animal health consulting

The major minerals, also called macrominerals, include calcium, phosphorus, magnesium, sodium, chloride, potassium, and sulphur. For horses, these minerals are required in amounts measured in grams per day.

The trace minerals, or microminerals, are discussed on the next page. For horses, they are required in amounts measured in milligrams per day. One milligram is one-thousandth of a gram, hence the term 'trace' minerals for those ones.

The content of these major minerals in pasture and hay depends on the type of plant (grass or legume) and the mineral content of the soil in which the plants grow. Good quality grass hays and pastures grown in fertile soil should contain adequate amounts and ratios of the major minerals for maintenance in adult horses, except for sodium, which is easily supplied in the form of a salt lick.

Good quality legume hays such as lucerne do not; their calcium-phosphorus ratio is way off. No matter how good the hay/chaff is, legumes are not suitable as the primary forage for horses of any age. If they simply must be used as the primary or sole forage for short periods (e.g., the only forage available in a drought), then supplemental phosphorus is needed, particularly in growing horses, pregnant or lactating mares, and working horses — in other words, in most horses.

The soil mineral profile in Australia is very inconsistent, and 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. Same goes for hay fields.

When it comes to mineral analysis (macro- and microminerals), testing your pasture or hay field once is usually enough, as the mineral profile won't change with the season anywhere near as much as dry matter, crude protein, the various fibre components, and the nonstructural carbohydrates do. If  you're making substantial amendments to your soil, then test your pasture or hay field again in several months; otherwise, once is usually enough.

If you're having to buy hay or chaff, it's not economical to have each load of hay or chaff tested when you're buying only a few bales or bags at a time. However, if you're buying hay or chaff in bulk, it is worth testing a sample before you do anything more than put a deposit down on it, particularly if you have any horses needing a low-NSC diet.

When the NSC content of a forage is high, no amount of soaking or 'diluting' that forage with low-NSC feeds can make the hay or pasture safe for at-risk horses — and there's no way to know if the NSC content of a forage is high unless you test it.

Another reason it's wise to forage-test bulk hay or chaff purchases is to make sure that the hay/chaff will meet the horse's mineral needs — and if not, then which minerals you may need to supplement. It's easy to over-supplement when you're guessing, and your best guess may be way off when the hay has come from interstate or a region with inconsistent soil mineral profiles.

Some of the hay growers who list their hays on Feed Central provide forage test results. These reports don't often include minerals, but it is encouraging that hay growers are starting to realise the importance of testing their hays and providing the results to buyers. I hope this trend continues and becomes standard practice. That will happen faster if we start requesting forage test results from hay suppliers.

Problems with a forage's mineral profile are generally manageable with appropriate supplementation, so mineral excesses, deficiencies, or imbalances usually aren't a deal-breaker as long as the forage is otherwise suitable (DM, CP, fibre, NSC). However, gross mineral excesses, deficiencies, or imbalances indicate a forage that may not be the best choice for horses. Cereal hays, for example, typically are not grown with horses in mind.

In addition to the waste of money, over-supplementing with minerals can cause problems. In excess, some minerals interfere with the absorption of others, creating a deficiency in the horse where one may not exist in the forage.

Calcium and phosphorus are one example. An excess of phosphorus can interfere with the absorption of calcium. An excess of calcium is better tolerated — as long as there is ample phosphorus in the diet.

***

To make this complicated topic as simple as possible, 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 bulk 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 if there's little or no grass in the paddock.

Pasture is always a 'wild card'. Although forage analysis can tell us precisely how much of each nutrient is in a sample of fresh pasture, and we can know the precise water or dry matter content, we have no way of knowing how much of the pasture a horse is eating each day.

The average adult horse on full-time pasture turnout grazes for about 16 hours per day and eats between 0.45 and 0.9 kg of dry matter (DM) per hour, depending on pasture quality and availability. However, the actual amount is so variable horse-to-horse and even hour-to-hour, and so many factors are at play, that this figure is rather meaningless for anything but the roughest of calculations.

So, let's get into it, with these assumptions and limitations in mind.

Calcium (Ca)

The recommended intake of calcium for the horse we're discussing is 18 grams per day.

To at least meet this requirement at the daily roughage intake we're discussing, the hay would need to have a calcium content of at least 0.20% (as-sampled), or at least 2 g/kg of hay (as-sampled) if you prefer those units.

To convert % to g/kg, multiply the value by 10 (i.e., move the decimal point one place to the right).

For example, 0.2% = 2 grams per kg. That's because % = grams per 100 grams, and we want to know how grams are in 1,000 grams (1 kg) of this feed, so multiply the % value by 10.

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 % or g/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 % or g/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%).

Phosphorus (P)

The recommended intake of phosphorus for this horse is 12.6 grams per day.

To at least meet this requirement at this daily roughage intake, the hay would need to have a phosphorus content of at least 0.14% (as-sampled), or at least 1.4 g/kg of hay (as-sampled).

Calcium to phosphorus ratio (Ca:P)

The actual amounts of calcium and phosphorus are important, but so too is the ratio of calcium to phosphorus, abbreviated Ca:P. That's because an excess of one can interfere with absorption of the other, potentially making a marginal deficiency worse.

The recommended daily intakes of calcium (18 grams) and phosphorus (12.6 grams) for maintenance in the horse we're discussing create a ratio of 1.4:1. It's a simple calculation, calcium ÷ phosphorus (18 ÷ 12.6 = 1.4). The result is usually written as 1.4:1, as in how much calcium per 1 unit of phosphorus, but it can also be written simply as 1.4.

Adult horses who are not pregnant or lactating can tolerate Ca:P figures as low as 1.1 (relatively low calcium or relatively high phosphorus) and well over 3 (relatively high calcium or relatively low phosphorus), although they are more tolerant of high Ca:P than of low Ca:P values.

Some grass hays have Ca:P values of 1 or even lower (i.e., more phosphorus than calcium), so it's always worth checking the Ca:P, even though the actual amounts of calcium and phosphorus in the hay may be adequate for the horse's needs.

In general, it's best to maintain a Ca:P between 1.4 and 2 — i.e., 1.4 to 2 times as much calcium as phosphorus. Values way out of this range require either calcium or phosphorus supplementation.

Note: these figures assume that the grasses contain little or no oxalate, a plant molecule that binds dietary calcium and makes it unavailable for absorption by the horse. Some of the 'tropical' or 'warm-season' grasses such as kikuyu, setaria, and buffel grass contain enough oxalate to cause calcium deficiency even in adult horses. Horses grazing such pastures or eating high-oxalate hay need extra calcium beyond what the forage analysis may indicate.

Magnesium (Mg)

The recommended intake of magnesium for the horse we're discussing is 6.8 grams per day.

To at least meet this requirement at the roughage intake we're assuming, the hay would need to have a magnesium content of at least 0.08% (as-sampled), or at least 0.8 g/kg of hay (as-sampled).

magnesium supplementation

Magnesium supplementation has been promoted for overweight horses with equine metabolic syndrome, and for various other conditions, including anxiety, 'tying up', and headshaking. But in all the years I've been testing hay and pasture for my patients, I have never encountered a good quality forage (grass or legume, hay or pasture) that was even close to being deficient in magnesium.

For example, Equi-Analytical reports that grass hays contain an average of 1.9 g/kg Mg as-sampled, and it ranges from 1.2 to 2.7 g/kg of hay. Legume hays tend to be a little higher; the average is 3.o g/kg Mg as sampled, ranging from 2.2 to 3.7 g/kg of hay.

This lab routinely tests hay and pasture samples from all over the US, a country as large and diverse in its soil types as Australia; in fact, they routinely test hays from all over the world, including Australia, so their database includes 'magnesium-deficient' regions. These averages and ranges are based on over 92,000 samples for grass hay and 213,000 samples for legume hay.

Perhaps a more relevant value is a good quality rhodesgrass hay I tested for a client with a laminitic pony in southeast Queensland a couple of years ago. The hay contained 0.2% Mg on a DM basis, which converted to 1.85 g/kg as-fed. (This hay was 92.3% DM.) When fed to our theoretical horse at a rate of 2% bwt/day, this hay would provide 16.7. grams of Mg per day, which is 245% of the horse's maintenance needs. And that's a pretty typical grass hay.

That may explain why I have yet to encounter a horse who has benefitted from the simple addition of magnesium, with no other changes to their diet or lifestyle. Typically, grasses and legumes (as pasture or hay) contain more than enough magnesium to meet the maintenance needs of an adult horse.

Indiscriminate magnesium supplementation can mess with the ratio of calcium, phosphorus, and magnesium (Ca:P:Mg), which for maintenance in adult horses is 2.7:1.9:1. Rounding up, that's approximately 3:2:1, or 3 times as much calcium as magnesium, and 2 times as much phosphorus as magnesium.

The rhodesgrass hay I mentioned earlier contained 0.3% Ca, 0.2% P, and 0.2% Mg, for a Ca:P:Mg of 3:2:2. Adding more Mg to this pony's diet would have been a mistake!

As with the Ca:P, messing with the ratio of these macrominerals by oversupplementing one can interfere with the absorption and utilisation of the others and create excesses or deficiencies that might not otherwise exist.

Small amounts of magnesium (and calcium) are lost in sweat, but a diet based on good quality forages can ably address such losses, except when the horse is sweating heavily for several hours with no opportunity to replenish its mineral stores by eating.

Sodium (Na)

I generally don't look too closely at the sodium content of hays, because it's always too low even for maintenance needs — unless the hay grower has tried to dry the hay by salting it. Instead, I simply advise providing the horse with free-choice salt.

Just as a point of reference, the recommended intake of sodium for the horse we're discussing is 9 grams per day.

Chloride (Cl)

Chloride intake is seldom an issue in horses on good quality forages. The recommended intake for the horse we're discussing is 36 grams per day.

To at least meet this requirement at the daily roughage intake we're assuming, the hay would need to have a chloride content of at least 0.40% (as-sampled), or at least 4 g/kg of hay (as-sampled).

According to Equi-Analytical, the average chloride content of grass hay is 0.55% (as-sampled), or 5.5 g/kg of hay (as-sampled). It ranges from 0.15% to 0.97%. At the low end, a diet based on this forage would be deficient in chloride, but at the high end it would provide almost 2.5 times as much chloride as needed.

Chloride is one of the electrolytes lost in sweat, but as salt is sodium-chloride (1 molecule each of Na and Cl), horses generally get more chloride than they need for maintenance while meeting their sodium needs at the salt lick.

Potassium (K)

I also don't bother looking at the potassium content of hays, because they've always been more than adequate.

The recommended intake of potassium in the horse we're discussing is 22.5 grams per day.

According to Equi-Analytical's database, grass hays have an average potassium content of 1.7% (as-sampled), or 17 grams per kg of hay (as-sampled). The range is 11 to 23 grams of potassium per kg of hay. I don't think I need to do the sums to show that horses get more than enough potassium from the forages in their diets.

Even very hard-working horses, losing a lot of potassium in their sweat, could meet their increased need for potassium (47.7 grams per day for this size racehorse or 3-day eventer) from less than 3 kg of the average grass hay.

So, are we overusing electrolyte supplements?  In most horses, probably so. Salt (sodium chloride) is needed, but potassium? Not in most cases.

Sulphur (S)

The recommended intake of sulphur for this horse is 13.5 grams per day.

To at least meet this requirement at the daily roughage intake we're assuming, the hay would need to have a sulphur content of at least 0.15% (as-sampled), or at least 1.5 g/kg of hay (as-sampled).

According to the Equi-Analytical database, the average grass hay has a sulphur content of 1.7 g/kg (as-sampled), and it ranges from 1.0 g/kg to 2.3 g/kg. The average for legume hays is 2.7 g/kg (as-sampled), ranging from 1.7 g/kg to 3.7 g/kg.

The rhodesgrass hay I mentioned earlier contained 0.2% S on a DM basis, which converted to 1.85 g/kg as-fed. When fed to our theoretical horse at a rate of 2% bwt/day, this hay would provide 16.7. grams of S per day, which is 124% of the horse's maintenance needs. And that's a pretty typical grass hay.

So, sulphur deficiency is possible but unlikely with good quality forages.

Next: Trace mineral content

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

back