Last week we looked at acid detergent fiber (ADF) and neutral detergent fiber (NDF) and how they are used to measure a hay sample’s fiber content, which affects digestibility, and forage intake, which help predict animal performance. This week, we will look at total digestible nutrients (TDN).
Often, the terms TDN 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. 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 values to know from a hay test. In many cases, TDN is overlooked. Diets may be lacking energy as much or more than crude protein because protein often receives more focus in diets.
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 50-65% 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. — Brad Schick, Nebraska Extension educator
