Effect of lasalocid, monensin and thiopeptin on rumen protozoa.

The effects of lasalocid, monensin and thiopeptin on the total number and the generic composition of rumen protozoa were determined in vivo and in vitro. Feeding lasalocid or monensin to cattle on either high grain or high roughage diets reduced total protozoal counts. Addition of lasalocid or monensin (6 to 48 micrograms ml-1) to the in vitro rumen fermentation resulted in marked reduction in protozoal numbers. The inhibition was dose dependent. Thiopeptin had no effect on rumen protozoa either in vivo or in vitro. Among the protozoal types, holotrichs (Dasytricha, Isotricha and Charonina) were unaffected by either lasalocid or monensin. Among the entodiniomorphs, Entodinium, Diplodinium and Ophryoscolex were more sensitive than the other types. Ophryoscolex purkynei was more sensitive to monensin than to lasalocid. Protozoal inhibition by lasalocid and monensin was transient because prolonged antibiotic feeding resulted in the selection of a resistant population in the rumen of cattle.