Field studies were conducted for 4 yr to investigate the effects of tillage and herbicide programs on weed control and wheat (Triticum aestivum L. ‘Potomac’ in 1981 and ‘Wheeler’ from 1982 to 1984) grain yields in a wheat-soy bean [Glycine max (L.) Merr.] double-crop rotation. Predominant weed species were common chickweed [Stellaria media (L.) Vill. # STEME], corn chamomile (Anthemis arvensis L. # ANTAR), and annual bluegrass (Poa annua L. # POAAN) at the onset of research and cheat (Bromus secalinus L. # BROSE) and soft brome (Bromus mollis L. # BROMO) at the conclusion of the study. Control of all species was excellent with conventional tillage and no-till plus nonselective herbicides but was significantly less with no-till without nonselective herbicides and with minimum tillage. After 4 yr, maximum wheat grain yields were significantly higher in conventional- than in minimum- or no-till systems.

Norflurazon [4-chloro-5-(methylamino)-2-(3-(trifluoromethyl)phenyl)-3(2H)-pyridazinone] persistence in five Georgia soils after application to the same plots in 1982 and 1983 was characterized by rapid initial degradation followed by slow loss of the herbicide. The rate of norflurazon loss at each location for each year was not affected by application rate (1.7 or 3.4 kg ai/ha). Relative rates of norflurazon loss were Dothan loamy sand ≥ Greenville sandy clay loam ≥ Rome gravelly clay loam = Appling coarse sanely loam > Bradson clay loam. The rate of dissipation was slower in 1983 than 1982 in the Greenville and Appling soils. Cool and/or dry environmental conditions combined with higher soil organic matter content caused slower herbicide loss. Norflurazon residue 1 yr after treatment in all soils was greater in 1983 than in 1982. Significant injury to grain sorghum [Sorghum bicolor (L.) Moench. ‘BR 64’) was observed in the spring of 1984 at all locations in all plots that were treated with norflurazon for the previous two seasons. The degree of injury corresponded to the concentration of norflurazon detected at that sampling date. Leaching did not appear to be an important method of norflurazon loss.

Response of grain sorghum [Sorghum bicolor (L.) Moench. ‘BR 64’] to norflurazon [4-chloro-5-(methylamino)-2-(3-(trifluoromethyl)phenyl)-3(2H)-pyridazinone] and fluridone {1-methyl-3-phenyl-5-[3-(trifluoromethyl) phenyl]-4(1H)pyridinone} and the effect of previous treatment on the persistence of herbicide activity in five soils were determined in the greenhouse. Relative phytotoxicity of fluridone in the soils was ordered as Greenville sandy clay loam > Appling coarse sandy loam > Dothan loamy sand = Rome gravelly clay loam > Bradson clay loam. Phytotoxicity of norflurazon was ordered as Greenville = Appling = Dothan > Rome > Bradson. Fluridone was more phytotoxic than norflurazon. Field soil that has been previously treated with norflurazon or fluridone (500 days earlier at 1.7 kg ai/ha) still had low concentrations of each herbicide present at the time of retreatment in the greenhouse. Previous treatment of norflurazon did not affect the rate of decline in activity in any of the soils compared to previously nontreated soils except for Appling. Significantly higher concentrations of norflurazon remained in all previously treated soils 297 days after treatment (DAT) compared to previously nontreated soil. Residual carryover of norflurazon was greater in the Bradson soil (2.8% O.M.) than soils having less than 2% O.M. Fluridone dissipation was significantly faster in previously treated soil than in previously nontreated soil, indicating enhanced microbial degradation of the herbicide.

Acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)acetamide], alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] were applied in 280 L of water/ha to plots covered with 0 to 6720 kg/ha of wheat (Triticum aestivum L.) straw. At straw levels of 1120 kg/ha or greater, 50% or less of the applied herbicides were received by the soil surface before irrigation. Sprinkle irrigation (1.3 cm) washed 15 to 20% of the originally applied herbicide into the soil regardless of straw level. More metolachlor was retained on the straw than acetochlor or alachlor. Analysis of the wheat straw indicated that little water-extractable herbicide remained for all herbicides. Initial herbicidal activity on grain sorghum [Sorghum bicolor (L.) Moench.] was reduced by the presence of wheat straw at the time of application, with acetochlor being least affected and alachlor most affected. Ten days after treatment, less than 10% of the original alachlor and acetochlor remained in the soil. When planted at this time, grain sorghum response was inversely related to the amount of straw mulch that was originally present. Metolachlor residues in the soil on day 10 were 11 to 26% of that on day 0 and there was comparably less reduction in activity on grain sorghum.

Field research was conducted in 1982 and 1983 to characterize the persistence of fluridone {1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)pyridinone} in five Georgia soils. Fluridone persisted less than 365 days in all soils, with shorter persistence upon reapplication in 1983 which indicated the potential for enhanced microbial degradation. A significantly higher rate of loss in 1983 compared to 1982 was recorded in the Greenville sandy clay and Dothan loamy sand soils. A higher rate of loss was recorded for the 1.7 kg ai/ha than the 0.6 kg/ha treatment in the Bradson clay loam and Rome gravelly clay loam soils. No grain sorghum [Sorghum bicolor (L.) Moench. ‘BR 64’) injury was observed in a field bio assay planted in the spring of 1984. Herbicide leaching did not appear to be an important method of loss.

Velvetleaf (Abutilon theophrasti Medik. # ABUTH) seeds were bioassayed on 241 microbial isolates to assess their antimicrobial activity. Seeds placed on agar plates inoculated with test microorganisms released a diffusible substance(s) that inhibited the growth of 117 of 202 (58%) bacteria and all of the fungi tested. Antimicrobial activity of the seeds appeared to be nonselective as the extent of inhibition was not related to type of microorganism nor their origin. Hard, water-impermeable seeds had greater inhibitory activity than imbibed (soft) seeds. The intensity of inhibition was affected by prior leaching of seeds with various solvents and by the stage of seed development. Chemical analysis of diffusion zones from agar plates and seed leachates revealed the presence of phenolic compounds. The presence of antimicrobial substances in velvetleaf seeds may contribute to the persistence of viable seeds in soil by inhibiting potential seed-deteriorating microorganisms.

Greenhouse and laboratory studies were conducted to determine the allelopathic potential of two sweet potato [Ipomoea batatas L. (Lam.)] cultivars, ‘Regal’ and ‘SC 1149-19’. Yellow nutsedge (Cyperus esculentus L. # CYPES) and alfalfa (Medicago sativa L.) plants grown in soil from sweet potato field plots accumulated less dry matter than plants grown in soil from adjacent weedy plots. Growth of yellow nutsedge and alfalfa plants was also reduced when grown in a potting medium containing decomposing sweet potato plants in comparison to plants grown in potting medium alone. When the sweet potato potting medium mixture was incubated at 25 C and tested weekly using an alfalfa growth bioassay, inhibition was high initially but decreased over time and was not observed after a 12-week incubation. Aqueous methanol (50%)-soluble extracts of sweet potato periderm were inhibitory to yellow nutsedge root growth in vermiculite and alfalfa seed germination on filter paper. Regal extracts were inhibitory to yellow nutsedge root growth at a concentration of 2.5 mg periderm extracted/ml, but SC 1149-19 extract was inhibitory only at a concentration eight times higher. Similar differences between cultivars were observed with the alfalfa germination bioassay. Preliminary separation of the Regal periderm extract by paper chromatography indicated the presence of phenolic compounds with a wide range of polarities. Several of these UV-visible compounds were inhibitory to alfalfa germination.

The survival of two Rhizobium japonicum strains was determined in soils treated with 13 herbicides used in soybeans [Glycine max (L.) Merr.]. Survival in soils treated with concentrations equivalent to or 10-fold greater than those from field applications at recommended rates was compared to survival in untreated soil under controlled temperature and moisture conditions. Nine of the herbicides had transitory or no effects, regardless of rate. Acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid}, bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide], fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)benzenamine], and dinoseb [2-(1-methylpropyl)-4,6-dinitrophenol] consistently reduced survival of one or both strains at rates 10-fold greater than recommended. At rates of application equivalent to suggested field use rates, effects on survival were reduced or eliminated. Reductions in survival attributable to herbicides were in some cases comparable to those obtained by a weekly wetting and drying of the soil, indicating that herbicide effects on R. japonicum may be equivalent in magnitude to effects due to environmental stress. The herbicides were not sufficiently toxic to reduce R. japonicum populations in soil to levels that would be likely to affect nodulation.

Field experiments were conducted in 1982 and 1983 in cotton (Gossypium hirsutum L.) to evaluate alternatives in grass weed management programs with the use of postemergence herbicides and to assess the economic potential for incorporating the new over-the-top grass weed herbicides into total cotton production practices. Twelve weed management systems using the grass herbicides sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} and fluazifop {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid} were compared to six standard systems. Differences in cotton production costs among the 18 systems were due to cost differences in total weed management. Equal or greater net return values were obtained when over-the-top herbicides followed fluometuron {N,N-dimethyl-N′-[3-(trifluoromethyl)phenyl)urea} preemergence or when these materials were applied twice compared to the over-the-top materials used in conjunction with trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine] preplant incorporated or with directed postemergence materials. High net returns were the result of both high seed cotton yields and low total production costs.