How Is Nutrient Digestibility Expressed?

Some of the studies conducted at Kentucky Equine Research are designed to evaluate the actual digestibility of various nutrients in the equine diet. If a particular form of a nutrient is contained in feed, but the horse’s body can’t absorb it, that feed ingredient isn’t helping the horse grow, perform, or stay healthy. Therefore, it’s important to find out how digestible each form of a nutrient is. By analyzing feed and grain eaten by the horse as well as urine and manure eliminated by the same horse, it is possible to find out the level of a nutrient that has been retained and used.

Nutrient digestibility is expressed as apparent digestibility. Apparent digestibility is the nutrient content found in the feces subtracted from the nutrient content in the feed. The resultant number is then divided by the total daily intake of the nutrient. This produces a percentage, called the apparent digestibility.

Apparent digestibility is the sum of nutrients that remain undigested during their journey through the digestive tract plus that which may have been sloughed off from the intestinal wall and then excreted in feces. Fecal nutrients that originate from within the horse are termed endogenous. Due to endogenous losses, it is possible for an underestimation of digestibility to occur.

True digestibility values do not include endogenous losses. True digestibility can be determined by using statistical procedures that test a range of nutrient intakes.