Sulfur in the body is an indispensable element for protein production, integrity of skin/hair/hooves and nails, enzyme action, some B vitamins, production of substances like chondroitin sulfate and elimination of toxins. However, you won’t find sulfur on an ingredient list for mineral supplements.
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.
Sulfur bonds/crosslinks maintain structural integrity in protein hormones like insulin as well as the keratin in skin, hair and hooves. These bonds don’t come from sulfur being inserted into the structure. They occur between the sulfur groups of two cystine amino acids. The sulfur needed to make sulfated compounds like chondroitin sulfate comes from the desulfurization of sulfur amino acids.
As an aside, ruminants like cows or goats can benefit from elemental sulfur because the organisms in their forestomachs will use it to make methionine just like plants do. When the bacteria pass into the stomach and small intestine, the methionine will be absorbed. Horses ferment their food in the hind gut, where absorption of amino acids is minimal – if it occurs at all.
The true equine requirement for methionine is unknown but is probably 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 and other sources recommend at least 0.2%. (Note: About 90% of the sulfur in a hay analysis is incorporated into plant protein amino acids.) 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 mounting problem 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. I have seen hay sulfur as low as 0.04% and levels under 0.15% are common. 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 things 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.
Eleanor Kellon, VMD