USE OF MICROHISTOLOGICAL TECHNIQUES AND INTERNAL MARKERS TO STUDY THE NUTRITION OF GRAZING CATTLE

Methods for determination of the quality and quantity of forage consumed by grazing animals have been difficult to achieve. Microhistological analysis of feces and internal markers may provide effective means for assessing the intake of grazing livestock. The purpose of this study was to quantify diet habits of cattle grazing improved pastures with well established stands of big bluestem (Andropogon gerardii), switchgrass (Panicum virgatum), and indiangrass (Sorghastrum nutans). A split-plot design was used with paddock (12 paddocks) as the experimental unit. Paddocks were incorporated in a 3-paddock rotation. Three completed cycles were grazed in the summers of 1983 and 1984. The study was conducted at the University of Nebraska Field Laboratory, near Mead, Nebraska. Fecal composition of warm-season grasses was determined by the microhistological analysis of feces (Holechek 1982). Forage and fecal samples were also analyzed for chemical composition and in vitro dry matter digestibility (IVDMD). Indigestible acid detergent fiber (IADF) was isolated from forage and feces by cellulase digestion (Penning and Johnson 1983b). Acid insoluble ash (AIA) was determined using the 2N HCl procedure (Van Keulen and Young 1977). Growth stage had significant (P \u3c 0.05) influence on fecal composition of cattle. Proportions of big bluestem and switchgrass in the feces were greater (P \u3c 0.05) than indiangrass early in the season. Percentages indiangrass in feces rose steadily (P \u3c 0.05) to reach its peak late August. Percent fragment identification averaged 87.9% (1983) and 84.9% (1984) but it declined (P \u3c 0.05) with plant maturity. IADF underestimated IVDMD by 1.1 (1983) and 1.7 (1984) percentage units. Coefficients of variation between sampling dates were in the range of 5.0. In contrast, AIA overestimated IVDMD by 1.7 (1983) and 4.1 (1984) percentage units. Estimates of IVDMD and digestibility estimates with AIA and IADF decreased (P \u3c 0.05) with plant maturation. Estimates of IVDMD were best predicted by 1-forage components models containing either AIA or IADF. The models accounted for over 92% of total variation in IVDMD. IADF was a better indicator for measuring for quality and intake of cattle grazing these warm-season grasses compared to AIA