Imazaquin {2-(4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl)-3-quinolinecarboxylic acid} site of uptake and toxicity from soil application were investigated in yellow nutsedge (Cyperus esculentus L. #3 CYPES) and purple nutsedge (C. rotundus L. # CYPRO). Imazaquin concentrations of 0.1 to 0.5 ppmw inhibited yellow nutsedge shoot emergence completely, while purple nutsedge shoots emerged at the lower concentrations. Herbicide placement above the tuber reduced shoot emergence and shoot and root dry weights of both species more than did placement below the tuber. Increasing herbicide rate increased the number of tuber buds that sprouted. Three-day-old nutsedge propagules absorbed 14C-imazaquin from both rhizome shoots and roots and the herbicide moved both acropetally and basipetally in nutsedge propagules.

Previous reports have suggested that bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] tolerance among soybean genotypes is the result of differential translocation or metabolism. The basis for tolerance was reexamined using susceptible and tolerant genotypes. Tolerant genotypes (‘Hill’ and ‘Clark 63’) were found to tolerate 100- to 300-fold more bentazon than susceptible genotypes (‘L78–3263’, ‘Hurrelbrink’, and ‘PI 229.342’). Minor differences in absorption and translocation occurred among the genotypes but they did not correlate with tolerance. Tolerant genotypes metabolized 80 to 90% of absorbed bentazon within 24 h, while susceptible genotypes metabolized only 10 to 15%. Two major metabolites, the glycosyl conjugates of 6- and 8-hydroxybentazon, were formed in tolerant genotypes. Susceptible genotypes did not form the hydroxybentazon conjugates but instead produced relatively low levels of two unidentified metabolites. It is concluded that differential bentazon tolerance among soybean genotypes is linked to the ability to form both the 6- and 8-hydroxybentazon conjugates.

A mathematical model describing the behavior of 14C-haloxyfop {2-[4-[(3-chloro-5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} and its methyl and ethoxyethyl esters on yellow foxtail [Seteria glauca (L.) Beauv. #3 SETLU] has been developed to evaluate volatility, penetration, ester transformation, metabolism, and transport of the chemical in the plant system. The effects of crop oil concentrate, temperature, structure of the molecule, and the addition of bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] to the application solution on the kinetic rate constants have been studied. Crop oil concentrate was found to dramatically increase penetration at an optimal level of approximately 0.3% (v/v). Significant increases in rate constants were observed at higher temperatures. Esters were rapidly converted to the acid in the plant with conversion of the ethoxyethyl ester being approximately twice as fast as the methyl ester. Subsequent metabolism of the acid proceeded at a moderate rate which was independent of the original ester form of the molecule. All kinetic processes were described by first-order rate constants with the exception of transport which appeared to slow a few hours after application even though acid levels in the plant were high. When bentazon was applied in combination with haloxyfop-methyl, penetration of the ester was somewhat inhibited, while transport of radioactivity associated with the molecule to other plant parts was dramatically reduced.

Atrazine metabolism was studied in four warm-season forage grasses to determine if metabolism was the basis for differential atrazine tolerance among the grasses. Big bluestem and switchgrass are atrazine tolerant while indiangrass and sideoats grama are atrazine susceptible in the seedling stage. Metabolism of atrazine in big bluestem and switchgrass occurred primarily by glutathione conjugation. The major metabolic product isolated from indiangrass and sideoats grama was the N-deethylated metabolite of atrazine. Glutathione conjugation by big bluestem and switchgrass occurred at a faster rate than N-dealkylation of atrazine in indiangrass and sideoats grama. Differential tolerance to atrazine among the grasses studied was probably due to the metabolic route by which they detoxify atrazine and the rate of metabolism for that specific route. Intraspecific differences in atrazine tolerance in indiangrass were due to the amount of metabolite produced in relationship to the amount of parent atrazine remaining in the shoot tissue. The more tolerant indiangrass lines had a higher metabolite to parent atrazine ratio than susceptible lines. This study confirmed differences in seedling atrazine tolerance of four indiangrass lines observed in previous greenhouse studies.

Hydrophobic exudate from roots of sorghum [Sorghum bicolor (L.) Moench. #3 SORVU ‘IS 8768’] contain four p-benzoquinones which in the dihydroquinone form are active as germination stimulants of witchweed [Striga asiatica (L.) Kuntz. # STRLU]. The three minor p-benzoquinones were partially characterized and found to be structurally similar to sorgoleone, the major p-benzoquinone of this exudate. Herbicidal activity of the hydrophobic exudate was due to concentration- and pH-dependent inhibition of root elongation in some but not all weeds tested. Witchweed has apparently adapted these “defense” compounds of sorghum as host-specific germination stimulants.

Greenhouse studies were conducted to determine the extent of translocation from the foliage to fleshy roots, the inherent toxicity, and the fate of radiolabeled and nonlabeled dicamba, picloram, and triclopyr in horsenettle. Roots of horsenettle acted as the major sink for photosynthate accumulation at the 0.2- to 0.5-bloom growth stages as determined by autoradiography. Dicamba, picloram, and triclopyr were translocated into the roots of horsenettle and accumulation continued for at least 16 days. 14C associated with each herbicide found in the roots ranged from 1.3% at 4 days to 3.8% at 16 days. After 16 days, slightly more 14C from plants treated with dicamba and triclopyr (3.8 and 3.6%) than picloram (3.0%) was translocated to roots. These compounds were metabolized slowly in the foliage and roots as determined by thin-layer chromatography (TLC) and autoradiography. In translocation studies with horsenettle shoots, picloram at 1.12 kg/ha killed the treated and untreated shoots and roots. Dicamba and triclopyr at the highest rates killed the treated shoots and partially destroyed the root system. Symptoms were noted on the untreated shoots, but full recovery occurred at 8 weeks. Since each of the herbicides was metabolized slowly and only slight differences in their translocation were observed, the relatively higher herbicidal effectiveness of picloram must be attributed to its greater inherent potency.

The relative activities of (R) and (S) enantiomers of the methyl ester of haloxyfop were determined on annual grasses. Samples enriched in the (S) enantiomer were markedly less active than the (R) in petri dish evaluations and foliar applications. The pure (S) enantiomer was estimated by regression to be 1000-fold or less active than the (R). The activity of the (S) enantiomer was found to be equivalent to that of the (R) following preemergence applications. Isolation and characterization of haloxyfop from soil treated with the methyl ester of haloxyfop indicated inversion of the (S) enantiomer to the (R) enantiomer within 7 days. Field trials confirmed the differential activity of enantiomers applied postemergence and their equivalence when applied preemergence. These findings indicate that inversion of the (S) enantiomer to the (R) occurs in soil following preemergence applications.

Structural studies were conducted to evaluate the effects of napropamide on the growth and development of corn roots. At 1.0 and 10.0 μM napropamide, root growth was inhibited severely within 3 days of seed germination. Root diameter within 1 mm of the root apex doubled and numerous lateral root primordia were observed within 10 mm of the meristem tip in treated roots. The number of cortical parenchyma cell files, xylem vessel, and phloem sieve tube strands also significantly increased. Average cortical cell size did not change, regardless of the treatment. A lateral expansion of the meristematic region of the root coincided with a slight reduction in meristem length but resulted in an overall increase in meristem volume. However, enlargement of the meristem occurred despite a reduction in the number of mitotic figures in the root meristem. Treatment of excised root tips for 24 h with 20 μM napropamide reduced the number of mitotic figures 84%.

A growth room study was conducted to evaluate the effect that timing of application has on the distribution of several herbicides in quackgrass. Uniformly labeled 14C-sucrose and the radiolabeled herbicides glyphosate, sethoxydim, the butyl ester of fluazifop, and the methyl ester of haloxyfop were applied to quackgrass (ranging from the three- to eight-leaf stage) propagated from six-bud rhizome segments. Five days after treatment the plants were harvested, lyophilized, and later sectioned, mapped, and oxidized in preparation for 14C quantification. In most cases, slightly more 14C was translocated to the shoots than to the rhizomes. 14C translocation to the rhizomes was similar at all growth stages. The 14C accumulating in the rhizomes exhibited a nonuniform distribution pattern with more 14C in the distal areas of new rhizomes than in the other areas of the rhizome system. Plants treated with haloxyfop had a more uniform distribution of 14C along their rhizomes than did those treated with fluazifop or sethoxydim.

A fungal pathogen, Colletotrichum gloeosporioides (Penz.) Sacc. f. sp. malvae, isolated from anthracnose symptoms of round-leaved mallow (Malva pusilla Sm.), was shown in greenhouse tests to be host specific to Malva spp. and velvetleaf (Abutilon theophrasti Medic. #3 ABUTH), and only with slight attack on hollyhock [Althaea rosea (L.) Cav. # ALGRO], Malope trifida Cav., and Venice mallow (Hibiscus trionum L. # HIBTR). Round-leaved mallow plants inoculated with a spore suspension of the fungus were killed after 17 to 20 days. It was less pathogenic on velvetleaf with 60 to 70% attack. The fungus can readily be cultured and field tests from 1982 to 1987 resulted in excellent control of round-leaved mallow under natural conditions. Therefore, it has good potential for biological control of round-leaved mallow in field crops.

In three replacement series experiments, winter wheat (Triticum aestivum L.), jointed goatgrass (Aegilops cylindrica Host. #3 AEGCY), and downy brome (Bromus tectorum L. # BROTE) were paired in all possible combinations to determine competitive relationships during vegetative growth. Under growth chamber conditions of ample fertility and soil moisture and day/night temperatures of 18/10 C, relative yield totals for the three species were similar, indicating that they compete for the same resources. Both winter wheat and jointed goatgrass had greater plant growth and higher relative crowding coefficients than downy brome, which indicated a hierarchy of relative competitiveness of winter wheat > jointed goatgrass >> downy brome. In other growth chamber studies, winter wheat was slightly more competitive than jointed goatgrass regardless of fertility levels. Winter wheat was the superior competitor at 18/10 C and −33 kPa (soil moisture), whereas jointed goatgrass was superior at 27/10 C and −300 kPa, conditions that are frequently encountered in the Pacific Northwest.

Photosynthesis and growth responses to irradiance level during growth were compared in soybean (Glycine max L. Merr. ‘Century’) and three broadleaf weeds to determine if these responses were associated with differences in shade tolerance among species. In response to reduced irradiance during growth, leaf thickness of all species decreased, while chlorophyll content per unit leaf volume and photosynthetic rate per unit leaf volume, measured at low irradiance, increased. Soybean and common cocklebur (Xanthium strumarium L. #3 XANST) also exhibited a decrease in soluble proteins on a leaf volume basis under reduced irradiance, and common cocklebur further exhibited a decrease in ribulose-1,5-bisphosphate carboxylase (RuBPcase) protein per unit leaf volume. Decreased irradiance during growth did not alter the content of RuBPcase or other soluble proteins per unit leaf volume in jimsonweed (Datura stramonium L. # DATST) or velvetleaf (Abutilon theophrasti Medic. # ABUTH). The superior shade tolerance of common cocklebur compared to the other species was attributed in part to the levels of RuBPcase and other photosynthetic proteins in leaves developed at low irradiance.

The distribution pattern of s-triazine-resistant biotypes of common lambsquarters (Chenopodium album L. #3 CHEAL) and smooth pigweed (Amaranthus hybridus L. # AMACH) in Virginia was determined. Seeds were collected from suspected triazine-resistant biotypes of both species. Triazine resistance was confirmed by measuring chlorophyll fluorescence in the presence of atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine]. Greenhouse bioassay with whole-plant material and a sinking leaf disc assay were also performed as further confirmation of triazine resistance. Triazine-resistant smooth pigweed was confirmed in 19 counties and common lambsquarters in eight counties in Virginia. Triazine-resistant smooth pigweed and common lambsquarters were located mostly in the northern and southwestern highlands of the state where there has been a long history of triazine use in no-till corn (Zea mays L.) production. S-triazine-resistant biotypes were also cross-resistant to other representative s-triazine and as-triazine herbicides but susceptible to the substituted urea herbicide diuron [N′-(3,4-dichlorophenyl)-N,N-dimethylurea].

Weed seed and seedling populations, and weed competition were compared in plots of continuous corn and corn/soybean rotation under ridge and conventional tillage. After 7 to 8 yr of standard chemical and mechanical weed control, from 1500 to 3000 weed seeds/m2 (to a 10-cm depth) were found in continuous corn with ridge tillage whereas about two-thirds fewer seeds were found in conventionally tilled corn. Soil from a corn/soybean rotation had from 200 to 700 seeds/m2 in both tillage systems. Annual loss of weed seeds from the soil through germination was from 3 to 12% in ridge tillage and 11 to 43% in conventional tillage. Additions to the seed pool were supplied by small weeds whose germination was stimulated by “layby” cultivation, with up to 10 times more emergence and 140 times more seed production in ridge than in conventional tillage. Withholding herbicides for 1 yr reduced yields of continuous corn by 10 to 27% in ridge tillage, only 2 to 4% in conventional tillage, and negligibly in corn/soybean rotations regardless of tillage. Reducing seed production of small layby weeds in ridge tillage may aid in solving the weed problem in this conservation tillage system.

Acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} and bentazon [3-(1-methylethyl-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] plus acifluorfen were applied through hydraulic flat-fan nozzles or controlled-droplet applicators (CDA) in water plus surfactant, soybean [Glycine max (L.) Merr.] oil and water emulsions, and soybean oil alone. Except for inadequate weed control with CDA applications at 7 L/ha, method of application did not affect weed control of common cocklebur (Xanthium strumarium L. #3 XANST) or smooth pigweed (Amaranthus hybridus L. # AMACH) at high rates of bentazon plus acifluorfen (560 plus 280 g ai/ha or above). With low rates (280 plus 140 g/ha or less), hydraulic flat-fan nozzles were more effective than CDA applications. Early CDA applications of acifluorfen in an oil carrier at a volume of 9 L/ha were as effective as hydraulic nozzle applications at a carrier volume of 47 L/ha. Later applications resulted in inadequate weed control. Increasing soybean oil concentration from 2.5 to 40% (v/v) in acifluorfen spray mixtures did not significantly increase the phytotoxicity of acifluorfen.

Field experiments were conducted to evaluate johnsongrass control in no-till grain sorghum with glyphosate treatments applied in mid-June, late July, and mid-September before planting grain sorghum the following spring. Mid-June applications provided the best johnsongrass control. Ammonium sulfate enhanced the activity of glyphosate for only the late-July applications. In separate experiments, herbicide systems comprised of fall applications of glyphosate and/or spring applications of foliar and residual herbicides were evaluated. Fall applications of glyphosate provided superior johnsongrass control and grain sorghum yields.

Transferability of lysimeter data to the field was investigated using the leaching and degradation behavior of 14C-labeled metamitron [4-amino-3-methyl-6-phenyl-2,4-triazin-5(4H)-one] and methabenzthiazuron [1,3-dimethyl-3-(2-benzthiazolyl)-urea]. Forty-one to 53 days after application about 4% of the applied metamitron was detected in the upper 20 cm of the field soil either as active ingredient or the major metabolite desamino metamitron, whereas about 8% was found in the lysimeter soil. Forty percent and 33% of the applied methabenzthiazuron were identified in the 0- to 10-cm soil layer in the field and lysimeter 127 to 133 days after application, respectively. It is demonstrated that lysimeter experiments using 14C-labeled compounds simulate actual field conditions and give further information about the amount of total 14C-residues in the soil and percolate.

Accelerated degradation of the herbicide diphenamid (N,N-dimethyl-α-phenylbenzeneacetamide) was investigated in Israeli soils. Repeated application of this herbicide in the soil in the laboratory enhanced its degradation, which increased with an increasing number of applications. After the fourth application nearly 100% of the herbicide was degraded within 5 days of incubation, whereas only slight degradation was observed during the first 25 days in a previously untreated soil. Accelerated degradation was also observed in a soil collected from a field with previous diphenamid treatments. Fumigation with methyl bromide or treating the soil with the fungicide fentin acetate (triphenyltin acetate) was effective in decreasing the accelerated degradation of diphenamid, whereas the fungicides TMTD (tetramethylthiuram disulfide) and TBZ [2-(4-thiazolyl)benzimidazole] were only partially effective in inhibiting accelerated degradation. In the laboratory, fentin acetate prevented the degradation of diphenamid applied to previously untreated soil. Several fungi capable of degrading diphenamid were isolated from soils with or without accelerated degradation.

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.

An investigation of the adsorption of chlorsulfuron by four selected soil constituents, i.e. humic acid, two iron oxides, and montmorillonite, was carried out under concentration and pH conditions similar to those in most natural soils. CaCl2 (0.01 M) was used as background electrolyte to suppress nonspecific adsorption. Negligible amounts of chlorsulfuron were adsorbed by montmorillonite, whereas humic acid and the iron oxides were found to be important adsorbents. For these adsorbents, chlorsulfuron adsorption decreased when pH increased from 4 to 8, with little adsorption occurring at pH 8. Adsorption by iron oxides was a function of their surface area. Chlorsulfuron adsorption was found to be closely related to the surface charge of the adsorbents, but in weakly acidic solution, also to the acid-base properties of chlorsulfuron itself.