Written by Dr. Kris Hiney
This month we will return to discussions of nutrient requirement for horses. Remember we discussed energy needs for horses in the earlier articles: Equine Energy Requirements, Energy for Work, and Broodmares and Babies. Now we will look more closely at other nutrient requirements, beginning with mineral requirements. Minerals are involved in a variety of functions in the body, including enzymes, structural components, energy transfer and acid base balance. Minerals are also incorporated into vitamins, amino acids, and hormones. Thus proper mineral nutrition is vital to have a healthy horse. The minerals that are needed in the largest quantities by horses are referred to as the macro-minerals. These include calcium (Ca), phosphorous (P), magnesium (Mg), potassium (K), sodium (Na) and chloride (Cl). These minerals are needed in the diet in concentrations of g/kg or percentages, versus ppm or mg/kg of micro-minerals. Today we start with those most commonly talked about in equine nutrition, Ca and P.
A note of cautionIt is important to remember that when creating diets for horses, we consider not only how much mineral is in the diet, but also the ratio of particular minerals in relationship to others. Minerals have very complex interactions with each other, and excesses or deficiencies of minerals can greatly affect the absorption, metabolism and excretion of others. Therefore horse owners who “tinker” too much with their horse’s diet through overzealous supplementation may be doing more harm than good for their horse.
Previously in Broodmares and Babies we had mentioned that the ratio of Ca and P is always important when looking at horse rations. If you recall, we would like to see a ratio of 2:1 Ca:P, with a range of 1:1 to 6:1 being acceptable. What you should avoid is a total diet which is less than 1:1 or in other words, offers more P than Ca. This is due to the fact that phosphorous competes with Ca for absorption in the gut. Remember to calculate the entire diet however! And while the proper ratio is important, it is possible to have the correct ratio, but still be deficient in these minerals if they are in insufficient quantities in the feed. Now let’s talk in more detail about these very important minerals.
How important is calcium?
Commonly most people think of calcium’s role as that of bone development and integrity. Certainly the skeleton does account for 99% of the calcium in the horses’ body. However, Ca is absolutely essential for neuromuscular function, blood clotting, cell signaling, and an array of enzymes. Because of its importance, calcium concentrations are very tightly regulated in the blood. When Ca in the diet is inadequate, the bone serves as a major reservoir. Thus the body will sacrifice calcium within the bone to maintain blood Ca homeostasis. A prolonged period of Ca deprivation can lead to a weakened skeleton. In addition, this means that blood values of Ca are relatively poor indicators of Ca status in the horse. Calcium deficiencies are especially detrimental to young growing horses as this can lead to osteopenia. Improper ossification can lead to enlarged joints or improper growth patterns of the long bones. Therefore, it is critical to look carefully at the diet of lactating mares and foals.
Adult horses which aren’t exercising are relatively easy to meet their calcium requirements as can be seen in Table 1. The increase in Ca requirements for exercising horses is presumably due to an increase in bone deposition. Horses undergoing intense exercise experience an increase in bone mass and thus have a greater need for calcium. (More on bone formation in upcoming articles) It is unlikely that light exercise, or exercise that the horse is already adapted to (essentially no change in work intensity) results in much change in calcium requirements. Additionally most studies of calcium and exercise have focused on the young, growing horse. However, in an effort to err on the side of safety, the National Research Council recommends higher intakes of calcium. There is some loss of Ca in the sweat of exercising animals which is also represented in the increase in requirements for work.
|Weight (lbs.)||Maintenance||Light work||Moderate work||Heavy work|
Table 1. Calcium requirements (in grams/day) for adult horses at maintenance or work. To determine which class your horse fits into, read Energy for Work.
For gestating mares, requirements of Ca increase the greatest for the 9th, 10th and 11th month of gestation, which is concurrent with the most rapid increase in fetal growth. However, there is still substantial fetal growth in the 7th and 8th month of gestation as well, and so Ca requirements are greater than maintenance. Lactating mares clearly have an increase in Ca demand in order to support the very rapid growth of their foal. Mares fed an inadequate amount of Ca actually experience a decrease in bone density (as detected through radiographic analysis of the cannon bone). If you compare Table 1 with Table 2, you can see that at least in terms of Ca horses at light to moderate work would be considered to be comparable to gestating mares. However, demands of lactation far outstrip the working horses in needs of calcium. Therefore, one should either choose a feed for lactating mares and babies, or a supplement designed to meet their needs. After the first three months of peak lactation, the calcium demands on the mare taper off as the foal derives more of his nutrition from the feed he consumes.
|Weight (lbs)||Month of Gestation||Month of Lactation|
Table 2. Calcium requirements (g/d) for gestating and pregnant mares. Remember to use Table 5 from Broodmares and Babies to determine how much your mare should weigh.
Obviously foals get much of their Ca from their mothers’ milk, but as they start to ingest new feeds and taper off their reliance on mom, it becomes your job to balance their diet. In the following table, I list the approximate Ca requirements of growing foals from 4 months until 2 years of age. Now, in just looking at the table, one might think that the Ca requirements appear low, but remember they are listed in grams per day! Because the foal is much smaller, he eats much less per day, thus the concentration of Ca in his feed must be greater. For example, a foal which consumed 2% of its body weight in alfalfa hay that was 2% Ca would meet its requirements, but if it was eating orchard grass hay that was 0.4% Ca would definitely not! There is not a very large decrease in overall Ca requirements as the foal matures. But as the foal matures and reaches a larger body size, it will consume more and thus the concentration of Ca needed in the diet will go down. Confusing, right? Don’t worry, next month we will discuss P requirements and then more importantly, how to put this all together with calculations from what you might be actually feeding.
|Estimated Mature Weight||4 mo-7 mo||8-14 mo||15-24 mo|
Table 3. Approximate Ca requirements (g/d) for growing foals based on their estimated mature weight.
How big will my baby be?
How do you know what size they will be? Look at both the mare and the sire, and use an average. However, foals from maiden mares and older mares, tend to be smaller. Don’t forget that the nutrition program and environment that the dam and sire were subjected to also played a large role in their final growth. Also, foals carried by recipient mares will have a large influence in their size due to their foster momma, and less of an extent by their genetic mother. That’s why most farms choose larger mares for embryo transfer programs to carry donor mare’s babies. In addition, the early a colt is gelded, the larger they may mature to be. All in all, there is no firm way to know, but we can use our best estimation!
How much is too much?
Calcium has been fed as high as five times the horse’s requirement without any ill effects provided that the P intake is adequate. The maximal concentration of Ca in the horse’s diet is 2%, however it would be hard to find feeds that reach that level. However, excess Ca has been implicated as a causative factor of ulcers (See Is My Horse Stressed Out) due to an increase in gastrin secretion. Alternatively, others have found that alfalfa diets (and thus higher Ca) may decrease the incidence of ulcers. Clearly more work regarding Ca and ulcer formation in the horse is needed.
Osteopenia – a decrease in bone mineral density below normal. In humans this is considered to be a precursor to osteoporosis. Horses don’t really suffer from osteoporosis.
Ossification – essentially proper bone formation replacing cartilage as the horse grows, not to be confused with calcification. While calcification is a normal process of ossification, abnormal calcification can also occur, for example the formation of splints.
Gastrin – a peptide hormone secreted by the parietal cells of the stomach which stimulates secretion of gastric acids (HCL) and increases gastric motility. Gastrin release is stimulated by the presence of protein in the stomach, as well as conditions of hypercalcemia.