TDN Translation: Understanding Energy of Forages

By Hannah Smith

Total digestible nutrients (TDN) are the common energy reference for both feed content and animal requirement, so how are the two connected and what can we know to better examine TDN of feedstuffs and use energy economically?

The initial measure of energy begins with gross energy (GE), which is the term for all energy contained within a feed and consumed by the animal. By subtracting all the energy that passes through to manure from GE, you get digestible energy (DE). Digestible energy is the measure of energy converted into TDN within feedstuffs.

Where does TDN intersect with cattle management decisions? Most commonly, when analyzing forage sample results, we can see the TDN value as a percent of the dry matter composition. First, understand that the percent TDN indicates forage quality and measures potential for digestibility. The average measure of meadow hay is 53% with a typical range from 49% to 57%. Sandhills range has potential to be 56% but can range from 49% to 68% throughout the year, when evaluating actual selected forages (Table 1).

Table 1. Examples of forages and their TDN and crude protein (CP) values. The value for native prairie hay and meadow hay sourced from The Nutrient Requirements of Beef Cattle, 2016. The value for Sandhills range and meadow sourced from actual cattle diets collected at UNL’s Gudmundsen Sandhills Laboratory.

The relationship between feedstuff digestibility and protein content are somewhat correlated. As plants mature and the protein value starts to decline, digestibility decreases and we will see a lower TDN value. Knowing this allows us to economically evaluate forages beyond just a protein value.

A 1,400 lb, Spring-calving cow currently in mid-gestation requires 11.6 lb/day of TDN. On a forage with 50% TDN, the cow’s daily intake would have to be 27.0 lb of hay, as-fed. If the hay is 60% TDN, that intake requirement drops to 22.2 lb/day. The December 1 Nebraska Hay Summary reported Good Quality (9 to 13% CP) prairie hay selling at $205.00/ton. For a cow during mid-gestation (90 days), she would consume 405 lb additional forage of 50% TDN, which costs $41.51/cow more for mid-gestation alone. Multiply that by 150 cows and the additional economic value during the second trimester is 60,750 lb of hay for an added cost of $6,227.

Protein is the first limiting nutrient when meeting range beef cow requirements because if crude protein falls below 7%, intake will decrease. However, energy is what the cow needs to live and do her job. Evaluating forages on both a protein and energy basis can ensure we know how to manage our feeds this winter, especially in a purchasing scenario, to ensure we get the most value with the resources we have going into a winter with elevated feed costs and inconsistent moisture.