Estimation of the Leaf Area Index in cereal crops for variable rate fungicide spraying

One method of optimising fungicide application in heterogeneous cereal fields is to apply the same concentration of active fungicidal substance per unit of crop canopy surface area. The crop canopy surface can be characterised by the Leaf Area Index (LAI). This index is used in precision farming for controlling the application rate in sensor-based fungicide spraying in cereal crops. For research purposes the LAI is measured using optical hand-held instruments. This method is not applicable in agricultural practice where rapid information about the LAI of the whole field is needed just before spraying. The CROP-Meter is a real-time sensor for measuring crop biomass density within cereal fields.The paper starts by analysing the correlation between the CROP-Meter records and LAI measurements obtained from hand-held optical instruments in farmers’ cereal fields. The sensor signal (deflection angle) of the CROP-Meter showed good linear correlation with the reference LAI measurements (R2 values from 15 errors-in-variable regression analyses: 0.45–0.86).As the CROP-Meter sensor signal is correlated linearly with the LAI, this mechanical sensor can be used for controlling a field sprayer for variable rate fungicide spraying. The paper goes onto present a calibration routine for variable rate fungicide application for use in conjunction with a CROP-Meter controlled field sprayer:
• The standard dosage of fungicide is prepared with water in the tank of the field sprayer.
• This standard dosage (upper application rate) is applied uniformly along a typical tramline of the field, which is representative of the variability in plant biomass density.
• The upper and lower deflection angle measured along this tramline are then recalled from the working display of the on-board terminal.
• On the calibration display, the upper application rate is assigned to the upper deflection angle and the lower application rate is assigned to the lower deflection angle on the calibration display of the on-board terminal. The proportion of the lower and upper application rates depends on the range of LAI found in the field.
• The application rate is adapted linearly according to the deflection angle of the CROP-Meter.To define the routine at the on-board terminal the farmer has to decide on the minimum and maximum application rate. As the variability of the LAI occurring in heterogeneous fields is a criterion for choosing the minimum and maximum application rates, the third part of the paper discusses a simple deterministic model to determine the LAI in the field. This was based on the results from 46 errors-in-variable regression analyses of the functional relationship between the product of crop height (m) times number of tillers (m−2) and the measured reference values of LAI. Crop height and the number of tillers were used as the only parameters for this model. The product of the two has to be divided by 100 to obtain an estimate of the LAI. There was a good correlation between the LAI* estimated by the model and the LAI measured by the optical hand-held instruments (Pearson's correlation coefficients: 0.71, 0.91, 0.80, 0.64) in four analysed growth stages in winter wheat and winter barley (shooting, ear emergence, flowering, ripeness). The method presented enables the farmer to obtain information about the range of the LAI occurring within his heterogeneous cereal field very rapidly without using hand-held optical instruments.