Evaluating a hand‐held crop‐measuring device for estimating the herbage biomass, leaf area index and crude protein content in an Italian ryegrass field

The objective of this study is to evaluate the ability of a newly developed hand-held crop-measuring device and vegetation indices (VIs) to estimate the herbage biomass (BM), leaf area index (LAI) and forage crude protein mass (CPmass) in an Italian ryegrass (Lolium multiflorum Lam.) field, Japan. The device uses bi-directional passive sensors (550, 650 and 880 nm) upward and downward to overcome the major drawback of optical remote sensing as influenced by weather conditions. The canopy reflectance and plant sample data were collected 11 times during two winter growing seasons in 2010–11 and 2011–12. Seven VIs were compared to estimate the forage parameters in the normal and logarithmic forms. The predictive ability of the VIs was assessed by the cross-validated coefficient of determination (R2cv) and the residual prediction (RPD) values. Acceptable RPD values (>1.5) were found in most of the log-transformed forage parameters with all of the VIs but not in most of the normal-form. The highest R2cv and RPD were obtained in the normalized difference vegetation index (NDVI) for the ln BM (R2cv = 0.76, RPD = 2.04) and ln LAI (R2cv = 0.80, RPD = 2.25), and the highest modified soil-adjusted vegetation index (MSAVI) was obtained for the ln CPmass (R2cv = 0.78, RPD = 2.16). By evaluating the limitation of NDVI or MSAVI sensitivity with ranges of cumulative data, the log-transformed forage parameters showed good R2cv values in the ln BM (0.76–0.79), ln LAI (0.80–0.84) and ln CPmass (0.71–0.84) with acceptable RPD values throughout most of the range, whereas in the normal-form, the NDVI or MSAVI was saturated at moderate-high BM, LAI or CPmass, causing the R2cv and RPD values to decrease with increasing plant parameters. These results suggest that this device is applicable in log-transformed BM, LAI and CPmass throughout the growing season without cloud effect.