The Sulphur Story and it’s Importance to Hoof Health
For many years, horse owners have been wetting or soaking hay for the ‘good-doer’. We should look to provide these horses and ponies with hay of less than 10% water-soluble carbohydrates. We would recommend having your regular source of hay, be it home grown or bought in, analysed so that you know exactly what you are feeding your horse or pony.
Fig 1: Elemental sulphur.
Many sulphur compounds are highly odoriferous, and many well-known strong smells are due to organosulfur compounds, including the smell of natural gas and garlic. For farriers and horse owners alike, the distinctive aroma of hot shoeing is due to burning sulphur. This sign alone tells us that sulphur containing compounds are a key part of hoof horn structure.
Sulphur in the Body
Sulphur is also a common element in the body, being the seventh or eight most abundent by mass, roughly equal with potassium. As an organic substrate it is most closely associated with the sulphur containing amino acids, methionine and cysteine, so we start to see that those nutrients we know to be important to hoof health, are themselves sulphur containing compounds. This association with methionine and cysteine explains why we find 70% of the body's total sulphur is in the muscles, as protein. The metabolism of nitrogen and sulphur are closely linked throughout both plant and animal systems, due to their protein association1. Other sources of sulphur in the horse include the B vitamins biotin and thiamin; herparin, insulin and chondroitin sulphate. The B vitamins are involved with metabolism, herparin is an anticoagulant in the blood, insulin is vital to regulating carbohydrate metabolism, while chondroitin sulphate is an important building block of the joint capsules2.
However, it is sulphur's role in the hooves that interests us most here, and while the largest store of sulphur by mass is in the muscles, the highest concentration of sulphur is certainly within the hoof horn3 (Fig 2).
Sulphur and Keratin
The reason for the concentration of sulphur in the hoof horn is simple, and that is due to it's amino acid role in keratin. The alpha-keratin of the equine hoof is a tough fibrous structrual protein found in the skin and it’s appendages, and, of course, it is the unique structrue of keratin that gives hoof horn it’s tough, yet flexible properties. The equine hoof is uniquely adapted, even down to the type of keratin it uses. Keratin types distinctive to the equine hoof, specifically the lamellar tissue, have been identified that are not found in other keratinized sites such as the haired skin, hoof coronet or cornea4, thus demonstrating the unique nature of the hoof.
The amino acids central to the structure of keratin are cysteine and it’s oxidised dimer, cystine, however the importance of methionine should not be overlooked. Methionine is classed as an 'essential amino acid', meaning it must be provided in the diet as the horse cannot produce methionine itself. Cysteine, and cystine, by contrast, are considered 'non-essential', as the horse can manufacture those internally. In fact it is the methionine itself that the horse uses to produce cysteine, which it does along a pathway of conversion initially to homocysteine, donating a sulphur molecule to serine production (another non-essential amino acid) through ultimately to cysteine, which, in turn, is then converted to cystine.
The strength and flexability of keratin owes much to the disulphide bonds formed when two cysteine molecules undergo a redox reaction, through a process known as 'oxidative folding' to form cystine5. (Fig 3). Therefore we can see how, in a very real sense, sulphur is the element right at the core of hoof horn strength.
Fig 3: Two molecules of Cysteine (C3H7NO2S) undergoing ‘Oxidative Folding’ to Cystine (C6H12N2O4S2), with the Disulphide bridge highlighted.
Once formed, keratin monomers arrange into bundles of intermediate filaments with substantial strength, yet retaining flexbility. It is this combination of properties that gives hooves their toughness – the ability to absorb energy, even changing shape if need be, without breaking. In nature almost no other biological tissue has this ability, with the only other matter approximating the toughness of keratin being ‘chitin’, the tough outer layer arthropods such as crustacea and insects6.
Dietary Sources of Sulphur
Traditionally the 'Flowers of Sulphur', that bright yellow, inorganic element sulphur would be used to supplement the diet, however the bioavailability is very poor, and therefore simply wasteful as a supplement. Rather we should consider sources of sulphur natural to their diet, how the modern equine life influences those, and how we can help.
The principle source of sulphur for horses is from the botanical proteins in their grazing and forages, and these proteins will include the sulphur containing amino acids methionine and cysteine discussed. The level of sulphur in the forage will depend on both type and treatment of the ley. Grazing with a high percentage of legumes, such as clover, will be higher in protein, while grass contains lower levels. Treatment with fertilizer can also significantly impact, as sulphur is often included in those applications, and certainly for fields near agricultural activity there may be some transfer of sulphur from crop treatments. However horse owners often choose not to fertilise, either through a desire to pasture manage 'organically', or due to conerns of the grazing becoming too 'rich'. However for most horses, certainly in average work, fresh grazing will meet their protein requirements, and through that their sulphur requirements, for general maintenance1.
While fresh grazing provides adequate sulphur, can we say the same when the dependence moves from fresh to preserved forage, either due to the time of year, or periods of stabling and travel? UK hay often fails to meet the expected protein levels, and so sulphur levels, of grass hay globally. In the UK we can expect an average hay value of 7% Crude Protein, and may be as low as 3 or 4%7. If we look at how that impacts on diet we can see how UK equines can, quite easily, be protein deficient.
Eg 1: 500kg Horse in Moderate work.
Protein from 10kg hay:
2% Bodyweight (BW), i.e. 10kg / day.
768g Crude Protein (CP) /day2
10,000g * 7% = 700g
Eg 2: 400kg I.R. Pony on restricted intake due to weight gain, and in Light exercise.
Protein from 10kg hay:
1.5% BW i.e. 6kg / day
6,000g* 7% = 420g
Therefore we can see how it is relatively easy for a horse to be deficient in plant protein, amino acids, and so, essential sulphur. We know a significant level of the forage methionine will be required for endogenous cysteine production, which, in turn, is required for keratin production; so low protein isn’t just a risk for general health, but has direct implications for hoof strength and growth.
Other sulphur sources include the B Vitamins, biotin and thiamine. Thiamine, is principally involved in carbohydrate metabolism, and so of these two it is biotin that is of interest as a sulphur donor for hoof health, although it is also used by almost all cell types in a range of actions. Biotin may not, strictly speaking, be considered a dietary source, as it is naturally produced within in the system, by microbial fermentation in the hindgut1. However it can be found in the diet, both naturally in feedstuffs including alfalfa and some cereals, and, specifically for hoof health, as a dietary complementary feed, or 'supplement'.
It is a valid question to ask why, if is naturally produced in the hindgut, do we also supplement biotin? In fact, the dietary route for biotin is often considered preferable to the natural production, as microbial synthesis takes place in the hindgut, so past the point of optimum absorption for vitamin like substances, i.e. the small intestine. Feeding biotin allows it to be absorbed in the small intestine and bioavailability is thought to be very high, as has been found in similar monogastrics, such as pigs8. Research in horses has found improvements in hoof quality with supplementary biotin9, with some trials also showing improvement in hoof growth rate10. The research shows that when supplementing with biotin patiences is required as an improvement takes many months, reflecting the length of time required for hoof growth. Once established on a daily biotin supplement it is advised that the horse stays on it for long term maintenance, as research shows when that is stopped the hooves return to pre-supplement condition11.
However useful and important the plant based amino acids and dietary biotin can be, if we consider these to be our only sulphur sources we are missing a very significant source. Methyl sulphonyl methane, goes under a number of different names, but we usually know it best simply as MSM. Fig 4.
Fig 4. MSM (Methyl sulphonyl methane, C2H6O2S)
MSM is an organosulphur compound, and is highly sulphur rich, being 34% sulphur, compared to, for example, methionine at 21.5% or biotin at 13.1% sulphur. Of course that sulphur is of no use if it is not bioavailable. However trials in mammals show MSM to be rapidly assimilated, and evenly distributed through the system12,13. MSM is part of the Earth's sulphur cycle, and will be found in both soil and plants, with fresh grazing being the natural source for horses. MSM not only has the ability to directly donate sulphur to the system, but has also been shown to be a source of sulphur for both the methionine and cysteine, particularly for the essential amino acid, methionine,14 which then feeds keratin through the cystine production discussed above.
Supplementary MSM has been shown to be associated with many benefits across species, and that includes the integumentary system, the skin and it’s appendages, such as hooves. A trial in middle aged women showed a significant improvement in hair and nail condition, based both on self-assessment by the subjects, but also independent scientific grading. The results showed the higher rate to be more effective, and a dose response result like this can give us confidence that the results are due to the MSM and not coincidental15.
Trials in feeding horses MSM tend to concentrate on it’s role in the musculoskeletal system and lameness, however this sulphur donation role strongly suggests that we should also be considering MSM’s role in hooves.
As with the plant based sulphur amino acids, the level of MSM drops markedly once fresh forage is preserved, either through drying to hay or ensiling to haylage. Therefore horses regularly stabled, or travelling regularly, with a greater reliance on preserved forage, may be sulphur deficient, both through lower levels of methionine and cysteine, and lower levels of MSM.
The U.K. has a relatively high reliance on stabling compared to many other countries, and it is the author’s opinion that this is partly due to our smaller landmass, so less available grazing particularly in certain areas, but also the traditional approaches to horse care in the U.K. This is a view shared by my colleague, Nick Larkins DSc, BVSc, MRCVS, who helped to revolutionise equine supplements for horses in the U.K, through NAF, by being the first to introduce MSM for horses. back in the early 1990s.
Field Trial to assess MSM on Hoof Quality
Rationale: The reliance on preserved forage and feed for horses in the U.K. leads to a depletion of natural MSM, as this organosulphur compound evaporates quickly and is driven off by feed conservation processes. Earlier trials had shown the average horse had an MSM range in the blood of 0.3 to 0.7ppm, whereas it is thought that a level of at least 1.0ppm is required for optimal health. Therefore the opportunity to improve horse’s health through MSM supplementation is clear.
Protocol: The trial was a field trial questionnaire of owners using an MSM complex for health, including for hoof health. Questionnaires were simply sent to horse owners and asked to complete and return. No payment, prize or product in kind was offered for taking part, in order to minimise the risk of bias.
Owners were feeding at a rate of 2-5g MSM per day, dependent on their horse’s bodyweight, and had been feeding for between 12-20 weeks prior to answering the questionnaire.
Of the two hundred replies, 104 were using an MSM based supplement for hoof health, with further results reported for MSM and it’s role in respiratory health, laminitis and management of inflammatory conditions.
Results: The results showed 86% of respondents reported an improvement in their horse’s hooves, with over half reporting a Significant Improvement15. (Fig 5)
No customers reported the hooves worsened. Additionally, although customers were not asked additional questions, they were given the chance to comment, and 36% of them commented they had also noticed an improvement in at least one of the following, coat condition, skin, mane or tail.
Trial Conclusion: It was concluded that supplementing with an MSM dietary complex gave self-evident responsiveness in improving the quality and the growth of hoof horn in the horse.
Case Studies – Feeding a Sulphur rich Hoof Supplement
For over thirty years NAF have continued to research and develop innovative evidence-based nutraceuticals, many of which aim to balance a sulphur deficiency in the modern horse through the use of MSM. For hoof health NAF formulae also utilise the recognised roles of the sulphur containing essential amino acid, methionine, and the B vitamin, biotin. Together with a complex of nutrients, outside the scope of this article, they form PROFEET supplement, available in a range of deliveries (powder, liquid or pellets) to suit both the horse and their owner. The following are just a couple of Case Studies.
Farrier reported Case Study.
"With many different supplements available it is often hard to decide which one or ascertain whether there is a significant difference. I can safely say since recommending PROFEET to my customers not only have we witnessed good hoof growth, but the quality of horn has appeared to be stronger and more resilient. Giving me more hoof to work with from visit to visit and the horse a great hoof to perform on."
- Andrew James AWCF Master Farrier.
Horse Owner reported Case Study.
"When I bought my first horse, Kingfisher, he was a very green five year old but had bags of potential and a great attitude. I had already decided I wanted to event and he may just be the horse to get me there. Unfortunately, being a thoroughbred cross he had poor, very dry feet and the Appaloosa in him meant he had spongy soles, this made no difference in winter but in summer I was very lucky to keep shoes on him more than two weeks at a time. Not only this, but every time he travelled and was on different ground it meant his shoes would either become loose or cast. It proved to be a nightmare, his fittening and schooling work was drastically reduced. I knew I need to improve his feet if I was ever going to event so I asked at the local tack shop, and was advised PROFEET. I have been feeding PROFEET for three years now and the difference my farrier and I have seen in his feet is just amazing; he now goes the full 6 weeks between shoeing even in summer. It has enabled us to train and compete regularly. I would highly recommend PROFEET to anybody who is having foot and shoeing issues!" Fig 6
- Mark Holmes.
Fig 6: Mark and Kingfisher. Photo Credit: Grossick Photography
Horse Owner reported Case Study.
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- Nutrient Requirements of Horses (2007) 6th Rev Ed. Pub: NRC National Academies.
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- Armstrong C et al (2019) The expression of equine keratins K42 and K125 is restricted to the hoof epidermal lamellae of Equus caballus. PLoS One. 2019 Sep 24:14(9) : e0219234.
- Sevier C.A & Kaiser C.A (2002) Formation and transfer of disulphide bonds in living cells. Nature review: Molecular Cell Biology. Nov 3(11) 836-47.
- Vincent J.F. V & Wegst U.G.K (2004) Design and mechanical properties of insect cuticle. Arthropod Structure and Development. Jul 33 (3) 187-99.
- Baker D.H (1995) Vitamin Bioavailability. Bioavailability of Nutrients for Animals. P.399. pub: New York : Academic Press.
- Josseck H et al (1995) Hoof horn abnormalities in Lipizzaner horses and the effect of dietary biotin on macroscopic aspects of hoof quality. Equine Veterinary Journal. 27. 175-182.
- Reilly J.D. et al (1998) Effect of supplementary dietary biotin on hoof growth and hoof growth rate in ponies: a controlled trial. Equine Veterinary Journal Supplement. 26, 51-57.
- Geyer H & Schulze J (1994) The long term influence of biotin supplementation on hoof horn quality in horses. Schweizer Archiv Fur Thierheilkunde 136 (4) 137-149.
- Wong T et al (2018) Small intestinal absorption of Methylsulfonylmethane (MSM) and accumulation of the sulfur moiety in selected tissues of mice. Nutrients. Jan 10(1) : 19.
- Butawan M, Benjamin R.L & Bloomer R.J (2017) Methylsufonylmethane: Applications and safety of a novel dietary supplement. Nutrients. Mar 9(3) 290.
- Richmond V.L. (1986) Incorporation of methylsulfonylmethan sulfur into guinea pig serum proteins. Life Sciences. 39(3) 263-268.
- Larkins N.J & Gaugas J (1993) Methylsulphonylmethane (MSM) resolves poor hoof condition in the majority of treated cases. Pers.comm.