Thirty years ago, I routinely checked the nitrogen-to-sulfur ratio in feedlot diets to see if there was adequate sulfur. Cattle diets deficient in sulfur result in decreased microbial populations, microbial protein synthesis and lactate utilization. Severe deficiencies can cause lethargy, weight loss and potentially death. Nowadays, however, it is far more likely that there is too much sulfur rather than too little. Allegedly, famed actress Mae West once postulated, “Too much of a good thing is marvelous.” Too much sulfur is not. Excessive sulfur intake by cattle decreases dry matter intake and average daily gain and can lead to potentially lethal diseases.
An essential macro mineral, sulfur is a necessary component for many organic molecules. Beef cattle require 0.15 percent dietary sulfur for adequate growth. That percentage, for the most part, comes from sulfur amino acids in protein. Rumen bacteria, especially cellulolytic bacteria, also require sulfur for adequate growth.
While the appropriate amount of sulfur is vital to normal performance and productivity, excessive amounts can cause sulfur toxicity. In the rumen, bacteria reduce sulfur to sulfide or hydrogen sulfide. The balance in the rumen between sulfide and hydrogen sulfide is pH-dependent. As ruminal pH decreases, the proportion of hydrogen sulfide increases. Large amounts of hydrogen sulfide is believed to cause the negative effects of excess dietary sulfur, including decreased cattle growth, reduced copper, diarrhea, muscular twitching, and polioencephalomalacia, a potentially fatal neurologic disease of ruminants. In cases of sulfur-induced polio, the excess ruminal hydrogen sulfide—which is similar to weak battery acid—“chews” on nerve tissue.
The National Academy of Sciences suggests that 0.3 percent sulfur is the maximum tolerable level in beef cattle diets with less than 15 percent forage. For diets containing greater than 40 percent forage, the maximum tolerable level is 0.45 percent. Higher concentrations may result in sulfur toxicity. Therefore, the risk can be decreased by increasing the forage fraction of the diet.
A feedlot ration commonly has 8 percent of its dry matter as forage. That’s why sulfur toxicity problems are often found in finishing animals or in situations where an excessive amount of corn co-products, such as corn gluten or distillers grains, are used in the diet. When corn co-products are fed at more than 1 percent of body weight, they are being overfed. The feeding value decreases, with reduced average daily gain and gain-to-feed ratio. Excessive sulfates in the water source can also be a problem.
Not only does decreased ruminal pH increase the risk of sulfur toxicity, but it also negatively affects dry matter intake, fiber digestibility and increased risk of acidosis, a common nutritional disorder in beef cattle. Acidosis is caused by rapid rumen fermentation of carbohydrates, which results in an accumulation of rumen and blood lactate/lactic acid The lactic acid load is hurtful, not helpful.
There are three common approaches to address acidotic challenges that can lead to sulfur toxicity: feed a buffer, increase forage and manage feed intake.
Including dietary forage increases ruminal pH, but not all sources have an equal effect. The neutral detergent fiber of roughage does not account for physical characteristics such as particle size and shape, moisture and density that affect digestion. These combined measurements are defined as effective neutral detergent fiber.
Increasing forage intake adds to time spent chewing, which stimulates saliva secretion and increases the amount of buffers in the rumen. Thus, more forage in the ration will increase rumen pH, decrease the daily fluctuation in ruminal pH and lower hydrogen sulfide.
However, feeding greater amounts of roughage and other prevention practices are not always economical for cattle feeders. Increasing forage lowers the energy level in the diet, which tends to reduce average daily gain and efficiency. The opposing economics related to management practices known to decrease the risk of acidic pH and the goal to increase cattle performance pose a question of how to best optimize forage concentration to avoid sulfur toxicity and maintain cattle performance.