Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/26/2003
Publication Date: 2/1/2004
Citation: Broderick, G.A. 2004. Effect of low level monensin supplementation on the production of dairy cows fed alfalfa silage. Journal of Dairy Science. 87:359-368. Interpretive Summary: Alfalfa silage is the most important forage fed to dairy cows in the U.S. Midwest. When preserved in the silo, more than half of the protein in alfalfa is converted to nonprotein nitrogen (NPN). Although 80-90% of this NPN is in the form of amino acids, the protein building blocks required by the cow, certain microbes in the rumen (the first compartment of the cow's stomach) degrade most of these amino acids for energy. Thus, the NPN in alfalfa silage is used poorly and much of this nitrogen ends up being excreted in the cow's urine and contributing to air and water pollution. Monensin is a compound that improves feed efficiency in beef cattle and has been fed in beef feed-lots for over 25 years. Although not now approved for milking cows, some microbiologists have reported that monensin will suppress those microbes that degrade amino acids in the rumen. The objective of this research was to see if feeding dairy cows a small amount of monensin would improve the protein value of alfalfa silage. Adding a high quality protein to the diet increased milk and protein production, indicating that the cows required more useful protein. However, feeding monensin at 250 mg per day did not alter milk production and actually reduced production of fat and protein. Studies done with rumen fluid from some of the cows did not show that monensin feeding reduced amino acid breakdown by rumen microbes. The only positive finding was a small improvement in protein efficiency when monensin was fed without the high quality protein, but this happened because intake of feed protein decreased more than milk protein yield. Although further study is needed to see if higher amounts of monensin are effective, this research indicates that feeding dairy cows 250 mg per day of monensin will not improve production or the protein value of alfalfa silage. This means that there are no economic or environmental advantages for Midwest dairy farmers for adding monensin to the diets of their cows.
Technical Abstract: Effectiveness of low level monensin supplementation on N utilization in lactating dairy cows fed alfalfa silage was assessed using 48 multiparous Holsteins. Cows were fed a covariate diet [% of dry matter (DM): 56% alfalfa silage, 39% ground high moisture corn, 3% soybean meal, 1% ground corn, 1% vitamin-mineral supplements, and 31% NDF] for two weeks, then grouped by days-in-milk in two sets into six blocks of four. Cows were randomly assigned within blocks to one of four diets that were fed for 10-wk: 1) control (covariate diet); 2) control plus 3% fish meal (replacing DM from high moisture corn); 3) monensin (10 mg/kg DM); and 4) monensin plus 3% fish meal. Diets 1 and 3 averaged 16.7% crude protein (25% from free AA in alfalfa silage); diets 2 and 4 averaged 18.5% crude protein. Monensin intake averaged 16 mg/d on diets 1 and 2 (due to contamination) and 248 mg/d on diets 3 and 4. There was no effect of fish meal or monensin on DM intake. However, weight gain and yield of milk, protein, and SNF increased with fish meal feeding, indicating metabolizable protein limited production. Feeding monensin increased blood glucose but reduced yield of 3.5% FCM, milk fat content and yield, and milk protein content and yield. Apparent N efficiency was greatest on monensin (diet 3) but lowest on monensin plus fish meal (diet 4). Fish meal reduced blood glucose and apparent N efficiency, and increased milk and blood urea. Monensin increased ruminal propionate, and decreased ruminal acetate, butyrate, and acetate: propionate in ruminally cannulated cows fed the experimental diets. However, these changes were small, suggesting that too little monensin may have been fed. Fish meal reduced ruminal total AA but monensin did not alter ruminal NH3 or total AA. Both fish meal and monensin increased NH3 formation form casein AA using ruminal inoculum from the cannulated cows. There was no evidence from this trial that monensin fed to lactating cows at 250 mg/d improved N utilization by reducing ruminal catabolism of the large amounts of free AA in alfalfa silage.