ADM: Feeding Rumensin to Lactating Cows

Feeding Rumensin ® to Lactating CowsRETURNTO STARTRumensin ® has been widely used in Western Based on our understanding of the nutrition ofCanadian feedlot rations for over 20 years. Based the lactating cow, improved energy efficiency,on its effectiveness in the feedlot and our enhanced propionate production, a lowerknowledge about is mode of action, interest has acetate:propionate ratio and reduced amino aciddeveloped in its use for lactating dairy cows. breakdown in the rumen should promote higherThe active ingredient, monensin, is approved by production accompanied by increased protein andAgriculture Canada for the improvement of feed decreased fat tests. The risk of acidosis should beefficiency in beef cattle intended for slaughter, for reduced as a result of lower lactic acid productionincreased weight gain in growing cattle on pasture and reproductive performance might improve dueand as an aid in the prevention of coccidiosis. to a more positive energy balance and lowerUntil very recently, monensin was not approved ammonia production. Reduction in the incidencefor use in lactating dairy cattle, although its use of ketosis has also been attributed to the effects ofcould be prescribed by a veterinarian. As a result monensin on rumen metabolism.of trials indicating that there are no unsafe drugEffects on feed intakeresidues in the milk of treated cows, approval forcoccidiosis control has been extended to include In feedlot rations, monensin stabilizes intake.lactating dairy cattle. However, coccidiosis is not When feed is offered ad libitum, cattle eat lessconsidered to be a significant concern in lactating during each meal, eat more times per day andcows and, therefore, the use of monensin for its consume more consistent amounts from day toprevention has been questioned.day. Total intake is usually lower but thecombination of reduced intake and improved feedWill monensin improve production efficiency inconversion efficiency generally results in a slightlactating cows? If so, can it be used for thisto significant improvement in daily gain.purpose when the only official non-veterinaryclearance is for coccidiosis control?In lactating cows, potential increases in milkproduction from the use of monensin will alsoMode of actiondepend on the balance between feed intake andMonensin is an antibiotic which inhibits the feed conversion efficiency. If improved energygrowth of specific rumen microbes resulting in efficiency outweighs the negative effect ofchanges in rumen fermentation and metabolism: reduced dry matter intake (see article 1I1), then• production of propionic acid increases, resulting increased output should be expected.in a decrease in the acetate:propionate ratio andMonensin toxicitya reduction in the amount of energy wasted ascarbon dioxide and methane;At the levels used in cattle feeds, monensin can• lactic acid production decreases, blunting the be very toxic to non-ruminants. Consumption ofsharp declines in rumen pH that often occur after these feeds by dogs and horses has, in many cases,large meals of rapidly fermentable starch; been fatal. If monensin premixes are used onfarm,it is very important to observe label• reduced feed protein breakdown results in adecrease in microbial protein and an increase in warnings which suggest “The use of protectivefeed protein passing from the rumen;clothing, impervious gloves and a dust mask…”• ammonia-producing bacteria which use amino and “…wash with soap and water after using.”acids and peptides as their sole energy sources Several cases of monensin toxicity in ruminantsare suppressed;have been reported. In one dairy herd, several• reduction in the incidence of bloat due to the cows died when a 1.65% monensin premix wasinhibition of bacteria which produce large fed rather than a much-diluted mineral mix. Inamounts of gas-trapping mucous.another, 7-8 kg of a concentrate containing 366mg/kg monensin was fed. Digestive disorders® registered trademark of Eli Lilly and Company resulted in a dramatic drop in production.1M3:1

Research resultsResearch has clearly demonstrated the value ofmonensin in improving feed efficiency and rate ofgain in growing cattle and in reducing the incidenceof coccidiosis. These results are the basis of theregulatory approval of monensin for these purposes.There are few published studies examining theeffects of monensin in lactating cows, although theresults of several trials currently underway areexpected soon. Experiments conducted in NewZealand and Australia have focussed on the bloatreducingeffects of a Rumensin ® controlled releasecapsule in grazing dairy cows. These capsules aredesigned to deliver approximately 270 mg/day ofmonensin for an average of 120 days. Although bloatreduction has been consistent and significant,production responses in these cows have been quitevariable; changes in milk yield have ranged fromnone to an increase of 1.5 kg/day.Results of a trial conducted at Agriculture Canada’sAnimal Research Centre in Ottawa are summarized intable 1. Fed to fresh cows at a dietary level of 33 mg/kg, monensin reduced the incidence of subclinicalketosis from 6 cases in 12 control cows to 1 case in 12treated cows. Although monensin provokedsignificant changes in dry matter intake (DMI) andmilk fat %, increases in milk yield at both levels ofinclusion were not statistically significant.Several preliminary reports on feeding monensin tolactating cows have been presented at recent scientificmeetings:MONENSIN LEVEL, mg/kg0 16.5 33first 3 weeks postpartum:DMI, kg/day 14.5 a 13.9 ab 13.3 bWt Change, kg/day -1.5 -0.9 -1.1first 4 weeks postpartum:Milk Fat, % 4.12 a 3.58 b 3.71 abMilk Protein, % 3.49 3.25 3.34weeks 12-16 postpartum:Milk Fat, % 3.40 3.29 3.83Milk Protein, % 3.25 3.18 3.55weeks 1-16 postpartum:Milk Yield, kg/day 29.9 32.8 31.4Table 1 : Production responses to monensin at 2levels of inclusion in diets fed to lactating cows.Values with different superscripts in the same row aresignificantly different from one another.MONENSIN INTAKE, mg/day0 150 300 450Milk Yield, kg/day 25.0 a 27.8 b 27.5 b 26.6Milk Fat, % 3.91 a 3.58 b 3.44 b 3.44 bMilk Protein, % 3.39 a 3.24 b 3.25 b 3.23 bTable 2 : Milk yield and component level responses to4 levels of monensin intake. Values with differentsuperscripts in the same row are significantlydifferent from one another.• In a trial conducted in Indiana, cows were fed 0,150, 300 or 400 mg/day monensin from 2-4 weeksprepartum until 12 weeks postpartum. Monensinproduced no detectable effect on milk yield or milkcomposition, at any level of inclusion. DMI wasdepressed in multiparous cows receiving 150 mg/day but, otherwise, intakes of treated cows did notdiffer from those of controls.• Cows in a Kentucky trial were fed diets containing15.1 or 19.2% ADF, with or without 300 mg/day ofmonensin. When monensin was added to the 15.1%ADF diet, milk production increased and milk fat %decreased significantly while DMI and milk proteincontent did not change. However, when added to the19.2% ADF diet, monensin significantly decreasedDMI, milk yield, fat % and protein %.• Results of a British study are summarized in table 2.Mature cows received 0, 150, 300 or 450 mg/day ofmonensin for 20 weeks postpartum. At all levels ofintake, monensin significantly increased milk yieldand decreased milk fat and protein content.• A South African study demonstrated significantincreases in milk yield with no significant changesin milk fat or protein levels when cows were feddiets containing 10 or 20 mg/kg monensin fromweeks 4 to 12 postpartum.Based on published research results, it is notpossible to predict whether monensin will improveproduction efficiency under any specific set offeeding and management conditions. Therefore, untilmore complete information on production responsesbecomes available, the use of monensin in lactationrations should be confined to applications where asolid body of research supports its efficacy, namely:the control of bloat, ketosis and coccidiosis.prepared by :Steve Mason, Ph.D.ProLivestock : Nutrition/Management SpecialistsCalgary : 284-5484The Dairy Extension Advisory Group encourages the distribution of Alberta Dairy Management articlesbut requests that permission for direct reference to or use of these articles be obtained in writing.1M3:2