Forage analysis and mineral balancing has a long history in animal production but is a relatively new concept in horsekeeping. The major forage laboratories in the U.S. have added equine divisions in response as horse owners become educated and more sophisticated about what and how they feed their horses. Balancing pays off in health, performance and cost savings.
At a minimum, your horse's forage - pasture, hay, haylage, chaff and hay pellets/cubes - should be analyzed as this makes up the largest portion of the ration. Averages can be used for beet pulp, hard feeds and pellets/cubes used in small amounts - cautiously if a horse is insulin resistant (IR) or has other metabolic issues.
Once you have the analysis results in hand, they are compared with the horse's requirements to ensure there is adequate digestible energy (DE), crude protein (CP) and calcium (Ca). The results are also checked for sugar and starch levels - important for horses with metabolic conditions - and excessively high iron. If you have breeding horses, your nutritionist may also have had you test for nitrite and nitrate levels and, if unsure of the selenium status in your area, a selenium level.
The mechanics of mineral balancing involve bringing the major minerals - Calcium, Phosphorus and Magnesium - into correct ratios, then adjusting the trace minerals to ensure they are at adequate levels and that an excess of one doesn't cause a relative deficiency of another.
The concept of relative deficiency is generally ignored by commercial feed manufacturers. Most quality feeds are "balanced to themselves" and will, when fed according to directions, fulfill at least minimum requirements for most minerals and vitamins but they will not balance the forage portion of the diet - which is, after all, the major foundation of your horse's ration.
Iron overload in horses is a fairly recent concern and is often disregarded by feed and supplement manufacturers, making it difficult to find feeds and supplements without added iron. Combined with the frequently excessive levels of iron present in many forages, these high levels of iron compete with copper for absorption which results in the "relative" deficiency as well as contributing to inflammation. Correcting the iron to copper ratio to 4:1 or lower by increasing supplemental copper has been shown to alleviate signs of copper deficiency such as coat fading. When the entire diet is correctly mineral balanced, there are usually improvements in overall condition and health.