Often, the terms Total Digestible Nutrients and energy are used interchangeably when discussing forages and feeds, but realistically TDN is one measurement of energy.
There are many different components that make up what we call TDN. TDN is a combination of digestible fiber, lipids and proteins. Acid detergent fiber or ADF, is used to help calculate TDN from the fiber component. The lower the ADF, the higher energy the forage contains in most situations.
When supplementation is being considered, especially on a diet that is primarily forage, TDN is one of the most important vales to know from our forage test. When comparing forages, protein is often looked at first while TDN can overlooked. This can easily lead to diets lacking energy to meet animal needs.
TDN values will be affected by maturity of the forage, weather conditions in which the forage was grown and harvested, and fertilization or other agronomic practices. For example, Sandhills meadow hay TDN ranges between 50% and 65% on a dry matter basis.
Understanding energy is important for the health and nutrition of livestock. For cows, TDN is the value that typically needs to be used to calculate their needs which changes drastically with different stages of production. Buy understanding TDN values and how they fit into livestock diets, we can ensure our animals receive adequate energy year-round.
Ben Beckman is an Extension educator in Engagement Zone 5 with the University of Nebraska-Lincoln.
