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If a little is good, is a lot marvelous?

Updated: Jan 1, 2022

Certainly the purveyors of the myriad of feed supplements in the marketplace today seem to think so. Added to already ‘balanced ‘feeds (as determined by the National Research Council - NRC), can additional mineral supplements have any beneficial effect based on science? Is their addition reflected in the Form Book results or are they doing actual damage & cause toxicity to equines ??

Of late I have been increasingly struck by yet MORE variations & new products in the equine supplements market. Innovation you might say? Marketing budgets increased to combat world economic pressures? Perhaps! So when are we helping & when are we hindering by adding more & varied supplements to the equine diet. Who is benefiting …? My own background as a Veterinarian of over 50 years has exposed me to ‘real life’ examples of mineral toxicity in equines specifically. To elaborate the point, consider the addition of Calcium & Iodine & Selenium in excess to equine diets.

They say that Calcium is good for bones…

Support for the addition of increased levels of calcium to the equine diet was proposed in 1978 by the National Research Council (NRC). At this time, the NRC suggested doubling the recommended calcium levels for horses.

The subsequential increase in levels of Osteochondrosis (OCD – failure of cartilage to mature to bone) and Osteopetrosis (too much bone tissue per unit volume of bone resulting in weakness) in the equine population was documented by Krook & Maylin (1989, Racehorses at risk: Over nutrition, drugs, breakdowns , Cornell University Press). Then, as now, my opinion is that some of the recommended supplemental mineral levels as proposed by the NRC were incorrect & based on suspect trials regarding racehorses. In correspondence with Professor Lennart Krook, my suspicions were substantiated. Feel free to review the correspondence.

My own father, Dick McCormick (1894 – 1963) a renowned race horse trainer was against the addition of excess calcium in the equine diet. Having trained the winner of the top two year old race in Ireland, the Phoenix Stakes (referred to as the 1500) for the second time (1959) with the filly GIGI (IRE), Dick was invited to inspect the yearlings at the well-known Dunboyne Stud Farm prior to going to auction. On viewing milk powder amongst the contents of the feed room, he declined to purchase any of them saying “milk powder makes their bones soft”. His opinion, developed on his day to day experience with the legendary H.S.’Atty’ Persse was later substantiated by Krook & Maylin that OCD fractures are ‘pathologic’ rather than traumatic (due to poor race track conditions) & as a consequence such injuries are ‘man made’ & preventable through better nutrition support.

Seaweed, a great source of iodine.

While access to iodine is necessary for optimum thyroid function, excess will cause damage to the gland & is invariably the result of human intervention . Seaweed, the best dietary source of iodine has become increasingly popular in the equine diet. The maximum tolerable dietary concentration of iodine has been estimated to be 5 mg/kg (PPM) of dry matter (NRC,1980), equivalent to 50 mg of iodine/day for a horse consuming 10 kg of dry matter daily. Driscoll et all (1978) noted that goiter (swelling of the thyroid gland) in horses can be caused by either too much or too little dietary iodine. Clinically, this results in obesity, poor coat condition, lethargy, intolerance to cold & excess gas formation.

My own personal experience of excessive iodine exposure in quality thoroughbred broodmares & their off spring was confirmed by hair analysis & had expensive consequences. I recall four broodmares at a particular stud. One had slipped a foal in May, another had foaled normally but the foal lacked thrive & the remaining two foals appeared clinically normal. In August, I reviewed their feed routine, noting feed used & method of administration (three different people participated in feeding). Hair samples were taken from all stock. Results, a week later revealed toxic levels of Iodine in the hair of all four brood mares & three remaining foals. It was discovered that while a feed supplement containing iodine was being added as a supplement to the daily feed, the amount was incorrectly calculated (by volume rather than by weight) resulting in double the recommended daily amount of iodine causing toxicity. Since iodine is concentrated across the placenta & in milk, the developing foetuses & subsequent nursing foals received much higher concentrations of iodine than were present in the mare’s samples.

More recently, while visiting a West of Ireland practice, a client arrived in with a 24 hr foal which had normal clinical signs yet was unable to stand & nurse .On enquiring what the owner had fed the mare he mentioned seaweed. Here was a case of suspected excess Iodine in the pregnant mare’s diet causing dysmaturity in the foal despite the foal being ‘full term’. Additional symptoms of excessive iodine exposure include weakness due to the carpal & tarsal bones not being fully formed. This can be confirmed using radiographs & hair analysis which will establish iodine excess. Cost prevented a full diagnostic analysis in this case.

Selenium, a trace mineral that can be deadly

Selenium is essential for cell function but chronic over exposure can be seen as hair loss (from the mane & tail), cracking of the hooves, lameness, excess salivation & respiratory failure. Excess selenium in equines can be both a man-made issue through excess feed additives or environmental where pasture contains excess selenium due to naturally high levels in the soil (a single field on a farm or a locality ….well documented in Ireland ) or contamination by man-made pollution (from cleaning ditches resulting in the spread of this material on grassland). The toxic results of selenium can be quite quick, dramatic & have disastrous consequences. It is imperative that time is taken to read labels & calculate how much, if any, excess selenium is contributing to your horses diet. Critical too is knowledge of where your hay comes from & if it is tested on a regular basis.

Supplements & performance

In our quest to provide a nutritious diet & support our equines daily performance, it is easy to think that providing more is better. But when you start with an already ‘balanced’ feed, it seems that the addition of supplements can do more harm than good. Excess levels of essential vitamins & minerals while being processed by the body have a direct impact on functioning.

Analysis of hair samples are used to confirm often toxic levels of minerals giving a direct ‘cause & effect’ link to the addition of supplements into an already balanced feed. Impact on performance is clearly documented in the form book. Since 1950, when the US Jockey Club began tracking the size of field & starts per horse (US & Canada) statistics, the numbers have decreased dramatically from 12 ‘starts’ (1950) versus the average of 3 ‘starts’ noted by leading US Trainers in 2020. Simply put, today’s owner needs four racehorses to enjoy what one racehorse was doing in 1950*

Author: Dr. Richard. J. McCormick, M.V.B, Dip.Eq.St, M.R.C.V.S. Licensed Veterinarian (Kentucky, USA, Ireland & United Kingdom). Contact @ or


National Research Council. 1989. Nutrient Requirements of Horses. Washington D.C.: National Academy Press.

Driscoll, J. et al. 1978. Goiter in foals caused by excess iodine. J. Am. Vet., Med. Assoc. 173:858

Web: (2017) Iodine in the Horse: Too much or too little? Available @

Web: Amanda House , DVM ,DACVIM, (2016) Selenium in the Equine Diet


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