**2.2 Sensor-based variable rate application (VRA)**

Data collection of weed presence and processing in sensor-based VRA are made fractions of seconds before herbicide application, avoiding the need to generate a previous map of the area. Sensor-based systems have the ability to vary application rate without any mapping or prior data collection. Sensors measure in real time the desired properties while they are in motion. The measurements made by the system are processed immediately and sent to the controller who will perform the application at a varied rate.

The use of sensors does not necessarily require the use of a positioning system, map generation, or extensive data analysis before making the VRA. Thus, it is an easier-to-use system, consumes less time, and has greater accuracy when compared to the map-based method. Its current limitation is related to the state of

**183**

*Variable Rate Application of Herbicides for Weed Management in Pre- and Postemergence*

the development of sensors and algorithms with sufficient accuracy to collect and

In **Figure 3**, there is an example of this type of method, where an optical sensor along with an infrared light source is implanted in the machinery spray bar. This set will be responsible for identifying weeds in the field by reflecting the green color of the leaves and indicating to the controller which sites will be necessary to carry out

The objective of an herbicide application in preemergency is to manage weeds that have not yet germinated, and the herbicide application is made directly in the soil so that as soon as the seeds/propagules germinate, they can absorb the herbicide. But for this to occur, the herbicide must be bioavailable in the soil solution. The application of herbicides in PRE follows different destinations due to the herbicide-soil interactions regulated by physical, chemical, and biological

The efficiency of chemical control is associated with several factors that will determine whether the herbicides will be in the soil solution, thus being absorbed by the vegetables; leached, including groundwater; transported by the process of erosion or runoff; and volatilized [13]. In addition, they can be sorbed by soil col-

The variability of soil properties can cause a differential sorption of herbicides,

which, in turn, reflects on the different availability of the herbicide in the soil solution, and may generate variation in weed control [14, 15], especially in large cultivated areas where herbicide application is made in a single dose. Thus, the VRA for herbicides in PRE should obtain the main data related to herbicide retention and availability in the soil solution in order to have the correct deposition of the

Herbicide sorption is dependent on the interaction of the molecules of the product with the soil, and the process is influenced by the management and climate, mainly soil temperature and humidity. The main physicochemical characteristics of the soil that affect herbicide sorption are organic matter (OM), texture, pH, and cation exchange capacity (CEC). Regarding the herbicide physicochemical characteristics, water solubility (Sw), acid/base dissociation constant (pKa/pKb), octanolwater coefficient (Kow) half-life degradation time (DT50), and mainly sorption/

Each herbicide will have a type of behavior in different soil classes. Therefore, to perform VRA in PRE, a previous study of sorption and desorption of the herbicide molecule in the soil type of interest is necessary for the VRA to be efficient. Currently, the technique for sorption and desorption studies of herbicides most

*DOI: http://dx.doi.org/10.5772/intechopen.93558*

process more detailed information of plants and soil.

*Acting of an optical sensor in the control of spray nozzles. Source: Grisso et al. [7].*

**3. Variable rate application (VRA) in preemergence (PRE)**

herbicide application.

**Figure 3.**

processes [12].

product.

loids, thus becoming unavailable to plants.

desorption coefficient (Kd) [10].

*Variable Rate Application of Herbicides for Weed Management in Pre- and Postemergence DOI: http://dx.doi.org/10.5772/intechopen.93558*

**Figure 3.**

*Pests, Weeds and Diseases in Agricultural Crop and Animal Husbandry Production*

tional difficulty of map-based systems is greater.

**2.2 Sensor-based variable rate application (VRA)**

application at a varied rate.

coordinate occupied by the machinery at the time of application. Thus, the opera-

the map method is very efficient when used correctly and with accurate equipment. **Figure 2** shows a mapping of weed distribution in a given area and correlated with the required amount of herbicide needed to control weeds according to their density. The result of this crossing of information is a varied rate application map. In the area, there were infestations ranging from 0 to >30 plants m−2; so, it is not necessary to apply the same dose at all levels of infestations [11]. Areas with higher infestation will receive more herbicide than areas with low infestation. In the specific case, the volume of syrup varies from 100 to 250 L ha−1, which corresponds to a variation of 150%. If the volume of syrup was kept constant, there would certainly be herbicide wasting due to excess or lack in certain places. In the example of **Figure 2**, the VRA allowed uniform yield of the crop that was implanted, reduced environmental impacts, and provided savings of 29% in the amount of herbicide.

Although it has some disadvantages referring to operating costs and complexity,

Data collection of weed presence and processing in sensor-based VRA are made fractions of seconds before herbicide application, avoiding the need to generate a previous map of the area. Sensor-based systems have the ability to vary application rate without any mapping or prior data collection. Sensors measure in real time the desired properties while they are in motion. The measurements made by the system are processed immediately and sent to the controller who will perform the

The use of sensors does not necessarily require the use of a positioning system, map generation, or extensive data analysis before making the VRA. Thus, it is an easier-to-use system, consumes less time, and has greater accuracy when compared to the map-based method. Its current limitation is related to the state of

*Weed density map (left) and variable rate application (VRA) of herbicide (right). Source: Carrara et al. [11].*

**182**

**Figure 2.**

*Acting of an optical sensor in the control of spray nozzles. Source: Grisso et al. [7].*

the development of sensors and algorithms with sufficient accuracy to collect and process more detailed information of plants and soil.

In **Figure 3**, there is an example of this type of method, where an optical sensor along with an infrared light source is implanted in the machinery spray bar. This set will be responsible for identifying weeds in the field by reflecting the green color of the leaves and indicating to the controller which sites will be necessary to carry out herbicide application.

## **3. Variable rate application (VRA) in preemergence (PRE)**

The objective of an herbicide application in preemergency is to manage weeds that have not yet germinated, and the herbicide application is made directly in the soil so that as soon as the seeds/propagules germinate, they can absorb the herbicide. But for this to occur, the herbicide must be bioavailable in the soil solution. The application of herbicides in PRE follows different destinations due to the herbicide-soil interactions regulated by physical, chemical, and biological processes [12].

The efficiency of chemical control is associated with several factors that will determine whether the herbicides will be in the soil solution, thus being absorbed by the vegetables; leached, including groundwater; transported by the process of erosion or runoff; and volatilized [13]. In addition, they can be sorbed by soil colloids, thus becoming unavailable to plants.

The variability of soil properties can cause a differential sorption of herbicides, which, in turn, reflects on the different availability of the herbicide in the soil solution, and may generate variation in weed control [14, 15], especially in large cultivated areas where herbicide application is made in a single dose. Thus, the VRA for herbicides in PRE should obtain the main data related to herbicide retention and availability in the soil solution in order to have the correct deposition of the product.

Herbicide sorption is dependent on the interaction of the molecules of the product with the soil, and the process is influenced by the management and climate, mainly soil temperature and humidity. The main physicochemical characteristics of the soil that affect herbicide sorption are organic matter (OM), texture, pH, and cation exchange capacity (CEC). Regarding the herbicide physicochemical characteristics, water solubility (Sw), acid/base dissociation constant (pKa/pKb), octanolwater coefficient (Kow) half-life degradation time (DT50), and mainly sorption/ desorption coefficient (Kd) [10].

Each herbicide will have a type of behavior in different soil classes. Therefore, to perform VRA in PRE, a previous study of sorption and desorption of the herbicide molecule in the soil type of interest is necessary for the VRA to be efficient. Currently, the technique for sorption and desorption studies of herbicides most

used and mentioned in the literature is liquid or gas chromatography. The chromatographic technique can identify individual compounds quantitatively and qualitatively even at small concentrations, being very useful to identify herbicide concentrations in a solution. However, sorption and desorption studies can also be performed with radioisotopes (14C and 3 H), in addition to bioassay with plant species sensitive to herbicide [16–18].

Data on soil characteristics are difficult to obtain with sensors in the field; so, most methods for applying herbicides in PRE are based on the generation of maps from laboratory analyses of soil samples. From soil information and herbicide sorption and desorption, a map is interpolated with application information at varying rate [10].

A study of sorption and desorption of the herbicide cyanazine was carried out in different soils (**Table 2**). From this study, the herbicide application was recommended based on soil texture and OM content. Herbicide doses increase as clay and OM contents increases.

Thus, for the application of PRE, herbicide is necessary to analyze the soil's physicochemical properties to interpolate the VRA map. **Figure 4** contains the VRA map in which the different colors represent doses of herbicide to be applied. In this study [15], the use of VRA in PRE decreased the total amount of herbicide by 13%. In addition to the herbicide economy, it should be considered that other benefits are obtained such as better efficiency in weed control, which can help in an increase in productivity, in addition to reducing environmental risks.

Laboratory analyses of soil characteristics are very efficient and accurate. The major disadvantage is the high costs of soil analysis, compromising its use for very large areas. An alternative to map the soil characteristics responsible for herbicide retention and availability without the need for labor collection and analysis is the use of electrical conductivity sensors in the field. The mapping of electrical conductivity with the aid of GPS is a simple tool, which is used to estimate soil texture, in addition to other properties [19]. This quantification considers the clay and ion contents in the soil, resulting in significant correlations [20].

An example of a sensor used to measure electrical conductivity is the VARIS 3100 platform (**Figure 5**). The operation of the equipment consists in the emission of an electric current by two intermediate discs, while two internal discs and two external discs detect the potential difference, which occurs in the electromagnetic field generated in the soil resulting from the applied electric current [21]. The spacing between the discs is calculated so that values of electrical conductivity are measured at depths of 0–0.30 m and 0–0.90 m. Data obtained in the field can be


#### **Table 2.**

*Recommendation of doses of cyanazine (L ha−1) according to the texture and organic matter content of the soil.*

**185**

**Figure 5.**

**Figure 4.**

*Source: Mohammadzamani et al. [15].*

*Variable Rate Application of Herbicides for Weed Management in Pre- and Postemergence*

visualized, recorded, and exported, since the sensor has a data logger. Data collection occurs with moving equipment, coupled to a tractor and the whole process can be georeferenced by a global navigation satellite system (GNSS) receiver. According to the manufacturer's instructions, two tests must be performed to confirm the correct calibration of the equipment. After data collection, the electrical conductivity

*Veris Platform® 3100 to measure the electrical conductivity of the soil. ESALQ/USP, Piracicaba, SP, Brazil.*

*Two-dimensional (I) and three-dimensional (II) maps for variable rate application (VRA) of cyanazine.* 

is correlated with the clay content for the generation of a textural map.

*DOI: http://dx.doi.org/10.5772/intechopen.93558*

*Variable Rate Application of Herbicides for Weed Management in Pre- and Postemergence DOI: http://dx.doi.org/10.5772/intechopen.93558*


#### **Figure 4.**

*Pests, Weeds and Diseases in Agricultural Crop and Animal Husbandry Production*

be performed with radioisotopes (14C and 3

species sensitive to herbicide [16–18].

rate [10].

OM contents increases.

used and mentioned in the literature is liquid or gas chromatography. The chromatographic technique can identify individual compounds quantitatively and qualitatively even at small concentrations, being very useful to identify herbicide concentrations in a solution. However, sorption and desorption studies can also

Data on soil characteristics are difficult to obtain with sensors in the field; so, most methods for applying herbicides in PRE are based on the generation of maps from laboratory analyses of soil samples. From soil information and herbicide sorption and desorption, a map is interpolated with application information at varying

A study of sorption and desorption of the herbicide cyanazine was carried out in different soils (**Table 2**). From this study, the herbicide application was recommended based on soil texture and OM content. Herbicide doses increase as clay and

Thus, for the application of PRE, herbicide is necessary to analyze the soil's physicochemical properties to interpolate the VRA map. **Figure 4** contains the VRA map in which the different colors represent doses of herbicide to be applied. In this study [15], the use of VRA in PRE decreased the total amount of herbicide by 13%. In addition to the herbicide economy, it should be considered that other benefits are obtained such as better efficiency in weed control, which can help in an increase in

Laboratory analyses of soil characteristics are very efficient and accurate. The major disadvantage is the high costs of soil analysis, compromising its use for very large areas. An alternative to map the soil characteristics responsible for herbicide retention and availability without the need for labor collection and analysis is the use of electrical conductivity sensors in the field. The mapping of electrical conductivity with the aid of GPS is a simple tool, which is used to estimate soil texture, in addition to other properties [19]. This quantification considers the clay and ion

An example of a sensor used to measure electrical conductivity is the VARIS 3100 platform (**Figure 5**). The operation of the equipment consists in the emission of an electric current by two intermediate discs, while two internal discs and two external discs detect the potential difference, which occurs in the electromagnetic field generated in the soil resulting from the applied electric current [21]. The spacing between the discs is calculated so that values of electrical conductivity are measured at depths of 0–0.30 m and 0–0.90 m. Data obtained in the field can be

Sand 0.60 0.75 1.25 1.50 1.75 2.00 Sandy loam 0.75 1.25 1.50 1.75 2.00 2.25 Loam, silty loam, silt 1.25 1.50 1.75 2.00 2.25 2.50

Sandy clay, silty clay, and clay 1.75 2.00 2.25 2.50 3.75 3.00

*Recommendation of doses of cyanazine (L ha−1) according to the texture and organic matter content of the soil.*

**<1.0 1.0 2.0 3.0 4.0 ≤5.0**

1.50 1.75 2.00 2.25 2.50 2.75

**Soil texture Soil organic matter content (%)**

Peat or muck Not recommended

productivity, in addition to reducing environmental risks.

contents in the soil, resulting in significant correlations [20].

H), in addition to bioassay with plant

**184**

**Table 2.**

Sand clay loam, clay loam, and

*Source: Mohammadzamani et al. [15].*

silty clay loam

*Two-dimensional (I) and three-dimensional (II) maps for variable rate application (VRA) of cyanazine. Source: Mohammadzamani et al. [15].*

**Figure 5.**

*Veris Platform® 3100 to measure the electrical conductivity of the soil. ESALQ/USP, Piracicaba, SP, Brazil.*

visualized, recorded, and exported, since the sensor has a data logger. Data collection occurs with moving equipment, coupled to a tractor and the whole process can be georeferenced by a global navigation satellite system (GNSS) receiver. According to the manufacturer's instructions, two tests must be performed to confirm the correct calibration of the equipment. After data collection, the electrical conductivity is correlated with the clay content for the generation of a textural map.

Studies show that the electrical conductivity measured by contact sensor adequately reflects the variation in clay contents of the studied soil, being efficient to generate soil texture maps, including in no-tillage areas [21]. **Figure 6** shows a conductivity map elaborated with the data collected in VARIS 3100; the lowest conductivity values correlated with lower clay contents. However, for high clay contents, the model was less efficient. Thus, the mapping of electrical conductivity can be a useful tool in the design of more homogeneous areas, which present more similar soil conditions.

Considering that other factors such as moisture, salt concentration, and total carbon remain in the same conditions, soils with higher clay contents conduct more electricity than those with sandier texture. However, these factors may vary and affect the correlation between electrical conductivity and soil texture. Therefore, as the electrical conductivity method does not quantify the CEC and soil OM contents, the use of the same may have reduced efficiency in some situations.

There are companies on the market that provide the VRA service for herbicides in PRE, one of which is APagri which has the HTV® method which consists of a process developed and patented for the application of herbicides in PRE at the varied rate based on maps (**Figure 7**), that considers the clay, OM, and CEC content of the soil [22]. The objective is to adjust the dose according to the soil ability to retain each type of herbicide so that the final concentration in the soil solution is equal regardless of the position in space.

Due to technological limitations, there is still no VRA available on the market for PRE herbicides based on sensors that read, process, and apply the herbicide without the need for the generation of maps. One of the great challenges of this market is precisely to eliminate this stage, in view of the costs of generating the maps.

#### **Figure 6.**

*Interpolated map of electrical conductivity measured with mobile contact measurement equipment. Source: Machado et al. [21].*

**187**

*Variable Rate Application of Herbicides for Weed Management in Pre- and Postemergence*

**4. Variable rate application (VRA) in postemergence (POST)**

*Variable rate application (VRA) map drawn up with the system HTV®. Source: APagri [22].*

The purpose of a POST application is to control weeds that have already emerged in the field. Thus, the target of the application is the aerial part of the plant species. For the VRA to be used in POST, it is necessary that the system has information about the weed population in the area. This information can be collected by the mapbased and sensor-based systems. Therefore, both methods can be used VRA in POST.

The literature mentions several methodologies for weed mapping, where each one has its specificity. Some have processing algorithms to differentiate monocot and eudicot plants [23]. Others use machine learning with deep neural network to identify weeds [24, 25]. However, all have the principle based on the quantitative and qualitative identification of the infested area, generation of the recommenda-

Remote sensing is generally considered one of the most important technologies for precision agriculture. This technology can be used in weed mapping. Remote sensing can monitor many crops and vegetation parameters through images at various wavelengths. Images can be acquired by satellites, manned aircrafts, or unmanned aerial vehicles (UAV). However, satellite imagery is often not the best option because of the low spatial resolution of images acquired and the restrictions of the temporal resolutions as satellites are not always available to capture the necessary images [26]. Considering the use of manned aircrafts, usually it results in high costs, and many times, it is not possible to carry out multiple flights to obtain more than a few crop images. UAVs' ability to fly at a low altitude results in ultra-high spatial resolution images of the crops (i.e., a few centimeters). This significantly improves the performance of the monitoring systems. Furthermore, UAV-based monitoring systems have high temporal resolution as they can be used at the user's will. This enhances the flexibility of the image acquisition process [27]. In addition, UAVs are a lot simpler to use and also cheaper than manned aircrafts. Moreover,

*DOI: http://dx.doi.org/10.5772/intechopen.93558*

**4.1 Map based: weed mapping**

**Figure 7.**

tion map, and integration with the VRA system.

*Variable Rate Application of Herbicides for Weed Management in Pre- and Postemergence DOI: http://dx.doi.org/10.5772/intechopen.93558*

**Figure 7.** *Variable rate application (VRA) map drawn up with the system HTV®. Source: APagri [22].*
