Christine King BVSc, MANZCVS (equine), MVetClinStud
Trace Mineral Content
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.
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 soil mineral profile in southeast Queensland 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.
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.
The recommended intake of copper for the horse we're discussing is 90 milligrams 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. But adding to this problem, a dietary excess of zinc, iron, manganese, molybdenum, phosphorus, potassium, or sulphur can interfere with copper absorption, turning what may be only a marginal insufficiency into a serious deficiency.
Trace mineral interactions are extremely complex and not always predictable. For this reason, it's important to...
Don't guess; know! Even your best guess may be way off and you could make things worse.
The recommended intake of zinc for this horse is 360 milligrams 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. Even when the zinc content of a forage is adequate, absorption of zinc by the horse may be blocked by an excess of calcium, chromium, cobalt, copper, iron, manganese, phosphorus, or selenium.
Zinc-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 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.
The recommended intake of iron for the horse we're discussing is 360 milligrams per day. 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.
The recommended intake of manganese for this horse is 360 milligrams per day. 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.
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. It's a fairly expensive add-on, but it's well worth the cost to know for sure whether the horse's diet needs supplemental selenium, and if so, how much.
According to the DPI, southeast Queensland is one of the selenium-deficient regions in Australia. So, it's probably safe to assume that forages grown in this area will be marginal or low in selenium. Forages grown elsewhere, though, are an unknown quantity without testing. Not all parts of Australia are selenium deficient, and some of the drier areas have selenium in excess.
The recommended intake of selenium for the horse we're discussing is 0.9 milligram per day or 900 micrograms per day. Let's round that up to 1 mg/ day or 1,000 mcg/day. (Some supplement manufacturers list the selenium content of their products in micrograms.) 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 mcg/kg of hay (as-sampled).
Don't be too heavy-handed with selenium supplementation; enough really is enough.
The recommended intake of cobalt for this horse is 0.45 milligram per day or 450 micrograms 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 mcg/kg of hay (as-sampled).
The recommended intake of iodine for this horse is 3.2 milligrams 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 mcg/kg of hay (as-sampled).
Forages grown in coastal regions may contain adequate iodine, but forages grown in inland regions may not. As with selenium, don't be too heavy-handed with iodine supplementation.
Free-choice trace mineral supplementation
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.
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. My working hypothesis is that horses will selectively consume specific minerals according to need, and will otherwise ignore them if offered. (I expect that some horses will try all available options out of curiosity, but after the novelty has worn off they will continue to consume specific minerals only as needed.) At least, that's what I'm wanting to examine.
I currently have:
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.