This paper provides a brief overview of precision farming, followed by an overview of the main components of variable rate equipment, then concludes with a summary of the current commercially available equipment for variable rate application of seeds and chemicals. The equipment reviewed includes: computer/controllers, liquid sprayers, granular fertilizer applicators, air sprayers and spreaders, and drills and planters.

Precision farming is a farming system concept which involves the development and adoption of knowledge-based technical management systems with the main goal of optimizing profit. This management system will enable micro- management concepts, that is, the ability to appropriately manage every field operation at each location in the field, if it is technically and economically advantageous to manage at that level. The system will likely include the ability to vary or tailor the rate of application of all inputs such as tillage, seeds, weed, insect and disease control, cultivation and irrigation.

It will be possible to implement precision farming at many different levels. In its most extensive form, it will include precise micro-management of every step of the farming process. It is expected that the advisability of micro-management will be dependent upon many factors, such as soil type, crop, seasonal weather, and other factors. For example, in a dry year, it may be possible to control insects by spraying only small areas where the insects are known to exist; in a wet year, it may be advisable to uniformly spray the whole field.

Technically, one important aspect of the development of precision farming concepts is the development of the hardware and software necessary to vary the rate of the application of agricultural inputs. A number of research projects have been conducted in this area, and several companies have been developing variable rate application equipment in recent years. The objective of this paper is to provide a brief overview of precision farming systems, then outline the main components which are usually found in variable rate application equipment, followed by a review of the commercially available equipment on the market today.

An overview of the precision farming system of the future is depicted in Figure 1 (right). The brain of the system is a geographic information system (GIS), which will form the knowledge base and decision making parts of the precision farming system. The technical and economic decisions related to the farming operation will be governed by this knowledge based GIS. A GIS will be made up of layers of related information, and the GIS will allow a quantitative study of the relationships between the layers. For example, the GIS may contain the following layers: (1) field topography, (2) soil types, (3) surface drainage, (4) sub-surface drainage, (5) soil testing results, (6) rainfall, (7) irrigation, (8) actual chemical application rates, and (9) yield. Some of these layers will be entered once; some will be entered annually or even more frequently. The GIS will then allow a study of the relationship between these layers of information to determine cause and effect and to base decisions upon this knowledge.

As indicated in Figure 1, each field operation may include variable rate technology. Tillage depth may be varied according to field location; for example, subsoiling depth may be dependent on field location. Seeding rates may vary according to field location, which may depend on factors such as topography and soil type. Fertilizer application rates may vary in relationship to factors such as soil type and the results from either real time or pre-application testing. Application of insecticides may be dependent on insect location from either scouting reports or from aerial imaging. In like manner, the application of all inputs to the crop production process may vary with field location.

The main components which make up a variable rate application system are shown in Figure 2 (right). Not all systems will necessarily contain all of the components shown. As variable rate technology develops, other system components may be included.

The central component of variable rate application equipment is the computer/controller. This device receives information from several sources which will in turn be used to control the application equipment. The controller may receive information from the application equipment and other sensors to maintain a database on the actual application rate as a function of field position.

A key component for all precision farming operations is the technology to determine the instantaneous position of equipment as it operates in the field, and to provide this information in a computer compatible format. The technology which has rapidly gained acceptance as the optimum system is the Global Positioning System (GPS). A stand-alone GPS receiver can have instantaneous errors as high as 100 m, which is unacceptable for precision farming. Fortunately, several systems to calculate what is known as differential corrections have been designed, which can allow the GPS system on a farm vehicle to achieve position accuracies in three basic accuracy ranges: (1) 2-5 meters, (2) sub-meter, or (3) in the sub-decimeter range, depending on the technologies used. These maximum error figures relate to horizontal position, and vertical position (elevation) error is usually 1.5-5 times the horizontal position error. Most precision farming operations do not require vertical position information; the main application requiring vertical as well as horizontal information is to develop topographic maps. Most precision farming operations will require real-time differential corrections so that vehicle position information will be accurate when the vehicle is operating in the field. For precision farming applications, the GPS positioning technology should be thought of as RT-DGPS, that is, the farmer should always be using real-time differential corrections to minimize position error.

Information contained in the geographic information system related to a specific field operation is downloaded to the system computer before field operations commence. The computer/controller will continuously control variable application rates based upon knowledge gained both from the geographic information system, from a knowledge of field location as provided by RT- DGPS, and perhaps from real time sensors. For example, assume that the desired fertilizer application rate is known to be a function of results from soil analysis tests, field location, and crop. The soil analysis test results as a function of field location would be entered into the GIS and downloaded to the computer/controller of the fertilizer applicator. If one crop is being grown in the field being fertilized, then the operator may simply enter the crop from the computer/controller keyboard. However, if two crops are grown in alternating strips, this information would be entered into the GIS as a function of field position, then also downloaded to the variable rate application (VRA) computer/controller. When the equipment is operating in the field, the VRA computer/controller will be receiving RT- DGPS receiver position information and will match required application rate and crop as a function of field location to control the applicator equipment. It may also be possible to have a real time soil sensor which will provide information on-the-fly about fertilizer application rate needed, rather than using pre-application soil sampling/analysis techniques.

The application equipment may also have sensors which provide quantitative information on the actual application rates. This information, along with RT- DGPS position, can be recorded to maintain a historical record of application rates. This historical information may allow the farmer to analyze cause and effect in the precision farming system, and perhaps can influence future decision making processes implemented in the computer/controller. For example, assuming that sufficient information has been gathered over several years, the farmer may have historical records on the effect of all of the inputs to his system for a specific field, including the crop yield. The GIS would then allow an analysis of cause and effect, based upon many factors, and allow fine-tuning of chemical application rates in subsequent seasons.

Eventually the RT-DGPS system may also be used for vehicle guidance. Most farm vehicle guidance systems today are visual prompting systems for the vehicle operator which can establish accurate vehicle position for application swaths. In the future, the guidance system may automatically guide the application vehicle.

In this paper the precision farming system of the future was briefly outlined. The brain of the system is a geographic information system which will enable knowledge-based farming decisions to optimize net profit. An important aspect of the technology is the ability to vary the rate of application of all inputs, that is, to tailor or prescribe the application to various sites throughout each field, including tillage, fertilizer and lime application, planting, cultivation, and spraying. The components usually found in variable rate application equipment were outlined and discussed in some detail. The paper appendix contains two summary tables which provide information on most companies involved in producing variable rate application equipment.

Most of the commercial ventures to date have focused on the variable rate equipment for application of liquid and granular materials. There remain many unanswered questions about how to implement this technology. It was pointed out that the GIS is the brain of the system, but this aspect of the technology is still in the infancy stage. A critical aspect of the electronic technologies is standardization, ranging from physical connections which can withstand the farming environment, to standardization of data format. It will be critical to develop the technologies to make them simple to use and user friendly, as well as economical. Much technical development work remains before the precision farming system of the future can be implemented. In the final analysis, it must be shown that precision farming pays- particularly economically, environmentally, and from the viewpoint of the conservation of our natural resources.