Unlike all other macro minerals, clinical deficiency of magnesium is not uncommon in U.S. dairy cattle. One reason is because cows have small labile pools of magnesium. If cows do not absorb adequate magnesium on an almost-daily basis, deficiency signs will develop quickly.
That’s according to Bill Weiss, dairy scientist, and Alex Tebbe, graduate research associate, with The Ohio State University.
The other reason is that absorbability of magnesium varies greatly among diets. The two major reasons for the variation in absorption are the 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, availability of magnesium from magnesium oxide can vary up to five-fold across sources.
Availability of magnesium from magnesium sulfate is less variable. It’s thought to be substantially more available than magnesium oxide.
The major site of magnesium absorption in cattle is the rumen. The major mechanism controlling magnesium absorption across rumen epithelial cells is an electrical gradient in cells. The inside of the cell is negative, which helps pull in positive ions such as magnesium. Potassium is also positively charged. It disrupts the electrical gradient and inhibits magnesium absorption. Because the concentration of potassium ions can be more than 10 times that of magnesium ions, the antagonism is substantial. A 50 percent reduction in absorption of magnesium is possible across 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 percent to 20 percent. Weiss and Tebbe 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 in that new study had about 2.1 percent potassium — about 1.3 percent from feeds and 0.8 percent from potassium carbonate. Diets had either magnesium sulfate or magnesium oxide; each increased dietary magnesium by about 0.15 percentage unit. And diets either included zero or about 360 milligrams per day of monensin.
On average the source of magnesium did not affect magnesium absorption, but there was a significant interaction between monensin and the 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, the researchers said.
When magnesium oxide was fed, monensin increased apparent absorption of magnesium by about 25 percent. However when magnesium sulfate was fed, monensin decreased magnesium absorption by about 30 percent.
When the most commonly used source of supplemental magnesium — magnesium oxide — was included in the diet, monensin substantially increased absorption of magnesium. Including monensin increases feed efficiency; enhancing magnesium absorption is an additional benefit.
Because of cost and potential negative effects of sulfur on fiber digestion, intake and trace-mineral absorption, magnesium sulfate is rarely fed to lactating cows. But because it can reduce the dietary cation-anion difference, it’s commonly fed to dry cows. Monensin is also commonly fed to dry cows.
The diets fed in the experiment were typical lactation-type diets. Weiss and Tebbe said results may not extrapolate to dry cows. However the data raise concerns regarding supply of available magnesium when cows are fed monensin and magnesium sulfate. Data suggest that dietary magnesium concentrations should be increased by about 15 percent when magnesium sulfate and monensin are fed. Data using dry cows fed typical dry-cow diets are needed to verify that recommendation.
Visit ansci.osu.edu for more information.