Simulating the grazing of a white clover 3-D virtual sward canopy and the balance between bite mass and light capture by the residual sward
Résumé
Background and Aims The productivity and stability of grazed grassland rely on dynamic interactions between the sward and the animal. The descriptions of the sward canopies by standard 2-D representations in studies of animal-sward interactions at the bite scale need to be improved to account for the effect of local canopy heterogeneity on bite size and regrowth ability. The aim of this study was to assess a methodology of 3-D digitized canopies in order to understand the balance between bite mass and light interception by the residual sward. Methods 3-D canopy structures of four white clover swards were recorded using a POLHEMUS electromagnetic digitizer and adapted software (POL95). Plant components were removed after digitizing to determine aerial dry matter. Virtual canopies were synthesized and then used to derive canopy geometrical parameters, to compute directional interception and to calculate bite mass. The bit masses of cattle and sheep were simulated according to their form, depth and placement on the patch, taking account of explicit sward architecture. The resulting light interception efficiency (LIE) of each organ was then calculated using a projective method applied to the virtual residual sward. This process enabled an evaluation of light interception based on Beer's law at the bite scale. Key Results The patterns of the vertical profiles of LAI appeared as bimodal, triangular or skewed parabolic functions. For a single bite of similar area and depth, the lowest mass was observed with half-spherical form and the highest for the cylindrical form, whatever the initial sward structure. The differences between the actual LIE and that calculated by Beer's law were marked for residual swards shorter than 8 cm. Bite mass and LIE values after grazing were more strongly affected by the initial structure of the sward than by bite form and placement. Conclusions 3-D digitizing techniques enabled a definition of the geometry of each component in sward canopies and an accurate description of their vertical and horizontal heterogeneities. The discrepancy between Beer's law results and actual light interception was reduced when the sward regrew rapidly and if the rest period was long. Studies on the biting process would greatly benefit from this method as a framework to formulate and test hypotheses in a quantitative manner.