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Equipments for site specific chemical application are being developed for precision farming to apply the chemical product in the recommended rate, based on the present spatial variability of weeds in the field. The purpose of this work was to evaluate the herbicide efficiency and economy by using a system for variable rate site specific chemical application based on the spatial variability of weeds. The boundary of weed patches were mapped using an AG-132 Trimble DGPS model carried in a backpack. A prescription map was created based on the weed map and using the SSToolbox GIS software. The rates of Glyphosate herbicide were selected considering the spatial variability of weeds and the stage of development of each species. The site specific chemical application was done using a self-propelled sprayer adapted with a Mid-Tech TASC 6600 chemical injection metering system. The management of chemical application was accomplished using the Farm Site Mate GIS software. The same DGPS already mentioned was used for vehicle positioning. The herbicide application was done one day after the weed mapping. The time response of the equipment was taken into account to correct the as-applied map. The Farm Site Mate GIS software created an as-applied map during the herbicide application. Fifteen days after the herbicide application a new weed mapping was obtained to confirm the efficiency control. It was observed the need to create a buffer around the field for equipments with long time responses. The as-applied map was consistent with the prescription map. Weeds were controlled efficiently. The equipment used allowed 31,6% of herbicide economy, when compared with the traditional method using a single rate for the whole field.