A negative aspect of glyphosate-based management systems is the potential for volunteer glyphosate-resistant (GR) crop plants to impact subsequent crops. Two independent studies were conducted to determine the effect of differing density (full-season competition) and differing duration of interference (constant density) of GR cotton (Gossypium hirsutum L.) on GR soybean (Glycine max (L.) Merr.) growth and yield. Cotton density and duration of interference effects were not observed on soybean height at harvest. Season-long competition with a cotton density of 1 plant per row m would be expected to reduce soybean yield 6%. A cotton density of 5.25 plants per row m would have to interfere with soybean for at least 6 wk following emergence before a yield reduction would be expected. Even then, yield reduction would be significantly lower than 1%. The cotton densities and duration of interference periods evaluated did have a significant impact on soybean yield. However, impact does not appear to be as significant as other common problematic grass or broadleaf weeds.
