Abstract
Field studies were conducted in 2016 and 2017 to determine if multispectral imagery collected from an unmanned aerial vehicle (UAV) equipped with a five-band sensor could successfully identify Palmer amaranth (Amaranthus palmeri) infestations of various densities growing among soybeans (Glycine max [L.] Merr.). The multispectral sensor captures imagery from five wavebands: 475 (blue), 560 (green), 668 (red), 840 (near infrared [NIR]), and 717 nm (red-edge). Image analysis was performed to examine the spectral properties of discrete Palmer amaranth and soybean plants at various weed densities using these wavebands. Additionally, imagery was subjected to supervised classification to evaluate the usefulness of classification as a tool to differentiate the two species in a field setting. Date was a significant factor influencing the spectral reflectance values of the Palmer amaranth densities. The effects of altitude on reflectance were less clear and were dependent on band and density being evaluated. The near infrared (NIR) waveband offered the best resolution in separating Palmer amaranth densities. Spectral separability in the other wavebands was less defined, although low weed densities were consistently able to be discriminated from high densities. Palmer amaranth and soybean were found to be spectrally distinct regardless of imaging date, weed density, or waveband. Soybean exhibited overall lower reflectance intensity than Palmer amaranth across all wavebands. The reflectance of both species within blue, green, red, and red-edge wavebands declined as the season progressed, while reflectance in NIR increased. Near infrared and red-edge wavebands were shown to be the most useful for species discrimination and maintained their utility at most weed densities. Palmer amaranth weed densities were found to be spectrally distinct from one another in all wavebands, with greatest distinction when using the red, NIR and red-edge wavebands. Supervised classification in a two-class system was consistently able to discriminate between Palmer amaranth and soybean with at least 80% overall accuracy. The incorporation of a weed density component into these classifications introduced an error of 65% or greater into these classifications. Reducing the number of classes in a supervised classification system could improve the accuracy of discriminating between Palmer amaranth and soybean.
Highlights
Weeds can reduce soybean (Glycine max [L.] Merr.) yield by ≥50% if not controlled [1]
Previous research conducted to identify wavebands useful for weed species discrimination in soybean found that areas between 490 and 500 nm and 600 and 700 nm within the visible range of the electromagnetic spectrum were proven to be useful for crop and weed discrimination [12,13,14]
All three experimental runs showed a significant date by species interaction across all wavebands, which suggests species discrimination via spectral reflectance may be influenced by temporal conditions
Summary
Weeds can reduce soybean (Glycine max [L.] Merr.) yield by ≥50% if not controlled [1]. The complexity of weed management in soybean has increased with the evolution of herbicide-resistant weeds [2,3]. Wats.) is a pervasive and ubiquitous weed in soybean and the difficulty in controlling the species is exacerbated by the evolution of resistance to most herbicides labeled for soybean [4,5]. Due to the already limited options for effective chemical control of Palmer amaranth and the potential for the evolution of additional herbicide resistance(s), site-specific weed management strategies have become necessary to reduce the amount of selection pressure exerted on weed populations [6,7,8]. Remote sensing research in soybean systems has been successful in identifying weed species via spectral characterization and has proven to be useful in performing speciesbased image classifications. Previous research conducted to identify wavebands useful for weed species discrimination in soybean found that areas between 490 and 500 nm and 600 and 700 nm within the visible range of the electromagnetic spectrum were proven to be useful for crop and weed discrimination [12,13,14]
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