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Ionophores : A Potential Feed Additive For Lactating Dairy Cows
by Ignacio R. Ipharraguerre and Jimmy H. Clark


  • Ionophores should not be fed to lactating dairy cows in the United States until they are approved by the Food and Drug Administration.
  • As a result of alterations in ruminal fermentation when ionophores are supplemented, the energy balance of dairy cows is improved, which enhances milk production and efficiency of milk production. The magnitude of these effects appears to be determined by several factors.
  • Improved energy balance and its favorable effects on health and milk production appear to be the primary benefits of supplementing ionophores to lactating cows.
  • Periparturient dairy cows, particularly over-conditioned cows, may benefit the most from the use of ionophores.


In 1968, the generic term ionophores was used for the first time to refer to all carboxylic polyethers that fit the classical definition of antibiotics. Since the mid-1970’s ionophores have been extensively used to manipulate rumen fermentation, to improve the efficiency of feed utilization, and to increase weight gain of growing cattle. Examples of ionophores that either are used commercially or have been investigated for use in growing cattle are monensin, lasalocid, tetronasin, salinomycin, lysocellin, narasin, nigericin, laidlomycin, and valynomycin. In recent years, ionophores were approved in several countries including Australia, Argentina, New Zealand, and South Africa for use in lactating dairy cows with claims for improved milk production and immune response. More recently, a monensin controlled-release capsule was cleared for use in dairy cows as an aid in the prevention of subclinical ketosis in Canada. However, to date the Food and Drug Administration has not approved or rejected the use of ionophores for feeding to lactating dairy cows in the United States. An application for approval for use of monensin (trade name Rumensin®) in lactating dairy cows has been submitted to the United States Food and Drug Administration. This ionophore ( monensin, trade name Rumensin®) or other ionophores should not be supplemented to lactating dairy cows until they are approved by the Food and Drug Administration. If ionophores are supplemented to lactating dairy cows after approval by the Food and Drug Administration, they should be used according to directions on the label.


Feed Intake

Data from several experiments in which ionophores, especially monensin, were administered to lactating dairy cows have become available in the last few years. These data suggest that when administered at effective doses, ionophores do not affect, or they slightly decrease, feed intake by lactating dairy cows. Administration of ionophores to cows fed high-grain diets is more likely to depress feed intake than administration of ionophores to cows fed high-forage diets.

Ruminal Effects

Ruminal microorganisms ferment carbohydrates and protein from feed to obtain energy and nutrients for growth. Endproducts of ruminal fermentation, like volatile fatty acids and microbial protein, are major sources of energy and protein for cows. In contrast, other fermentation endproducts, like heat, methane, and ammonia, can represent a loss of energy and protein in feed from the cow into the environment. Ruminal gram-positive bacteria are involved in fermentation processes that produce, among other endproducts, acetate, butyrate, formate, lactate, hydrogen, methane, and ammonia. In contrast, ruminal gram-negative bacteria are engaged in fermentation pathways associated with the production of propionate and succinate. Ionophores inhibit gram-positive bacteria, which subsequently alters ruminal fermentation. Therefore, when ionophores are fed to dairy cattle the number of gram-positive bacteria in the rumen are decreased and the number of gram-negative bacteria are increased. When the gram-negative bacteria predominate in the rumen, less methane is produced and more propionate is produced. Therefore, a major benefit of feeding ionophores to dairy cattle is the potential increase in production of propionate and the associated decrease in production of methane, which conserves energy.

Milk Production, Milk Composition, and Efficiency of Production

As a result of altered ruminal fermentation, energy balance is improved, which enhances milk production and efficiency of milk production. The magnitude of these effects appears to be determined by several factors, many of which are poorly understood. The response in milk production appears to be independent of the stage of lactation in which the supplementation of ionophores is initiated. However, the type of diet fed and body condition of cows may affect the milk production response to ionophores. For instance, supplementing diets that contained more than 50% forage with monensin increased milk production more than providing monensin in diets that contained less than 50% forage. Likewise, monensin had no significant effect on milk production of cows classified as thin but increased milk production of cows classified as fat.

Ionophores may slightly depress milk fat and milk protein percentages; however, because of increased milk production, efficiency of milk production is improved. The improved efficiency of milk production by cows fed ionophores has been attributed to the increase in availability of energy from feeds caused by the ionophore effects on ruminal fermentation (increased propionate and decreased methane). Other factors that potentially could contribute to improved efficiency are the ionophore-induced reduction in ruminal protein degradation as well as decreased ketosis caused by increased propionate production in the ionophore-fed dairy cows.

Immune Response

Monensin-supplemented cows have been reported to be less likely to experience clinical ketosis, abomasal displacement, and multiple illnesses in some but not all experiments. Multiple illnesses were defined as having more than one of the following: retained placenta, milk fever, metritis, endometritis, mastitis, ketosis, displaced abomasum, digestive disorder, lameness, respiratory disease, or other disease. Monensin also has been reported to be efficacious for the prevention of pasture bloat. Monensin has not been shown to be effective for improving reproductive performance of dairy cows possibly because the improvement in energy balance caused by monensin may not be large enough to enhance the cows fertility. Data suggest that cows at greater risk of negative energy balance, such as transition cows, and (or) cows at greater risk of metabolic disorders, such as over-conditioned periparturient cows, may benefit the most from the supplementation of ionophores.


Scientific data indicate that meat and milk produced from animals fed monensin is safe for human consumption. Likewise, monensin is biodegradable in manure and soil, and is not toxic for crop plants.

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