximum concentration
once in the chamber, the bird has to be conveyed to the point of maximum
concentration of the gas mixture within a maximum period of 10 seconds
a mechanism must be in place which stops the entry of birds into the chamber if gas
concentrations:i)rise above 2% for oxygen ORii)above 30% for carbon dioxide
there must be a means of visually monitoring birds in the chamber
visible and audible warning systems must indicate when:(i) oxygen levels rise above 5% for more than 30 seconds in the inert gas mixture OR(ii) carbon dioxide levels rise above 30% for more than 30 seconds in the other mixture
there must be a means of flushing the chamber with atmospheric air with the minimumof delay
birds must be killed by the gas to prevent them being shackled before they are deadPrinciples of gas killing
The killing of poultry with gas mixtures does not result in an immediate loss ofconsciousness, therefore it is important to ensure that the induction of unconsciousness
does not cause distress to the birds. In addition to this, the type of gas used must be
suitable for a commercial environment. The characteristics of an ideal humane andefficient gas mixture include that it must:zzzzzz

be non-aversive (causes no detectable signs of distress)
induce loss of consciousness in birds as rapidly as possible
be capable of killing birds within the system (in the UK)
be reasonably cheap and safe to use in industrial conditions
be relatively easy to contain within an open container
have no side effects on meat quality

Gas mixtures
From these specifications and further research, potentially suitable gases have so far been
narrowed down to carbon dioxide, argon and nitrogen.
Carbon dioxide is heavier than air (therefore easy to control), relatively cheap and ananaesthetic gas which produces rapid unconsciousness when inhaled at high concentrations.
However, it is acidic, induces a sense of breathlessness, and is thought to be highly aversive
to birds at concentrations above 30%. Argon is an inert gas (colourless, odourless andtasteless) that is heavier than air, kills birds by anoxia and is non-aversive. Argon is more
expensive than both carbon dioxide and nitrogen. Nitrogen has the same welfare
Humane Slaughter Association

advantages and properties as argon except it is slightly lighter than atmospheric air, somay be more difficult to contain in high concentrations.
Research into the behaviour of chickens, during exposure to a number of different
gas mixtures, looked at the time to loss of posture (an indicator of unconsciousness) and thenumber of gasping and head shaking episodes (an indicator of discomfort) occurring during
this time. The two mixtures which resulted in rapid induction of unconsciousness andproduced the least number of gasps and head shakes were:
(i)
(ii)

any mixture of argon, nitrogen or other inert gases with a maximum of 2% total
oxygen by volume OR
carbon dioxide with any mixture of argon, nitrogen or other inert gases witha maximum of 2% oxygen by volume, provided that the carbon dioxide does
not exceed 30%.

Signs of effective and ineffective killing processes
Once birds have left the CAS system, they must be checked to ensure they have been
killed before they are shackled. If any indicators of consciousness are seen, the bird(s)must be killed immediately using a back-up system, eg electrical stunning or captive-bolt
equipment, and the loading of new birds into the system stopped until the problem has
been rectified. Any birds still within the system should be removed immediately and killedusing a back-up method.
The signs that a bird has been killed are:z no breathingz wings droopingz the absence of a third eyelid (nictitating membrane) reflex when the surface of theeye is touchedz dilated pupils
The signs that a bird has not been killed are:z rhythmic breathing (look for movements in the vent area)z tension in the neck (ability to control the movement of the head)z signs of co-ordinated movement
If any of these indicators are seen, the bird(s) should be killed immediately andeffectively, before the neck is cut.

Bleeding
Although CAS systems within the UK must kill the birds within the system, for meat quality
purposes their necks will still have to be cut to drain as much blood as possible from thecarcase before any further processing. Blood loss can be achieved by a ventral neck cut
across both carotid arteries and both jugular veins. A sharp, clean blade must be used tocut across the front of the neck just below the head (see Figure 1).
JugularVeins
Carotid Arteries

Figure 1 Diagram of ventral neck cut across carotid arteries and jugular veins
Humane Slaughter Association

Advantages of gas killing
The use of CAS systems for poultry killing has a number of welfare and commercial
advantages in comparison to conventional electrical waterbath stunning. These include:zzzzz

birds are not inverted and shackled live, thus reducing handling stresses and the riskof injury to both the bird and the hanger-on
birds are killed within their crates, therefore there should not be a risk of recovery
before slaughter if the system is functioning correctly
the possibility of pre-stun shocks is eliminatedthe possibility of birds receiving inadequate current levels is eliminatedthe killing of birds using gas mixtures is associated with a lower incidence of brokenbones compared to electrical waterbath stunning, which can improve carcase quality

Disadvantages of gas killingSome disadvantages of gas killing also need to be taken into account when considering
fitting this system. These include:zzz

more moving parts in the system compared to electrical waterbath stunners, therefore
there is an increased chance of a system breakdown
the initial cost of fitting the equipment is highalthough argon is an excellent gas to use for these systems, it is more expensive
than other gases available

Further reading
The Welfare of Animals (Slaughter or Killing) Regulations 1995 (as amended) HMSO 1995
Full details of all legislation can be found on www.tso.co.uk
Practical Slaughter of Poultry, A Guide for the Small Producer (2nd Edition) HSA 2001
Technical Note No. 10 Electrical Waterbaths
HSA 2005

WARNING: DISCLAIMER OF LIABILITY
In no circumstances can the HSA accept liability for the way in which the information in this leaflet
is used: or for any loss, damage, death or injury caused thereby, since this depends
on circumstances wholly outside the HSA’s control.

The only charity committed
exclusively to the welfareof animals in markets,
during transport and to thepoint of slaughter

Taking a rational, practicalapproach, making real,lasting improvements tothe welfare of food animals

The Old School . Brewhouse Hill . Wheathampstead . Herts AL4 8AN . UK
Tel: +44(0)1582 831919

Fax: +44(0)1582 831414

Email: info@hsa.org.uk

Registered Charity No. 209563

June 2005

Website: www.hsa.org.uk