Agriculture Reference
In-Depth Information
specific conditions within discrete areas of a field. Efforts devoted to identifying
spatial variability and developing variable rate maps will have limited value unless
equipment can automatically regulate application rates as it travels across a field.
Variable rate application equipment is available for a variety of substances including
granular and liquid fertilizers, pesticides, seeds, and irrigation water. Several com-
panies are marketing variable rate application equipment.
The major components of a typical map-based variable rate control system include
an in-cab computer (or controller) loaded with application software and variable rate
application maps, a DGPS receiver that provides vehicle position information to the
computer, and an actuator that regulates material rates under direction of the com-
puter. When the equipment is operating in the field, the computer receives position
information, matches the required application rate as a function of vehicle position,
and then sends a set-point signal to the controller that adjusts the application to the
desired rate. A variable rate system may also record actual application rates along
with GPS position. This information serves as a record of what was applied to the
field and allows for review of application for future recommendation considerations.
Among the earlier studies, Schueller and Wang (1994) described the concept
of variable rate fertilizer and pesticide application, described commercial applica-
tors using GPS, and suggested more research to understand various error sources.
Al-Gaadi and Ayers (1999) developed a site-specific variable rate herbicide applica-
tion system, and reported that the system produced an actual application rate map
and that its highest application rate error was 2%. Carrara et al. (2004) implemented
a variable rate herbicide application system for durum wheat and reported a savings
of 29% of herbicides compared to the conventional spray operation. Tumbo et al.
(2007) evaluated a commercial variable rate controller for properly applying aldi-
carb outside the buffer zones around water wells in citrus groves and determined the
dynamic performance of the system using two common drive mechanisms.
Variable rate application will not affect the basic functions of most application
machinery. The required changes will be necessary to accommodate the addi-
tion of sensors and controllers. Most of the current liquid systems can be modified
for variable rate application. Yang (2001) adapted a FALCON variable rate con-
trol system (Ag-Chem, AGCO Corp., Duluth, GA) to an existing liquid fertilizer
applicator to vary rates of nitrogen and phosphorus simultaneously. Ess and Morgan
(2010) described ACCU-RATE (Rawson Control Systems, Inc., Oelwein, IA) and
SOILECTION (Ag-Chem, AGCO Corp.) as examples of commercial variable rate
application systems. The ACCU-RATE is a variable rate seed metering drive system,
which can be added to planters to implement variable rate seeding. It can vary the
application rate of seeds and fertilizers. The SOILECTION system is used for vari-
able rate dry and liquid fertilizer application. It can variably apply up to 10 different
products in a single pass and can record actual application rates for future use.
Sensor-based variable rate application can be used to apply fertilizers as a side
dressing or foliar spray during the growing season. Biermacher et al. (2009) devel-
oped a site-specific nitrogen fertilizer application system that uses optical reflec-
tance measurements of growing wheat plants to estimate N requirements. Another
example of sensor-based liquid fertilizer application is the GreenSeeker selective
spraying system (Trimble Navigation Limited). The system uses an optical sensor to
