LAUSR

Feedlot performance is predicted reliably from DMI and diet NE. Tabular NE values in turn commonly are calculated from TDN either measured in digestion trials or predicted from nutrient compositions. When estimated from nutrient composition (NRBC, 2016), TDN values for feeds were related imprecisely (R2 = 0.83) to in vivo TDN values (NRC, 2000); deviations ranged from -29% to 162%. Traditionally, DE has been calculated as TDN percentage times 0.044, an assumed calorie content of carbohydrate (4.4 kcal/g). Surprisingly, this value is 5% greater than the heats of combustion of DM from isolated starch, cellulose, and glycogen (4.1788, 4.1810, and 4.1868 kcal/g; Blaxter, 1962; The Energy Metabolism of Ruminants). Being “carbohydrate-equivalent energy,” TDN includes the extra calories from lipid whereas urinary energy (UE) lost during catabolism of digested protein automatically is deducted. When determined calorimetrically, ME equals DE minus both gas energy (GasE) loss and UE. But when DE is being calculated from TDN, UE from digested CP already has been subtracted. Double deduction for UE underestimates ME content of high protein feeds. When DE is calculated from TDN, ME equals DE minus only GasE. Using data assembled by Galyean et al. (2016), multiple regression revealed that GasE was predicted more accurately from dietary NDF and lipid concentrations (GasE, % of DE = 4.87 + 0.131 x NDF - 0.331 x EE; R2 = 0.69) than from diet DE (R2 = 0.27). From this formula, GasE loss as a fraction of DE of feeds ranges from 5 to 15% except for isolated fats than have a negative value; non-additivity across feeds complicates formulation of least cost diets. Correcting discrepancies in calculating ME from TDN should improve reliability of predicting the cost of ME from various feeds and for quantifying feed requirements for maintenance and gain of cattle fed diverse diets.