Weed Management in Glyphosate Resistant Soybean: Weed Emergence Patterns in Relation to Glyphosate Treatment Timing1

Knowing when weed species are likely to emerge can aid in developing effective integrated weed management programs. When using nonresidual herbicides such as glyphosate for weed control, treatment timing is critical. This study characterized the emergence patterns of common lambsquarters, common sunflower, common waterhemp, eastern black nightshade, ivyleaf morningglory, shattercane, and woolly cupgrass in soybean, in relation to common glyphosate application timings. Approximately 90% or more of common lambsquarters, common sunflower, and common waterhemp seedlings emerged before the end of May, both in 2000 and 2001. Both ivyleaf morningglory and shattercane emerged from late April to mid-August, allowing these species to avoid glyphosate applications timed to prevent early-season weed competition. Avoidance through periodicity in emergence underscores the importance of integrating multiple tactics to ensure that difficult to manage weeds are not selected for in this management system. Nomenclature: Glyphosate; common lambsquarters, Chenopodium album L.#3 CHEAL; common sunflower, Helianthus annuus L. # HELAN; common waterhemp, Amaranthus rudis Sauer # AMATA; eastern black nightshade, Solanum ptycanthum Dun. ex DC. # SOLPT; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. # IPOHE; shattercane, Sorghum bicolor (L.) Moench # SORVU; woolly cupgrass, Eriochloa villosa (Thunb.) Kunth # ERBVI; soybean, Glycine max (L.) Merr. ‘Asgrow 3003RR’. Additional index words: Cohorts, germination, growing degree days, heat units, integrated weed management, periodicity. Abbreviations: DAP, days after planting; IWM, integrated weed management; POST, postemergence.