Manitoba Agriculture, Food and Rural Initiatives

Feature Article

Monensin In Lactating Dairy Cow Diets

Monensin (trade name Rumensin), manufactured by Elanco, belongs to a class of feed additives called ionophores. Ionophores have been widely used in the beef industry since 1977 and their effects on improving feed efficiency are well known. It is only recently though that monensin has been approved for use in lactating dairy cow diets. In 1997 Elanco released Rumensin CRC (controlled release capsule) as an aid in the prevention of subclinical ketosis in lactating dairy cows. More recently, the CFIA has approved three new claims for its use as a feed ingredient in lactating dairy cow diets.

Mode of Action

The mode of action of monensin is complex but well studied. Simply put, monensin causes a shift in rumen microbial populations favoring the production of propionic acid over the production of acetic and butyric acids. This improves glucose synthesis and the efficiency of energy metabolism. It is this positive effect which results in the health benefits (decreased ketosis, retained placentas and displaced abomasums) typically associated with the Rumensin CRC (Duffield, 2001). Other effects of monensin include changes in feed intake, improved nitrogen metabolism and reduced methane production. For a detailed review of monensin’s mode of action and its impact on the health of transition dairy cows, please refer to the article by Dr. Todd Duffield in the 2001 Proceedings of the Western Dairy Seminar.

New Feed Claims for Lactating Dairy Cows

1. Rumensin is approved for the reduction of milk fat percentage when fed at a rate of 16 – 24 mg/kg in the complete diet dry matter (forage plus concentrate). In a summary of 9 feeding trials, Elanco reported a drop in milk fat from 3.67% (control) to 3.49% and 3.35% at the 16 mg/kg and24 mg/kg addition rate, respectively. Because of increases in milk production, the total milk fat yield was reduced only marginally at 16 mg/kg  and significantly at the 24 mg/kg inclusion rate. However, the impact on milk fat appears to be variable. Dr. Mike Hutjens (2005) indicated only one-third of dairy managers attending the 2005 Illinois Dairy Days reported a drop in milk fat when feeding monensin.. Holstein herds with a milk fat average over 3.8% experienced marginal or no effect on milk fat. Herds with a low milk fat (<3.5%) reported drops of 0.1 to 0.4%. CFIA addresses this variability with the following note on the claim – “The expected efficacy of this product for reduction of milk fat percentage may be affected by dietary factors. Reduced efficacy may be expected with diets higher in fibre or lower in unsaturated oils.”
2.  Rumensin is approved for minimizing loss of body condition during lactation when fed at a rate of 8 to 24 mg/kg in the total diet dry matter. In the 9 feeding trials, cows fed monensin lost less body condition in early lactation. This result was consistent across all dose levels.
3. Rumensin is approved for improving feed efficiency of milk protein production when fed at a rate of 16 to 24 mg/ kg of total diet dry matter. The improvement in overall feed efficiency is twofold. In early lactation, cows fed monensin produced more milk on the same amount of dry matter.  After 50 Days in Milk, dry matter intake was reduced but milk production remained higher in cows fed monensin. Improvements in feed efficiency of 2.5 – 4 % have been reported. The benefit to cost ratio for monensin for lactating cows has been estimated at 5 to 1.

Cautions and Concerns

1. CFIA lists the following cautions on the use of Rumensin in lactating dairy cow diets:
   a. “The 24 mg/kg monensin treatment in primiparous cows may result in the increased incidence of udder edema and increased number of inseminations per full term conception”. The 9 feeding trials in Canada and the U.S. included 357 first lactation cows (primiparous) and 609 second and greater lactation cows (multiparous). Udder edema increased in first lactation cows from 2.2% in control cows to 3.4 and 5.6% at the 16 mg/kg and 24 mg/kg feeding rate respectively. Inseminations per full term conception for multiparous cows were not affected by monensin feeding. First calf heifers fed 24 mg/kg required 2.13 inseminations compared to 1.68 and 1.52 for the control group and 16 mg/kg group respectively.
   b. “The continuous use of monensin in dairy cows may be associated with increased rates of twinning and stillbirths, and heavier birth weights for heifer calves”. Twinning rates and stillbirths were numerically (but not statistically) higher for cows fed all levels of monensin. Although the birth weight of heifer calves was significantly higher than that of the controls, this difference was not noted in the birth weight of bull calves.
2. There is no milk or meat withdrawal associated with the label use of monensin.
3. Ensure cows are not receiving multiple sources of monensin (eg. other feedstuffs containing monensin or the Rumensin controlled release capsule).
4. Follow all cautions, notes and instructions listed on the label.

New Research

Decreased feed intake and erratic feed consumption are the first signs of SARA (sub-acute ruminal acidosis) in a herd. This leads to decreased milk production and long term consequences such as laminitis, weight loss and losses in body condition. The economic consequences of SARA are mind boggling with estimates of losses to the North American dairy industry between $500 million and $1 billion annually. A recent trial conducted by Lunn et al (University of Guelph, 2005) showed that monensin may have a positive impact on feed intake among cows suffering from SARA. Cows suffering from induced SARA were shown to have increased meal frequency when the diet was supplemented with monensin. Increasing meal frequency is believed to produce a more stable rumen environment by minimizing starch intake per feeding. Cows fed monensin consumed 1.5 – 2.8 kg more dry matter per day and produced 3.6 to 4 kg more milk compared to control cows.

References

Compendium of Medicating Ingredient Brochures. Canadian Food Inspection Agency.

Duffield, Todd. 2001. Impact of Rumensin on the Health of the Transition Dairy Cow. Proceedings of the Western Canadian Dairy Seminar, 2001.

Elanco Publications – Dairy Advisor Reference Guide, Understanding the Impact on Dairy Production Efficiency, Elanco Rumensin Improves Production Efficiency in Dairy Cows. www.dowagro.com/ca/index.htm

Hutjens, Mike. Most experiences with monensin are positive. Hoard’s Dairyman, May 25, 2005.

Lunn et al. 2005. Effect of monensin on meal frequency during sub-acute ruminal acidosis in dairy cows. Can. J. Anim.Sci. 85:247-249.