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Several studies have shown that most weed species occur in an aggregated form and often in patches, resulting in a large within-field variation not only in terms of occurrence, but also in density. Therefore, a site-specific weed management can reduce the amount of herbicides applied on the field, bringing economical and environmental benefits. This article presents the results obtained in field tests using a platform mounted on a tractor to evaluate a computer program developed for site-specific weed patch chemical application at fixed rates using the on/off strategy. A hypothetical area of infestation was marked on the ground and georeferenced. The weed patch contour was used as a prescription map to control where the chemical should be applied. A DGPS was used to determine the position of the tractor in the field. A communication driver interface was constructed and linked to two leds, representing a left and a right half boom of a sprayer. The tests showed that the software was able to identify when the tractor entered the hypothetical weed patch, turning the two leds on. When the tractor was outside the patch, the program was also able to identify if a hypothetical right or left boom was on a weed patch, allowing the chemical application on it. Further studies may use this software to control a conventional sprayer with a boom divided into two sections adapted with solenoid valves, for site-specific weed patch control, using the on/off strategy.