Prediction of stem extension of lucerne (Medicago sativa L.) in early spring

Abstract

The ability to predict the time of rapid stem extension in spring can be used to assist the development of lucerne grazing management plans. Therefore, the aim of this work was to validate the APSIM_Lucerne height function generated by Yang et al. (2021) for early spring. Two experiments in 2022 and 2023 used four winter defoliation dates (01/06, 03/07, 17/07, 01/08) to measure the time and rate of initial stem extension (height) in spring. The data were used to validate the APSIM_Lucerne height function which is based on thermal time accumulation targets modified by photoperiod. The current exponential decay curve in ASPIM showed a poor agreement (RMSE = 2.1) because it had limited data through the winter period. This was modified using the 2022 and 2023 field data, which showed the start of stem extension occurred at a photoperiod of 11.1 h. This was consistent with the original dataset. The change in photoperiod direction was accounted for calculating a weighted mean photoperiod from the start of the regrowth rotation (in a decreasing photoperiod) up to the 11.1 h increasing photoperiod until the end of rotation. This improved (RMSE = 1.49) the prediction of plant height and is recommended for implementation into the APSIM_Lucerne framework. In the field, the start of maximum stem extension occurred at an 11.1 h in both years, which suggests this is the critical photoperiod required before rapid stem extension occurs. However, this remains to be validated in other environments.