Potassium is an essential mineral for dairy cows, and diets without adequate potassium will result in an almost immediate drop in milk production. The NRC requirement is about 1 percent of dietary DM, but because forages often contain very high concentrations of potassium, most diets have concentrations of potassium much greater than the requirement, and thus, supplementation is uncommon. Some newer information has revealed that feeding supplemental potassium (from potassium carbonate) to lactating cows at rates in excess of NRC may increase yields of milk and fat. Cows in heat stress also often benefit from supplementation of potassium above NRC requirements. Although feeding extra potassium is common (and often unavoidable) and may have benefits, it also clearly reduces magnesium absorption.
Potassium and magnesium
The negative effect of feeding excess potassium on magnesium absorption can be overcome by increasing the concentration of magnesium in the diet. Two studies (one from Ohio and one from Europe) developed equations that can be used to estimate how much additional magnesium should be fed based on the potassium concentration of the diet. The European equation was developed mostly from cows fed grass-based diets; whereas, the Ohio equation was developed from cows fed mostly alfalfa and corn silage-based diets.
- Ohio Equation (J. Dairy Sci. 87:2167): Increase dietary magnesium by 0.08 percentage units above NRC for every 1 percentage unit that dietary potassium is greater than 1 percent of diet dry matter.
- European equation (J. Dairy Sci. 91:271): Increase dietary magnesium by 0.02 percentage units for every 1 percentage unit that dietary potassium is greater than 1 percent of diet dry matter.
For example, if cows were consuming a diet with 1.8 percent potassium (not uncommon with high alfalfa or grass diets), and the NRC requirement for magnesium is 0.15 percent, you should formulate a diet with either 0.21 percent magnesium (Ohio equation) or 0.17 percent magnesium (European equation). Because magnesium deficiency is more costly than slightly overfeeding magnesium, 0.21 percent magnesium is the preferred concentration.
For both equations, the supplemental magnesium was assumed to come from a source with an availability coefficient of 70 percent for the magnesium (i.e., the availability of magnesium from high quality magnesium oxide). The availability of magnesium from magnesium oxide can vary greatly depending on particle size (particles should be less than about 400 microns) and the temperature used to make it (1470 to about 2000 degrees F). You will not know the manufacturing temperature, so magnesium oxide should be purchased from a reputable manufacturer.
On many farms, minerals are over fed because nutritionists and farmers want to avoid a mineral deficiency. Deficiencies are almost always more costly than slight overfeeding; therefore, a reasonable safety factor should be applied to NRC requirements (perhaps 10 to 20percent extra). However, when diets (including contributions from water) are high in sulfur, substantial modification of copper and selenium supplementation may be needed. If diets are high in potassium, magnesium supplementation should be increased above the standard safety levels.