Mortality in broiler chicken flocks represents both a significant welfare indicator and an economic concern. Understanding the causes of mortality and implementing evidence-based prevention strategies reduces suffering and improves flock welfare outcomes.
Leading Causes of Broiler Mortality
Sudden death syndrome (SDS, or flip-over disease) and ascites are the two most prevalent non-infectious mortality causes in fast-growing broiler breeds. SDS causes sudden, apparently healthy birds to die with spasmodic wing-beating, linked to cardiac arrhythmia associated with the high oxygen demands of rapidly growing breast muscle. Ascites — fluid accumulation in the body cavity — results from right-sided heart failure caused by insufficient pulmonary vascular capacity relative to metabolic demand. Both conditions are more prevalent in fast-growing breeds, underscoring the welfare connection between breed selection and health outcomes.
Infectious causes of mortality including Gumboro disease (infectious bursal disease), Newcastle disease, infectious bronchitis, and bacterial infections contribute substantially to flock mortality in inadequately vaccinated or biosecurally compromised flocks. Disease outbreaks cause both mortality and significant morbidity, with affected birds experiencing respiratory distress, diarrhea, and systemic illness before death.
Monitoring as a Welfare Tool
Daily mortality recording and analysis is a fundamental welfare management practice. Elevated mortality — particularly early in the flock cycle or with unusual age distribution — is an early warning signal of disease or management problems requiring investigation. Farms that systematically record and review mortality patterns detect problems earlier and intervene more effectively than those treating mortality as an inevitable cost.
Flock daily checks should identify not just dead birds but also sick and moribund birds that require welfare assessment and prompt veterinary attention or humane euthanasia where recovery is not expected. Leaving moribund birds without intervention is a welfare failure that extends suffering unnecessarily.
Breed Selection and Mortality
Slow-growth and improved conventional breeds show substantially lower rates of SDS and ascites than the fastest-growing conventional breeds. The metabolic adaptations that enable rapid growth impose cardiac and pulmonary constraints that generate these mortality causes. Breed selection is therefore a welfare intervention that directly reduces mortality from these conditions.
Environmental Management
Thermal management, ventilation, and stocking density all affect broiler mortality rates. Adequate ventilation reducing ammonia and carbon dioxide concentrations supports respiratory health. Appropriate temperature gradients allowing thermoregulation reduce heat stress mortality. Stocking density management — reducing density as birds grow — prevents crowding-related mortality from smothering and reduces disease transmission.