Horses need a number of minerals to build and maintain skeletal and soft tissue and support the function of various body systems. Although the horse can synthesize some vitamins within its own body, it must get its entire requirement of minerals from the forage and grain it ingests. Some minerals are more easily absorbed than others, and an oversupply of selected minerals can interfere with absorption of others, so it’s important to include the correct mineral amounts and ratios in equine diets.

Mineral digestibility can be expressed as either apparent digestibility or true digestibility. Apparent digestibility, which is calculated by figuring the amount of a mineral recovered from the feces and subtracting it from the total daily intake, is sometimes an underestimation of true nutrient digestibility. Researchers can work toward a more accurate figure by studying a range of intake levels and regressing the digested amounts against the corresponding levels of intake. This method eliminates the influence of endogenous losses, or amounts of a previously ingested mineral that are shed from the horse’s body.

Kentucky Equine Research has conducted a number of studies on mineral digestibility. Results showed some interactions and other factors that affected how well a particular mineral was digested.

• Calcium digestibility was not affected by the concentration of dietary protein, fat, magnesium, iron, zinc, copper or manganese. Factors that inhibited calcium digestibility were abnormally high levels of neutral detergent fiber and phosphorus.
• Phosphorus digestibility, as a function of intake, was much more variable than calcium digestibility. There was no relationship between protein, calcium or calcium:phosphorus ratio and phosphorus digestibility. True phosphorus digestibility was also negatively correlated with fiber content.
• There was no relationship between protein, fat, magnesium, iron, copper, or manganese and true magnesium digestibility. There was a significant negative correlation between fiber and phosphorus levels in the diet and magnesium digestibility. Calcium content and calcium:phosphorus ratio were positively correlated with magnesium digestibility.
• The only nutrient that showed a significant positive correlation to zinc digestibility was magnesium. None of the trace minerals, including iron, affected zinc digestibility.
• Protein and calcium were negatively correlated to copper digestibility. Iron level in the diet did not affect copper digestibility.

In these studies, iron content did not affect the digestibility of any of the minerals, though dietary iron levels were fairly high. Most of this iron was not supplemented and was probably in the form of iron oxide, so it remains to be determined whether supplemental sources of iron might have a greater effect.

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