'Whole House' killing of layer hens may be a relatively humane slaughter method, but there are variables that need to be tightly controlled.

Those who are less familiar with animal agriculture may think that cows farmed for their milk or hens used for their eggs live a life of less suffering. They may also believe that dairy products and eggs do not involve killing the animals that made those products. Unfortunately, nothing could be further from the truth. These animals may live longer, but when they are “spent” – meaning they no longer produce milk or eggs at an acceptably productive rate – they are often slaughtered and turned into meat to extract one last bit of “value.”

What’s more, the longer lives endured by milk-producing cows and egg-laying hens mean more confinement and suffering. And the slaughter process can be even harder on them than for animals raised directly for food. For hens, in particular, researchers note that “some end-of-lay flocks are not suitable for traditional slaughter and transport due to disease problems, poor plumage condition or poor skeletal strength.” Having spent so much of their life energy producing eggs, they are not even fit for transport or slaughter.

Some factory farms are looking at an alternative: bringing the slaughterhouse to the farm. The most common slaughter techniques – CO, CO2, “whole-house” and “mobile killing devices” – are actually methods that were originally used in avian influenza outbreaks in the early-to-mid 2000s. Researchers noted that the main welfare issues associated with these techniques were “i) the catching and handling of birds for slaughter or killing outside the barn; and ii) the time to unconsciousness when killing birds inside the barn.” The researchers here note that birds from the Gallus family (which includes hens) seem to be “able to detect CO at concentrations of 5.0–7.5% which is lower than concentrations used for the purposes of killing. Furthermore, these birds actively avoid atmospheres with CO2 levels above 7.5%.”

Despite these known shortcomings, CO2 is “commonly used for stunning/killing animals at slaughter plants and for emergency killings during disease outbreaks, possibly because it is nevertheless considered a ‘lesser evil’ when compared to other methods available.” The purpose of this study by Swedish researchers was to outline the exact techniques of CO2 killing and “assist legal authority decision-making as regards the welfare of hens during euthanasia.” The authors wanted to know how the birds’ environment changes during the process, how the birds react, and what welfare indicators are most relevant. Also, and importantly, they wanted to see if the CO2 method met legislated welfare requirements.

By studying two trials of the technique (in instances that were undertaken by egg producers and not the researchers) as well as a veterinarian’s analysis of the trials, the researchers formed a strong sense of the welfare implications of CO2 “whole-house” killing as a method. Perhaps unsurprisingly, considering the above information, the researchers found that the environment of the birds becomes “hostile” during CO2 killing. The injection of the gas creates a fog inside the barns, and the researchers describe how the hens react:

(The hens) react with avoidance, i.e., moving away from the gas inlet. Thereafter, there is a brief period of relative calmness, before the respiration of the birds is affected. Gasping, vocalisation and convulsions have been proposed as bird welfare indicators and their suitability is confirmed by this study.

Although the study found that the majority of cases were carried out in compliance with legislation, about one-fourth of them were not, and there seems to be some “systematic underreporting” to the Swedish Board of Agriculture.

Of course, animal advocates know – and the researchers themselves note – that killing animals involves some meaningful amount of stress or suffering regardless of the method used. That being said, the use of CO2 “whole-house” killing no doubt represents what could be seen as a systematic failure of animal agriculture to address welfare. If hens are so “spent” after being used to produce eggs that we cannot even safely transport them to be killed, then the system itself needs to be reconsidered rather than just the method of killing. We certainly shouldn’t rely on methods that induce even more suffering to end their lives.