The following was provided by Bill Weiss, dairy scientist at Ohio State University.
Q: How can excessive intakes of sulfur and potassium cause problems in dairy cows?
A: Proper mineral nutrition of dairy cows is essential for high milk yields, efficient reproduction, and good health. Meeting the 2001 NRC requirements for dairy cows in most situations will result in adequate status for most minerals. However, for some minerals and some situations, NRC requirements are inadequate and additional supplementation will be necessary.
Experiments conducted after the NRC was published have shown that the NRC requirement for manganese (approximately 15 ppm) is inadequate to prevent a deficiency, and the actual requirement is probably 30 to 50 ppm. Other newer data suggest that the NRC requirement for cobalt (approximately 0.11 ppm) may not result in optimal vitamin B-12 status, and cows may need at least twice as much cobalt (approximately 0.25 ppm).
However, for all other minerals, no evidence is available suggesting that the NRC requirements are not adequate in most situations. The NRC requirements assume a typical cow, fed a typical diet, and housed and managed under typical conditions.
Two situations which are not ‘typical,” but occur very frequently, are excessive intakes of sulfur and potassium. Sulfur greatly reduces absorption of copper and selenium, and excess potassium reduces the absorption of magnesium. The NRC recommendations for copper, selenium, and magnesium are often not adequate because of the presence of mineral antagonists.
Common mineral antagonists
The absorption of many minerals (probably all of them) can be reduced when other minerals or compounds are present in the diet. For many minerals, the concentrations of the potentially interfering minerals usually are not high enough to be a concern; however, for copper, selenium, and magnesium, antagonism (or interference with absorption) commonly occurs under field conditions. For copper and selenium, excess sulfur from diet and water can cause problems, and for magnesium excess dietary potassium can reduce absorption. Excess iron from water and diet can also be an issue but that will be discussed in a later article.
High concentrations of dietary sulfur or the consumption of water that contains high concentrations of sulfate will greatly reduce the absorption of copper and selenium.
Most diets, without any supplemental sulfur, will contain enough sulfur (0.20 percent) to meet the NRC requirement. Dietary sulfur concentrations of 0.20 percent are normal and will not cause absorption problems for other minerals. However, distillers grains can contain substantial, and highly variable, amounts of sulfur. For dried distillers grains, the average sulfur concentration is 0.63 percent with a standard deviation of 0.18 (results from DairyOne feed composition database). Using the average sulfur concentration, a diet with 20 percent distiller grains (dry basis) will have about 0.08 percentages units more sulfur than a diet without distillers grains (i.e., 0.28 vs. 0.20 percent sulfur). This higher concentration of sulfur probably is not enough to cause substantial problems with copper and selenium, but many diets with more than 20 percent distillers grains, and because of the highly variable sulfur concentrations in distillers grains, some diets with 20 percent distillers grains may have more than 0.35 percent sulfur, which is high enough to cause problems.