Effects of monensin-feeding regimens on flock immunity to coccidiosis.

Feed medication with monensin caused delays in development of immunity in two floor-pen experiments which simulated commercial broiler production. Development of immunity was retarded with the higher level of monensin (120 p.p.m.) but was progressively less delayed as the monensin level was decreased (100, 60 or 0 p.p.m.). Delay was greatest with Eimeria tenella, but also occured with intestinal species including E. acervulina, E. brunetti, E. maxima, E. mivati, and E. necatrix. Drug withdrawal permitted earlier development of immunity. Plans involving use of monensin on layer replacements which later will be maintained on the floor without medication should consider: 1) prevalence of infective oocyst exposure in the area, 2) the lowest possible level of drug required for protection, and 3) its use for the shortest possible period of time.