Adding monensin sodium, also known as Rumensin, to a milking ration can cost you 2-4¢/cow/day. But it will raise energy-corrected milk yield by 8-12¢/day and may increase feed efficiency by 0.05 to 0.1 point, saving as much as 20¢/cow/day.

Classified as an antibiotic and produced naturally by a bacteria strain (Strept cinnamonensis), monensin is fed as a sodium salt.

How does it work? Monensin inhibits lactic acid-producing strains of bacteria, such as Streptococcus bovis, while not inhibiting major strains of lactic acid-utilizing bacteria. In beef cattle, monensin also has been found to reduce variation in feed intake with smaller and more frequent meals. Both responses could reduce the risk of sub-acute rumen acidosis (SARA) in dairy cows.

Because monensin affects forage and fiber digestion in the rumen, dairy producers must be sure to provide adequate levels and forms of fiber to maintain balanced fermentation and rumen pH. The following forage guidelines can help you make the most from monensin:

  • Maintain total NDF in the ration dry matter over 28%.

  • Include 5 lbs of forage dry matter that is over 1” in particle length.

  • When using the Penn State Particle Separator, strive for more than 10% in the top box and more than 40% in the second box.

  • Monitor your milk fat test before adding monensin, which can slow fiber-digesting microbes and, as a result, lower the butterfat content. If it's low (3.4% or 3.5% rather than a normal butterfat test of 3.7% for Hol-steins), adding monensin can be risky.

  • Cows fed high-forage-based rations respond positively to monensin supplementation.

The amount of monensin fed to lactating cows can vary from 11 to 22 grams/ton of TMR dry matter per day. Nutritionists targeting 11 grams/ton add 250-300 mg/cow for lactating cows. Stepping up the level is legal for component-fed herds. Allowing rumen fermentation to adjust to lower monensin levels can reduce the potential negative impact on milk fat test. Because dry cows consume half of the dry matter of lactating cows, the higher level of 22 grams/ton is recommended, giving them 250-275 mg/day.

Feed efficiency, calculated as pounds of 3.5% fat-corrected milk per pound of dry matter, can increase with monensin. An increase of 0.1 feed efficiency point is worth 20-22¢ today. Most farmers, unless they have been really working on feed efficiency the last couple of years, can improve it by one to two tenths of a point, maybe through feed or forage quality, optimal rumen function or getting cows bred on time.

Research under pasture and confinement feeding systems reported an average increase of 2.2 lbs of milk using monensin.

When looking at milk components, monensin maintains protein and should maintain butterfat test if all things are correct in the rumen. If they are not optimal, you could see a slight decline or a very large decline. A tenth of a point is breakeven.