Simulating the effect of conditioning on the drying of cut grass

Field wilting and drying of hay is an important agricultural process upon which the quality of material subsequently conserved is dependent. The research described here aims at translating the biophysical processes that affect field drying of grass into parameters sets to be used for decision support systems that would aid effective management of field wilting for silage making. A mechanistic model of field drying of grass was implemented, largely based on Thompson's (1981) multi-layer canopy representation. Given the meteorological conditions at a 10 minutes interval, an iterative algorithm calculates the profile of evaporation within a drying swath. The time course of moisture content is calculated by integration. The model is shown to reproduce in detail changes of swath moisture content, including dew interception at night time. Three input parameters (i.e. swath resistance per unit tissue moisture content, minimum initial resistance and water content at stomatal closure) were adapted in order to simulate different conditioning treatments. Model predictions and measurements on reference plots were compared with experimental drying rates of perennial rye-grass (Lolium perenne), either lightly or heavily conditioned. The results make clear that coupling grass handling procedures to parameters of a mechanistic model of the drying process can provide tools for use in optimising such procedures