Excess Sulfur and Potassium Can Cause Mineral Nutrition Problems with Dairy Cows

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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 that 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 (agents that interfere with absorption).

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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%) to meet the NRC requirement. Dietary sulfur concentrations of 0.20% 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% with a standard deviation of 0.18 (results from DairyOne feed composition database, www.dairyone.com/forage/feedcomp). Using the average sulfur concentration, a diet with 20% distiller grains (dry basis) will have about 0.08 percentage units more sulfur than a diet without distillers grains (i.e., 0.28% vs. 0.20% sulfur). This higher concentration of sulfur probably is not enough to cause substantial problems with copper and selenium. However, producers should take note of potential problems if they are feeding diets with distillers grains. Because of the highly variable sulfur concentrations in distillers grains, some diets with 20% distillers grains may have more than 0.35% sulfur, which is high enough to cause problems.

Cows may also consume excess sulfur because of water contamination. For a lactating cow consuming an average amount of water, water that contains 350 mg/L of sulfate-sulfur (see “Note” below on converting sulfate readings to sulfate-sulfur) is approximately equivalent to adding 0.20 percentage units of sulfur to the diet. A cow drinking water with 350 mg/L of sulfate-sulfur and fed a diet with 20% distillers grains is consuming the same amount of sulfur as if she was eating a diet with about 0.48% sulfur (0.20 from the water plus 0.28 from the diet). This level of consumption will reduce copper and selenium status, and if mineral supplementation is not adjusted, status of those two minerals for the animal will decline.

NOTE: Some labs report concentrations of sulfate, not sulfate-sulfur. To convert sulfate to sulfate-sulfur, multiply sulfate content by 0.333; for example 1,051 mg/L of sulfate is equal to 350 mg/L of sulfate-sulfur.

Copper and Sulfur

To account for reduced absorption of copper when excess sulfur (~0.40% of dietary DM) is consumed, additional copper, above NRC requirements, will be needed. The approximate NRC copper requirement for lactating cows is 12 ppm, but when diets (including sulfur from water) contain more than about 0.35 to 0.40% sulfur, twice as much copper should be fed (~20 to 25 ppm). In addition, nutritionists should consider feeding at least some of the supplemental copper from commercial sources designed to have greater bioavailability than copper sulfate.  

Selenium and Sulfur

Cows fed diets (including sulfur from water) with 0.40% sulfur also have lower absorption of selenium than cows fed diets with 0.20% sulfur. Because of U.S. Food and Drug Administration regulations, the concentration of supplemental selenium in diets cannot be increased above 0.3 ppm, even when sulfur can interfere with selenium absorption. Therefore, increasing the concentration of dietary selenium above 0.3 ppm to counter the reduced absorption is not a legal option. Some feed ingredients that are grown in areas with high selenium soil are naturally high in selenium (e.g., brewers grains or linseed meal) can increase selenium concentration of the diet. If cows are fed high sulfur diets, cows should be supplemented with the maximum amount of selenium permitted by law, and a substantial amount of the selenium should come from a high-quality selenium yeast product. If feeding the maximum legal amount of selenium from a mix of selenium yeast and inorganic selenium is not adequate to maintain selenium status (e.g., plasma concentration of selenium greater than about 0.075 mg/L), selenium may have to be injected.


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% 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.

Estimate Equations

  • 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% 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% of diet dry matter.

For example, if cows were consuming a diet with 1.8% potassium (not uncommon with high alfalfa or grass diets), and the NRC requirement for magnesium is 0.15%, you should formulate a diet with either 0.21% magnesium (Ohio equation) or 0.17% magnesium (European equation). Because magnesium deficiency is more costly than slightly overfeeding magnesium, 0.21% magnesium is the preferred concentration.

For both equations, the supplemental magnesium was assumed to come from a source with an availability coefficient of 70% 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 (1,470° to about 2,000°F). You will not know the manufacturing temperature, so magnesium oxide should be purchased from a reputable manufacturer.


On many farms, minerals are overfed 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 20% 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.

Author Information

Bill Weiss
Department of Animal Sciences
Ohio Agricultural Research and Development Center
The Ohio State University