Ionophore antimicrobial drugs have been used for modification of the rumen microbiome since the 1970s to improve feed efficiency, becoming the largest antimicrobial class by weight sold for cattle. Ionophores affect the movement of sodium, potassium, and hydrogen ions into and out of bacteria, slowing the growth of gram-positive bacteria. Associated increases in feed efficiency are believed to be due to a combination of several effects: decreased production of methane, increased production of propionate, and improved use of dietary protein. The decrease in methane production appears to be due most likely to a decrease in hydrogen, a key substrate for methanogenic archaea. Additional benefits include a decrease in lactate production, which minimizes the risk of rumen acidosis associated with high-starch diets.
Ionophores are also effective as coccidiostats and are commonly used for this purpose in calves.
