Chromatographic profiles of nonstructural carbohydrates contributing to the colorimetrically determined fructan, ethanol-soluble, and water-soluble carbohydrate contents of five grasses

Accurate estimates of forage fructan and mono- and disaccharide content may help with feeding management decisions for horses with increased risk for pasture-associated laminitis. In this study, five forages expected to differ in soluble carbohydrate concentrations were extracted with water or 800mL/L ethanol. Forages were Festuca arundinacea (tall fescue) with (E+) and without (E−) the common endophyte, cold-stressed and clipped, vegetative; Poa pratensis (Kentucky bluegrass), cold-stressed and clipped, vegetative; Dactylis glomerata (orchardgrass), air-dried, early reproductive stage; and Cynodon dactylon (bermudagrass), air-dried, 21 days of regrowth. Extracts were analyzed colorimetrically for water-soluble carbohydrates (WSC, mono- and disaccharides plus fructans) and ethanol-soluble carbohydrates (ESC, mono- and disaccharides). Soluble carbohydrates were separated and quantified by high-performance liquid chromatography (HPLC) with pulsed amperometric detection (PAD) to compare HPLC-based calculations of WSC, ESC, and fructan to the colorimetrically obtained concentrations. HPLC-PAD analysis of WSC extracts confirmed the presence of fructans in tall fescue and orchardgrass. The fructan content of orchardgrass and E− tall fescue did not differ between colorimetric and HPLC-PAD determinations (P=0.21 and 0.10, respectively), but E+ tall fescue fructan was about 36g/kg DM higher in colorimetric than in HPLC determinations (P=0.0026). Colorimetric analysis of bermudagrass WSC and ESC indicated a low fructan content, whereas HPLC-PAD indicated a lack of quantifiable fructans. Colorimetrically determined WSC and ESC were greater than the corresponding chromatographically determined values. An exception was bluegrass, for which the colorimetric and chromatographic ESC content did not differ, although both were greater than the chromatographically determined mono- and disaccharide content of water extracts. The results suggest that forage soluble carbohydrate concentrations tend to be higher in the colorimetric assays used than in HPLC-PAD determinations. However, HPLC-PAD analysis of unhydrolyzed water extracts may suffice for quantifying both ESC and WSC, besides providing information on individual sugars.