Chapter 12

Variable-Rate Technology in Precision Agriculture

Variable-Rate Application Methods

Variable-rate application (VRA) is a technology used in precision agriculture that allows farmers to adjust the rate at which inputs like fertilizers, pesticides, and seeds are applied across a field based on the specific needs of different areas. There are two basic methods of implementing site-specific management for the VRA of crop production inputs: map-based and sensor-based. While each method has unique benefits and limitations, some variable-rate application systems have been developed to take advantage of the benefits of both methods. Regardless of the precision agriculture approach utilized, the goal should be an improvement in the economic and environmental sustainability of the production system.

Map-Based Variable-Rate Application

In a map-based method, the crop and soil sensors are used to gather georeferenced data (e.g., yield, soil properties, etc.), which needs to be processed (filtered, normalized, etc.) and interpolated to create prescription maps representing the geo-spatial distribution of the variables measured throughout a field (Figure 12.1). A prescription map guides farm machinery (e.g., sprayers, seeders, spreaders) to apply inputs (seeds, fertilizers, pesticides, water, etc.) at variable rates across a field based on spatial variability in soil and crop conditions.

Advantages and Disadvantages of Map-Based VRA

The main advantages of a map-based system include the ability to harness multiple sources of information, the wide availability of application systems for various agricultural inputs, and the adequate time interval between input sampling and application, which contributes to enhancing system accuracy. The multiple sources of data that are necessary to facilitate map-based applications can also be used in other decision-making processes for a farming operation. A farm manager using GIS software can examine all yield, soil properties, pests, and as-applied data.

Prescription Map

The first step to successfully creating a VRA prescription map is to identify and understand the field variability. Once the variation is identified, the field can be divided into “management zones” that can be managed similarly. These zones can be created within a given field based on various data layers such as historical yields, soil properties, topography, and/or aerial imagery. If using yield data, it is important to use multiple years of data from the same crop that will be planted. Accurate yield data is critical when using it for VRA, so it should be recorded from a calibrated yield monitor, and the data should be cleaned prior to using it for creating prescriptions.

Process of Creating a Prescription Map

A prescription map (or VRT map) defines how inputs like fertilizer, seed, or pesticide should be applied variably across a field based on spatial variability.

Data Collection. Prescription maps may be constructed from yield, topography, soil, plant, or weed data. These prescription maps generally contain the application rate based on the spatial variability in the field. Lastly, these prescription maps are transferred to the variable-rate control panel system to deliver the proper rate of agricultural inputs at different locations in the field.
Data Analysis and Zone Delineation. Zone delineation is a critical step in creating a VRT prescription map. It involves analyzing spatial data to divide a field into management zones that share similar characteristics, allowing for tailored input applications (e.g., fertilizer, seed).
Decision Criteria for Input Rates. Decision criteria for input rates in VRT revolve around optimizing input application based on site-specific conditions and crop needs.
Prescription Map Creation in GIS or VRT Software. Once data has been analyzed and management zones defined, the next step is to create a prescription map that assigns specific input rates to each zone.

Sensor-Based Variable-Rate Application

The sensor-based method provides the capability to vary the application rate of inputs with no prior mapping or data collection involved. The sensor-based method utilizes multiple sensors and data acquisition and processing systems to collect and interpret real-time information, determine the application rate using a predetermined algorithm, and control the actuator to apply the desired amount of input (e.g., fertilizer, pesticide, or water) as the applicator travels across the field, enabling truly dynamic input control. The sensor-based variable-rate application process involves the following steps:

Advantages and Disadvantages of Sensor-Based VRA

Sensor-based systems offer key advantages, such as the capability to adjust the application rate of agricultural inputs without the need for prior mapping or extensive field data collection. They also enable real-time monitoring of crop and soil conditions and immediate application of agricultural inputs following measurements, eliminating delays compared to map-based approaches.

Sensor-Based Technologies

One example of an active sensor that emits red and infrared light is the GreenSeeker® sensor. It calculates the NDVI values based on the light the plant reflects. The sensor continues to sample the scanned area and provide an average NDVI reading (ranging from 0.00 to 0.99). The NDVI generated by the sensor provides an input data source for the electronic circuit, which consists of a microcontroller system and transfers the data based on the sensor algorithm to a proportional solenoid valve.

Errors in Variable-Rate Application

In precision agriculture, the goal of variable-rate application is to apply the desired amount of a product at each location in a field. Electronic rate control systems are designed to automatically adjust instantaneous application rates resulting from changes in machine and field operating parameters. Typically, machine-operating parameters such as ground speed, application swath width, and site-specific target application rates are used to calculate the rate at each point in the field. The ability to apply a VRT depends on the capacity of the equipment. The main responsibility of the application control unit is to link the prescription map or sensor algorithm rate with the machine's current location, orientation, and speed. The resulting control signal is often a rotation rate, flow rate, or conveyor rate that will achieve the desired product placement.

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