Application of convolutional neural networks in weed detection and identification: a systematic review

Citation: García-Navarrete, O.L.; Correa-Guimaraes, A.; Navas-Gracia, L.M. Application of Convolutional Neural Networks in Weed Detection and Identification: A Systematic Review. Agriculture 2024,14, 568. https://doi.org/10.3390/ agriculture14040568 Academic Editors: Ruirui Zhang and Francesco Marinello Received: 14 January 2024 Revised: 26 March 2024 Accepted: 29 March 2024 Published: 2 April 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). agriculture Review Application of Convolutional Neural Networks in Weed Detection and Identification: A Systematic Review Oscar Leonardo García-Navarrete 1,2,* , Adriana Correa-Guimaraes 1and Luis Manuel Navas-Gracia 1,* 1 TADRUS Research Group, Department of Agricultural and Forestry Engineering, Universidad de Valladolid, 34004 Palencia, Spain; [email protected] 2Department of Civil and Agricultural Engineering, Universidad Nacional de Colombia, Bogotá111321, Colombia *Correspondence: [email protected] or olgar[email protected] (O.L.G.-N.); [email protected] (L.M.N.-G.) Abstract: Weeds are unwanted and invasive plants that proliferate and compete for resources such as space, water, nutrients, and sunlight, affecting the quality and productivity of the desired crops. Weed detection is crucial for the application of precision agriculture methods and for this purpose machine learning techniques can be used, specifically convolutional neural networks (CNN). This study focuses on the search for CNN architectures used to detect and identify weeds in different crops; 61 articles applying CNN architectures were analyzed during the last five years (2019–2023). The results show the used of different devices to acquire the images for training, such as digital cameras, smartphones, and drone cameras. Additionally, the YOLO family and algorithms are the most widely adopted architectures, followed by VGG, ResNet, Faster R-CNN, AlexNet, and MobileNet, respectively. This study provides an update on CNNs that will serve as a starting point for researchers wishing to implement these weed detection and identification techniques. Keywords: precision agriculture; weed classification; machine learning; machine vision; image processing; CNN 1. Introduction According to the United Nations, the world population is estimated to reach 9.7 billion inhabitants by 2050 [ 1 ]. Against this backdrop, facing the challenge of feeding this growing population with high quality and sustainable products becomes an imperative task. Increasing crop productivity emerges as a measure to address this need. Thus, a strategy that contributes to improving productivity is proper management of weeds, given their direct impact on crop yields. Integrated weed management is essential to preserve agricultural productivity [ 2 ]. Plants considered as weeds are fast-growing and actively compete for vital resources, such as space, water, nutrients, and sunlight. This competition not only affects resource availability but also has a negative impact on crop yield and quality [ 3 ]. According to [ 4 ], damage due to weeds can represent up to 42% of agricultural production. Currently, diverse weeding techniques are used, such as pre-and post-emergence herbicides, whose application not only generates environmental impacts but also affects the health of the workers who apply them [ 5 ]. Mechanical weeding applies mechanized or manual techniques, whose effectiveness in eliminating weeds is not always the desired one, depending on their stage of development [ 5 ]. Other weeding alternatives are still under development, or their feasibility has not been fully demonstrated. One is physical weeding using plastic covers [ 5 ] and micro-biological weeding involving micro-organisms [ 6 ]. Traditional weeding methods present environmental challenges or economic disadvantages, creating the need to explore innovative solutions based on new technologies to increase treatment efficiency. For instance, precision weeding uses image sensors and computational Agriculture 2024,14, 568. https://doi.org/10.3390/agriculture14040568 https://www.mdpi.com/journal/agriculture Agriculture 2024,14, 568 2 of 19 algorithms to apply herbicides only when weeds are identified [ 7 ]. Precision weeding is done by applying herbicides in a variable way, applying a specific amount in the exact place where the weeds are located. Examples of this are commercial developments, such as Blue River Technology from John Deere, WeedSeeker from Trimble, or Bosch Smart Agriculture, among others, which use algorithms based on artificial intelligence (AI) to recognize weeds in the field. In addition to variable rate herbicide application methods, new precision weeding techniques have been introduced commercially, such as the use of lasers to eradicate weeds by burning. For example, LaserWeeding from Carbon Robotics uses AI to identify weeds and eliminate them with a high-power laser in real time; the development of this type of technology reduces environmental impacts and lowers production costs. Among the AI techniques used in precision weeding is Deep Learning (DL), which is an advanced branch of machine learning (ML) that uses multi-layered artificial neural networks (ANNs) to model and learn complex patterns in data. DL techniques are widely used in agriculture, and their applications are increasing as algorithms are improved [ 8 ]. Several studies compiling DL applications in weed detection are presented in the works of [ 9 – 12 ]. Among the different types of DL neural networks is Convolutional Neural Networks (CNN), a type of ANN architecture specially designed to process visual data, such as images and videos. CNNs efficiently detect spatial patterns in digital images by using convolution layers that apply filters to local regions of the input image [ 9 ]. These convolution layers allow the network to automatically learn hierarchical and complex features, such as edges, textures, and shapes, instead of relying on predefined features. The basic structure of a CNN model consists of three layers (Figure 1), a convolutional layer, a clustering layer, and a connection layer [10]. Agriculture 2024, 14, x FOR PEER REVIEW 2 of 19 to increase treatment efficiency. For instance, precision weeding uses image sensors and computational algorithms to apply herbicides only when weeds are identified [7]. Precision weeding is done by applying herbicides in a variable way, applying a specific amount in the exact place where the weeds are located. Examples of this are commercial developments, such as Blue River Technology from John Deere, WeedSeeker from Trimble, or Bosch Smart Agriculture, among others, which use algorithms based on artificial intelligence (AI) to recognize weeds in the field. In addition to variable rate herbicide application methods, new precision weeding techniques have been introduced commercially, such as the use of lasers to eradicate weeds by burning. For example, LaserWeeding from Carbon Robotics uses AI to identify weeds and eliminate them with a high-power laser in real time; the development of this type of technology reduces environmental impacts and lowers production costs. Among the AI techniques used in precision weeding is Deep Learning (DL), which is an advanced branch of machine learning (ML) that uses multi-layered artificial neural networks (ANNs) to model and learn complex patterns in data. DL techniques are widely used in agriculture, and their applications are increasing as algorithms are improved [8]. Several studies compiling DL applications in weed detection are presented in the works of [9–12]. Among the different types of DL neural networks is Convolutional Neural Networks (CNN), a type of ANN architecture specially designed to process visual data, such as images and videos. CNNs efficiently detect spatial patterns in digital images by using convolution layers that apply filters to local regions of the input image [9]. These convolution layers allow the network to automatically learn hierarchical and complex features, such as edges, textures, and shapes, instead of relying on predefined features. The basic structure of a CNN model consists of three layers (Figure 1), a convolutional layer, a clustering layer, and a connection layer [10]. • The convolutional layer extracts features from the image using mathematical filters; the features can be edges, corners, or alignment patterns, which give the output a feature map that serves as input to the next layer. • The pooling layer reduces the resolution by reducing the dimension of the feature map in order to minimize the computational cost. • The connection layer sends the feature maps obtained from the previous layer to the fully connected neural network layer, which contains the activation function used to recognize the final image. Figure 1. The basic structure of a CNN model. The application of CNNs involves a series of steps: first, data preparation (image acquisition and labeling); second, CNN selection and configuration (hyperparameter tuning); third, CNN training (through GPU processing Graphics Units); fourth, evaluation of CNN performance (usual metrics: average precision (mAP), recall, F1-score, and Confusion Matrix, among others); and fifth, model deployment (real-world applications). Figure 1. The basic structure of a CNN model. • The convolutional layer extracts features from the image using mathematical filters; the features can be edges, corners, or alignment patterns, which give the output a feature map that serves as input to the next layer. • The pooling layer reduces the resolution by reducing the dimension of the feature map in order to minimize the computational cost. • The connection layer sends the feature maps obtained from the previous layer to the fully connected neural network layer, which contains the activation function used to recognize the final image. The application of CNNs involves a series of steps: first, data preparation (image acquisition and labeling); second, CNN selection and configuration (hyperparameter tuning); third, CNN training (through GPU processing Graphics Units); fourth, evaluation of CNN performance (usual metrics: average precision (mAP), recall, F1-score, and Confusion Matrix, among others); and fifth, model deployment (real-world applications). CNNs have gained popularity as an effective image classification method [ 9 ]; the success of CNNs is attributed to their ability to process images effectively and extract relevant features automatically, which allows them to generalize new images due to their supervised learning, which has driven their wide use in practical applications and computer Agriculture 2024,14, 568 3 of 19 vision research. Although CNNs are efficient in object detection and classification, they present some limitations, e.g., large, labelled datasets are required for training. Lack of training data can lead to deficient performance or over-fitting problems. In addition, computational equipment with high processing capabilities, specifically GPUs, is required, since CNNs have a deep architecture with many parameters, which implies a higher need for processor and memory power. To improve the efficiency of CNNs, transfer learning (TL) techniques are adopted; these techniques take advantage of the knowledge acquired from pre-trained CNNs to improve their performance in new training [ 13 , 14 ]. TL aims to transfer knowledge from the source domain to the target application by improving its learning performance [ 15 ]; this is quite useful when the dataset for the target application is small or limited, as the pre-trained model can provide useful representations of the data and speed up the training process. The application of CNNs in agriculture, specifically for weed identification, has significantly improved traditional weeding, which employs classical computer vision, where the analysis is carried out pixel by pixel, and is computationally expensive in processing, especially in execution time, so the use of CNNs in weed identification has improved the detection, localization and recognition of weeds, besides, being faster in achieving real-time applications [ 8 , 9 , 16 ]. Although, in practice, weed detection faces several problems, such as similarities in color, textures, and shapes, as well as occlusion effects and variations in lighting environments, CNNs supported by large-scale datasets have shown great robustness to biological variability and diverse imaging conditions [ 13 ], leading to more accurate classification or detection [ 17 , 18 ], which allows for much more accurate and efficient automation of weeding processes [7,16]. One of the examples of using TL with CNN is AlexNet, which was trained on the ImageNet dataset [ 19 ]. In agriculture, the TL approach has been implemented in weed detection and classification, helping to minimize the need for large-scale image data collection and reduce the computational costs associated with the training hours of a new CNN model [ 20 – 22 ]. In addition to the use of TL in agriculture, some generative adversarial network (GAN) techniques have been applied to generate artificial images in order to augment the training set with TL [ 23 ]. The evolution of CNNs has been marked by significant advances in terms of architecture, training techniques, efficiency, and applications, starting from the need to implement fast solutions in image analysis; some of the most used CNN architectures in weed detection are mentioned below: - AlexNet: Developed by [ 19 ] in 2012, consists of eight convolutional layers and five fully connected layers, with the deep architecture allowing learning of complex hierarchical features from images. It introduced the effective use of Rectified Linear Unit (ReLu) activation functions, which helped mitigate the gradient fading problem and accelerated training. It won the ImageNet Large Scale Visual Recognition Challenge (ILSVRC) competition, demonstrating the impact of CNNs in computer vision, and is considered one of the most influential architectures in the field. Its main advantage is efficiency in training by using GPUs, allowing faster and more efficient training compared to previous methods. Among its disadvantages is that it requires large datasets to learn the complex hierarchical features of the image; therefore, it is also computationally demanding to train and run. - GoogLeNet (Inception): Developed in 2014 by the Google Research team, it introduced the concept of Inception modules with multiple filter sizes in parallel [ 24 ]. The modules are complex network structures with multiple convolution operations operating in parallel with different filter sizes. These modules allow the capturing of features at different scales and hierarchies, significantly improving the model’s ability to recognize complex image patterns. One of the most significant advantages is its high performance with fewer parameters than other architectures, making it computationally efficient. In addition, this architecture captures complex features at different scales, which improves its generalization capability and helps to reduce the risk of Agriculture 2024,14, 568 4 of 19 overfitting in large datasets. As for drawbacks, Inception modules can be complex to implement and understand, making them difficult to use for some developers, and they require large amounts of memory to train and execute, due to their depth and complexity. Although the original GoogLeNet architecture (Inception v1) has been widely used, there have been several versions incorporating improvements; Inception v2 and v3 integrate Batch Normalization techniques, Inception v4 incorporates larger and more complex modules, as well as residual connections inspired by ResNet, and opens the door to versions such as Inception-ResNet and Inception-ResNet v2, improving efficiency and accuracy. - VGG: Developed in 2014 by the Visual Graphics Group (VGG) at Oxford University, it is known for its simplicity and depth [ 25 ]. It is characterized by its depth and uniform structure, mainly composed of 3 × 3 convolutional layers and 2 × 2 pooling layers, followed by fully connected layers. The VGG architecture can have different configurations, from VGG16 (with 16 layers) to VGG19 (with 19 layers), depending on the number of convolutional layers. As an advantage of VGG, this architecture allows efficient feature extraction at different scales and abstraction levels due to its simple structure of 3 × 3 convolutional layers, and its easy implementation due to its uniform and symmetric architecture. Among the disadvantages of VGG is the computational cost in training and execution, due to the depth and number of parameters; also, the large number of parameters in VGG models can increase the risk of overfitting, especially in small datasets. - ResNet (Residual Network): Developed by [ 26 ] in 2015, this architecture was highlighted as using residual blocks, which allow the training of very deep networks. The residual blocks are based on facilitating the flow of information by creating a direct connection path (known as “skip connection” or “shortcut connection”) that prevents the gradient from disappearing during training. ResNet has undergone several versions where the number of layers is increased to improve prediction. The main ones are: ResNet-50, ResNet-101, and ResNet-152, variants with different depths, having 50, 101 and 152 layers, respectively. Their main advantage is based on the ability to train deeper networks by their residual connections, which decreases the degradation in performance, in addition to minimizing overfitting and improving the generalization capability of the model, which allows taking full advantage of the power of deep learning. Despite its efficiency, a disadvantage of ResNet is a higher requirement of significant resources in terms of memory and computational power, especially when training very deep architectures. - Fast R-CNN: an object detection model introduced in 2015, significantly improving its predecessor’s speed and efficiency, is the R-CNN (Region-Based Convolutional Neural Network) model. It was developed by the Microsoft Research group to address the speed and computational efficiency limitations associated with R-CNN, providing a faster and more practical solution for object detection in images [ 27 ]. Fast R-CNN uses a pre-trained CNN network to extract features from the input image. Then, regions of interest (ROIs) are generated using a technique called region proposal (e.g., using algorithms such as Selective Search), and these regions are transformed into a fixed region to be input to the CNN network. Finally, bounding boxes of these proposed regions are classified and fitted using classification and regression layers, respectively [ 27 ]. The advantage of Fast R-CNN is the runtime efficiency of using a single CNN network to extract features and perform region classification rather than passing each proposed region through a separate network. A disadvantage is that it may have difficulty detecting small objects, or in cases of object overlap. Likewise, this architecture has variants, such as Mask R-CNN, which adds an additional branch to the network to perform semantic segmentation of objects in the image, object detection, and classification. - DenseNet (Densely Connected Convolutional Network): In 2017, proposed by [ 28 ], this is notable for its densely connected structure, where each layer is directly con- Agriculture 2024,14, 568 5 of 19 nected to all subsequent layers. This dense connectivity can potentially improve information flow and mitigate the problem of gradient fading. It has influenced the design of subsequent architectures and continues to be a popular choice in research and practical implementation in computer vision tasks. Although DenseNet has been used primarily in its original form since its introduction, there have been some proposed extensions and variants, such as DenseNet 121, 169, and 201, each with a different depth. These numbers represent the total number of layers in the network, including convolutional layers, pooling layers, fully connected layers, and normalization layers. The main advantage of DenseNet is the direct flow of information from the input layers to the output layers, facilitating the learning of complex features and the propagation of gradients through the network. In addition, having a direct data flow mitigates the problem of gradient fading, which facilitates the training of deeper networks. As a disadvantage, it has a higher computational cost, mainly in memory, due to its dense connections requiring more training and inference computations. - MobileNet: Proposed in 2017 by [ 29 ], it is specially designed for implementations on mobile devices and uses lightweight and efficient operations to balance performance and resource consumption. The main feature of MobileNet is its ability to strike a balance between network accuracy and computational efficiency through a series of building blocks called “Depthwise Separable Convolution” that significantly reduce the number of parameters and the amount of computation. The building blocks divide the standard convolution into two separate stages: a depthwise convolution followed by a pointwise convolution; this allows for drastically reduced computational cost without sacrificing too much accuracy [ 29 ]. The advantages of MobileNet are computational efficiency and low resource consumption, which makes it ideal for running on resource-constrained devices, such as cell phones and IoT devices. Its main disadvantage is lower accuracy compared to other larger and more complex architectures in certain computer vision tasks. Attempts have been made to improve the architecture by obtaining versions such as MobileNetV2 and MobileNetV3, which improve accuracy and performance. - YOLO (You Only Look Once): Developed in 2016 by [ 30 ], it is a fast and efficient object detection architecture, as it approaches this task as a regression problem; instead of a separate classification for each region, this feature allows several versions from YOLOv1 up to YOLOv8 in 2023. Starting with the fifth version, released in 2020, known as YOLOv5, this was built on PyTorch [ 31 ], maintaining the original YOLO approach of dividing the image into a grid and predicting bounding boxes with class probabilities for each cell. The overall architecture includes convolutional layers, attention layers, and other modern techniques; it is important to mention that this version was developed by the Ultralytics team, not by the original authors. In 2022, the YOLOv6 and YOLOv7 versions were developed, presenting improvements in their architecture and training scheme, and improving object detection accuracy without increasing the cost of inference, a concept known as “trainable feature bags” [ 32 ]. Finally, in 2023, YOLOv8 was presented; its improvements included new features, better performance, flexibility, and efficiency. Additionally, it includes improvements for detection, segmentation, pose estimation, tracking, and classification [33]. The following is a systematic review of the latest work on the detection and identification of weeds with CNN techniques to serve as a starting point for researchers wishing to implement this technique; for this purpose, the following sections are introduced: (a) Methods, using the PRISMA statement; (b) Results: developing the research question and producing statistics for the literature analysis, of the CNNs used for the detection and identification of weeds, of the sources of image acquisition for training and the study species in the articles reviewed; (c) Discussion: analysis of the articles found based on the proposed objectives; and (d) Conclusions: conclusions and proposals for future work are developed. Agriculture 2024,14, 568 6 of 19 2. Methods In this study, a systematic review was carried out to identify and analyze the scientific literature published on weed detection using CNNs. The guidelines of the PRISMA statement [34] were followed in this review. 2.1. Research Question and Review Objectives 1. Research question: What are the Convolutional Neural Network (CNN) architectures most frequently used for weed detection and what are the most commonly used image acquisition sources for training these CNNs?? 2. Main Objective: Analyze the different CNN architectures used for weed detection and identify the sources of image acquisition for CNN training. 3. Specific Objectives: • To identify and analyze the most commonly used CNN architectures for weed detection in different crops. • To determine the image acquisition sources most commonly used in CNN training for weed identification in different forms of production. 2.2. Sources of Information The following databases were used for the systematic search: Web of Science and Scopus. 2.3. Search for Keywords A primarily search was carried out to establish the relevant words for the systematic search; for this, the Scopus database was used with the words “weeds detection deep learning,” establishing the “search within all fields,” (ALL (weeds AND detection AND deep AND learning); in this search, 6096 results were obtained. With the filtering options in “Filter by keyword,” we found the five most commonly used keywords with their number of matches: “Deep Learning” (1800), “Machine Learning” (988), “Remote Sensing” (704), “Crops” (647), and “Convolutional Neural Networks” (638). The initial search covered the topics of importance for this systematic review and took “Weed detection,” “Deep Learning,” and “Convolutional Neural Networks” as keywords for the search. 2.4. Inclusion and Exclusion Criteria The following inclusion and exclusion criteria were used and implemented through the filters of each database: 1. The search field is selected where the search is directed through titles, abstracts, and keywords, among others; this is specific to each database: •In Scopus, “search within Article title, Abstract, Keywords” was established. • In Web of Science, the search was established in “Topic”; this includes title, abstract, author keywords and keywords plus. 2. The date range of the search is the last five years, from 2019 to 2023. 3. Document type: “Document type: Article”. 4. Excluded are reviews, book chapters, narrative articles, conference or congress articles, unofficial notes or communications, and studies from other areas, such as social, human, biological, chemical, legislative, social and economic impacts. 5. Language: “English Language”. 2.5. Search String in Bibliographic Databases The search equation is established by restrictively connecting all the results containing the keywords “weeds detection” AND “Deep learning” AND “Convolutional Neural Networks.” With this, the search equation is established according to each platform: • Scopus: TITLE-ABS-KEY (“weed detection” AND “deep learning” AND “Convolutional Neural Networks”) Agriculture 2024,14, 568 7 of 19 • WOS: TS = (“weed detection” AND “deep learning” AND “Convolutional Neural Networks”) 2.6. Initial Search Results 1. Initial records: Scopus 104 and WOS 40. Initial results obtained, 144. 2. Records eliminated by exclusion criteria: 65 results eliminated and 79 results remained, 61 from Scopus and 18 from WOS. 2.7. Duplicates and Screening The free access tool Zotero 6.0.30 was used to eliminate duplicate results, and 14 duplicate results were eliminated, obtaining a total of 65 results. 2.8. Additional Records 16 articles obtained from reading book chapters and reviews were added to the results, obtaining 81 results. 2.9. Records Excluded A total of 20 records were excluded because they do not meet the objective of this review or cannot be accessed. Finally, 61 articles that meet the established criteria were gathered and analyzed. Figure 2illustrates the process in a flow chart using the PRISMA methodology. Agriculture 2024, 14, x FOR PEER REVIEW 7 of 19 2.5. Search String in Bibliographic Databases The search equation is established by restrictively connecting all the results containing the keywords “weeds detection” AND “Deep learning” AND “Convolutional Neural Networks.” With this, the search equation is established according to each platform: • Scopus: TITLE-ABS-KEY (“weed detection” AND “deep learning” AND “Convolutional Neural Networks”) • WOS: TS = (“weed detection” AND “deep learning” AND “Convolutional Neural Networks”) 2.6. Initial Search Results 1. Initial records: Scopus 104 and WOS 40. Initial results obtained, 144. 2. Records eliminated by exclusion criteria: 65 results eliminated and 79 results remained, 61 from Scopus and 18 from WOS. 2.7. Duplicates and Screening The free access tool Zotero 6.0.30 was used to eliminate duplicate results, and 14 duplicate results were eliminated, obtaining a total of 65 results. 2.8. Additional Records 16 articles obtained from reading book chapters and reviews were added to the results, obtaining 81 results. 2.9. Records Excluded A total of 20 records were excluded because they do not meet the objective of this review or cannot be accessed. Finally, 61 articles that meet the established criteria were gathered and analyzed. Figure 2 illustrates the process in a flow chart using the PRISMA methodology. Figure 2. Flow chart illustrating the number of articles included in the systematic review according to the PRISMA process. 3. Results 3.1. Literature Analysis A detailed analysis of the 61 bibliographic articles was conducted. The analysis indicated that, in the last two years, there has been a growth in the number of articles published on weed detection (Figure 3). The increasing amount of research in this area Agriculture 2024,14, 568 8 of 19 is mainly due to the development of new and more efficient CNN architectures, the increase in the processing capacity of computers, and the reduction in the price of cameras and GPU. Agriculture 2024, 14, x FOR PEER REVIEW 8 of 19 Figure 2. Flow chart illustrating the number of articles included in the systematic review according to the PRISMA process. 3. Results 3.1. Literature Analysis A detailed analysis of the 61 bibliographic articles was conducted. The analysis indicated that, in the last two years, there has been a growth in the number of articles published on weed detection (Figure 3). The increasing amount of research in this area is mainly due to the development of new and more efficient CNN architectures, the increase in the processing capacity of computers, and the reduction in the price of cameras and GPU. Figure 3. Number of publications for specific years. 3.2. Source of Images for the Training of the CNNs In the review of the selected articles, it was found that the authors used different types of sources for the acquisition of the images used in the training and validation of the CNNs, such as digital cameras, professional Reflex type, industrial high-speed and lowcost cameras, such as those using Raspberry cards. UAVs had various types of cameras, both RGB and multi-spectral. Smartphones with high-resolution cameras were also used. In addition, it was found that some researchers did not acquire images and used free databases or previous works. Figure 4 shows the number of publications according to the source used, where 49.2% used digital cameras, 29.5% used UAV as a mean of acquisition, 11.5% used smartphones, and 9.8% used already built datasets. 3 89 19 22 0 5 10 15 20 25 2019 2020 2021 2022 2023 Number of publication Year of publication Figure 3. Number of publications for specific years. 3.2. Source of Images for the Training of the CNNs In the review of the selected articles, it was found that the authors used different types of sources for the acquisition of the images used in the training and validation of the CNNs, such as digital cameras, professional Reflex type, industrial high-speed and low-cost cameras, such as those using Raspberry cards. UAVs had various types of cameras, both RGB and multi-spectral. Smartphones with high-resolution cameras were also used. In addition, it was found that some researchers did not acquire images and used free databases or previous works. Figure 4shows the number of publications according to the source used, where 49.2% used digital cameras, 29.5% used UAV as a mean of acquisition, 11.5% used smartphones, and 9.8% used already built datasets. Agriculture 2024, 14, x FOR PEER REVIEW 9 of 19 Figure 4. Number of publications by image source for training of CNNs. 3.3. CNN Architecture Used Table 1 summarizes relevant information extracted from the selected reviewed studies on CNN architectures. Table 1. Summary of articles describing CNN architecture and source of image. No. Reference CNN Architecture Source of Image 1 [35] Specifically designed CNN Database 2 [36] Design-specific CNN with SVM Digital Camera 3 [37] AlexNet, GoogLeNet and VGGNet Digital Camera 4 [38] SegNet, UNet, VGG16 and ResNet-50. Digital Camera 5 [39] CRowNet is based on SegNet and the Hough transform UAV multi-spectral camera 6 [40] YOLOv3 and YOLOv3-Tiny Digital Camera 7 [41] ResNet50, VGG16, VGG19, Xception and MobileNetV2. Digital Camera 8 [42] Mask R-CNN Digital Camera 9 [43] YOLOv3, Mask R-CNN, and CNN with SVM-HOG (histograms of oriented gradients) UAV multi-spectral camera 10 [44] YOLO-WEED, based on YOLOv3 UAV Camera 11 [45] Faster R-CNN and Single Shot Detector (SSD) UAV Camera 12 [46] CNN-LVQ-specific design based on Learning Vector Quantization (LVQ) UAV Camera 13 [47] YOLOv3, YOLOv3-Tiny and YOLOv3-Tiny-PRN Digital Camera 14 [48] Faster R-CNN and ResNet50 Digital Camera 15 [49] Detectron2, EfficientDet, YOLOv5, and Faster R-CNN. Digital Camera 16 [50] Faster R-CNN, ResNet-101, VGG16 and Yolov3 UAV Camera 17 [51] VGG-Beet based on VGG16 UAV multi-spectral camera 18 [52] YOLOv5 and Classic K-Nearest Neighbors, Random Forest, and Decision Algorithms tree UAV multi-spectral camera 19 [53] Faster R-CNN and Mask R-CNN Digital Camera 20 [54] Faster R-CNN and VGG16 Digital Camera 21 [55] VGG16, ResNet-50 and Inception-V3 Digital Camera 22 [56] AlexNet vs. VGG-16 UAV Camera 23 [57] YOLOv4, YOLO- sesame. Digital Camera 24 [58] VGG16 and ResNet16 Digital Camera 25 [59] YOLOv3-Tiny Digital Camera 30 18 7 6 Digital Camera UAV Cameras Smartphones Image Database Number of publications Figure 4. Number of publications by image source for training of CNNs. 3.3. CNN Architecture Used Table 1summarizes relevant information extracted from the selected reviewed studies on CNN architectures. Agriculture 2024,14, 568 9 of 19 Table 1. Summary of articles describing CNN architecture and source of image. No. Reference CNN Architecture Source of Image 1 [35] Specifically designed CNN Database 2 [36] Design-specific CNN with SVM Digital Camera 3 [37] AlexNet, GoogLeNet and VGGNet Digital Camera 4 [38] SegNet, UNet, VGG16 and ResNet-50. Digital Camera 5 [39] CRowNet is based on SegNet and the Hough transform UAV multi-spectral camera 6 [40] YOLOv3 and YOLOv3-Tiny Digital Camera 7 [41]ResNet50, VGG16, VGG19, Xception and MobileNetV2. Digital Camera 8 [42] Mask R-CNN Digital Camera 9 [43] YOLOv3, Mask R-CNN, and CNN with SVM-HOG (histograms of oriented gradients) UAV multi-spectral camera 10 [44] YOLO-WEED, based on YOLOv3 UAV Camera 11 [45]Faster R-CNN and Single Shot Detector (SSD) UAV Camera 12 [46] CNN-LVQ-specific design based on Learning Vector Quantization (LVQ) UAV Camera 13 [47]YOLOv3, YOLOv3-Tiny and YOLOv3-Tiny-PRN Digital Camera 14 [48] Faster R-CNN and ResNet50 Digital Camera 15 [49] Detectron2, EfficientDet, YOLOv5, and Faster R-CNN. Digital Camera 16 [50]Faster R-CNN, ResNet-101, VGG16 and Yolov3 UAV Camera 17 [51] VGG-Beet based on VGG16 UAV multi-spectral camera 18 [52] YOLOv5 and Classic K-Nearest Neighbors, Random Forest, and Decision Algorithms tree UAV multi-spectral camera 19 [53] Faster R-CNN and Mask R-CNN Digital Camera 20 [54] Faster R-CNN and VGG16 Digital Camera 21 [55] VGG16, ResNet-50 and Inception-V3 Digital Camera 22 [56] AlexNet vs. VGG-16 UAV Camera 23 [57] YOLOv4, YOLO- sesame. Digital Camera 24 [58] VGG16 and ResNet16 Digital Camera 25 [59] YOLOv3-Tiny Digital Camera 26 [60] YOLOv5 Digital Camera 27 [61]Faster R-CNN, SSD, YOLOv3, YOLOv3-tiny and YOLOv4-tiny Smartphone 28 [62] Faster R-CNN and ResNet Smartphone 29 [63] UNet based on ResNet50 Digital Camera 30 [64] MobileNetV2, ResNet50 UAV Camera 31 [65] YOLO-v3 and faster R-CNN Digital Camera Agriculture 2024,14, 568 16 of 19 and the required computational load. As future work, it is expected that the review will be extended to focus on the search for the most appropriate CNN architectures for weed detection and classification. Finally, it is hoped that this review article will help researchers to create new technological developments that will improve weed detection. Author Contributions: Conceptualization, O.L.G.-N., A.C.-G. and L.M.N.-G.; methodology, O.L.G.- N., A.C.-G. and L.M.N.-G.; software, O.L.G.-N.; validation, O.L.G.-N. and L.M.N.-G.; formal analysis, O.L.G.-N., A.C.-G. and L.M.N.-G.; investigation, O.L.G.-N. and L.M.N.-G.; resources, L.M.N.-G.; writing—original draft preparation, O.L.G.-N. and L.M.N.-G.; writing—review and editing, O.L.G.- N., A.C.-G. and L.M.N.-G.; supervision, O.L.G.-N., A.C.-G. and L.M.N.-G.; project administration, A.C.-G. and L.M.N.-G.; funding acquisition, A.C.-G. and L.M.N.-G. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the European Union through the Horizon Europe Program (HORIZON-CL6-2022-FARM2FORK-01) under project Agro-ecological strategies for resilient farming in West Africa (CIRAWA). Oscar Leonardo García-Navarrete has been financed under the call for University of Valladolid predoctoral contracts, co-financed by Banco Santander. 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