Horses evolved grazing over large areas eating a wide variety of plants from the mountains to the valleys. During the last 50 years their environment has changed. The typical horse is confined, eating 2 or 3 species of plants.These plants have been produced by intensive farming methods. This creates a situation prone to nutritional deficiencies. Here is why:
1. Horses eat only a few species of plants. This lack of variety increases the likelihood of deficiency. Different feeds contain varying levels of nutrients. For example, humans have hundreds of food sources from all over the world (including meats which are full of necessary vitamins and minerals.) Horses are strict vegetarians eating only a few types of plants from a limited geographical area. This lack of variety increases the probability that horses will be deficient.
2. Plant products that are farmed using high production methods are increasingly deficient.
3. For example, in many areas, five cuttings of alfalfa are produced off the same ground year after year. This decreases the trace minerals available to the plants. Crops are fertilized with sulfur containing fertilizers which greatly decreases the availability of trace minerals like selenium.
4. Storage decreases the vitamins in feeds. Typically hay is cut in the summer and stored until the winter, often nearly a year, sometimes longer. Hay loses 50 percent of its vitamin A content after six months of storage.