Silicon in the form of silicon oxide (silica) is the second-most abundant element on earth (oxygen is the first). Its most familiar natural form is sand, and its purified, colorless form is glass. There is an organic silicon cycle in the oceans, where silicon exists as orthosilicic acid and is used by algae as a component of their cell walls. Silicon is also found in fresh water, in the form of orthosilicic acid.
The silicon in soils is also taken up by plants. Plants secrete organic acids at the tips of their roots. These acids dissolve oxides and other inorganic mineral salts, freeing the minerals in their ionic, pure form so that they can be absorbed. The silicon level in plants depends on species (grasses are higher than legumes) and rises when soil levels are higher.
Like algae, plants concentrate silicon in their cell walls. The slippery feeling of blades of grass, straw and the outer coats of grains is caused by silicon crystals (phytoliths). Some silicon is also found associated with the hemicellulose portion of the cell wall, with pectin and other soluble fibers in feeds, legumes and beet pulp. High fiber foods are rich in silicon.
Essential, Yes
It’s unclear if silicon is an essential nutrient for all plants. In some laboratory experiments it has increased growth, while under field conditions it might not. Additional silicon has been used to protect plants from some mineral toxicities, such as manganese. However, in excess, it can compete with essential minerals — such as iron, copper, zinc and manganese — for uptake by plants.
In animals, a clear dietary requirement has only been established in rats and chicks (see sidebar). This is due in large part to the difficulty in creating truly deficient diets.
Deficiency in those species causes bone deformities and poor growth. Inside the body, silicon is found in highest concentrations in the lining of the aorta (which in health is smooth and slippery), in joint cartilage and to lesser extent in other connective tissues, hair and bone.
The 2007 NRC Nutritional Requirements of Horses addressed the issue of silicon as an essential element for horses and concluded that it likely is required in small amounts but that finding a silicon-deficient diet would be difficult.
Deficient, No
If we accept that silicon is likely an essential nutrient, the need for supplementation depends entirely on whether or not the diet is deficient.
As our chart on page 8 of silicon levels in common feeds shows, horses normally take in considerable amounts, 100 to 1000 times higher than in fruits and vegetables.
Silicon tightly bound to poorly fermented fiber fractions (cellulose) or to lignan (higher in mature cuttings of hay) may be liberated to some extent on contact with stomach acid, but the most highly available silicon will come from young pasture grasses, early cutting hays and feeds high in soluble fiber such as beet pulp.
Equine Silicon Study 1
The most frequently cited study that started the silicon supplementation craze (presented at the 13th Equine Nutrition and Physiology Society Symposium in 1993) was performed on Quarter Horses. Researchers divided 53 horses into four groups at age of 6 months and began diets supplemented with either 0, 0.66, 1.32 or 2% sodium zeolite (a silicon source).
When the test horses reached 18 months of age, they began training using a typical Quarter Horse regimen of nine weeks of breaking, then eight weeks of conditioning, during which they worked every other day, gradually increasing distances and speeds. They found horses supplemented with all levels of zeolite trained and/or raced further before developing an injury. They found that the blood level of silicon was directly correlated with how far the horses traveled before injuries occurred. So what’s going on’
For one thing, these horses were on a diet of 70% concentrate (4.56 kg = 10 pounds/day) and only 30% hay (2.45 kg = 10 pounds). The calcium and phosphorus levels in the concentrate were given, but not the hay. Calcium and phosphorus from the concentrate only didn’t meet the requirements for 18-month-old Quarter Horse in work, assuming an adult bodyweight of at least 500 kg (1,100 lbs.). If average analysis figures for the Bermudagrass hay are used to estimate calcium and phosphorus from that portion of the diet, the horses still came up short for calcium, using the 2007 NRC recommendations. The study also does not specify what the base was in the 1.12 kg of ”supplement” (containing the zeolite). Silicon is known to increase the retention/decrease urinary excretion of calcium. It may be that the zeolite supplementation in this likely borderline, if not calcium-deficient diet was actually helping to compensate for the inadequate calcium.
Equine Silicon Study 2
A study published in 2001 in the Journal of Animal Science looked at the effect of supplementing lactating mares with silicon. They measured silicon levels in the blood and milk of mares, silicon in blood of foals and biochemical markers of bone and connective tissue metabolism in mares and foals. A control group got the same diet but no supplemental silicon.
The control diet contained 10.79 grams of silicon. Supplemented mares received an addition 44.29 grams, from sodium zeolite. Total calcium and phosphorus intakes would have been adequate for early lactation in a 400 kg mare (the horses used in this study were Arabians).
However, the calcium:phosphorus ratio was lower than ideal, with excess dietary phosphorus. As above, blood levels of silicon increased in the mares. Their milk level also increased, leading to higher blood silicon levels in the foals of supplemented mares.
However, there were no differences in the markers of bone metabolism in the foals of supplemented mares. There was a slight, but not statistically significant, difference in bone breakdown noted in the supplemented mares, which again may reflect less calcium loss in the urine of the supplemented mares.
Equine Silicon Study 3
A 2007 study published in Equine and Comparative Exercise Physiology examined the effect of supplemental silicon on osteochondrosis lesions in two-year-old Standardbreds. All horses were sound but had radiographic evidence of osteochondrosis. Osteochondrosis is a bone disease and also involves glycosaminoglycans, both areas where silicon is believed to be actively involved.
A control group received no supplementation. Treatment group received 200 grams/day of zeolite for four months. There was no change in the lesions in the supplemented group.
Equine Silicon Study 4
A 2001 study in Journal of Equine Veterinary Science fed 10 yearlings 2% of their total diet as sodium zeolite, while another 10 were a control group. After 45 days, levels of one marker of bone breakdown were lower in the supplemented group, but other markers of bone metabolism were unchanged. Whether this actually means better bone quality remains unanswered.
Botto m Line
Silicon’s importance in bone and connective-tissue health is real. However, we’re not sure you should supplement it.
Silicon from food sources is bioavailable. If your horse is getting generous amounts of his natural source of silicon, there’s little chance that supplementing silicon will help.
High-performance horses receiving diets low in hay and high in concentrates, with borderline calcium levels, may benefit but much more work needs to be done to confirm this. For now, we’re not going to pay extra for supplements with silicon.
Article by Eleanor Kellon, VMD, our Veterinary Editor. She graduated from the University of Pennsylvania Veterinary School magna cum laude and has extensive experience with high-performance horses. With her husband, she breeds, races and trains Standardbred harness horses in Pennsylvania. Eleanor has written countless articles and several books, including ”The Older Horse” and ”Horse Journal Guide to Equine Supplements and Nutraceuticals.”
