Ash-free indigestible acid detergent fiber as an internal marker to estimate digestibility with grazing ruminants

Abstract

Two experiments with wethers and one with steers were conducted to compare extraction methods of ash-free indigestible acid detergent fiber (IADF) from feed, and to determine variation across days in fecal IADF content. In trial 1, 4 wethers were fed 4 maturities of intermediate wheatgrass (Agropyron intermedium) in a 4 x 4 Latin square digestion trlal. Methods of IADF extraction from feed were analytically defined as the ash-free acid detergent fiber residue remaining after: (1) 48-hour ruminal-fluid-buffer incubation, 24-hour pepsin-HCi hydrolysis, then a 96-hour in vitro incubation; (2) pepsin-HCI digestion then a 96-hour in vitro incubation; and (3) 96-hour in vitro incubation. Method of IADF extraction and fecal IADF recovery were not affected by, and did not interact (P>.1) with wheatgrass maturity. Fecal IADF reeovery averaged 96.6, 80.6, and 77.2% for extraction Methods 1, 2, and 3, respectively, and Method 1 differed from Method 2 (P<1 and 3 (P<.05). Four steers were fed 6 maturities of fresh bromegrass in a repeated measures design with a factorial arrangement in the subplot. Main effects were method of IADF extraction (1, 2, or 3) and endpoint of the incubation (96 or 120 h). No main effect of endpoint was detected. Method 1 differed (P<.01) from Method 3, but not Method 2, in forage IADF content and fecal recovery of IADF. Organic matter digestibility determined by total collection differed (P<.01) from that calculated from feed to feces ratio using IADF extracted by Method 3, but not by Methods 1 and 2. In a grazing trial, fecal IADF content varied little among sampling days within a period. Ash-free IADF extracted from feed by Method 1 appeared to be a suitable internal marker to calculate digestibility by forage-fed or graxing ruminants.