Image Recognition of Male Oilseed Rape (Brassica napus) Plants Based on Convolutional Neural Network for UAAS Navigation Applications on Supplementary Pollination and Aerial Spraying

Zhu Sun,Xiangyu Guo,Qiong Hu,Songchao Zhang,Xiaohui Cheng,Yang Xu,Xinyu Xue,Wenxiang Wang

doi:10.3390/agriculture12010062

Abstract

To ensure the hybrid oilseed rape (OSR, Brassica napus) seed production, two important things are necessary, the stamen sterility on the female OSR plants and the effective pollen spread onto the pistil from the OSR male plants to the OSR female plants. The unmanned agricultural aerial system (UAAS) has developed rapidly in China. It has been used on supplementary pollination and aerial spraying during the hybrid OSR seed production. This study developed a new method to rapidly recognize the male OSR plants and extract the row center line for supporting the UAAS navigation. A male OSR plant recognition model was constructed based on the convolutional neural network (CNN). The sequence images of male OSR plants were extracted, the feature regions and points were obtained from the images through morphological and boundary process methods and horizontal segmentation, respectively. The male OSR plant image recognition accuracies of different CNN structures and segmentation sizes were discussed. The male OSR plant row center lines were fitted using the least-squares method (LSM) and Hough transform. The results showed that the segmentation algorithm could segment the male OSR plants from the complex background. The highest average recognition accuracy was 93.54%, and the minimum loss function value was 0.2059 with three convolutional layers, one fully connected layer, and a segmentation size of 40 pix × 40 pix. The LSM is better for center line fitting. The average recognition model accuracies of original input images were 98% and 94%, and the average root mean square errors (RMSE) of angle were 3.22° and 1.36° under cloudy day and sunny day lighting conditions, respectively. The results demonstrate the potential of using digital imaging technology to recognize the male OSR plant row for UAAS visual navigation on the applications of hybrid OSR supplementary pollination and aerial spraying, which would be a meaningful supplement in precision agriculture.

Highlights

The oilseed rape (OSR, Brassica napus) is the third most important oil crop in the world [1,2,3,4,5]
Finding a new navigation method is important for unmanned agricultural aerial system (UAAS) application on the OSR, especially the hybrid OSR seed production
This study proposed an image recognition method based on convolutional neural network (CNN) to recognize the male OSR plants

Summary

Introduction

The oilseed rape (OSR, Brassica napus) is the third most important oil crop in the world [1,2,3,4,5]. The seed production is a guarantee for its large-scale planting of OSR. That is, cross-pollination generally, is an important means for high yield of the OSR seed because, in this way, genetic variability is ensured. Spreading the pollen from the stamens of the male OSR plants to the pistils of the female OSR plants and the stamen sterility of the female OSR plants are necessary during the OSR seed production [7,8,9]. The anthericide is sprayed on the female OSR plants to make them sterile, and the pollen spreads from the male OSR plants to the female ones to achieve cross-pollination during the flowering period, forming siliques usually. Carried different airborne mission equipment, the UAAS can realize remote sensing [10,11], aerial spraying [12,13,14], particle fertilizing [15], aerial seeding [16], etc

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