With increased reliance on glyphosate for weed control, weed species composition shifts are likely. Changes in relative abundance could arise from differential tolerance or resistance to, or avoidance of, glyphosate. This study characterized the potential changes in seedbank composition of a mixed population of summer annual weeds due to glyphosate application timing. Seven weeds common in soybean fields in the Midwest were introduced at known densities and treated with glyphosate at several treatment timings. Common lambsquarters, common sunflower, common waterhemp, eastern black nightshade, ivyleaf morningglory, shattercane, and woolly cupgrass were selected to represent a range of emergence patterns and tolerances to glyphosate. Seedling emergence and seed production were monitored. Differences in herbicide tolerance and avoidance, through markedly different weed emergence patterns, were strong contributors to projected changes in weed species composition. Lessened sensitivity to glyphosate allowed some emerged species, such as ivyleaf morningglory, to better survive the herbicide. Late emergence allowed others, such as shattercane and ivyleaf morningglory, to avoid glyphosate applications. Such mechanisms may allow specific species to increase in a continuous glyphosate 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: Differential sensitivity, fecundity, herbicide tolerance, population dynamics, relative abundance.

Abbreviations: DAP, days after planting; POST, postemergence; SB_t, seedbank at time = 0; SB_{t 1}, seedbank at time = 1 yr.