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Commodities: Field Crops: Forages

Common Terms Used in Animal Feeding and Nutrition

Glossary: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


Total Digestible Nutrients (TDN):  a measure of the energy value in a feedstuff. The term TDN has its origins in an older system of measuring available energy in feeds and is very hard to measure directly. Today, reported TDN values are calculated, not measured values. Formulas for calculating TDN originally were based on ADF and frequently varied by region and the nutritionist doing the calculation. The National Research Council (NRC) suggested a more accurate and robust procedure of estimating TDN than those based solely on ADF (NRC, 2001). Their procedure is based on the assumption that forage classes (legumes, cool season grasses, warm season grasses, etc.) have more uniform and predictable digestion coefficients. So, they proposed that the TDN for alfalfa, clovers and legume/grass mixtures be calculated as follows:

  • TDNlegume = (CP × 0.93) + (FA × 0.97× 2.25) + [NDFn × (NDFD ÷ 100)] + (NFC × 0.98) – 7


NDFn = nitrogen free NDF = NDF – NDFICP, also estimated as

NDFn = NDF × 0.93

NDFICP = neutral detergent fiber insoluble crude protein

NDFD = 48-hour in vitro NDF digestibility (% of NDF)

NFC = non fibrous carbohydrate (% of DM) = 100 – (NDFn + CP + EE + ash)

EE = ether extract (% of DM)

The TDN for warm and cool season grasses are calculated as:

  • TDNgrass = (NFC × 0.98) + (CP × 0.87) + (FA × 0.97 × 2.25) + [NDFn × (NDFDp ÷ 100)] – 10


 All other terms are as defined previously and NDFDp = 22.7 + 0.664 × NDFD

Though TDN is a widely used measure of energy, it is not without its weaknesses. The most significant issue with TDN is that it does not account for additional energy losses, particularly heat increment and, to some extent, gaseous losses, particularly with regards to ruminant systems. Consequently, TDN is known to over-estimate the energy value of roughages in comparison to grains.

Total Mixed Ration (TMR): a homogenous mixture of mechanically mixed ration ingredients that typically combine roughages (forages) and concentrates such as grains to optimize animal performance. TMRs are commonly used in large dairy or beef feedlot operations.

Toxicity : refers to the extent to which a substance can exert a poisonous effect on animals.