The effect of growth stage and plant counting accuracy of maize inbred lines on LAI and biomass prediction

Accurate maize plant counting plays an essential role in prediction of leaf area index (LAI), aboveground biomass (AGB) and yield. Plant counting of maize inbred lines at early growth stage will result in counting bias caused by death and growth of small seedlings. Therefore, the estimation of LAI and AGB might be negatively affected by plant counting bias at early growth stage. In this study, morphologic discrimination model (MDM) and interpolation discriminant model (IDM) were proposed for plant counting of maize inbred lines at second to fourth (V2–V4) leaf and fourth to sixth (V4–V6) leaf stages with different uncrewed aerial vehicles (UAV) flight heights. Automatic optimum angle calculation of each row, location-based plant cluster segmentation and mosaic method were presented to improve the estimation accuracy of plant counting. Then, the impact of accurate plant counting was evaluated in LAI and AGB prediction at the two growth stages. The results indicated that germination rate difference of some inbred lines could reach up to 38% between V2–V4 and V4–V6 leaf stages. The proposed method accurately estimated the plant counting in the UAV images during V2–V4 leaf stage (R2 = 0.98, RMSE = 7.7, rRMSE = 2.6%) and V4–V6 leaf stage (R2 = 0.86, RMSE = 2.0, rRMSE = 5.5%). The estimated LAI and AGB with plant numbers calculated at V4–V6 leaf stage correlated better with the field measurements (R2 = 0.85 and R2 = 0.9, respectively) compared with those estimated at V2–V4 leaf stage (R2 = 0.8 and R2 = 0.86, respectively). This research indicates that better estimation of LAI and AGB in the field were obtained by accurate plant counting in the late growth stage using UAV images and provides valuable insight for more accurate prediction of yield and crop management and breeding.