Contributions of shading, soybean (Glycine max) row width, and planting green on horseweed (Conyza canadensis) management compared with soil-applied residual herbicides

• Published in: Weed technology Vol. 37; no. 4; pp. 383 - 393
• Main Authors: Fisher, Justine L., Sprague, Christy L.
• Format: Journal Article
• Language: English
• Published: New York, USA Cambridge University Press 01-08-2023
• Subjects: 2,4-D; Biomass; Canada fleabane; Conyza canadensis; Crop yield; Crops; Glycine max; Glyphosate; Herbicides; Marestail; Metribuzin; Planting; Rosette; Row width; Rye; Seeds; Shade; Shading; Soybeans; Summer; Weeds; United States > US; Michigan
• ISSN: 0890-037X; 1550-2740
• DOI: 10.1017/wet.2023.49

Glyphosate-resistant (GR) horseweed is a problematic weed for Michigan soybean growers. Additionally, rosette- and upright-horseweed growth types have been observed co-emerging during mid- to late summer in several Michigan fields. In the greenhouse, shade levels from 35% to 92% reduced rosette- and upright-horseweed biomass 31% to 99% compared with the upright growth type grown under 0% shade. Greater reductions in biomass occurred under 69% and 92% shade. Thus, increased shading by planting in narrow rows and/or planting green into cereal rye may improve horseweed suppression. A field experiment conducted over 3 site-years compared the effect of fall-planted cereal rye terminated with glyphosate 1 wk after planting (WAP; planting green) with a preemergence residual herbicide program (glyphosate + 2,4-D + flumioxazin + metribuzin) on horseweed control in soybean planted in three row widths (19, 38, and 76 cm). Planting green or applying a residual herbicide program across all row widths reduced horseweed biomass 86% to 91% and 95% to 99%, respectively, compared with soybean planted with no cover in 76-cm rows, 4 to 6 WAP. At soybean harvest, when a noneffective postemergence herbicide (glyphosate) was applied, horseweed biomass was 42% and 81% lower by planting green or applying a residual-herbicide program compared with no cover, respectively. Similarly, planting soybean in 19-cm rows reduced horseweed biomass compared with 38- and 76-cm rows. When an effective postemergence program was applied, similar horseweed biomass reductions were observed by planting green or applying a residual herbicide across all row widths. Additionally, soybean yield and economic returns were similar between planting green and applying a residual herbicide in 1 of 2 site-years. Integrating planting green and an effective postemergence herbicide program offers an alternative horseweed management strategy to applying a residual preemergence herbicide program. Nomenclature: 2,4-D; flumioxazin; glyphosate; metribuzin; horseweed, Conyza canadensis (L.) Cronq.; cereal rye, Secale cereale L.; soybean, Glycine max (L.) Merr.