Manganese (Mn) is essential for correct bone formation because it is a part of chondroitin sulfate. It is also required to activate a number of important enzymes, and is essential in the digestion of carbohydrates and lipids.
Overall, this mineral is found in lower concentrations in the horse's body compared to other minerals. This mineral, unlike others, does not appear to gather in tissues such as the liver when dietary sources are high.
It also has a low absorption rate, ranging somewhere around 20-40%, depending on what source you read. The inability to figure out an exact absorption rate has made it somewhat difficult to figure out exactly how much horses need.
Grains contain a concentration anywhere from 15-45 mg/kg dry matter (except corn, which contains around 10 mg/kg dry matter) while forages usually contain 45-140 mg/kg dry matter.
Manganese toxicity is not a concern in horses, because this mineral is one of the least toxic of all the minerals.
However, excessive amounts of this mineral in the diet have been shown to reduce the absorption of phosphorus, so care should be taken to keep the levels within a reasonable level. For this reason, the maximum tolerable intake has been set at 400 mg/kg of ration.
Likewise, a deficiency is not a concern for most horses. However, in Oklahoma, a deficiency as been associated with "smelter smoke syndrome" due to the excessive liming required to offset the acidic effects of the smelter. This amount of liming is thought to decrease the availability of Mn.
A deficiency in this mineral will show as bone abnormalities, since chondroitin sulfate cannot be properly created in the absence of the mineral. If chondroitin sulfate is not properly formed, then the bones cannot be properly formed either.
Mn is an essential mineral in bone formation. Thankfully, both deficiency and toxicity are of little concern in the horse, except in rare circumstances.
Knowledge of this mineral will help you more clearly understand exactly how your horse uses it and why it is important in his diet.