Effect of ionophores on ruminal fermentation.

In the mid-1970s, ionophores were approved by the Food and Drug Administration for addition to the rations of ruminants in the United States. Monensin has been used most extensively, but others, including lasalocid, tetronasin, lysocellin, narasin, and laidlomycin, either have been investigated or are used commercially. lonophores generally have improved the efficiency of feed utilization; with monensin, the most commonly used ionophore, food intake often decreases while the rate of body weight gain either is unaffected or improves slightly (40, 47). The total slaughter population of beef cattle in the United States is more than 25 million. Assuming that virtually all of the slaughter cattle are fed a daily dose of 350 mg at 2¢ per dose for 140 days, the yearly sales of ionophores as feed additives for ruminants is estimated at 70 million dollars. At an efficacy of eight to one, the overall benefit in terms of feed savings would be approximately 560 million dollars. This value does not include the use of ionophores for grazing animals. Monensin was originally developed as a coccidiostat for poultry (38), but later work showed that it could also affect ruminal fermentation. Effects included a decrease in methane production (55), an increase in the ratio of propionic to acetic acid production (38), and a decrease in protein degradation to ammonia (17). Although the effect of monensin on animal performance has been the subject of several recent reviews (1, 40, 47), there has been much less discussion concerning its effect on ruminal microbial ecology, microbial metabolism, and membrane physiology.