Chapter 12

Variable-Rate Technology in Precision Agriculture

Applications of Variable-Rate Technology in Precision Agriculture

In precision agriculture, the main types of variable-rate applications include variable-rate fertilizer, variable-rate seeding, variable-rate irrigation, variable-rate pesticide application, and variable-rate herbicide application, all of which involve adjusting the application rate of inputs like fertilizer, seeds, water, and pesticides on specific areas of a field according to their varying needs, often using data from soil maps and sensor technology.

Variable-Rate Application for Seeding

Variable-rate seeding (VRS) is a precise agricultural technology that can properly and accurately adjust the seeding rate according to the variability of soil properties, terrain, meteorological conditions, and other factors. VRS not only provides better opportunities for using variable soil nutrient and water storage capacity characteristics, but it can also increase crop yields by reducing seeding rates. Seed germination, crop development, and yield potential may vary in different areas of a field, and thus, VRS is a method of linking seed quantities to a specific area, thereby increasing crop yields and production profits.

Variable-Rate Application for Fertilizer

Fertilizer application is an essential and critical practice in agricultural production systems. Fertilization is one of the greatest cost inputs in crop production. Taking fertilizer nitrogen (N) as an example, the use of agricultural inputs has increased significantly during the past few decades.

Map-Based VRT for Fertilizer Application

The map-based VRT fertilizer application requires a prescription map, VRA controller, software, and GPS receiver. The GPS receiver identifies the VRA’s location in the field when it travels across the field. The VRA controller provides an electrical signal that controls a mechanical actuator to apply fertilizer at a specified rate to that specific location in the field based on spatial information from the GPS receiver and data from the prescription map. The prescription map for variable-rate fertilizer should include spatial coordinates of each location and the fertilizer application rate associated with each location within the field.

Sensor-Based VRA for Fertilizer

No prescription map is needed for sensor-based variable-rate fertilization. Sensor-based applicators use sensors to collect information to determine the nutrient application rate in real time and in situ. While implementing the sensor-based nutrient application, an on-the-go sensor, and control system is required for quantifying the fertilizer needs of crops and applying the required fertilizer rate at each specific location. Several sensors are commercially available for variable-rate fertilizer application. Crop sensors such as Crop Circle and GreenSeeker® are routinely used to monitor plant canopy reflectance.

Variable-Rate Application for Weeds

There are two very different approaches for weed detection: the map-based approach and the sensor-based or real-time approach. Both methods require the application rates to be determined based on site-specific conditions, though only the map-based method requires a prescription map. In map-based approaches, sensor data are stored and processed off-line, with images stitched together or sampled sensor data being interpolated. Decision algorithms may integrate other types of data, such as agronomic and economic data or preferences for a weed control strategy over several years, generating a prescription map.

Sensor-Based VRA for Weeds

Sensor-based weed control spraying herbicides use sensors to detect weeds and apply herbicides in real-time. No prior mapping or data collection is required. Photoelectric sensors emit light and then detect differences in wavelengths of the light reflected off targeted surfaces. This information is used for automated object recognition. For example, green plants absorb most red light while reflecting light in near-infrared regions, which is useful to distinguish green plants from the soil background. More sophisticated photoelectric sensors can use differences in reflective wavelengths to differentiate weeds from crops. With the relatively low cost of photoelectric sensors and the benefits of real-time feedback, several variable-rate sprayer systems using this technology are commercially available.

Machine Learning. Weed detection using machine vision technology can be broadly categorized into two methods. The first method is traditional machine learning (ML) algorithms, and the second uses deep learning algorithms. Machine vision technology uses a series of image-processing methods to extract the important features of weeds and plants, and then applies a classifier to those features to identify the specific weed and plant type.

Variable-Rate Application for Irrigation

Variable-rate irrigation (VRI) technologies are designed to site-specifically apply irrigation water at variable rates within the field to adjust the temporal and spatial variability in soil and plant characteristics. VRI is normally implemented on self-propelled center pivot and linear move sprinkler irrigation systems. Similar to other VRA systems in precision agriculture.

Irrigation Options

Variable-rate irrigation (VRI) is a technology that provides the opportunity to vary the irrigation rate. The primary goal behind these systems is to increase water use efficiency by avoiding over- or under watering of the crop throughout the growing season. Different VRI options exist, typically based on the manufacturer, but systems are generally grouped into two categories: speed or duty-cycle control. Some manufacturers offer a combination of speed and duty-cycle control to affect application rates.

Speed Control. Speed control systems allow the pivot to run at different speeds throughout the field, creating pie-shaped zones (Figure 12.13). The flow rate of the pivot is not modified with speed control. As the other operation parameters of the irrigation system remain constant, the water application depth is inversely proportional to the travel speed of the system in the field. This means the higher the travel speed, the lower the water application depth.
Duty-Cycle Control. The duty-cycle control method, often referred to as the zone method, allows the pivot to change the duty cycle of individual sprinklers or groups of sprinklers, creating irrigation management zones (IMZs) (Figure 12.13). As the VRI system moves at a constant speed, the GPS receiver determines the system’s position in the field. Then, using a preloaded VRI prescription map or the real-time information collected from the field, the VRI controller adjusts the on/off time of the sprinklers to achieve the desired water application rate.

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