Sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} and RO-13-8895 {acetone-O-[d-2-[p-[(α,α,α-trifluoro-p-tolyl)-oxy]phenoxy]propionyl]oxime} were evaluated for quackgrass [Agropyron repens (L.) Beauv. ♯ AGRRE] control in ‘Norcen’ birdsfoot trefoil [Lotus corniculatus (L.)], ‘Monarch’ cicer milkvetch [Astragalus cicer (L.) Hook.], and ‘Arlington’ red clover [Trifolium pratense (L.)]. None of the herbicide treatments applied to weed-free legumes 3, 15, and 30 cm tall reduced crop ground cover or seed yield. Postemergence treatments of sethoxydim and RO-13-8895 applied to quackgrass in the one- to four-leaf stage, at rates from 0.56 to 1.1 kg ai/ha, gave control of 57 to 91% in September of the seedling year. Legumes in all postemergence herbicide-treated plots produced more seed than legumes in untreated plots, regardless of time of application. Sequential applications of sethoxydim, RO-13-8895, or fluazifop {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid} at 0.14 plus 0.14 kg ai/ha gave more effective quackgrass control and greater legume ground cover than single applications at 0.28 kg ai/ha.

Soils did not remain enhanced for EPTC degradation when EPTC was applied in a biennial rotation with cyanazine, cycloate, or alachlor as indicated by laboratory analysis and early-season weed control in the year of EPTC use. Control of late-season weeds was less when EPTC was rotated with alachlor than with a first-time use of EPTC. EPTC degradation remained enhanced in an EPTC-butylate rotation. The reversion rate of soil from an enhanced condition to normality was gradual and varied with location, year, and number of prior EPTC applications. At Clay Center and Scottsbluff, soils reverted to a nonenhanced EPTC degradation rate 18 months after initial EPTC application. At Scottsbluff, soil was not enhanced for EPTC degradation 18 months after the second of two annual EPTC applications. Soil from Clay Center was partially enhanced for EPTC degradation 18 months after the second of two annual EPTC applications but was not enhanced after 30 months.

The influence of buthidazole {3-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-4-hydroxy-1-methyl-2-imidazolidinone}, diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], and atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] on fluorescence emission and post-luminescence decay in chlorella (Chlorella vulgaris Beyer) was studied. All three herbicides blocked photosynthetic electron transport and thus enhanced fluorescence. Buthidazole was the most active, a 79% increase in fluorescence being observed when algal cultures were exposed to 1 × 10-6 M herbicide for 5 min, and a 206% increase with an exposure time of 60 min. Postluminescence in chlorella exposed to 1 × 10-6 M buthidazole for 5, 10, 20, and 60 min was inhibited 15, 34, 58, and 69%, respectively. Diuron and atrazine inhibition of postluminescence was much less than that of buthidazole. The ratio of relative fluorescence to relative postluminescence can be sued to give a numerical value (C) for the phytotoxicity of this type of herbicide to a given plant species.

Addition of a surfactant (alkylaryl polyglycol ether) increased the rate and quantity of 14C-bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] absorbed by leaves of 3-week-old hydroponically grown sunflower (Helianthus annuus L.). Autoradiographs and radioassays of plant parts indicated that the surfactant enhanced acropetal movement of bentazon in sunflower, whereas bentazon applied alone remained in the treated leaves. Basipetal movement of 14C-bentazon was insignificant in sunflower. High-pressure liquid chromatography showed that 3 days after treatment 40% of the absorbed 14C-bentazon was metabolized to a methanol-soluble metabolite. Thinlayer chromatography substantiated the occurrence of a 14C-bentazon metabolite 3 days after treatment.

Field and laboratory studies were conducted in 1985 and 1986 to determine the effect of herbicide application timing and level of corn residue on metribuzin plus metolachlor for weed control in soybean. Herbicides were applied early preplant (35 days before planting), sequentially (early preplant and preemergence), or preemergence. Giant foxtail control was better with sequential application than early preplant herbicide application in both years. Control of velvetleaf, smooth pigweed, and jimsonweed was independent of application timing, except in 1986, when jimsonweed control was less from herbicides applied early preplant compared to sequentially. Metribuzin reaching the soil was less when applied to high levels of corn residue compared to low residue levels. However, corn residue level had no effect on weed control.

Field and greenhouse studies conducted in North Carolina in 1981 and 1982 evaluated the effect of the dimethylamine salt of 2,4-DB [4-(2,4-dichlorophenoxy) butyric acid] on soybeans. In the field, 2,4-DB at and above 0.07 kg ae/ha reduced soybean shoot mass, leaf nitrogen levels, and nodule activity. Soybean seed yields were reduced by 2,4-DB at and above 0.07 kg/ha. In greenhouse studies, total nodule activity was reduced by 0.07 and 0.14 kg/ha of 2,4-DB. Soybeans treated with 2,4-DB at initiation of flowering (R1 stage) or when pods were 1.9 cm long (R4 stage) yielded progeny with visible injury symptoms. Seedling emergence was also reduced in progeny of soybeans.

Rice (Oryza sativa L., var. Bluebonnet) bioassay was utilized to evaluate activity of several herbicide combinations at concentrations of each individual compound causing a 50% reduction in root length. Synergistic combinations were trichloroacetic acid (TCA) plus 1,1-dimethyl-3-phenylurea (fenuron); TCA plus 3-(p-chlorophenyl)-1,1-dimethylurea (monuron); TCA plus 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron); TCA plus 3-amino-s-triazole (amitrole); TCA plus 2,2-dichloropropionic acid (dalapon); and dalapon plus monuron. Additive results were obtained from combinations of TCA plus 1,2-dihydro-3,6-pyridazinedione (MH); dalapon plus fenuron; and dalapon plus diuron. MH and (2,4-dichlorophenoxy) acetic acid (2,4-D) exhibited antagonistic responses when combined with fenuron, monuron, diuron, dalapon, amitrole, and with each other. Antagonistic responses were obtained from combination of 2,4-D and TCA.

The efficacy of the safeners naphthalic anhydride (NA), R-29148, benoxacor, and dichlormid as protectants of corn against injury from the methyl ester of tribenuron was evaluated under greenhouse conditions. Tribenuron applied alone reduced the root length of both ‘Pioneer 3180’ and ‘Northrup-King 9283’ corn. The concentration of tribenuron causing 50% reduction of corn root length (GR50) was 1 μM. NA applied as a seed dressing at 0.5 and 1.0% (w/w) was the most effective of the four safeners tested and provided excellent protection to both corn hybrids against 1, 2, and 4 μM tribenuron. Soaking corn seeds in an aqueous solution of R-29148 containing 0.5,1, or 2 mM protected ‘Pioneer 3180’ against 1 μM of tribenuron. The safening efficacy of R-29148 against tribenuron injury to ‘Northrup-King 9283’ corn was limited. Dichlormid and benoxacor, were less effective as tribenuron safeners and offered partial protection to both corn hybrids. Individual applications of the synergist piperonyl butoxide (PBO) at 0.1 and 0.2 mM did not affect the growth of corn seedlings. PBO synergized the activity of all concentrations of tribenuron, but only on the ‘Northrup-King 9283’ hybrid. These results demonstrate that the tolerance of corn to tribenuron can be enhanced with the use of safeners such as NA and R-29148, and that oxidative reactions may be involved in the metabolism of tribenuron by corn.

Two separate experiments were conducted from 1974 through 1980 and from 1976 through 1982 at an experimental farm in southwestern Ontario, Canada, to determine the pattern of temporal and spatial distributions of velvetleaf seedlings from seed produced by uncontrolled plants. For each experiment, a soybean field with no previous record of velvetleaf infestation was selected and planted with four patches of 10 velvetleaf plants. The plants were allowed to grow to reproductive maturity for 1 yr, and the density and spatial distribution of velvetleaf seedlings within the field were mapped over the next 6 yr. Although velvetleaf seedlings emerged each year after seeding, the highest percentage emerged the second year after seeding. Velvetleaf seedlings emerged at locations with or without a seeding plant, but a high density of emerged seedlings was often observed at locations some distance away from the seeding plants. The results suggest that under conventional harvesting methods there may not always be a close spatial relationship between the distribution of parent plants and their offspring, depending on the scale of the land and biotic and abiotic environmental conditions.

Two field experiments were conducted in 1984 and 1985 to determine the effects of fluazifop, haloxyfop, and sethoxydim on the suppression of water uptake and competition of wheat to seedling alfalfa. In all cases, suppression of wheat growth and stand led to significant reduction in soil water use. The amounts of conserved soil water significantly correlated with increased numbers of alfalfa plants; the correlation coefficients, respectively, for mid- and late-summer plantings were 0.47 and 0.41 in 1984 and 0.90 and 0.86 in 1985. Conservation of soil water was attributed to growth reduction or cessation of wheat growth by the herbicides and, to some extent, suppression of wheat leaf transpiration. All of the three herbicides would be effective in controlling volunteer wheat in a new alfalfa planting.

Research was conducted to formulate a temperature-dependent population-level model for rhizome johnsongrass flowering. A nonlinear poikilotherm rate equation was used to describe development as a function of temperature and a temperature-independent Weibull function was used to distribute development times for the population. Johnsongrass flowering data were collected under constant temperature conditions to parameterize the poikilotherm rate equation and Weibull function. Coupling the poikilotherm rate equation with the Weibull function resulted in a population level temperature-dependent model. The model was validated against independent field data sets. The model accurately predicted rhizome johnsongrass flowering from plants emerging in the spring. The model performed poorly for plants emerging in summer. Adjustments to the high-temperature inhibition parameter of the poikilotherm rate equation improved model performance in the summer without affecting spring predictions.

Methazole [2-(3,4-dichlorophenyl)-4-methyl-l,2,4-oxadiazolidine-3,5-dione], metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one], and combinations of MSMA (monosodium methanearsonate) with either methazole or metribuzin were applied to bare ground at 0, 1, 2, and 4 weeks before planting with bermudagrass [Cynodon dactylon (L.) Pers. ‘Common’] as sprigs, stolons, and seed. In most instances, the growth of newly planted bermudagrass was as good or better from combination of MSMA (2.2 kg/ha) with either methazole or metribuzin at 0.1 kg/ha than with methazole or metribuzin applied alone at higher rates. MSMA + methazole can be applied safely on day of planting with sprigs and seed, but a 1-week interval between treatment and planting was needed with stolons in 1 of 2 yr. The MSMA + metribuzin combination can also be applied safely to sprigged bermudagrass on day of planting, but a 1-week interval was needed between treatment and planting with stolons and a 2-week interval between treatment and planting with seed to insure minimum bermudagrass injury.

Benzyladenine (BA), kinetin, and gibberellic acid (GA) at concentrations of 6 × 10−4 and 4 × 10−3 M, respectively, were used to induce formation of basal bulbs above ground in rhizomes of purple nutsedge (Cyperus rotundus L.). BA plus GA induced a tropistic response in all of the rhizomes and only above ground bulbs were produced, while kinetin plus GA induced a tropistic response and bulbs formed above and below the soil surface. Control plants as well as those treated with GA, kinetin, or BA produced underground bulbs and rhizomes but no bulbs above ground. Height of above ground secondary bulbs induced by kinetin plus GA was significantly greater than those induced by BA plus GA. Transformation of primary rhizomes to shoots in the control plants did not differ significantly from transformation of those originating from plants of the various treatments. Height of primary plants of all treatments differed significantly from the controls.

The phytotoxicity of seven herbicides after soil application was determined in the field and greenhouse. Clopropoxydim {(E,E)-2-[1-[[(3-chloro-2-propenyl)oxy] imino] butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one}, sethoxydim {2-[1-(ethoxyimino)butyl]-5-(2-ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one}, the methyl ester of haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid} applied to soil at 0.21 kg/ha, and the methyl ester of diclofop {(±)-2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid}applied to soil at 1.12 kg/ha were phytotoxic to oats (Avena sativa L. ‘Moore’) in the field and greenhouse. These rates are equivalent to postemergence use rates of these herbicides. DPX-Y6202 {2-[4-[(6-chloro-2-quinoxalinyl)oxy] phenoxy] propanoic acid} and the butyl ester of fluazifop {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy] phenoxy] propanoic acid} applied to soil at 0.84 kg/ha or greater were phytotoxic to oats. The ethyl ester of fenoxaprop {(±)-2-[4-[(6-chloro-2-benzoxazolyl)oxy] phenoxy] propanoic acid} soil applied exhibited very limited phytotoxicity to oats. Oat stands were reduced when oats were seeded into soil treated with clopropoxydim, sethoxydim, haloxyfop, diclofop, DPX-Y6202, fluazifop, and fenoxaprop, with the amount of stand reduction decreasing from clopropoxydim to fenoxaprop. Oat fresh weight reductions were greatest with all herbicides when the herbicides were in contact with all underground parts (shoots, seeds, and roots). Oat fresh weight reductions were greater when either the shoot or seed of the germinating oat plants were exposed to the herbicides compared to root exposure. Foxtail millet (Setaria italica L. ‘Siberian’) was more susceptible to injury than oats or wheat (Triticum aestivum L. ‘Len’) when seeded into soil treated with any one of the seven postemergence grass control herbicides.

Eight substituted dinitroaniline herbicides were studied for relative effectiveness in controlling weeds and for tolerance of peas (Pisum sativum L.). Greenhouse trials comparing 10-5 M herbicide solutions added to washed silica sand indicated that trifluralin (α,α,α-trifluroro-2,6-dinitro-N,N,-dipropyl-p-toluidine) and dinitramine (N4,N4-diethyl-α,α,α-trifluoro-3,5-dinitrotoluene-2,4-diamine) were the most phytotoxic to peas. Field trials conducted over a 3-yr period using 0.84 and 1.68 kg/ha application rates demonstrated that all eight of the herbicides successfully controlled the weeds present. Use of trifluralin resulted in the lowest shelled pea yields at both rates of application. Significant reductions in yield occurred from the 1.68 kg/ha rate of trifluralin in 1973 and 1975 when compared with the untreated checks. While major differences did not occur between the other chemicals, the greatest average pea yields resulted from the use of oryzalin (3,5-dinitro-N4,N4-dipropylsulfanilamide), dinitramine, and fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)analine].

14C-chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} and its metabolites were extracted from flax (Linum sativum L.), field penny cress (stinkweed) (Thlaspi arvense L. # THLAR), and wheat (Triticum aestivum L.) with an average efficiency of 94% using an aqueous extraction procedure. Chlorsulfuron and its metabolites were separated on a reverse-phase preparative chromatography column by eluting with a step gradient of aqueous 0.1% (v/v) formic acid and methanol. Major peaks were eluted at 35 and 45% methanol, and minor peaks at 10, 25, and 100% methanol. The 45% methanol peak represented unmetabolized chlorsulfuron or its hydroxylated derivative and was the only fraction that exhibited biological activity. In wheat and flax, 75 and 62%, respectively, of the extracted activity were eluted by 35% methanol and, in the case of wheat, this fraction was shown to be a glycosylated derivative of chlorsulfuron. Although 14C-chlorsulfuron was metabolized to a lesser extent in field pennycress, our data indicate that over 50% of the absorbed 14C-activity was recovered in forms other than 14C-chlorsulfuron 5 days after treatment. The separation procedure can be used readily to assess the amount of chlorsulfuron detoxification that occurs in plants.

Dimethyl tetrachloroterephthalate (DCPA) caused swelling of a localized region of the hypocotyls of cucumber (Cucumis sativus L., var. Wis. SMR-15), tomato (Lycopersicon esculentum Mill., var. Heinz 1370), and jimsonweed (Datura stramonium L.). DCPA caused stem swelling in jimsonweed by affecting dividing and/or differentiating cells in the vascular cambium. As swelling proceeded, xylem tissue was increased over the controls and xylem cells were disorganized and no longer continuous. Swelling was induced at the site of DCPA application, and was reduced when DCPA concentration per unit of stem was lowered, even though the same amount per plant was applied. Uptake of 14C-DCPA from the treated stem region was slow, but the treated area had the highest specific activity.

Rapid strides are being made in understanding the fundamental regulation of plant growth, development, and responses to the environment due to recent advances in molecular biology. Current questions in weed science such as herbicide mechanisms of action, biodegradation, and mechanisms of weed resistance are equally approachable using such methodology. Efforts to introduce herbicide resistance into agronomically important crops are possible because of successful isolation and transfer of genes. Investigations of weed survival and competitive strategies based on developmental processes, such as seed dormancy, are currently underway using techniques designed to monitor and characterize differential gene expression. Molecular methodology also plays a key role in taxonomic studies of weed populations using restriction fragment length polymorphism (RFLP) mapping. The future potential for these and other techniques such as nucleic acid hybridization, polymerase chain reaction (PCR), gene transfer, and the use of transgenic plants is described.

Field experiments were conducted from 1994 through 1996 in dry-seeded and water-seeded Oryza sativa to compare weed control and O. sativa yield in conventional tillage and reduced tillage (stale seedbed and Triticum aestivum cover crop) systems with no in-season herbicide, propanil applied early postemergence (EPOST), and propanil EPOST followed by propanil plus molinate (dry-seeded production) or granular molinate (water-seeded production). The T. aestivum cover crop reduced Echinochloa crus-galli, Heteranthera limosa, Ammania coccinea, and Cyperus esculentus infestation and reduced the need for in-season herbicides in some but not all experiments. Oryza sativa grain yield was affected by both tillage systems and herbicide programs; however, these treatment factors influenced yield independently. Increasing the number of in-season herbicide applications increased weed control and O. sativa yield in some but not all experiments.

Laboratory studies were conducted to determine the effect of SAN 582 on the absorption, translocation, and metabolism of sethoxydim in johnsongrass and soybean. 14C-sethoxydim absorption in johnsongrass and soybean was enhanced 4 h after application when tank mixed with SAN 582. Absorption in johnsongrass increased from 18% with 14C-sethoxydim applied alone to 65% when applied with SAN 582. Absorption in soybean increased from 30% with 14C-sethoxydim applied alone to 83% when applied with SAN 582. The addition of crop oil concentrate also enhanced foliar absorption of 14C-sethoxydim. Eight hours after application, foliar absorption in johnsongrass increased from 22% with 14C-sethoxydim applied alone to 61% when 14C-sethoxydim was applied with crop oil concentrate. Foliar absorption in soybean increased from 38% with 14C-sethoxydim applied alone to 59% when 14C-sethoxydim was applied with crop oil concentrate. When 14C-sethoxydim was applied in combination with crop oil concentrate and SAN 582, foliar absorption was 84% or more in both johnsongrass and soybean 4 h after application and did not increase over time. Minor differences in translocation were observed, but these differences were not sufficient to account for the increase in foliar activity observed when SAN 582 is tank mixed with sethoxydim. SAN 582 did not affect metabolism of sethoxydim in either species. Based on these data, it appears that SAN 582 increases the efficacy of sethoxydim by enhancing foliar absorption.