Before adding minerals to your horse's diet, it's important to have an idea of the levels in the hay (grass or alfalfa) and other diet components you're feeding.  Otherwise, you will be adding minerals blindly.  It is not always practical or feasible for riders to have their hay analyzed (due to variability & changing sources), BUT, you can get some regional average analyses from your university extension office.  Another source for average hay (grain and by-products also) analyses can be found at www.DairyOne, go to "Forage Labaratory Services," and then"Feed Composition Library."  Here you can compare the relative compositions of grass vs. alfalfa hay, oats, beet pulp, rice bran, wheat brain, etc.  When you look at these tables, the components (minerals, protein, starch) are listed as %.  The easiest way to convert these %'s to grams is the following.  (Hang in there, it's not very difficult!)  First, 10 kilograms of hay = 22 lbs.  If you multiply the % in the chart by 100, you'll get the grams in 22 lbs. of hay.  (I'm metrically challenged, but I can do this.) 

 

Still with me?  If you know how many lbs. of hay you're feeding, you can figure out your current levels of protein, starch, sugars, minerals, electrolytes, etc.  Wouldn't you like to know what you're feeding before you start adding supplements? 

 

With regard to the questions about calcium, phosphorus, and magnesium, my source is the information I've learned from Dr. Eleanor Kellon, VMD (www.drkellon.com )     Her recommendations are based on the NRC 2007 data, which list minimum requirements, but she increases the amounts by 50%, which she believes is still conservative.  Below are shown recommendations for these 3 minerals for a 1000 lb horse (455 kg.)  in light vs.heavy exercise (like an endurance ride). 

 

                                   Light Exerc.                 Heavy Exer.

Calcium                           41.0 g                         54.6 g

Phosphorus                   24.6 g                         39.6 g

Magnesium                    20.5 g                         27.3 g

 

Dr. Kellon recommenda a 2:1 calcium:magnesium ratio, and calcium:phosphorus at or above 1.2:1.

 

As an example, I have analyzed my grass hay, and I know that in 22 lbs., my horses get 37.6 g of calcium, 7.0 g of phosphorus, and 4.6 g of magnesium.  You can see that my hay falls below the levels recommended for even light exercise.  So I can add supplements accordingly.  Also, as Angie mentioned, alfalfa hay is an excellent source of calcium, and is also the most bioavailable form of calcium. 

 

I would HIGHLY recommend Dr. Kellons's nutrition courses to anyone.  They're the best source of information on equine nutrition that I've found.  And what you save on buying expensive supplements will easily pay for the course!