Rumensin® Approved for Lactating Dairy Cows

Dr. Maurice Eastridge, Dairy Nutrition Specialist, Ohio State University

Ionophores (monensin and lasalocid) have been approved as feed additives for use in dairy replacement heifers for several years but not for lactating dairy cattle. However, the Food and Drug Administration approved on November 3, 2004 the use of Rumensin® (monensin sodium) for increased milk production efficiency in dairy cows. The product is produced by Elanco Animal Health, a division of Eli Lilly and Company, Greenfield, Indiana. It is already approved in feed for therapeutic and production uses in feedlot cattle, pasture cattle (beef and dairy heifers, and slaughter, stocker feeder cattle), beef cows, and calves excluding veal calves. The FDA concluded that the meat and milk derived from dairy animals fed monensin sodium are safe when the animals are fed according to the approved labeling; therefore, there is no withdrawal period for the product. Previous caution statements on the label will remain, including not feed it to horses or other equines because ingestion of monensin sodium by horses has been fatal.

Rumensin® can be fed to dry and lactating dairy cows, with the suggested initial feeding rate being 11g/ton (DM basis) and the continual feeding rate between 11 and 22 g/ton). The base product released by Elanco is Rumensin 80®, which contains 80 g/lb of monensin; carefully follow mixing instructions. The generally expected responses are as follows:

DM intake Decrease (no change in transition period) Body condition score No change to increase
Milk yield No change to increase Body weight No change to increase
Milk fat (%) Decrease Rumen acidosis No change to decrease
Milk protein (%) No change to decrease Ketosis No change to decrease
Milk fat yield No change to decrease Feed efficiency Increase
Milk protein yield No change to increase    

Determining the economic impact of using this product in a specific herd can be done by using the MS Excel file on the OSU dairy web site:, click on the link titled "Monitoring Economics of Ration Changes for Lactating Cows".