We’ve all heard it. People talking about putting out mineral/vitamin supplements of various sorts and allowing the horse to choose what they need. They will often have stories about how the horses vary their consumption over time, again presumably depending on what they know they need.
A Stan Schapp photo – horses stripping bark.
Whether it’s choosing from a selection of supplements or feral behavior like bark stripping, the presumption is that the horse instinctively knows what they need on an individual nutrient basis. Is there any support for this idea?
On one level, it is safe to assume that the species evolved to have an appetite for foods that will support their dietary needs; in the case of the horse, a diet of primarily grasses. These grasses take up minerals from the soil in high amounts for those they need to grow and passively depending on soil levels for minerals that are not critical. The feral horses range over a wide swath of land taking in varying amounts of minerals depending on where the grasses grow and meet their needs because of this variety.
The only mineral horses have a documented drive to ingest is salt – sodium chloride. Sodium is very low in the basic grass diet but is absolutely critical to life. In all areas where horses evolved there are natural salt/sodium chloride deposits which they will seek out and use to fulfill their sodium needs. Otherwise, there is no evidence that horses with other mineral deficiencies will “know” to seek out sources of those minerals to supplement themselves.
Companies that sell free choice minerals for horses invariably have those minerals and/or vitamins in a palatable base of some type, or linked to salt intake. One company famous for pushing free choice minerals has a zinc supplement that is actually 92 to 98% salt with multiple other flavorings. There is no evidence, either formal study or even anecdotal reports, showing that a horse with a copper, zinc or other deficiency would be able to select the mineral they need from a smorgasbord that included options of pure minerals with no other ingredients to tempt them added.
Feral horses eating odd things like tree bark have been suggested to be after minerals but studies have shown the only consistent finding in the barks they eat versus those they don’t is a higher sugar content.
Does this mean that horses don’t really know what they need in terms of minerals? On one level, yes. There is no way a horse, or any other animal, with one or more deficiencies could know what those specifically are and what to eat to correct it. However, sick horses do often change their eating habits, including eating dirt and digging around tree roots where acids secreted by the roots make minerals more available.
A horse that is ill or injured may instinctively know they need more minerals to support their immune system and healing but won’t know which specific minerals. They will seek general mineral sources, like dirt, to try to fill this need. This may work for animals ranging over a large area of land but for domestic horses with limited diets and limited foraging ranges, not likely.
To meet the nutritional needs of our horses we need to know what is present in their base diet, preferably by analysis, compared to what their documented requirements are. In this way we can provide them with precisely what they need to get and stay healthy.
Eleanor Kellon, VMD