The other common reason cows consume excess sulfur is because of water contamination. For a lactating cow consuming an average amount of water, water that contains 350 mg/L of sulfate-sulfur (NOTE: Some labs report concentrations of sulfate, not sulfate-sulfur. To convert sulfate to sulfate-sulfur, divided by 0.333; for example 1051 mg/L of sulfate is equal to 350 mg/L of 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 percent distillers grains is consuming the same amount of sulfur as if she was eating a diet with about 0.48 percent sulfur (0.20 from the water plus 0.28 from the diet). This will reduce copper and selenium status, and if adjustments in mineral supplementation are not made, status of those two minerals for the animal will decline.
Copper and sulfur
To account for reduced absorption of copper when excess sulfur (~0.40 percent 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 percent 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 percent sulfur also have lower absorption of selenium than cows fed diets with 0.20 percent sulfur. Because of U.S. FDA 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.