Cultivation is an exacting task that is often tiring for the tractor operator. It causes crop loss if not done carefully. These problems are multiplied when using tools that have to travel at a precise position relative to the crop row. Consequently, some rear-mounted implements are designed to be steered by a rider (Geier & Vogtmann, 1987; Melander, 1997), but this is a laborintensive solution. Fortunately, automation of implement and tractor guidance is advancing rapidly.
The simplest approach is purely mechanical. Wheels mounted on the cultivator guide the implement by rolling along the sides of raised beds or ridges, or else travel in furrows laid down by the planter. These systems are sufficiently accurate for cultivating at high speeds with in-row tools (Mohler, Frisch & Mt. Pleasant, 1997), or for cultivating very close to small plants with a conventional inter-row shovel cultivator (Melander & Hartvig, 1997). They are best adapted to rear-mounted machines since the implement must have some lateral sway relative to the tractor. Mechanical guidance is inexpensive relative to the electronic guidance systems discussed below, and can be used when the crops are too small to sense electronically. However, furrow guidance requires implements six rows or wider, since two wheels are needed for stability and the tractor tires must not obliterate the furrows. Also, the furrow made by the planter must be preserved during any early-season passes with a rotary hoe or harrow, and recreated with a furrowing tool on the cultivator for subsequent cultivations. Parish, Reynolds & Crawford (1995) found that cultivating cotton using a mechanical guidance system substantially reduced costs by allowing a narrower band of herbicides over the crop row.
Electronic guidance systems usually have wands that sense the presence of the crop row. The simplest merely alert the operator when the implement strays to one side, but most models control a steering device. Various steering devices correct the cultivator's position by (i) shifting the lift arms of the three-point hitch, (ii) shifting the cultivator laterally relative to the hitch, (iii) rotating the cultivator slightly on the hitch, (iv) turning it slightly using disk coulters, or (v) turning the tractor steering wheel (Cramer, 1988; Bowman, 1991, 1997, pp. 30-3). The last approach results in a longer delay between error and correction on rear-mounted machines, but it allows the driver to watch for jamming and other problems. It is also the only approach that is well adapted to belly- or front-mounted machines, although in principle, the second approach could work if the implement was attached to the tractor by a laterally sliding carriage.
Generally, crops like maize and sorghum that are flexible when young cannot be reliably sensed until 12-15 cm tall; beans can be detected at 7-10 cm. Some systems can guide electronically off a planter-made furrow when the crop is too small to be sensed. Tian, Slaughter & Norris (1997) recently developed software for detecting rows of tomato seedlings by computer interpretation of a video image. When incorporated into a guidance system, this should allow guided cultivation of very young crops. Van Zuydam, Sonneveld & Naber (1995) used a laser beacon to provide precision tractor guidance for several types of field operation, including cultivation.
Rapid progress in the computer industry opens possibilities for further development of automatically guided cultivators. These could potentially operate under the control of artificial intelligence software, distinguish weeds from crops using a variety of electronic sensors and image processing devices (Zhang & Chaisattapagon, 1995), and selectively destroy weeds with a flexible array of tools. Development of such machines does not require new technological breakthroughs, but may be unnecessarily sophisticated for most cropping systems.
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