Christine King BVSc, MANZCVS (equine), MVetClinStud
Don't guess; test!
I'm a lover of words, so great phrases tend to stick in my brain. One such phrase is the name of a trace mineral supplement for horses, "Best Guess." That's a great name for a product and, generally speaking, a sound concept. But as it turns out, it's also a cautionary tale...
I finally got around to taking a sample of the typical pasture grass in my practice area, in Park Ridge, QLD. It's primarily couch (Cynodon dactylon), growing in the thin, grey, sandy topsoil, over a deep bed of grainy yellow clay, that's typical of this area. Here are two photos from late September, 2020, the one on the right taken down at grazing level:
I sent the sample to Southern Cross University Environmental Analysis Laboratory and requested their plant mineral package (PA-PACK-001) plus selenium (SS-SING-132). I decided against adding iodine, owing to the expense, but I wish I had, just for completeness.
The results surprised me...
I expected this poor soil to grow grass that is low in pretty much all of the minerals needed by horses. However, here is what the results showed, in relation to the daily mainenance needs of adult horses:
A horse would need to graze for about 34 hours a day to meet his or her daily copper needs if this grass was the only source of minerals. (That's not a typo; a horse would need to graze steadily for about 34 hours in order to meet the daily copper requirement from this grass.)
Not only is this grass grossly deficient in copper, but there is enough manganese and zinc, and potentially enough iron as well, to block the absorption of what little copper there is in this grass.
Feeding a supplement that contains iron, manganese, and/or zinc could make things worse, unless it contains a superabundance of copper to not only meet the shortfall but also make up for the grossly unbalanced ratio of copper to zinc and other trace minerals capable of blocking its absorption.
What a horse grazing such pasture would need in the way of trace minerals is simply a good supply of copper and selenium. Supplemental iodine is also a good idea.*
Providing a salt block is wise, too. (Plain salt is sodium chloride.) It's generally best to allow horses and other grazing animals to meet their sodium needs by providing plain salt free-choice.
As for the marginal amounts of phosphorus and sulphur in this grass, feeding a good quality grass hay grown somewhere else would meet the shortfall in most adult horses. (Hay doesn't grow well in this area anyway, so almost all hays sold here come from somewhere else.)
* UPDATE, July 2021: Since writing this article, I have tested a few other pastures in this region (southeast QLD, northeast NSW) for their mineral content. The trends are similar, particularly with regard to copper deficiency. But now I know for sure that pastures in this region are also marginal or deficient in iodine.
So, the moral of the story is this: Don't guess; test!