Field studies were conducted at three sites in Missouri in 1986 and 1987 to evaluate the performance of below-label rates of bentazon, acifluorfen, and chlorimuron tank mixed with sethoxydim and applied to soybeans 7 and 14 days after planting to evaluate broadleaf and grass weed control and weed seed production. Sequential applications of 0.25X-labeled rates of all three broadleaf herbicides tank mixed with 0.5X sethoxydim rates controlled giant foxtail, velvetleaf, and common cocklebur equivalent to one application of standard-labeled rates of the same tank mixes. Single applications of below-labeled rates of the postemergence herbicides did not control weeds, and soybean yields were not equal to sequential 0.25X or single full-rate treatments. A preemergence treatment of clomazone plus imazaquin applied at labeled rates controlled weeds, and soybean yields were equal to a handweeded check in both years at all test locations. Weeds survived and produced seed to reinfest the plots the following year unless nearly 100% control was achieved.

The potential of preventing seed production and reducing seed viability of weeds that commonly grow taller than soybeans [Glycine max(L.) Merr. ‘Williams 82’] was evaluated. Chlorflurenol (2-chloro-9-hydroxy-9H-fluorene-9-carboxylic acid), chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide}, and glyphosate [N-(phosphonomethyl)glycine] were evaluated against sparse stands of velvetleaf (Abutilon theophrastiMedik. # ABUTH) and giant foxtail (Setaria faberiHerrm. # SETFA) growing in soybeans. Simulated-roller applications of all three herbicides significantly reduced seed production and germination of both weeds, although glyphosate was more effective than were the other two. Applications during early flowering of velvetleaf and early heading of giant foxtail reduced seed production more than later applications when some seeds were present. With the early application of glyphosate, 99% prevention of velvetleaf and 96% prevention of giant foxtail seed production were attained. With the early glyphosate application, germination of seeds produced was reduced by 50% in velvetleaf and by 95% in giant foxtail. Soybean yields were not reduced by either glyphosate or chlorflurenol but were drastically reduced by chlorsulfuron.

Annual weed seeds persist in cultivated soils due to seed dormancy and possibly to resistance to microbial attack. The relationships between microorganisms ·and velvetleaf (Abutilon theophrasti Medic.) seeds were investigated to elucidate the deterioration resistance mechanisms in this species. Microorganisms were isolated from velvetleaf seeds matured on the plant and from seeds dispersed on the soil surface. An association of sporulating fungi comprised of Alternaria altemata, Cladosporium cladosporioides, Epicoccum purpurascens, and Fusarium spp. was consistently found on the seed surface. This association persisted on seeds after dispersal to the soil. Seed deterioration was infrequent despite abundant microbial growth on the seed surface. About 80% of the bacteria isolated from within seeds were antagonistic to externally seedborne fungi.The proportion of surface-sterilized seeds with internal fungi was 10%, indicating the seed coat acts as a barrier to most fungal invasions. When the seed coat was punctured, however, seed deterioration by the seedbome microorganisms readily occurred. Seed coat extracts contained tannin-like compounds that inhibited in vitro growth of several bacterial and fungal isolates. Histochemical examination of seed sections revealed localization of tannin-like compounds in the palisade layer of the seed coat and the chalazal area of the embryo. It is suggested that deterioration of velvetleaf seeds by microorganisms is limited by the dense layer of palisade cells in the seed coat, chemical compounds within the seed coat that are inhibitory to certain microorganisms outside the seed, and antagonistic bacteria located within the seed. Resistances to microbial attack may act together with seed diJrmancy to maintain velvetleaf seed longevity in soil.----------------Additional index words: Abutilon theophrasti Medic., Antagonism.Antimicrobial compounds, Microbial ecology, Seed dormancy, Weed seeds.