Modelling Spring Growth of Timothy and Meadow Fescue by an Expolinear Growth Equation

The applicability of an expolinear growth equation for describing dry matter yield was investigated in seven field experiments for spring growth of timothy (Phleum pratense L.) and meadow fescue (Festuca pratensis Huds.) under two levels of N application. The equation was expanded by a growth index (GI) correcting for variations in radiation, temperature and plant-available soil moisture, and an ageing function describing the decrease in growth rate caused by advance in phenological development. The field sites covered a wide range of climatic conditions and the yield was recorded at five phenological stages from leaf stage to anthesis. The expansion of the equation appeared to be adequate for a combined analysis of the dry matter yield in meadow fescue and timothy. The estimated maximum growth rate during the linear phase (C _m) did not differ significantly between species. C _m increased with higher N application. It was concluded that C _m of the expanded model represented a potential rate, whereas the relative growth rate of the exponential phase (R _m) could not be considered as a potential rate. It varied more among locations and years, e.g. it was strongly affected by the length of the period from growth onset to the start of the linear phase.