OSYAQ, an organ-specific growth model for forage grasses

The effects of different stern-to-leaf proportions on the yield and quality of forage grasses is a question of importance to plant breeders. This pape r describes the growth submodule of OSYAQ, an organ-specific yield and qual ity model. The model input includes daily average air temperature, precipit ation, potential evapotranspiration and incoming global radiation, The majo r processes implemented in the growth submodule are photosynthesis, mainten ance and growth respiration, recycling of senescent plant material and remo bilization of reserves. The model calculates separately yield formation for stein, leaf and ear. Available assimilates are partitioned to the various plant parts according to the concept of relative sink strength. Calibration and validation were based on a 2-year and a 1-year field trial, respective ly, carried out under long- and short-day conditions with Lolium multifloru m ssp. gaudini. These tests indicated a good performance of the model for t he long-day growth but showed some deficiencies for regrowth after defoliat ion under conditions of extreme water shortage. The paper discusses these r esults and possible improvements. It concludes that the growth module of OS YAQ provides a suitable tool for the analysis of the effects of varied stem -to-leaf ratios on yield.