Effects of Sampling Time, Cultivar, and Methodology on Water- and Ethanol-Soluble Carbohydrate Profiles of Three Cool-Season Grasses in Central Kentucky

Abstract

Cool-season grasses (CSGs) accumulate variable amounts of water-soluble carbohydrates (WSCs, monosaccharides and disaccharides, and fructans), depending on climate, time of day and year, and genotype. Fructan concentrations in CSG are sometimes estimated as the difference between concentrations of WSC and ethanol-soluble carbohydrates (ESCs, monosaccharides and disaccharides, and variable amounts of fructan). Characterizing both WSC and ESC may improve understanding of soluble carbohydrate profiles in pastures and inform grazing management decisions, particularly for horses at risk for laminitis. Three CSG cultivars from Kentucky pastures were collected in the morning and afternoon on two springtime dates. Water-soluble carbohydrates and ESC were extracted with water or 80% ethanol, respectively, and analyzed by high-performance liquid chromatography (HPLC) and a colorimetric assay. Method of analysis (HPLC or colorimetry), and extraction solvent, affected the composition of WSC or ESC determined in CSG samples, demonstrating the need to consider methodology when interpreting results. Total soluble carbohydrate (mean of WSC and ESC) concentrations across cultivars and harvests were generally higher in the afternoon than in the morning, based on both HPLC (P = .0023) and colorimetric (P < .0001) analysis. The diurnal variation was mainly due to sucrose (P < .0001). Among cultivars, monosaccharides and disaccharides constituted the majority of the averaged WSC and ESC concentrations. The proportions of water- and ethanol-soluble fructan (P = .0101), and fructan chain lengths detected in water and ethanol extracts (P < .0001), differed among CSG cultivars. In choosing CSG cultivars for pastures, both soluble carbohydrate composition and concentrations should be considered.