total digestible nutrients

Total digestible nutrients, or TDN, is a measure of the energy content in animal feed that your animal can genuinely extract and use. It is expressed as a percentage or as pounds per ton, and accounts for the fact that not all of what goes into an animal's mouth becomes available energy. A feed that is 65% TDN means that 65% of its dry matter can be broken down and absorbed in the animal's digestive tract; the remaining 35% passes through as waste.

TDN incorporates four components: crude protein, crude fiber, ether extract (fat), and nitrogen-free extract (carbohydrates). Each is weighed by its actual digestibility in the target animal species. A grain like corn may have a TDN value around 80%, while poor-quality hay might be 45% to 50%. These values are not fixed: digestibility varies with the animal's age, health, and digestive efficiency, and with how the feed was processed, stored, and ground.

Why TDN matters for ration design

Feed formulation depends on TDN because it tells you how much energy an animal will actually receive. A dairy cow producing 60 pounds of milk daily needs roughly 11 to 12 pounds of TDN per day; meeting this with low-TDN forage alone may be impossible without underfeeding the animal or generating feed waste. Conversely, over-feeding high-TDN concentrate wastes cost and can cause metabolic disorders. Nutritionists use TDN as the primary energy unit (often converted to megacalories of metabolizable energy) to balance rations.

TDN values are determined by laboratory feed analysis, typically using in vivo digestibility trials where the feed is fed to a standardized animal and the fecal output is measured against feed intake. These trials are expensive, so commercial feed tags and reference tables rely on prediction equations based on crude fiber, crude protein, and other components. Always verify the source and age of a published TDN value; values from USDA feedstuffs tables are generally reliable, but field variation is real.

Common sources of confusion arise from the difference between gross energy (total calories in the feed) and digestible energy (what is absorbed) and metabolizable energy (what is available after accounting for losses in urine and gas). TDN is an older standard that predates modern metabolizable energy systems, but it remains embedded in feed tags, sales documents, and many farm management software programs, especially in beef and hay trading.