Many dairy producers use Rumensin® (Elanco Animal Health) in dairy heifers as an aid to controlling coccidiosis and for improved feed efficiency. Rumensin® also is approved for use in lactating cows as a tool for improving milk production efficiency. Monensin, the active compound in Rumensin®, has a very wide safety margin for humans and cattle. But, it can be toxic if not fed according to the FDA-approved label. In other species, such as horses, monensin can be extremely toxic.

Monensin is a carboxylic polyether ionophore produced by a naturally occurring strain of Streptomyces cinnamonensis. The basic cellular function of ionophores is to create a flux of ion transport across cell membranes. Monensin binds to bacterial cell membranes and causes an efflux of potassium from the cell and an influx of hydrogen ions into the cell. Subsequently, the increased hydrogen ions are exported out of the cell either by active transport involving ATP or passively via sodium entry into cells in exchange for hydrogen. In order to maintain inner cell equilibrium, the bacterial cell expends energy. This ultimately results in death or reduced growth of the bacterium. Since gram-negative bacteria have complex outer cell membranes, they are more resistant to the action of ionophores than are gram-positive bacteria. Ionophores, therefore, selectively inhibit gram-positive bacteria rather than gram-negative bacteria. This shift in rumen microbial population is responsible for most of the effects of monensin.

Three general areas of animal metabolism influenced by monensin are energy metabolism, improved nitrogen metabolism, and general digestive effects, including reductions in both bloat and lactic acidosis. Several modes of action have been described including modified volatile fatty acid production, modified feed intake, changes in gas production, modified feed digestibilities, and alterations in both rumen fill and rate of passage. Other effects of monensin include a reduction in 3-methylindole production, a reduction in face fly and horn fly numbers and control of bovine coccidiosis.

This case report describes a situation of acute monensin toxicity in a Michigan dairy farm. The problem occurred in the summer of 2008 in a group of 264 dairy replacement heifers between the ages of 6 and 14 months. The heifers were housed in a single sloped three-sided barn on a bedded pack. They were fed a totally mixed ration (TMR) consisting of haylage (80%), corn silage (10%), straw (10%) and a mineral/vitamin supplement that was formulated to meet the specific needs of the heifers. Included in the mineral mix was Rumensin® added at a concentration formulated to provide 50 mg/head per day when mixed in the TMR.

The initial sign that something was wrong in this group of heifers was an acute drop in appetite. It was noted by the morning feeder that the heifers had not cleaned up their TMR from the previous day. Not suspecting any major problem, the employee feeding these heifers mixed and fed a new batch of TMR. Again the heifers had no appetite at all, even for the new feed. The feed was being mixed and delivered to the heifers by a relatively new and inexperienced employee who had been on the job for about 3 weeks. Upon being informed of the lack of appetite, the herdsman investigated the feed and how it was mixed and discovered that concentrated Rumensin® 80 was being mixed into the TMR instead of a mineral premix which should contain the Rumensin®. The mineral premix was designed to deliver 50 mg/head perday of monensin when mixed into the TMR. The approved label dose of monensin for replacement heifers is 50 to 200 mg/head per day. The concentrated Rumensin® 80 was being added accidently to the TMR at the same rate as the mineral premix. By calculation, it was estimated that heifers were actually fed 20,000 mg/head per day, or about 400 times the formulated rate.

The Mix-up

Rumensin® is available to provide different concentration of monensin. Rumensin® 80 contains 80 grams/lb of monensin and is designed to be added to TMRs. Rumensin® is often mixed into custom mineral mixes at various concentrations depending on how they are to be delivered (e.g. free choice or mixed into rations). Upon further investigation of this case, it was discovered that the feed supplier had mistakenly delivered a pallet of Rumensin® 80 instead of the mineral premix containing Rumensin® at a much lower concentration. The inexperienced feeder did not recognize the mix-up and incorporated the concentrated Rumensin® 80 into the TMR at the same rate as the mineral mix was to be incorporated.