Estimating forage yield and quality changes during field drying for hay. 2. Model of forage drying

A forage drying submodel (HayDry) has been developed for use in an operational model of weather-induced forage quality losses during field drying for hay. HayDry simulates changes in bulk swath forage moisture content during field drying periods that may or may not be prolonged by rain. Model inputs are hourly solar irradiance (or net radiation), air temperature and humidity, wind speed and rainfall. Forage drying is estimated by the Penman-Monteith combination equation, with canopy resistance estimated as an empirical function of surface water, forage moisture content and forage dry-matter yield. Forage wetting and rehydration are estimated by a distributed rewetting submodel. HayDry was developed and validated in three steps: (1) a sensitivity analysis identified key model parameters; (2) established values for key parameters were evaluated by comparing the simulated forage moisture contents with field measurements during two rain-free and two prolonged drying periods; (3) the model was independently validated during a third prolonged drying period and at a second site. The simulated changes in forage moisture matched closely the measured sequence of drying, rewetting by dew and rewetting by rain, for forage mixtures that were predominantly alfalfa. The mean forage moisture content during drying was simulated to within 20% during rain-free and prolonged drying periods at both sites.