Diurnal changes in the concentration of water-soluble carbohydrates in Phalaris aquatica L. pasture in spring, and the effect of short-term shading

Abstract

The concentrations of water-soluble carbohydrate (WSC) and its components, starch, total nitrogen, and dry matter of phalaris (Phalaris aquatica L. cv. Australian) pasture were varied by shading for periods ranging from 38.5 to 46.5 h. In unshaded pasture, WSC concentrations were lowest at sunrise [103 mg/g dry matter (DM)] and increased until early afternoon (to 160 mg/g DM). Sucrose and starch increased in concentration during daylight, whilst the concentrations of glucose, fructose, fructan, and a component of WSC considered to be mainly the carbohydrate moiety of glycoside(s) were relatively constant. The concentrations of starch, and all components of WSC except sucrose, were reduced by shading, but increased to the concentrations observed in the unshaded pasture within 2–4 h after removal of the cover. The fructans present in phalaris were determined to be oligosaccharides of degree of polymerisation (DP) 3 and DP 4 and high molecular mass fructans with DP >10. Nitrogen concentration of shaded pasture was initially higher (4.7% DM) than in unshaded pasture (3.9% DM), but decreased after removal of the shade cover. Dry matter content was reduced in shaded pasture, partly due to increased retention of water on the exterior of plants. The experiment was a precursor for a grazing trial in which the WSC content of pasture was to be altered by shading. It indicated that shading would potentially alter WSC and N concentrations, and DM content, but would have only a relatively small impact on the digestibility of the pasture.