Additive and replacement series studies were conducted to investigate the effect of Glycine max cultivar canopy characteristics, interference, and weed-free period on the vegetative and reproductive growth of Solanum ptycanthum. Solanum ptycanthum established 0, 2, and 4 wk after planting (WAP) did not reduce seed yield of either Group II (Pioneer 9273) or Group IV (Asgrow 4715) varieties. The Group IV variety intercepted a greater percentage of light throughout the season. However, both varieties reduced growth of S. ptycanthum established 2 and 4 WAP, suggesting no disadvantage of Group II in suppressing S ptycanthum growth. The greenhouse and field replacement series experiments indicated that S. ptycanthum plants experienced greater intraspecific than interspecific competition; that is, S. ptycanthum grew more in mixed species competition than in monoculture. Under field conditions, G. max grew equally well in competition with S. ptycanthum or in monoculture. However, in the greenhouse G. max accumulated more biomass and fruit weight in monoculture vs. competition with S. ptycanthum. Relative yield total values (RYT) in the field supported greater productivity per unit area for mixed species competition when compared to G. max or S. ptycanthum grown in monoculture. Data from both studies suggested that S. ptycanthum did not reduce G. max yield when grown in the field. Regardless of maturity group, a fully developed G. max canopy reduced S. ptycanthum growth.

Nomenclature: Solanum ptycanthum Dun. SOLPT, eastern black nightshade; Glycine max (L.) Merr. ‘Pioneer’ 9273, ‘Asgrow’ 4715, soybean.