Predicting Digestibility of Different Diets with Internal Markers: Evaluation of Four Potential Markers

Abstract

Four independent digestion trials with four diets were conducted using four Hereford × Angus steers per trial to evaluate the ability of four internal markers to predict in vivo dry matter digestibility. Diets were 1) cubed alfalfa hay; 2) tall fescue (pre-boot stage, harvested .5 h before feeding); 3) tall wheatgrass hay plus .9 kg soybean meal (SBM)/d and 4) prairie hay. Forages were fed once daily at 90% of ad libitum intake. Separation of internal markers was accomplished by 1) 6-d in vitro fermentation followed by acid detergent fiber (ADF) extraction (IVADF); 2) 6-d in vitro fermentation followed by neutral detergent fiber extraction (IVNDF); 3) ADF extraction followed by 3-h digestion with 72% H2SO4 (ADD and 4) ADF extraction followed by 10-d cellulase incubation (ADFIC). Dry matter digestibility (DMD) was calculated from the feed:feces ratio of each internal marker and compared with in vivo apparent DMD within each diet. When compared within both cubed alfalfa hay and tall wheatgrass/SBM, digestion coefficients from IVADF ratio and IVNDF ratio were similar (P>.10) to in vivo coefficients. However, digestion coefficients from ADL ratio and ADFIC differed (P<.01) from in vivo apparent digestibilities. When compared within tall fescue and prairie hay, digestion coefficients from all internal markers evaluated were different (P<.01) from in vivo digestion coefficients. Fecal marker recoveries explained observed differences between in vivo DMD and DMD from marker ratios. The relationship between in vivo DMD and DMD determined by IVADF and IVNDF ratio was highly variable. Caution should be exercised in using IVADF and IVNDF as internal markers, particularly with immature, freshly harvested or grazed forage. Acid detergent lignin and ADFIC were least acceptable for use as internal markers in all diets evaluated.