An empirical statistical model for predicting the yield of herbage from legume-grass swards within organic crop rotations based on cumulative water balances

Multi-species legume-grass swards play a key role in organic farming systems as a source of forage and of symbiotically fixed nitrogen (N). Forage and N balances, based on site-specific data on dry matter (DM) yields of herbage, are useful planning tools in describing forage and N supply at the field and farm level. In practice, measurements of DM yield are often imprecise or missing, and thus a weather- and site-specific assessment of DM yield is required. An empirical model to predict DM yield from legume-grass swards was developed based on weather and soil data commonly available at field and regional scales. The main underlying hypothesis was that water use, calculated from cumulative water balances, can be used as a predictor of DM yield. The model was calibrated with data from a multi-year field experiment in Muncheberg, north-east Germany and was tested with data from other countries of Europe. In the calibration data set, highly significant linear relationships were found between water use and DM yield for DM yield of single harvests and for annual DM yield. The only additional variable significantly improving the prediction of DM yield was cut number. For the validation data set the DM yield for single cuts and annual yields was predicted with a similar accuracy as found with other models requiring the use of more information. The models described offer a straightforward weather- and site-specific means of predicting DM yield with a satisfactory level of precision, especially for annual DM yields, and thus can help to reduce planning failures concerning forage and N supply in organic farming systems in Europe.