Toughness, particle size and chemical composition of meadow fescue (Festuca pratensis Hud.) herbage as affected by time of day

While herbage chemical composition varies diurnally, it is not known if this variation affects herbage biomechanical properties. The objective of this study was to evaluate changes in herbage toughness and particle size in relation to diurnal fluctuations of herbage chemical composition. Vegetative (i.e, tillers with three fully expanded leaves) micro-swards of Festuca pratensis Hud. were sampled at four times of the day being: 06:50 (sunrise), 11:10, 15:30 and 19:25 h. Cut herbage was analysed for dry matter (DM), crude protein (CP), total non-structural carbohydrates (TNC), neutral and acid detergent fibre (aNDF, ADF), in vitro true digestibility (IVTD), toughness and particle size. The experiment was repeated on two consecutive days. Diurnal variation of temperature, relative humidity and photosynthetic radiation were recorded every 5 min with an automated weather station. To characterize the relationship between dependent variables, Pearson correlations were performed. From 06:50 to 19:25 h, DM, TNC and the proportion of small particles (<1 mm) increased (P<0.05), CP, aNDF, ADF, toughness and proportion of large particles (>4.75 mm) decreased (P<0.05) while IVTD remained constant (P>0.05). Toughness was negatively correlated (P<0.05) with DM and TNC concentrations, and positively correlated (P<0.05) with proportions of aNDF and ADF. The proportion of small particles was positively correlated (P<0.05) with DM and TNC concentrations, and negatively correlated (P<0.05) with proportions of aNDF, ADF and toughness. Results suggest an effect of time of day on herbage toughness and particle size as a function of increases in DM concentration, and reductions of aNDF and ADF concentrations, due to an increase in TNC. Diurnal fluctuations in chemical composition of herbage not only result in differential nutrient supply to grazing ruminants during the day, but also in temporal fluctuations in herbage biomechanical features.