Christine King BVSc, MANZCVS (equine), MVetClinStud
Major Mineral Content
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 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.
The soil mineral profile in southeast Queensland 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.
Regardless of where the hay was grown, 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 having a sample tested before you do anything more than put a deposit down on it, particularly if you have any horses needing a low-NSC diet.
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 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 part of Queensland with inconsistent soil mineral profiles.
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.
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.
So, let's get into it, with these assumptions and limitations in mind.
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.
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-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' grasses such as kikuyu and little pasture plants such as oxalis (soursob) 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.
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 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 for my patients, I have never encountered a good quality grass or legume hay or pasture that was even close to being deficient in magnesium. And I have yet to encounter a horse who has benefitted from the simple addition of magnesium. 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. 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.
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 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).
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.
I also don't bother looking at the potassium content of hays, because they've always been more than adequate. For example, 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 1.1% to 2.3%, or 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.
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).