The effects of feeding monensin and yeast culture, alone or in combination, on the concentration and generic composition of rumen protozoa in steers fed on low-quality pasture supplemented with increasing levels of concentrate

Sixteen zebu-cross steers, kept under grazing conditions, were supplemented with increasing levels of concentrate (0.4, 0.7 and 1.0% of body weight) over three periods of 41, 41 and 42 days, respectively. The animals were divided into four groups, a control plus groups supplemented with either monensin, yeast culture or a combination of monensin and yeast culture. Samples of rumen contents were taken on Day 0 and at the end of each period to determine the concentration and generic composition of rumen protozoa. After 124 days on treatment, protozoal concentrations increased in the controls (p<0.01), yeast culture treatment (p<0.03) and monensin treatment (p<0.06). However, protozoal concentrations did not change in the steers fed the combination of monensin+yeast culture. No marked differences were observed in the generic composition of protozoa among the four treatments. Entodinium was the predominant genus in all groups: control (87.7%); monensin (78.1%); yeast culture (69.6%) and the combination of both, (89.3%). The lower percentage of Entodinium in the monensin and yeast culture treatments was primarily replaced by an increase in the percentage of Dasytricha. The rest of the ciliate population, which belonged to ≈9 genera, was observed in similar proportions for all treatment groups.