Ask The Equine Nutritionist

I was wondering if you could clarify something things about selenium and supplementing for selenium that has confused me?

First of all, I'd like to say that the metric system is somewhat confusing to me. I'm quite sure that some of my confusion here is because of my unfamiliarity with the metric system and how the grams, milligrams and kilograms compare to each other.

Okay lets start here. The metric system is very logical, everything is in increments of 10 or 100
or 1000. 1000 milligrams = 1 gram 1000 grams = 1 Kilogram.

Thus there are 1000 X 1000 milligrams in 1 kilogram. Or 1 million milligrams
(mg) in 1 kilogram (kg) thus mg/kg = parts per million. (Remember this, you'll need it at the end)

The conversion back to Imperial measurements is a good bit harder.

1 oz = 28.35g
1 kilo = 2.2lbs
1 lb = 453.6g

Its easiest if you stay in the metric system for the whole calculation and just convert at the end if you have to.

2nd Question

I saw this on the internet,
http://www.ruralheritage.com/vet_clinic/selenium.htm .
H.F. Hintz is a Professor of Animal Science at Cornell University in
Ithaca, New York, where he teaches equine nutrition and conducts nutrition
research. In his article on selenium, he recomend supplementing 1 to 3
mg of selenium per day if your horse is deficient. 50mg per day
can be toxic.

I see that you recommended to feed 10 to 15 g per day of a selenium salt, and that 454g or above is considered toxic. Is selenium salt something different than pure selenium, for example is it a salt carrier that has selenium added? If so, is this number (454g) for the selenium salt, or pure selenium?

2nd Answer

He is talking about the amount of pure selenium. 1-3milligrams per day Remember a milligram is 1 thousandth of a gram. You would never actually feed pure selenium; it always comes as a salt that is selenium bound to something else, you need to feed enough of the salt to get the equivalent of 1 -3 mg of selenium. Therefore, I gave amounts of sodium selenite, not pure selenium, since that is what is actually used for supplementing in horses.

I still would say that 1-3 mg (pure) is a bit low, my recommendations would be for a slightly higher amount, 5-10mg.

Selenium salt contains 0.06% selenium. A percent is parts per hundred.
So 15g of sodium selenite = 15/100 X 0.06 = .009g selenium. 1 g = 1000mg so ..009 x 1000 = 9mg of selenium.

10g of sodium selenite is 6mg of (pure) selenium.

So I recommended a slightly higher amount than Dr Hintz, but no where near the 50mg that he considers to be toxic.

Although my number of a toxic level is a bit higher, there is a lot of variation in what is considered the toxic level, and I was maybe a bit generous. 454g of pure selenium would DEFINITELY be toxic!

454g /100 X .06 = 0.27g = 270mg

50mg(pure selenium) = 90.8g sodium selenite.

You could well say that 90g of selenium salt is close to the level of pure selenium that Dr Hintz feels is toxic.

90/100 X 0.06 = 0.054g
0.054 X 1000 = 54 mg

Very close, indeed.

This is why it is not a good idea to feed too many different supplements all containing selenium. Even in very deficient parts of the country it is possible to give too much selenium.

3rd Question

Finally, you mention that an overall intake of 5mg/kg (ppm) is considered toxic. Is this amount comparable to Dr. Heinz's 50mg?

3rd Answer

The figure ppm or mg/kg is the amount in the total feed intake.
Eg if a 1000 lb horse eats 22lb per day that is = to 10kg (2.2lb =1 kg). 5ppm = 5mg/kg. if the horse eats 10kg of a feed containing 5ppm then he would take in 50mg which Dr Hintz feels is the toxic level. So, yes, that is comparable.

This is the most important measure (the overall intake) as it takes into account what the feed contains as well what you are supplementing.
One thing to remember is that different parts of the country may well have different selenium levels in the soil. So, different amounts get recommended in different parts of the country.

Basically, I and Dr. Hintz really are not that different, I had slightly higher recommendations than Dr Hintz. And I was less cautious about the toxic level. Its very confusing when they are presented in different formats, and if you are not sure of the format and how to convert between them then it is even more confusing.

And rest assured even after all these years I still have to stop and think about it, and do the math just to make sure I don't have it wrong! Even after 12 years of grad school and many years in the industry it is confusing so,

I realize that it must be overwhelmingly hard for lay people to follow.

Dr. Melyni Worth Ph.D. - 2004 (c)

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