FACTORS AFFECTING MORTALITY OF CROSSBRED AND EXOTIC CHICKENS KEPT UNDER BACKYARD SYSTEMS IN YUCATAN, MEXICO

Chicken mortality was studied in 24 randomly selected smallholder floc ks in one village in Yucatan, Mexico between July and December 1993. E ach family received a package of 10 to 12 chicks of 3 weeks of age. Ap proximately half of the chicks were purebred and the remainder were cr osses produced by mating exotic with local breeds. All smallholders we re visited twice a week. Feeding and management (except vaccination an d medication) were left to smallholders. Data were processed by Chi-sq uare, Mantel-Haenzel test and survival analysis, Before leaching 140 c lays of age 43.2% of the birds died. The highest mortality?, was obser ved during the 113 to 140 days of age period and the lowest was in the period between 22 and 56 clays of age. Of all birds, 10.5% died from coccidiosis and 7.6% from Marek's disease, Of the risk factors investi gated only medication and genotype showed significant effects on morta lity. The effect of genotype was significant rip to 112 days of age (P < 0.05). Crossbred birds lived longer than purebred; independently, m edicated birds lived longer than non-medicated birds.