Joy A. Mench
Department of Animal Science
University of California
Davis, California 95616-8532
Paul B. Siegel
Department of Animal and Poultry Sciences
Virginia Polytechnic Institute and State University
Blacksburg, Virginia 24061-0306
Birds being sent to slaughter are hand-captured, crated, and transported by road over varying distances to the processing plant. Many elements of the transport process can be harmful to the bird (15,16). These include handling by humans, air temperature changes, removal of food and water, novelty, confinement, noise, motion, and mixing with unfamiliar birds. Improper handling and transport may also result in mortality, bruising, and bone breakage, with the latter representing a particular problem with spent laying hens. Mechanical harvesting may be less stressful to the bird than human handling (17). However, many problems have been encountered with regard to the maneuverability of harvesting machines in commercial houses. Whether harvesting is done by hand or machine, care should be taken to handle birds gently during capture and crating and uncrating. Stress should also be minimized during transportation. USDA has developed guidelines for air transport of chicks and hatching eggs (18).
Meat birds are slaughtered by being shackled and electrically stunned in a brine-water-bath stunner, followed by the severing of the vertebral and/or carotid arteries with an automatic knife. The stunning currents used are intended to render the bird insensible temporarily until bleed-out is completed. Laying hens are usually not stunned, because their bones, which may be osteoporotic due to lack of exercise and the high rate of calcium usage for egg formation, break during the application of an electrical current. Stunning is also not used for some religious slaughter methods.
Surveys in Europe have shown that approximately 30 percent of birds processed using a bath stunner are inadequately stunned before slaughter (19); no comparable surveys have been conducted in U.S. processing plants. Research is currently being conducted in England on the welfare aspects of stunning to induce cardiac arrest and on the use of gas (carbon dioxide plus argon) stunning (20). Additional research is needed on the commercial utility of alternative stunning methods. At present, electrical stunning should be carried out carefully to ensure maximum efficacy (21).
Because they have little market value, spent hens may now be slaughtered on-farm. Carbon dioxide delivered via a mobile killing unit with an on-board delivery system, cervical dislocation, or instant maceration using a specially designed high-speed grinder, are acceptable on-farm slaughter methods when properly performed (22,23). In the past, unhealthy chicks or surplus male chicks were killed at the hatchery by suffocation. This practice has essentially been abandoned by the U.S. poultry industry in favor of more humane methods like maceration, which result in instantaneous death.