Analysis of growth and nutrition value in timothy using a dynamic model

`The growth and changes in crude protein concentration and digestibility of timothy grass (Phleum pratense L.) were studied during two years with the aim of determining how differences between the years could be explained by variation in climatic conditions. The study was done in Sweden in 1995 and in 1996. The QUAL-model. which simulates growth and dynamics of nitrogen (N ) and indigestible fibre in forage grass crops as a function of climate, wa s used to evaluate the importance of different climatic factors. The model was calibrated for the season, using field measurements. Dry matter (DM) yi eld, nitrogen concentration, ash concentration and in vitro digestible orga nic matter concentration were measured to give initial values of canopy cov er, N mineralisation capacity and initial indigestible fibre concentration. When using the model for prediction, one calibration sample was taken earl y in the season (at about 25 cm canopy height). When using the model as a t ool to explain the differences between the two years, all measured samples before lodging were used to minimise the effects of eventual analysis error s. Before 19 May in both years, low temperature limited both growth and cha nges in nutritive value. After 19 May incoming solar radiation was less in 1995 than in 1996, but the canopy cover was higher in 1995 resulting in abo ut the same intercepted radiation per unit area in both years. Low temperat ure, water deficit and N deficiency limited growth mon in 1996. Maintenance respiration was higher in 1995, because of higher plant density. Soil N mi neralisation was higher in 1995 because of warmer and moister conditions, r esulting in higher N yield in the fodder. The model, calibrated with one ca libration sample, predicted the changes in digestibility better than using observed phase development. The model fitted the observed data with root me an square errors of 16g m(-2) for DM production, 0.2% units for crude prote in concentration and 0.3 MJ (kg DM)(-1) for metabolisable energy concentrat ion. (C) 2001 Elsevier Science B.V. All rights reserved.