The Net Energy of Soybean Mill Run for Growing Cattle

An energy balance (EB) study was conducted with six steers (288 kg BW) that were paired by weight and allotted within pairs to two groups. These groups were alternately fed diets containing 96.6% alfalfa (ALF) or 46.6% alfalfa and 50% soybean mill run (ALFSMR) at intakes varying from 3.6 to 9.4 kg/d in a two period crossover design. Soybean mill run (SMR) was estimated from chemical composition to contain 90.9% soyhulls. Intake and digestibilities were measured by total collection. Daily heat production of the steers was determined by indirect respiration calorimetry during collection periods when fed and again on d 4 and 5 of a fast which occurred at the end of the study. Dry matter, NDF, and ADF digestibilities were 14.8, 20.6, and 30.1% greater for ALFSMR than ALF, respectively (P\u3c.05). Crude protein digestibilities of the diets were similar (P\u3c.20). Fecal energy loss was greater for ALF than ALFSMR (41.8 and 34.9% of GE intake; P\u3c.01), while urine and methane energy losses did not differ (2.6 vs. 2.3%, and 5.1 vs. 4.8% of GE intake, respectively; P\u3e.20). As a result, DE and ME (Mcal/kg of DM) were greater for ALFSMR than ALF (2.83 vs. 2.56, and 2.51 vs. 2.21 respectively; P\u3c.01). Despite the greater metabolizability of ALFSMR, partial efficiencies of ME used for maintenance (Km) and gain (Kr) were similar between diets (P\u3e.20). This suggests that prediction equations based on metabolizability of a wide range of feeds may overpredict Km and Kr for highly digestible feeds containing considerable amount of fiber. Using the Km (.73) and Kr (.37) estimates from analysis of data pooled across diets, NEm­ and NEg were 1.61 and .83 Mcal/kg of DM for ALF, and 1.81 and .93 Mcal/kg of DM for ALFSMR diets respectively. The NEm and NEg for soyhulls were 1.80 and .90 Mcal/kg of DM, respectively