Root development and the associated acquisition of water and nutrients are an important part of weed competitiveness. Characterization of root morphological development, however, is inherently problematic because of the complexities of soil–plant interactions. In this study, we used hydroponically grown plants and digital imaging to examine root characteristics of Glycine max and the competing weeds Senna obtusifolia and Amaranthus palmeri. The purpose was to define inherent differences in root length and surface area that would contribute to growth responses during the establishment phase in the field. The methodology involved growing plants for 16 to 22 d, dissecting and staining root segments, mounting subsamples on slides, and imaging using a stereomicroscope and digital camera. Microscopy was required because of the small diameters of a significant proportion of the weed roots. With plants of similar root fresh weights (4.5 to 5.0 g), counting of individual roots revealed that 5. obtusifolia and A. palmeri had 2 and 3.7 times more roots than G. max (4,616 and 7,781 vs. 2,120, respectively). The imaging analyses indicated that roots of S. obtusifolia and A. palmeri had 2.9 and 5 times more length than G. max (10,042 and 17,192 cm vs. 3,418 cm, respectively). Furthermore, the analysis of length in different root diameter classes indicated that weed roots were noticeably finer then those of G. max. Approximately 84% of S. obtusifolia root length was contributed by roots in the 0.1- to 0.25-mm range, whereas 45% of the G. max roots were in the 0.1- to 0.25-mm range and 48% were in the 0.25- to 0.75-mm range. In contrast, 68% of A. palmeri length was contributed by roots smaller than 0.1 mm in diameter with 26% in the 0.1- to 0.25-mm range. Based on the expression of root characteristics observed here, root systems of these weed species would have finer roots with much greater length that would occupy a much larger volume of soil than those of G. max. Presumably, this would result in a competitive advantage in the acquisition of water and nutrients, especially when availability is limited.

An experiment was conducted at two locations in Georgia and two locations in North Carolina during 1994 and 1995 to evaluate weed management in conventional-tillage bromoxynil-resistant Gossypium hirsutum L. (cotton). The weed management systems evaluated included different combinations of fluometuron preemergence (PRE), bromoxynil or bromoxynil plus MSMA early postemergence (EPOST), bromoxynil postemergence (POST), and cyanazine plus MSMA late post-directed (LAYBY). Fluometuron PRE improved control of Acanthospermum hisptdium DC. (bristly starbur), Cassia occidentalis L. (coffee senna), Chenopodium album L. (common lambsquarters), Desmodium tortuosum (Sw.) DC. (Florida beggarweed), Sida spinosa L. (prickly sida), Jacquemontia tamnifolia (L.) Griseb. (smallflower morningglory), and Anoda cristata (L.) Schlecht. (spurred anoda), compared to system that did not use fluometuron PRE. It also improved G. hirsutum yields at three four locations. Bromoxynil-containing systems provided better weed control and higher G. hirsutum yields than systems without bromoxynil. Bromoxynil EPOST controlled A. hispidium, C. occidentalis, C. album, D. tortuosum, S. spinosa, J. tamnifolia, and A. cristata. Control of these species was frequently improved by a second application of bromoxynil POST. Bromoxynil EPOST, POST, or EPOST plus POST did not control Senna obtusifolia (L.) Irwin and Barneby (sicklepod), but the addition of MSMA to bromoxynil EPOST improved S. obtusifolia control. Control of all dicotyledonous weeds was improved by a LAYBY treatment of cyanazine plus MSMA, and yields were improved at three of four locations with this treatment. Gossypium hirsutum was not injured by POST treatments of bromoxynil, and only temporary injury resulted from POST treatments of MSMA.