What is Total Digestible Nutrients (TDN) comprised of?

Total Digestible Nutrients (TDN) is an important measure used in livestock nutrition to assess the energy content of feedstuffs. It is calculated as the sum of all digestible nutrients that contribute to the energy available to an animal. This includes digestible fiber, digestible protein, digestible nitrogen-free extract (which predominantly consists of sugars and starches), and digestible fat.

The inclusion of digestible fat is particularly significant because fat is a concentrated source of energy, providing more calories per gram than carbohydrates or protein. This comprehensive approach allows for a more accurate evaluation of the energy content of feeds, reflecting what the animal can actually utilize for maintenance, growth, reproduction, and lactation.

The other options fail to encompass all components that are part of the TDN calculation. For instance, focusing solely on protein and fiber excludes significant energy contributions from nitrogen-free extracts and fats. Understanding TDN thus provides a holistic view of the feed's nutritional value, which is crucial for formulating rations that meet the dietary needs of livestock effectively.