Site-specific weed management is the part of precision agriculture (PA) that tries to effectively control weed infestations with the least economical and environmental burdens. This can be achieved with the aid of ground-based or near-range sensors in combination with decision rules and precise application technologies. Near-range sensor technologies, developed for mounting on a vehicle, have been emerging for PA applications during the last three decades. These technologies focus on identifying plants and measuring their physiological status with the aid of their spectral and morphological characteristics. Cameras, spectrometers, fluorometers and distance sensors are the most prominent sensors for PA applications. The objective of this article is to describe-ground based sensors that have the potential to be used for weed detection and measurement of weed infestation level. An overview of current sensor systems is presented, describing their concepts, results that have been achieved, already utilized commercial systems and problems that persist. A perspective for the development of these sensors is given.

Site-specific weed control techniques have gained interest in the precision farming community over the last years. Managing weeds on a subfield level requires measuring the varying density of weeds within a field. Decision models aid in the selection and adjustment of the treatments, depending on the weed infestation. The weed control can be done either with herbicides or mechanically. A site-specific herbicide application technology can save large amounts of herbicides. Mechanical weed control techniques adapting to the weed situation in the field are applicable to a wide spectrum of crops.Site-specific techniques for the detection and management of weeds are presented. A system for the discrimination of different weed species and crops from images is described, which generates weed maps automatically. Models for the yield effect of weeds are developed and applied in onfarm-research experimental setups. Economic weed thresholds are derived and used for a herbicide application with a patch sprayer.

Field experiments were conducted in western Atakora, Benin, to determine the critical time period of weed competition in hand-weeded corn. Weeds were removed until different crop growth stages and then allowed to reemerge. Other treatments began weed control at different growth stages (four-, eight-, and ten-leaf stages and flowering) and were maintained until harvest. One treatment was permanently kept weed-free and one treatment was uncontrolled until harvest. Yields without weed competition ranged from 2.8 to 3.4 t ha−1. As expected, yield loss increased with duration of weed infestation and ranged from 38 to 65% compared to permanent weed-free plots. In three out of four site-years, the critical period for weed control started at the four- to six-leaf stage and continued until ten-leaf stage or flowering of corn. Approximately four hand-weeding applications were required in this critical period of weed control.

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