Association between cereal contents in the diet and incidence of necrotic enteritis in broiler chickens in Norway

The quarterly cumulative incidence of necrotic enteritis in the broiler chicken population of south-eastern Norway varied between zero and 34.8% during the years 1969-1989. Two epidemics appeared during the study period. The first and second epidemics lasted approximately 5 and 2 years and had peak incidences of 34.8% and 25.3%, respectively. The median incidence of all quarters was 1.2%, reflecting the generally low incidence during non-epidemic quarters. Univariable regression analysis indicated that necrotic enteritis occurred more often during the months October-March than during the months April-September. The inclusion levels of maize, barley, oats and wheat in broiler starter feed varied between zero and 38.0%, zero and 20.0%, zero and 26.0%, and 6.0% and 38.6%, respectively. A single primary feed variable was specified, based on a ratio comprising weighted values of maize, barley and wheat. This variable showed a striking covariation with disease incidence, but tended to increase and decrease a few quarters earlier than the disease incidence. Analyses of the associations between disease incidence and each of the cereal grains maize, wheat and barley, as well as the association between incidence and the primary variable comprising the latter three cereal types, were performed. Season and dietary animal proteins modified the associations between disease incidence and each of the feed variables. At above-median levels of animal proteins, significant associations were restricted to the primary variable, barley and maize, during the cold season. At below-median levels of animal proteins, the primary variable, barley and wheat, was associated with disease incidence. Above-median levels of maize protected against necrotic enteritis, whereas barley and wheat were risk factors.