animal health consulting

Don't guess; test!

why forage test your pasture

Christine King  BVSc, MANZCVS (equine), MVetClinStud

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...

In September 2020, I finally got around to taking a sample of the typical pasture grass in my (former) practice area, in Park Ridge, Queensland. 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 the area. Here are two photos from late September, 2020, the one on the right taken down at grazing level:

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.)

In case you're into this sort of thing, here are the sums:

* the daily copper requirement for maintenance in the average (450-kg) adult horse is 90 mg

* the average adult horse on full-time pasture turnout grazes for about 16 hours/day, and eats between 0.45 and 0.9 kg of dry matter (DM) per hour, depending on pasture quality and availability

* fresh grass at the time of year this sample was collected is only about 40% dry matter (or about 60% water)

* with short grass such as this, the hourly intake would be at the low end (0.45 kg DM/hr), so a horse would need to graze for about 34 hours to consume 90 mg of copper 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.

Amounts of each mineral matter, but so do the ratios or relationships between key minerals. For example, the optimal ratio of zinc to copper is 4 to 1, meaning that there should be about 4 times as much zinc as copper in the horse's diet. In this grass, the ratio is 30 to 1! There is 30 times as much zinc as there is copper 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 copper 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.

My personal preference is one of those beautiful, tasty pink salts such as Himalayan pink salt or Redmond salt. I'm also rather partial to sea salt. But a plain white salt block is fine, too.

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 that area anyway, so almost all hays sold there have come from somewhere else.)

* UPDATE, July 2021: Since writing this article, I tested a few other pastures in the 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 the region are also marginal or deficient in iodine.

So, the moral of the story is this: Don't guess; test!

© Christine M. King, 2020, 2022. All rights reserved.


I sent the sample to Southern Cross University Environmental Analysis Laboratory and requested their plant mineral package plus selenium. Owing to the expense, I decided against requesting iodine as well, but I wish I had. (More on iodine later.)

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, colour-coded in relation to the daily maintenance needs of adult horses: