THE EFFECT OF A POLYETHER IONOPHORE ON PROTEIN AND FIBER UTILIZATION IN THE RUMINANT

The influence of levels of monensin, a polyethor ionophore, on digestibility, rumenal diet protein degradation and bacterial protein synthesis when steers are fed high fiber diets was investigated in two metabolism trials. In addition, three performance trials were used to evaluate monensin levels when protein or energy was the first limiting nutrient and to evaluate possible interactions of forage quality and monensin level.;In a calf performance trial when energy was first limiting no interaction of forage quality and monensin was observed. Monensin did linearly decrease feed intake (P = .09) and had a quadratic effect on daily gain (P = .03) and feed/gain (P = .07). The 100 mg level, which corresponds to the 18.3 ppm level in the metabolism trials, was superior to either the 0 or 200 mg level in this trial. In calf and lamb performance trials, when protein was first limiting, no significant interactions of protein and monensin were observed. However, the animals on the low levels of protein did tend to respond more to monensin which may indicate that protein sparing occurred. The animals also responded to increasing levels of protein in a linear manner and to monensin in a quadratic manner as was observed in the other performance trial.;The level of monensin influenced organic matter, neutral detergent fiber (NDF) and ruminal NDF digestibilities in a quadratic manner, with the intermediate levels of monensin being superior to either high levels of monensin or no monensin. Monensin decreased (P = .10) the bacterial protein concentration and increased (P = .02) the ratio of total N:diaminopimilic acid in the rumen. In duodenally cannulated steers, a linear tendency for bacterial N flow to decrease (P = .17) and for diet N flow to increase (P = .27) was observed. This resulted in no overall difference in duodenal N flow or flow of individual amino acids. The data indicate that it may be possible to obtain both improved fiber digestion and increased diet N to the lower tract with supplementation of monensin at 18.3 ppm.