Unlike all other macrominerals, a clinical deficiency of magnesium is not uncommon in the U.S. dairy cattle population. One reason is because cows have very small labile pools of magnesium; therefore, if cows do not absorb adequate magnesium on an almost daily basis, deficiency signs will develop very quickly. The other reason is that absorbability of magnesium varies greatly among diets. The two major reasons for the variation in absorption are source of supplemental magnesium and the dietary concentration of potassium. Magnesium oxide is the most common source of supplemental magnesium, but because of differences in manufacturing practices (for example, calcination temperature and particle size), the availability of magnesium from magnesium oxide can vary up to 5-fold across sources. The availability of magnesium from magnesium sulfate is less variable and has been thought to be substantially more available than magnesium oxide. The major site of magnesium absorption in cattle is the rumen, and the major mechanism controlling magnesium absorption across rumen epithelial cells is an electrical gradient in cells (inside of the cell is negative which helps pull in positive ions such as magnesium). Potassium is also positively charged and disrupts the electrical gradient and inhibits magnesium absorption. Because the concentration of potassium ions can be more than ten times that of magnesium ions, the antagonism is substantial. A 50% reduction in absorption of magnesium is possible over the range of potassium concentrations commonly observed in diets. An additional dietary factor that could affect magnesium absorption is monensin. Based on studies conducted about 30 years ago with beef cattle, monensin supplementation can increase magnesium absorption by 15 to 20%.
We conducted an experiment to determine whether monensin affected magnesium absorption in dairy cows fed high potassium diets and whether that effect differed by magnesium source. All diets had about 2.1% potassium (about 1.3% from feeds and 0.8% from potassium carbonate). Diets had either magnesium sulfate or magnesium oxide (each increased dietary magnesium by about 0.15 percentage units) and either 0 or 14 mg/kg of diet of monensin (approximately 360 mg/day). On average, source of magnesium did not affect magnesium absorption, but there was a significant interaction between monensin and magnesium source. Without monensin, magnesium sulfate was a better source of available magnesium than magnesium oxide, but when monensin was in the diet, magnesium oxide was better. When magnesium oxide was fed, monensin increased apparent absorption of magnesium by about 25%. However when magnesium sulfate was fed, monensin decreased magnesium absorption by about 30%.
When the most commonly used source of supplemental magnesium (i.e., magnesium oxide) was included in the diet, monensin substantially increased absorption of magnesium. Inclusion of monensin increases feed efficiency and the enhancement of magnesium absorption is an additional benefit. Because of cost and the potential negative effects of sulfur on fiber digestion, intake and trace mineral absorption, magnesium sulfate is rarely fed to lactating cows. However, because it can reduce the dietary cation anion difference (DCAD), it is commonly fed to dry cows. Monensin is also commonly fed to dry cows. The diets fed in this experiment were typical lactation type diets and results may not be extrapolatable to dry cows. However, these data raise concerns regarding supply of available magnesium when cows are fed monensin and magnesium sulfate. These data suggest that dietary magnesium concentrations should be increased by about 15% when magnesium sulfate and monensin are fed. Data using dry cows fed typical dry cows diets are needed to verify this recommendation. (Note: Full details regarding this experiment are available in the Journal of Dairy Science; Tebbe et al., 2018)