Field experiments were conducted from 1997 through 1999 to evaluate interspecific interference between pitted morningglory at 0, 10, 16, and 62 plants m⁻² and drill-seeded, glyphosate-resistant soybean as influenced by soybean population and a single glyphosate application of 1.12 kg ai ha⁻¹. Photosynthetic rate of soybean was not influenced by pitted morningglory density or glyphosate use 2 wk after treatment (WAT). Photosynthetic rate of soybean 12 WAT was reduced by 21 and 91% with 62 treated and untreated pitted morningglory plants m⁻², respectively, whereas 10 treated and untreated pitted morningglory plants m⁻² had no effect on the soybean photosynthetic rate. Pitted morningglory photosynthetic rate 2 and 12 WAT was reduced by 64 and 80%, respectively, when treated with glyphosate. The reduction in the photosynthetic rate of glyphosate-treated pitted morningglory was partially attributed to shading by soybean, whereas untreated plants were fully exposed to sunlight. Glyphosate-treated pitted morningglory at 10 and 16 plants m⁻² did not reduce the rate of soybean leaf area index (LAI) accumulation; however, when the density was increased to 62 pitted morningglory plants m⁻², soybean LAI decreased from 1.19 to 0.88 for each accumulated 100 growing degree days. Pitted morningglory produced a maximum of 24 million seeds ha⁻¹ in the absence of glyphosate with 217,000 soybean plants ha⁻¹. Pitted morningglory seed production declined with increasing soybean seeding rate in the absence of glyphosate, with a 41% reduction occurring when soybean population increased from 217,000 to 521,000 plants ha⁻¹. Seed production of treated pitted morningglory ranged from 380,000 to 700,000 seeds ha⁻¹. Soybean seed yield was not influenced by pitted morningglory density when treated with glyphosate. Untreated pitted morningglory at 10, 16, and 62 plants m⁻² reduced soybean seed yield by 47, 62, and 81%, respectively. Competitiveness of untreated soybean increased with soybean seeding rate, resulting in 22% less yield loss with 521,000 than with 217,000 plants ha⁻¹. Soybean seed yield was not reduced by 10 and 16 glyphosate-treated pitted morningglory plants m⁻², but a 9% loss in yield occurred with 62 pitted morningglory plants m⁻². Averaged over all pitted morningglory densities, glyphosate-treated pitted morningglory failed to reduce soybean seed yield at each of the three soybean densities. Following a single application of glyphosate, no apparent benefit was noted from increasing the soybean population above 217,000 plants ha⁻¹.

Nomenclature: Glyphosate; pitted morningglory, Ipomoea lacunosa L. IPOLA; soybean, Glycine max L. Merr. ‘Delta King 5961 Roundup Ready^®’.