Disposal of spent hens: options evaluation

Appendix 1 – Literature Review

1. A review of various scientific and other papers was undertaken to obtain a clearer understanding of the impact of the health and welfare of spent hens being transported for slaughter. Most of the available research highlighted focusses on broilers and hence it would be unrealistic to draw direct comparisons for spent hens given the different genetics and rearing practices. The researchers could not find any specific academic research on spent hens transportation.

Pre slaughter conditions and transport

2. Birds will be exposed to a variety of potential stresses during transit, including the thermal demands of the transport microenvironment, acceleration, vibration, motion, impacts, fasting, withdrawal of water, social disruption, and noise (Jiang et al. 2016). According to some literature the handling process itself could be more stressful than the transport journey (Dos Santos et. al. 2020).

3. Although catching, crating, and loading are the activities most likely to cause physical injuries, transportation has also been reported to be stressful. Regarding the length of transport itself results are mixed. Studies by Ehinger (1977) found that broilers’ meat tenderness and water holding capacity were reduced after 2 hours and improved after 4 hours of transportation. Other work from Cashman (1987) assessed pH and meat colour which showed poorer results when the birds were transported for a 2-hour journey compared to shorter ones. This research suggests the influence of transport length is one factor alongside others which influence stress.

4. Theoretically, at long distances the heat produced in broilers tends to accumulate inside the load and increases the internal temperature of the bird. Similarly, this increase of internal temperature occurs in shorter journeys and the travel and lairage time combination could be too short to allow an adequate return to homeostasis (a state of balance), producing even greater stress in birds. 1-2 hours is the range of lairage time stipulated by DEFRA, but due to the flow of trucks in a slaughterhouse this can be challenging to control. This shows another pre-slaughter factor important to manage alongside distance/time, loading density and temperature in the truck.

5. The temperature and humidity at transport are also factors. They are endorsed by recommended temperature and relative humidity values in the different standards. Studies to better understand the bioclimatic variables for bird shipment and location in the truck are also necessary as the microclimate within the trailer can be key in terms of understanding animal welfare.

Meat quality

6. Pre-slaughter factors can also affect the process of converting muscle to meat and compromise consumer acceptability. Meat appearance (colour) for example is a critical factor influencing customer purchase desire. Other quality traits such as water holding capacity are also relevant but more impactful in further processing.

7. In the study by Bianchi et al. (2006), no difference was found on the redness and yellowness of the meat with a variable transport length between 40km and 210 km, while paleness was increased by longer transport journeys. Larger differences in colour were seen between different genetic lines and bird liveweight. Other parameters that heavily influenced the meat colour were temperature and stress and excitement.

8. In the study by Zang et al. (2009), neither transport time nor recovery time and their interactions affected pH, water holding capacity or the paleness of meat. However, there was a higher accumulation of lactate (a product from the breakdown of skeletal glycogen) in birds who were transported for longer. Extended recovery reduced the effects of transport induced stress by decreasing the lactate concentration in the muscle. This helped to maintain this meat quality trait.

9. These results are only partly transferable to the spent hens as the research was conducted with broilers. Broilers have a different muscle fibre composition and anatomy and are grown for shorter periods and to higher weights.

Blood metabolites

10. The effect of transport stress on blood metabolism in broilers is extensively documented. The concentration of plasma hormones, enzymes and metabolites (cortisol, corticosterone, creatine kinase, and glucose) are suggested as parameters to indicate stress and muscle damage. In addition, pre-slaughter and postmortem glycogen metabolism and lactate accumulation are also influential in meat quality. In blood, the ‘H:L’ (heterophils and lymphocytes) ratio is also recognised as a stress indicator.

11. Zang et al. (2009) researched glucose and corticosterone levels. This showed an increase of glucose concentration in the first 45 minutes of transport which then decreased dramatically in the birds transported longer (more than 3 hours). It is important to note that the glucose concentration in blood is a parameter of the metabolism of the liver, breaking down its energy reserves to cope with stress. The response after the first hour seems to stabilise indicating adaptation to the environment. Regarding the H:L ratio, the combinations of transport and recovery time did not influence the H:L ratio but longer transports increased it slightly.

12. The presence of corticosterone in plasma is common in transport stressed broilers, which in this study showed higher values for short transport and short recovery time and long-term recovery, suggesting that birds might experience heightened stress at the beginning of transport. They then become adapted to the perceived adverse environment later during transport, or they recover better during a longer recovery period. Even if the length of transport is increased, a long-term recovery after transport can help the reduction of stress metabolites in blood and lessen the impact on meat quality. Also, preparation factors such as feed withdrawal can be managed to provide adequate metabolic reserves for the birds to avoid stress due to hunger.

13. In summary, physical activities (catching, crating, etc), lairage time, temperature and humidity exerted the most important effects in broiler meat. Other factors such as genotype, live weight and transport also have an impact.

14. Reports on the effect of transport in meat quality are sometimes contradictory, ranging from no differences to decreased muscle glycogen and increase pH levels. Undoubtedly, transport can alter both the metabolism and physiological state of the birds and this can impact on product quality for broiler meat.

15. The research, particularly for broilers, suggests that longer animal transport times can affect their welfare compared to shorter journeys. However, it is important to recognise it is not necessarily the duration per se, but the conditions of the birds and all of the associate factors above that may induce stress and other issues. It is also worth highlighting that broilers are raised in a different environment to laying hens, such as in heated housing and are more sensitive to change. The researchers could not find equivalent studies for spent hens; hence it would be unrealistic to draw direct comparisons.

References include:

• Ali, MS 2008, A review: Influences of pre-slaughter stress on poultry meat quality. Asian Australian Journal of Animal Science, Vol. 21, No. 6: 912-916
• Bianchi, M., 2006. The influence of Genotype, Market Live Weight, transportation and holding conditions prior to slaughter on Broilers breast colour. Poultry Science 85:123-128
• Cashman, PJ. An assessment of the fear levels of broilers during transit. Thesis, University of Bristol, Bristol, UK
• dos Santos VM, Dallago BSL, Racanicci AMC, Santana AˆP, Cue RI, Bernal FEM (2020) Effect of transportation distances, seasons and crate microclimate on broiler chicken production losses.
• Ehinger, F. 1977. The influence of starvation and transportation on carcass quality of broilers. The quality of poultry meat, pp 117-124, (Munich, Germany, European Poultry Federation)
• Zhang, L, 2009. Transport stress in broilers: Blood metabolism, glycolytic potential and meat quality. Journal of Poultry Science 88:2033-2041