A comparison of visual and photographic estimates of weed biomass and weedcontrol

The objective of this study was to compare the consistency and accuracy of visually estimated weed biomass and weed control data to data obtained thro ugh image analysis. Weed biomass and weed control were evaluated in soybean herbicide efficacy trials conducted at the University of Nebraska-Lincoln during 1992 and 1993. Measurements were based on visual estimates and on ae rial photographs taken at a height of 3.5 m above the soil surface, Photogr aphs were digitized and classified, producing pixel values for broadleaf we eds, grass weeds, soybean, and soil. Percent weed cover was calculated in r elation to the crop canopy, based on the respective number of pixels per im age. Visual and photographic ratings of weed biomass and of weed control we re not closely correlated. In the first year the visual method discriminate d between more treatments than the photographic method, but the opposite oc curred in the second year. The photographic method predicted yield more clo sely than the visual estimates, We concluded that visual estimates were les s consistent and more subject to observer bias than measurements obtained w ith the photographic method.