Sulfur in the form of sulfate is essential for life in organisms from bacteria to plants to horses. Sulfate is an ion with a negative charge, SO4-2
Bacteria in the soil form sulfate from sulfur and oxygen. Soil sulfate may complex with other minerals or be absorbed by plants in its free form. Most of the sulfate in plants is used to manufacture the sulfur containing amino acids. Plants also need sulfate to make chlorophyll and chemicals that protect them from insects.
Sulfate forms of trace minerals like zinc and copper are highly bioavailable. The intestinal tract can also absorb the sulfate and has a special receptor for it called NaS1 – the sodium sulfate cotransporter. The same receptor is found in the kidney and reabsorbs sulfate from filtered blood to retain what the body needs. Sulfate absorption decreases when supply is abundant and vice versa.
While the body can, and does, absorb sulfate ions, sulfate can be generated from sulfur amino acids and this is the major source. Sulfate is formed in the terminal breakdown of those amino acids. Studies have shown that a completely sulfur/sulfate free diet will still support life if it supplies the required amounts of sulfur containing amino acids.
Free sulfate ions combine with ATP to form a compound called PAPS which supplies the sulfate for all body functions which require it – and there are a lot of them:
- sulfation – adding a sulfur group – is one of the main pathways for detoxifying and then excreting things like heavy metals and toxins of many different kinds including xenobiotics
- production of bile salts
- glycosaminoglycan synthesis (sulfates of chondroitin, heparin, dermatan)
- elimination of steroid and thyroid hormones
- activation of some hormones and cellular receptors
- mucus production, including the mucus which protects the stomach from ulceration
- essential for the formation of myelin sheaths which protect the nerves
The level of sulfur amino acids and sulfate in the diet is decreasing in many areas. Air pollution control legislation has dramatically reduced the amount of sulfur dioxide in the air. This used to be a rich source of sulfur for the soil, where bacteria converted it to sulfate. Since sulfur is a key nutrient for plant growth, fertilizer mixtures increasingly need to contain sulfur.
You can check for the adequacy of sulfur amino acids in your horse’s hay by making sure the analysis includes the percentage of sulfur then check the N:S – nitrogen sulfur ratio. To determine nitrogen, divide the percentage of crude protein by 6.25. The N:S should be between 10:1 and 15:1. Also, to support normal amino acid production in the hay the sulfur should be at least 0.2%. These recommendations are from research on cattle but since most sulfur in the diet is in the form of sulfur amino acids it should work as a minimum guideline for the horse as well.
In summary, sulfate is an important nutrient for the horse. Some comes directly from sulfate in the diet but most is derived from the metabolism of sulfur amino acids. Levels in the diet are dropping as sulfate in soil becomes less plentiful. Guard against this by supplementing 2.5 to 5 grams of methionine/day for the average size horse.
Eleanor Kellon, VMD