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February 17, 2005

Use Care When Adding Monensin

The official clearance earlier this year of Rumensin (monensin sodium) by the Food and Drug Administration, has provided dairy producers with a long-awaited tool to increase milk production efficiency. However, in some cases this has resulted in a loss of milk components.

“Efficiency is measured in terms of production of marketable solid-corrected milk per unit of feed intake,” says J.W. Schroeder, North Dakota State University Extension Service dairy specialist. “When milk yield goes up, milk protein and fat can decline. The economic impact depends on the final yield of components,” he says.

The label specifies feeding monensin continuously to dry and lactating cows in a total-mixed ration (TMR) or “complete feed.” “This is where challenges emerge,” Schroeder says. “When delivered to component-fed cows, some dairy producers have seen a definite drop in milk fat and protein tests. If not implemented correctly, producers will witness a depressed component yield.”

The main reason for the butterfat depression is because monensin inhibits hydrogen-producing bacteria, such as cellulolytic ruminococci. This explains the decrease in methane production and also explains the decrease in cellulose digestion and feed intake. The negative effect of monensin on fiber digestion will depress the milk fat percentage.

“In the literature, you see reviews across North America that show milk fat is depressed up to 0.3 percentage points with an increase in the monensin concentration in the diet,” Schroeder says.

The net result is more gain on forage and pasture diets or lower feed consumption on grain-based diets. Rumensin (Elanco) and Bovatec (Alpharma) work similarly. Both are cleared for heifers and steers, but only Rumensin has received clearance for lactating cows. Both are excellent additives for replacement heifers and calves that will increase average daily gain as well as prevent coccidiosis.

Many herds on good rations will have a transient drop (7-10 days) in fat test when Rumensin is added, followed by a recovery during the next two to three weeks. If your herd goes onto Rumensin and suffers a sustained large fat test drop, your ration needs to be examined for too much grain, too little fiber, too much fat, or any combination of these factors.

To deliver the correct amount of Rumensin or Bovatec supplement, it is important to know the animals feed intake. Read and follow label directions and consultant a nutritionist or feed representative to customize your herd’s needs.


Source: J.W. Schroeder, (701) 231-7663,
Editor: Rich Mattern, (701) 231-6136,



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