Interpreting hay analysis results
Major Minerals - Calcium, Phosphorus, Magnesium, etc.
Most good quality grass hays in the US contain adequate amounts and ratios of the major minerals (also termed macrominerals) for maintenance in adult horses. Even so, it's a good idea to check each batch of hay to make sure.
The following figures are assuming that hay is the primary source of nutrition for the theoretical 1,000-lb adult horse, and that the hay is being fed at a rate of approximately 2% of the horse's ideal body weight per day (approximately 20 lbs of hay per day).
The recommended daily intake of calcium for maintenance in a 1,000-lb horse is 18 grams. To at least meet this requirement, the hay would need to have a calcium content of at least 0.20% (as sampled), or at least 0.91 g/lb of hay if you prefer those units.
The recommended daily intake of phosphorus for maintenance in a 1,000-lb horse is 12.6 grams. To at least meet this requirement, the hay would need to have a phosphorus content of at least 0.14% (as sampled), or at least 0.64 g/lb of hay if you prefer.
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 an adult horse 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 1.1:1 and well over 3:1, although they are more tolerant of high Ca:P than low Ca:P values. Some grass hays have Ca:P values of 1:1 or even lower (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 1.4:1 and 2:1. The hay in this example has a Ca:P of 1.32:1. However, as both the calcium and phosphorus amounts are more than adequate, and the Ca:P is not far off the mark, I would not supplement this hay with calcium to bump up the Ca:P of the horse's diet.
The recommended daily intake of magnesium for maintenance in a 1,000-lb horse is 6.8 grams. To at least meet this requirement, the hay would need to have a magnesium content of at least 0.08% (as sampled), or at least 0.34 g/lb of hay if you prefer.
Magnesium supplementation has been promoted for overweight horses with equine metabolic syndrome. However, in all the years I've been testing hay for my patients, I have never encountered a good quality grass hay that was even close to being deficient in magnesium. Typically, grass hay contains more than enough magnesium to meet the maintenance needs of an adult horse. For example, the hay we're examining here would supply a 1,000-lb horse with 20 grams of magnesium per day when it's fed at a rate of 2% bodyweight (20 pounds of hay) per day. The recommended daily intake of magnesium for maintenance is less than 7 grams.
I don't bother looking at the potassium content of grass hays anymore because they've always been more than adequate. For example, the recommended daily intake of potassium for maintenance in a 1,000-lb horse is 22.5 grams. This hay contains 11.4 grams of potassium per pound (as sampled). Just 2 pounds of this hay would meet the horse's daily potassium requirement. 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 a 1,000-lb horse) from less than 5 pounds of this hay.
I also don't bother looking at the sodium content of grass hays anymore, but for the opposite reason: they're always way too low (unless the hay grower has tried to cure the hay by salting it). Instead, I simply advise providing the horse with salt free-choice. Just as a point of reference, the recommended daily intake of sodium for maintenance in a 1,000-lb horse is 9 grams. If relying only on this hay, the horse would have to eat almost 210 pounds of hay per day to meet its basic sodium needs.
Major minerals - Calcium, Phosphorus, Magnesium, etc.