Effects of level of feeding on energy utilization by Angora goats

Twelve mature Angora does were used in a replicated 3 × 3 Latin square to determine effects of feeding level on energy utilization. Fiber growth and change in tissue (nonfiber) mass were determined in the first 4 wk of 6-wk periods, preceded by 14 or 18 d of adaptation. Determination of ME intake and gas exchange measures occurred in wk 4, followed by feeding near maintenance, then fasting in wk 5 and 6 to determine the ME requirement for maintenance (ME(m)). A 60% concentrate diet was fed at levels to approximate 100, 125, and 150% of assumed ME(m) [low, medium (med), and high, respectively]. Digestibilities and diet ME/GE were not affected by treatment with different amounts of feed offered and subsequent intake near ME(m). Heat energy during fasting (261, 241, and 259 kJ/kg of BW(0.75); SEM = 8.7) and efficiency of ME used for maintenance (71.6, 69.6, and 69.2%; SEM = 2.29) were similar among treatments, although ME(m) differed (P < 0.04) between med and high (365, 344, and 377 kJ/kg of BW(0.75) for low, med, and high, respectively; SEM = 10.3). Tissue gain was less (P < 0.01) for low than for the mean of med and high (MH; -0.6, 23.7, and 29.8 g/d), although clean fiber growth only tended (P < 0.09) to differ between low and MH (5.60, 6.57, and 7.36 g/d for low, med, and high, respectively; SEM = 0.621). Intake of ME was greater (P < 0.01) for MH than for low (6.87, 8.22, and 8.41 MJ/d for low, med, and high, respectively). Total heat energy was less (P < 0.02) for low vs. MH and tended (P < 0.07) to be greater for high than for med (6.03, 6.31, and 6.77 MJ/d); mobilized tissue energy was low but greater (P < 0.02) for low vs. MH (0.16, 0.01, and 0.04 MJ/d for low, med, and high, respectively). Efficiency of ME use for fiber growth was similar among treatments (17.2, 16.3, and 17.7% for low, med, and high, respectively; SEM = 1.61). In conclusion, efficiency of ME use for fiber growth was similar to the NRC recommendation regardless of feeding level, although ME(m) was decreased perhaps because of experimental conditions used. Energy appeared partitioned to fiber growth, but preferential usage was not complete possibly because energy metabolism for tissue accretion reached a plateau with the greatest feeding level.