Rumensin is not a new product. The compound was first cleared by the FDA for use in cattle in 1975. The product is very safe, working only in the rumen and never entering the bloodstream. Because of this, it never enters the muscle products or the food chain for humans. In addition to being cleared for use in beef cows, the product is also cleared for use in the dairy industry.
When most beef cattle producers think about the ionophore Rumensin, they think about using it for stocker or backgrounding cattle for improving feed efficiency and weight gain. While this is a correct label claim and usage, they don’t realize that it is also cleared for and can be used for the brood cow as well. To understand how Rumensin can help beef cow production, an understanding of exactly what an ionophore is will be necessary. An ionophore is a compound that causes a change in the rumen microbial population. In other words, it causes a shift in the rumen bacteria population that increases the number of bacteria which allows more energy to be produced from any feedstuff. Cows get more energy out of everything that they eat!
What does this greater energy intake mean for the beef cow producer? As stated above, Rumensin is cleared for the dairy industry where the increased energy intake means an increase in milk production. For the beef cow, milk production is a major driver determining pounds of calf weaning weight.
Table 1. The Effect of Rumensin fed to Beef Cows on Calf Weaning Weight
Rumensin, 200 mg/cow/day
Number of pairs
Initial Calf Weight, lbs
Weaning Weight, lbs
Trial Gain, lbs
Increased Weaning Weight, lbs
Kansas State University, 2005
In the Kansas study shown above, cow-calf pairs were allocated to treatment based on cow age, calf age and weight. They were provided equal quality forage for the trial. The cows receiving 200 mg of Rumensin per head daily weaned calves weighing on the average, 19 pounds more than calves for the cows that received no Rumensin.
Table 2. The Effect of Rumensin on Beef Cow Performance and Reproduction, 4 Trial Average
Rumensin Intake, mg/hd/day
Initial Wt. lbs
Final Wt. lbs
DM Intake, % Control
Days Calving to Conception
Elanco 4 Trial Data
Feeding Rumensin to beef cows has also been shown to increase the number of cows that become pregnant, thus allowing more calves to be born. The data shown in table 2 is an average of 4 separate trials. Note that the control cows, those receiving no Rumensin had a 91 percent conception rate indicating that the herds were very well managed. The cows receiving 200 mg of Rumensin per head daily had a 97% pregnancy rate or 6 extra calves per one hundred cows in the herd.
Looking at the Kansas State University and the Elanco studies together, an amazing effect of Rumensin for the beef cow can be seen. For each one hundred cows in the herd, feeding 200 mg of Rumensin per head daily gave 6 more calves and 19 more pounds of weaning weight for all calves.
The economics of providing 200 mg of Rumensin per cow per day is shown in table 3. With Rumensin costing a maximum of 2 cents per cow per day for the drug, an excellent net return per one hundred cows can be obtained.
Table 3. Economics of Feeding Rumensin Year Round to Beef Cows
Steer/Heifer Value, $/lb for 500 lb Calves
Increased Income for 6 More Calves, $
Increased Income for 19 lbs Increase Weaning Weight, $
Total Increased Income/100 Cows, $
Rumensin Cost/Cow/Day, $
Rumensin Cost/100 Cows/Year, $
Net Return/100 Cows, $
Prices from KY Livestock Market Weekly Report for week ending 2/6/16
No Kentucky beef cow producer interested in maximizing profit from their herd should neglect providing Rumensin to their cows on a year round basis. Burkmann Nutrition provides both a high magnesium mineral with Rumensin that may be used during Grass Tetany season as well as a mineral for the non-Grass Tetany season. No Kentucky cow should be without Rumensin.