animal health consulting

The trace minerals, also called microminerals, include copper, zinc, iron, manganese, selenium, cobalt, and iodine. For horses, these minerals are required in amounts measured in milligrams (mg) per day, and for a few in micrograms (mcg) per day.

A microgram, abbreviated µg or mcg (not to be confused with mg), is one-thousandth of a milligram (mg), and one-millionth of a gram (g). So, 1 g = 1,000 mg = 1,000,000 µg.

The trace mineral content of pasture and hay depends primarily on the mineral content of the soil in which the plants grow. As with the major minerals, the trace mineral profile in Australian soils is very inconsistent, and trace mineral deficiencies are common.

For this reason, it's a good idea to have your pasture tested at least once for horses whose daily forage requirement is mostly met by grazing. It's also a good idea to have a mineral analysis done on bulk purchases of hay and chaff.

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As with the major minerals, I'll limit the discussion to the maintenance needs of the average-size adult horse weighing approximately 450 kg. Growing horses, pregnant or lactating mares, and horses in anything more than light work need more nutrients, but forages should still meet the majority of their needs, including the minerals.

I'll also assume a daily roughage intake of 2% bodyweight per day, which is optimal for most adult horses. (The range is between 1.5% and 3% bodyweight.) For the average-size adult horse, that's about 9 kg of hay per day if the horse has little or no pasture access, or there's little or no grass in the paddock.

The following figures for forage mineral content also assume an unsupplemented diet. That's a rarity these days, but unsupplemented forage analysis is an important starting point.

Horses do best when good quality forages form the basis of their diet, from weaning onward. When enough good quality forage (pasture and/or hay in any form) is fed, there's less need for concentrates and supplements and therefore less potential for mineral excesses, deficiencies, and problematic imbalances that we've created. Forage mineral analysis shows us what, if anything, we need to supplement.

As I discussed in detail in my article, Don't guess; test!, the assumptions I made about my own pasture in southeast Queensland were way off for all minerals except copper.

Regarding conversions:

There is no need to convert from ppm (parts per million) to mg/kg, because the two units are equivalent.

A milligram is one-thousandth of a gram, which is one-thousandth of a kilogram, so a milligram is one-millionth of a kilogram. In short, 1 ppm = 1 mg/kg.

To convert DM to as-sampled or as-fed, first convert % DM to its decimal value (divide by 100) and then multiply the nutrient value by the decimal DM value.

For example, if the hay is 90% dry matter, its decimal DM value is 0.90. Multiply 0.9 by the particular nutrient value (reported either as ppm or mg/kg DM); the result is the amount of that nutrient in the hay as it was sampled and as it is fed (which includes its water content of 10%).  

If the pasture sample is 60% dry matter, its decimal DM value is 0.60. Multiply 0.6 by the particular nutrient value (reported either as ppm or mg/kg DM); the result is the amount of that nutrient in the pasture as it was sampled (fresh, alive, growing) and as it is being grazed by the horse (which includes its water content of 40%).

Copper (Cu)

The recommended intake of copper for the horse we're discussing is 90 mg per day.

To at least meet this requirement at the daily roughage intake we're assuming, the hay would need to have a copper content of at least 10 ppm (as-sampled), or at least 10 mg/kg of hay (as-sampled).

Copper deficiency is common in forages. Adding to this problem, a dietary excess of several other minerals, including zinc, iron, manganese, molybdenum, phosphorus, potassium, and sulphur can interfere with copper absorption, turning what may be only a marginal insufficiency into a serious deficiency.

I ran into this problem on my own farm in North Carolina, in a herd of miniature goats I was managing. We lost one lovely young buck and nearly lost another before I finally figured it out. I've since seen similar problems in southeast Queensland and northeast New South Wales. I published a slide show detailing the complex of trace mineral deficiencies and excesses involved (some inadvertently created or worsened by me!).

Trace mineral interactions are extremely complex and not always predictable. For this reason, it's important to...

* forage test — know the mineral profile of the principal forages in your horse's diet

* avoid oversupplementing with trace minerals

Don't guess; know!  Even your best guess may be way off and you could make things worse.

While we're at it, the Equi-Tech package available through Feed Central in Toowoomba reports on the molybdenum (Mo) content of forages. There is no recommended daily intake for molybdenum in horses. However, it is included in the analysis because very high concentrations (above 20 ppm) may interfere with the absorption of copper.

Zinc (Zn)

The recommended intake of zinc for this horse is 360 mg per day.

To at least meet this requirement, the hay would need to have a zinc content of at least 40 ppm (as-sampled), or at least 40 mg/kg of hay (as-sampled).

Zinc and copper deficiencies are a common duo in forages, but they don't always go hand-in-hand. So, don't guess; test!

Even when the zinc content of a forage is adequate on paper, absorption of zinc by the horse may be blocked by an excess of several different minerals, including calcium, chromium, cobalt, copper, iron, manganese, phosphorus, or selenium.

Test! Know what's in your forages, and where problems may be lurking that you could be making worse by indiscriminate supplementation.

Zinc to Copper ratio (Zn:Cu)

As with calcium and phosphorus, the ratio of zinc to copper (Zn:Cu) is also important. Both can interfere with the absorption of the other when one is in excess, even when the other is present in a seemingly adequate amount.

The ideal Zn:Cu in adult horses is 4:1, which may be written simply as 4. While both minerals must be present in adequate amounts, there should be about 4 times as much zinc as copper in the total diet. And not a whole lot more than that.

For example, the Zn:Cu in my pasture in southeast Queensland was 30! It contained 30 times as much zinc as copper. A supplement containing both zinc and copper (which is pretty much every commercial supplement and concentrate sold for horses) would not do much to address this deficiency of copper, unless it contained a superabundance of copper and not much zinc.

Iron (Fe)

The recommended intake of iron for the horse we're discussing is 360 mg per day.

The same as zinc, and likewise 4 times that of copper.

To at least meet this requirement, the hay would need to have an iron content of at least 40 ppm  (as-sampled), or at least 40 mg/kg of hay (as-sampled).

I don't recall ever seeing a good quality hay, grass or legume, that is deficient in iron. It's certainly possible, but in my experience an excess of iron is common and can be high enough to interfere with the absorption of many other minerals, including calcium, chromium, cobalt, copper, manganese, phosphorus, and zinc. An excess of iron can also interfere with the absorption and utilisation of vitamins B12, D, and E.

I start to worry about interference when the iron content is at least 5 times higher than needed (i.e., above 200 ppm). If the hay is otherwise suitable, I simply make sure that the horse is receiving a bit more of the minerals iron can interfere with, particularly copper and zinc. How much extra depends on the mineral profile of the forage.

Manganese (Mn)

Not to be confused with magnesium (Mg), the recommended intake of manganese (Mn) for this horse is 360 mg per day.

The same as iron and zinc, and likewise 4 times that of copper.

To at least meet this requirement, the hay would need to have a manganese content of at least 40 ppm (as-sampled), or at least 40 mg/kg of hay (as-sampled).

As with iron, I can't remember a single instance in which manganese was low in a forage, but I can recall plenty of times when manganese was in excess. An excess of manganese can interfere with the absorption of calcium, chromium, cobalt, copper, iron, magnesium, and phosphorus. An excess of manganese can also interfere with the absorption and utilisation of vitamins B1, B12, D, and E.

Excesses of iron and manganese, and even of zinc, are quite common in forages. Maddeningly, many mineral supplements and concentrates (processed feeds) for horses are high, and some are really high, in these minerals, potentially creating complex trace mineral imbalances which result in deficiencies and excesses that can be a challenge to unravel without a spreadsheet and forage mineral analyses.

Selenium (Se)

Unfortunately, selenium is not included in any of the routine forage testing packages I've been able to find, but it can be ordered separately from most labs offering forage mineral analysis. Through SCU-EAL, for example, it's an inexpensive test to order separately or add to a testing package.

For pastures, hay fields, and bulk orders of hay or chaff, it's well worth the trouble of testing for selenium content, to know for sure whether the horse's diet needs supplemental selenium, and if so, how much (i.e., how serious is the deficiency).

Selenium is another mineral for which it's not good to guess. As with all of the trace minerals, too much can be as bad as too little, and there are parts of Australia that contain an excess of selenium in the soil, and even some plants that accumulate selenium to toxic levels.

For the most part, selenium-deficient regions in Australia are found around the south and east coast, although the region may extend a good way inland, to include important pastoral areas and hay-growing regions. Pastures and hays grown in these areas will likely be deficient in selenium, in relation to the needs of horses.

The recommended intake of selenium for the horse we're discussing is 0.9 mg per day or 900 µg per day. Let's round that up to 1 mg/day or 1,000 µg/day.

Some supplement manufacturers list the selenium content of their products in micrograms (µg or mcg), so read labels with care.

To at least meet this requirement, the hay would need to have a selenium content of at least 0.1 ppm (as-sampled) or 0.1 mg/kg of hay (as-sampled). In micrograms, that's a selenium content of at least 100 ppb or 100 µg/kg of hay (as-sampled).

When quantities in ppm drop below 0.10, it's time to multiply by 1,000 and convert the units to ppb (parts per billion), which is the same as saying micrograms per kilogram (µg/kg).

Don't be too heavy-handed with selenium supplementation; enough really is enough.

Cobalt (Co)

The recommended intake of cobalt for this horse is 0.45 mg per day or 450 µg per day.

To at least meet this requirement, the hay would need to have a cobalt content of at least 50 ppb (as-sampled) or 50 µg/kg of hay (as-sampled).

I don't see cobalt deficiency very often in horses.

Iodine (I)

The recommended intake of iodine for this horse is 3.2 mg per day.

To at least meet this requirement, the hay would need to have an iodine content of at least 0.36 ppm (as-sampled) or 0.36 mg/kg of hay (as-sampled). In micrograms, that's an iodine content of at least 360 ppb or 360 µg/kg of hay (as-sampled).

Forages grown in coastal regions may contain adequate iodine, but it really depends on the geology of the area and thus the mineral profile of the topsoils. I have seen forages grown in coastal regions of southeast QLD and northeast NSW that were marginal or low in iodine when tested.

As with selenium, don't be too heavy-handed with iodine supplementation.

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Free-choice trace mineral supplements

Trace mineral-salt blocks have been used for many years by livestock producers and horse owners. However, while they are designed to offer the minerals free-choice, they are not the best way of ensuring adequate trace mineral intake in horses (or any livestock species, for that matter). They're better than nothing, but we can do better.

For one thing, these blocks are seldom formulated with regional mineral deficiencies in mind, let alone the individual animal's particular needs. For another, most contain too much salt, which limits the intake of minerals in the block.

I have begun experimenting with single trace minerals to be offered free-choice as salt-free, loose minerals in separate pans. The limited feedback to date indicates that animals selectively consume specific minerals according to need, and will otherwise ignore them.

I currently have:

* copper (as a copper-lysine complex)

* zinc (as a zinc-methionine complex)

* selenium (as selenium-yeast, which is at least 70% selenium-methionine)

* kelp as a natural source of iodine

... and an iron-free combination of copper, zinc, manganese, cobalt, and chromium (as it was cost-prohibitive to buy the Mn, Co, and Cr separately).

Please contact me if you want to try this approach. I'd like to collect some basic data and maybe publish the findings as a pilot study.

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

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