Spatial weed distribution determined by ground cover measurements

A portable dual-camera video system was used to evaluate the potential for using total projected green cover as an indirect measure of weed infestations in a wheat crop during early growth stages. The video system would have applications in mapping weed infestations to assist precision farming operations. The two cameras provided a real-time composite image of reflected light measured in red (640 nm), and near-infrared (860 nm) wavelengths. A simple ratio of reflected light intensity in each wavelength was used to isolate the growing plants from the background. Software was developed to automatically adjust for varying ambient light conditions and calculate the percentage of the image occupied by growing plants. Total green cover was measured at randomly selected sites prior to direct seeding wheat and at four growth stages following wheat emergence. The portion of green cover observed was compared to crop and weed dry matter at each location. Weed infestations at each location were estimated by measuring the total green cover and subtracting the projected green cover due to the crop alone. A minimum weed dry matter of 20 g/m2 and 30 g/m2 could be detected by the video system at the 3-leaf and 5-leaf growth stages, respectively. Weed dry matter less than 20 g/m2 could not be detected reliably due to the variability of the wheat crop. Detection of weeds within the crop beyond the 5-leaf stage using this method was difficult due to crop canopy closure