Identifying dead-on-arrivals (DOA) at shackling in a slaughter line gas stunning system for end-of-lay hens: Part I - Hock flexion resistance and wing position

Abstract

This study of end-of-lay fowl characterized the development of rigor mortis over time after euthanasia by testing the force required to flex a hock joint and by evaluating wing position as a test to differentiate euthanized birds from stunned birds. Using a force gauge, it was demonstrated that measurable resistance to hock flexion to the point of activation of the perch reflex develops rapidly after death and consistently increases over time, plateauing at 90 min. Based on a minimum detection threshold of 300 gram-force (gf) for hock stiffness, birds that developed rigor quickly, in the 50th percentile and higher, were detectable by 5 min postmortem, and all birds in the study population were detectable by 21 min postmortem. In addition, the sensitivity and specificity of wing position to differentiate dead birds from stunned birds was high (87.5 and 100%, respectively). We conclude that the development of palpable hock joint stiffness is sufficiently rapid to be useful as a means of differentiating between hens unconscious after CO2 gas stunning from hens that died during transport or lairage. Wing position can be used as a secondary assessment as the hen is lifted to the shackle line, such that the presence of wings held tight to the body would identify the hen as a DOA.