Disclaimer: This article is presented for informational purposes only and is not intended to endorse any product.
You have probably read at least one article talking about sulfur in the body as indispensable for protein production, for the integrity of skin, hair, hooves and nails, its enzyme action, its presence in important B vitamins, for the production of substances like chondroitin sulfate, and its role in the elimination of toxins. All of that is true, and more, but confusion reigns about how you should supply it.
If you are from Australia or South Africa you may have been told horses should be supplemented with inorganic elemental sulfur. Regardless of where you are from, you probably have read that MSM or DMSO are sources of organic sulfur that will be available to perform all the important roles of sulfur in the body. Both are incorrect.
Horses may utilize small amounts of the sulfate ion present in their diet and water but their main source of sulfur, and the only form utilized by proteins and insulin, is the sulfur containing amino acids, the most important of which is methionine, which can also be converted to cysteine and from there to cystine — the other two structurally important sulfur amino acids. The horse cannot make methionine from sulfur or MSM/DMSO — it has to be present in the diet.
The true equine requirement for methionine is unknown, but is thought to be between 1/4 to 1/3 of the lysine requirement. Forage is the major source of methionine. The National Research Council has recommended a sulfur intake of approximately 0.15% of the diet dry matter, although there is evidence this may be inadequate. Good quality hay grown on soil with adequate sulfur should meet the requirements of at least maintenance and low-level exercise if there are no special needs, but there is a growing problem developing with this.
Sulfur was routinely incorporated into plant fertilizers until increasing industrialization began sending large amounts of sulfur into the air. This "acid rain" provided an excellent source of free sulfur for plants, but caused many other problems. Sulfur emissions have been tightly regulated since the 1980s and 90s, with the result that soil sulfur is dropping. A hay analysis crossed my desk this week that had only 0.04% sulfur. These hays will have low protein, low methionine, and the potential for high nitrate levels.
Taurine is another sulfur amino acid ultimately derived from methionine that plays many important roles in the nervous system, detoxification, liver function and metabolism. Increased levels may be needed by horses with abnormal glucose metabolism to support the body in avoiding harmful interactions of glucose with body tissues, including nerve damage. Taurine also helps maintain neurotransmitters responsible for a stable, happy mood.
When methionine intake is known to be low, or suspected from issues like poor hoof quality, supplementation of 5000 to 10,000 mg (5 to 10 grams) per day for the average size horse is reasonable. For situations that may benefit from taurine support, this can be supplemented directly in similar amounts.