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Virginia Cooperative Extension - Knowledge for the CommonWealth

Rumensin - A new management tool for dairymen

Dairy Pipeline: April 2005

Robert E. James
Extension Dairy Scientist,
Dairy Nutrition
(540) 231-4770 email:

In October, the FDA approved the use of the ionophore, Rumensin, for use in lactating and dry dairy cattle. Many producers have used Rumensin in dairy heifers as an aid to controlling coccidiosis and for improved feed efficiency. However, it's now approved for use in the milking herd with claims for improved milk production efficiency. Rumensin is an antibiotic that's not used in the human food industry. It alters fermentation of carbohydrates in the rumen by changing microbial populations resulting in more glucose precursors (that's good for lactose production) and less waste gas production. Effective dairy managers should evaluate this new technology by weighing expected costs against expected benefits and estimating the likelihood that a beneficial response will be observed. The good news is that inclusion of Rumensin in the ration costs pennies per day. Therefore, the amount of increased milk production necessary to generate a net positive return is small. Inclusion of Rumensin in the ration should result in an improvement in milk production of 1.8% to 3.9%. In addition, since it improves glucose production by the cow, it would be expected to lessen the risk of ketosis when fed to dry and transition cows. What are the risks? Rumensin has a very wide safety margin for humans and animals. Lowered milk fat percentage has been observed, particularly when effective fiber levels are marginal in the ration.

Nutritional management when Rumensin is included in the ration.

  1. Include Rumensin in the ration at 11 g/ton of total ration dry matter to begin. This is equal to 5.5 mg/lb. of ration dry matter. Increases in feeding rates should only be made in consultation with a qualified nutritionist.
  2. Feed Rumensin by adding it to a concentrate or mineral carrier mixture provided by your feed mill to assure accurate mixing. Feed this mixture as part of a total mixed ration. Feeding Rumensin in a concentrate mixture during milking or separate from a TMR is not approved at this time.
  3. Rations must contain adequate effective fiber
    1. If milk fat % is less than 3.5% for Holsteins, correct the fiber deficiencies before using Rumensin.
    2. Rations should contain the following minimum levels of fiber
      1. 19 - 20% Acid Detergent Fiber
      2. 28- 30% Neutral Detergent Fiber
      3. 21% NDF from forage
      4. 10% of the ration should reside on the top screen if the Penn State Particle Separator is used.
      5. More than 50% of the cows should be chewing their cud when observed.
      6. If rations don't contain long forage, consider adding 1 to 2 lb. of ground straw per cow per day.
    3. Consider using buffers such as sodium bicarbonate at a rate of .3 lb. per day.
  4. Give it time. Once Rumensin is added keep it in the ration for at least one month before making a judgment on it's efficacy in your herd.

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