Pronounced differences in the tolerance of tomatoes and bell peppers to clomazone observed in field studies were confirmed in a greenhouse experiment. In greenhouse studies, preemergence clomazone rates causing 50% visible injury on bell pepper and tomato seedlings 10 days after application were 9.4 and 0.1 kg/ha, respectively. Based on growth inhibition, bell peppers were 40-fold more tolerant of clomazone than tomatoes 20 days after clomazone application. In laboratory studies investigating the basis for differential clomazone tolerance, no differences in uptake of 14C-clomazone from nutrient solutions between tomato and bell pepper plants were observed after 24 h. Minor differences were observed in the distribution of 14C label within plants; a higher percentage of 14C was recovered in bell pepper roots than in tomato roots, while the opposite was true for the shoots. Clomazone was metabolized to two products in roots of both bell peppers and tomatoes within 48 h after treatment. Tomato shoots were more active in converting clomazone to these metabolites than were tomato roots. Bell pepper roots converted more clomazone to metabolites than did tomato roots 24 h after treatment. However, by 72 h, differences in clomazone metabolite levels between species were negligible in both roots and shoots. Enzymatic and acid hydrolysis of soluble, polaf clomazone metabolites indicated that these metabolites may be sugar conjugates of clomazone.

‘Cargill 921′, ‘Great Lakes 422′, ‘Northrup King 9410′, ‘Pioneer 3901′, ‘Pioneer 3737′, and ‘Stauffer 5650’ corn hybrids were tested in the greenhouse for imazaquin tolerance. Imazaquin rates that reduced shoot growth 50%, when compared to the untreated hybrid check, ranged from 17 to 50 g/ha. When averaged together, the three most tolerant hybrids (Cargill 921, Pioneer 3901, and Great Lakes 422) were approximately one-half as sensitive to imazaquin as the three least tolerant hybrids (Northrup King 9410, Pioneer 3737, and Stauffer 5650). Studies were conducted to determine if the observed differential tolerance was caused by differences in acetolactate synthase (ALS, EC 4.1.3.18) levels and sensitivity of ALS to imazaquin. Differential imazaquin uptake, translocation, and/or metabolism were also studied as a basis for the tolerance range as was seed size and seedling growth. There were differences among hybrids in the physiological and growth parameters studied; however, these differences did not correlate with imazaquin tolerance. None of the factors studied could alone account for the differences in imazaquin tolerance.

Antidotes were evaluated under greenhouse conditions for their ability to prevent injury to corn and sorghum from imazethapyr. Corn was more tolerant to imazethapyr and more effectively protected from imazethapyr toxicity than sorghum. Naphthalic anhydride (NA)3, CGA 92194, or flurazole treatment of corn seed reduced plant injury from preemergence applications of imazethapyr. Corn injury from postemergence applications of imazethapyr was decreased by seed treatment with NA or CGA 92194. Sorghum injury from preemergence applications of imazethapyr was not reduced by seed treatment with the antidotes. NA treatment of sorghum seed was the most effective antidote treatment for decreasing injury from postemergence applications of imazethapyr but the level of sorghum protection was much less than that achieved with corn. Corn seedlings grown from NA-treated and untreated seed absorbed equal amounts of 14C into the roots from nutrient solution containing 14C-imazethapyr. The NA-treated corn plants translocated less of the absorbed 14C to the shoots than the untreated plants. NA treatment of corn seeds increased the rate of imazethapyr conversion to soluble metabolites 2-and 10-fold in the corn seedling roots and shoots, respectively. The increased rate of imazethapyr metabolism in corn following NA seed treatment may be responsible for the protection from imazethapyr toxicity.

Phytotoxins from lantana leaves were extracted in aqueous media adjusted to pH 4, 7, or 10. All three leaf extracts showed considerable phytotoxic activity in duckweed growth bioassay. The acidic and the neutral extracts of lantana leaves at 10 g fresh weight/L were more phytotoxic to duckweed growth than the alkaline extract at the same concentration. Phenolic compounds present in lantana leaves were isolated by alkaline hydrolysis of the crude leaf extracts followed by separation of the various components in ether or aqueous media. The fraction containing the phenolics showed considerable phytotoxic activity against duckweed growth. In addition, the fraction containing the nonpolar substances separated from the acidic and the neutral crude extracts also showed significant phytotoxic activity against duckweed growth. No attempt was made to isolate and identify the phytotoxic component from the nonpolar fraction. Characterization of the phenolic fraction by HPLC revealed the presence of at least 14 phenolic compounds. All three leaf extracts contained the same array of phenolic compounds. However, the quantity of phenolic compounds extracted varied with the pH of the extraction medium. The acidic extract contained p-hydroxybenzoic acid (0.09 mM) as the most abundant phenolic compound, whereas the neutral and the basic extracts contained p-coumaric acid (0.11 and 0.26 mM, respectively) as the most abundant phenolic compound. All phenolic compounds, except p-hydroxybenzoic acid, proved phytotoxic to duckweed growth at a concentration of 1 mM or lower. Salicylic acid was the most phytotoxic of the phenolic compounds detected in lantana leaf extracts. The phytotoxicity of lantana leaf extracts is probably partly due to a complex interaction of all phenolic compounds present.

Common cocklebur root and shoot interferences were investigated as factors causing soybean yield reductions in the field. Significant decreases in soybean yield resulted from common cocklebur shoot interference when the two were grown together with the common cocklebur roots confined in plastic bags. Soybean yield decreased further when common cocklebur roots were not confined, but this decrease resulted, in part, from increased common cocklebur shoot growth. Wire mesh cylinders of varying diameters, placed in gaps in the soybean rows and designed to spatially restrict soybean shoot growth, reduced yield more than did gaps alone. When similar cylinders lined with black plastic were placed next to the soybean rows to restrict both within-canopy light and space for soybean shoot growth, soybean yield was reduced most (44%) with concomitant changes in stem height, stem diameter, lodging, and branches, nodes, and pods/plant. Interference of common cocklebur with soybeans resulted primarily from shoot interference, and competition for light within the soybean canopy implicated as the major factor causing the soybean yield reduction.

Greenhouse experiments were conducted to determine effects of watering frequency, drought stress, and a combination of watering frequency and glyphosate on growth of field bindweed. Watering daily, twice weekly, or weekly to field capacity affected field bindweed grown from seed differently than plants grown from root runner segments. Shoot and root dry matter of plants grown from seed increased when plants were watered frequently but dry matter allocated to root runners did not change when plants were watered less frequently. Plants grown from root runner segments increased their shoot and root runner dry matter when watered frequently, but root dry matter was not affected by watering frequency. Plants grown from seed had highest regrowth when watered weekly and plants grown from root runner segments had higher regrowth when watered daily. Drought reduced survival rate and dry matter of surviving plants. Frequency of watering and whether plants began from seeds or root runner segments influenced glyphosate action on field bindweed. No regrowth was observed 6 months after 1.5 kg ai/ha of glyphosate was applied to plants that had been grown for 8 weeks under three different watering frequencies.

Experiments were conducted to compare the influence of production site and seed source on the germination of big sagebrush seeds in relation to incubation temperatures. Seeds of big sagebrush were collected from five plant communities located on the margin of the Carson Desert in western Nevada. Seedlings were grown from the seeds and transplanted back to the sites of collection in reciprocal gardens. Seeds from 1-yr-old gardens were tested for germination at a constant 15 C for three consecutive years. The greater quantity of seeds produced by 2- and 3-yr-old gardens were tested at 55 constant or alternating temperature regimes. Seeds of big sagebrush produced in reciprocal gardens had lower germination than seeds collected from native stands at the same locations. The reduced germination disappeared as the reciprocally transplanted plants grew older and larger, but seeds from these plants did not reach the same levels of germination as those collected from native stands at the same location. Moisture stress and/or freezing temperature conditions during flowering may have directly influenced the quality of seeds produced or interacted with the inherent genotypes of big sagebrush to trigger seed production of variable quality in relation to germination characteristics.

Black nightshade plants were controlled by hoeing in the same cotton plots each year (1982 to 1986) for 3 to 15 weeks after crop emergence to evaluate the influence of several black nightshade-free periods on cotton yield, reproduction of black nightshade, and longevity of weed seeds in soil. Similar information, although limited, was also collected for Palmer amaranth that escaped the initial herbicidal treatment each year. Except for 1982, black nightshade competing with cotton for the duration of the growing season in nonhoed plots severely reduced yields (60 to 100%), with greatest yield reductions (82 to 100%) occurring in 1983 and 1984 when 0.5 to 0.7 cm of rain fell within 10 days after cotton planting. When combined with cultivation, a 3-week nightshade-free period at cotton planting was of sufficient duration to protect cotton yields. Weed seed production for all hoed treatments was less than 1% of the nonhoed treatment, and after five consecutive cotton crops (1982 to 1986), the amount of both black nightshade and Palmer amaranth seeds in soil was similar for all hoed treatments. These populations were 60 to 80% and 95 to 97% less than beginning populations of black nightshade and Palmer amaranth in 1982, respectively. After 5 yr of continuous treatments, cotton was grown in 1987, with standard cultivation as the only method of weed control, to evaluate how the weed-free periods in 1982 to 1986 influenced weed seed populations in the soil and the resultant effect on weed competition and cotton yields. Reduction of cotton yields in 1987, in the absence of weed-free periods, indicated that black nightshade seed survival in soil appears to be sufficiently long for ample establishment of this weed to compete with cotton. Thus, fields will have to be kept weed free for greater than 5 yr to reduce black nightshade populations to a level that will not reduce cotton yields.

Dormant barnyardgrass seeds imbibed in darkness for 8 days at 38 C displayed a very low fluence response to phytochrome, and very little germination occurred in darkness. However, if seeds were imbibed at temperatures >42 C or exposed to 46 C for only 0.5 h, considerable germination was evoked in the absence of a light stimulus. It is proposed that the high temperatures caused a transition in dormancy from a light-requiring to a light-independent state. The finding may have relevance to field behavior of barnyardgrass seeds where burial could prevent reception of the required light stimulus.

Several weed-free and johnsongrass competition periods were established and maintained in cotton to evaluate their influence on cotton yield and reproduction of johnsongrass. When johnsongrass was sprayed with fluazifop in field plots 3, 6, 9, or 12 weeks after cotton emergence, only plots sprayed at 3 weeks yielded as much cotton as weed-free plots. Johnsongrass permitted to compete for 6, 9, 12, and 25 weeks reduced cotton yields 20, 60, 80, and 90%, respectively. When johnsongrass was transplanted into weed-free plots 3, 6, 9, and 12 weeks after cotton emergence, a weed-free period of 9 weeks was required to prevent significant cotton yield reduction. Cotton in plots weed free for 3 and 6 weeks yielded 81 and 89% as much seed cotton, respectively, as weed-free controls. Johnsongrass competing with cotton for 9 to 12 weeks produced 20 and 40% as many seeds and rhizomes, respectively, as weedy control plots. Some seeds and rhizomes were produced (1% of weedy check plots) in plots weed free for 9 to 12 weeks. Cotton grades were reduced in plots weed free for 3 and 6 weeks, and in plots where johnsongrass competed for 9 to 25 weeks.

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.

In 1987, two applications of the methyl ester of diclofop failed to effectively control Italian ryegrass in some growers' fields. As a result of these observations, studies were initiated to determine if the surviving ryegrass is more tolerant to diclofop than previously untreated ryegrass. Growth chamber studies on five accessions of ryegrass demonstrated that collections from two growers' fields were highly tolerant of diclofop. Diclofop GR50 (rate required to reduce shoot weight by 50%) values of accessions A and B and commercial ryegrass, when diclofop was applied to seedlings at the two-leaf stage, were 2.9, 3.4, and 0.0056 kg ai/ha, respectively. Diclofop-tolerant biotypes were not more tolerant to postemergence barban or preemergence-incorporated triallate plus postemergence diuron or metribuzin. These herbicides are possible alternative treatments in fields where diclofop is no longer effective.

Experiments were conducted in 1985 and 1986 to determine and compare the efficacy, phytotoxicity, and soil persistence of imazquin, imazethapyr, and clomazone in no-till double-crop soybeans. All herbicides controlled 93% or greater jimsonweed, velvetleaf, and giant foxtail. Imazaquin and imazethapyr controlled more common cocklebur and ivyleaf morningglory than clomazone. Soybeans were injured from imazaquin and imazethapyr applications in 1986 but yields were not reduced. First-order kinetics described the dissipation of each herbicide. Imazaquin and imazethapyr were more persistent in the soil than clomazone. Averaged over both years, half-lives of 10, 43, and 60 days were calculated for clomazone, imazaquin, and imazethapyr, respectively. Clomazone was not detected 10 to 20 cm in the soil profile. More imazethapyr was detected 10 to 20 cm in the soil profile than imazaquin in 1985. Slight corn injury (≤10%) was observed in 1987 following previous year applications of imazaquin to no-till double-crop soybeans but corn yields were not reduced.

The efficacy of glyphosate as affected by rate, time of application, and addition of surfactant3 was evaluated on blackberry, Japanese honeysuckle, poison ivy, sericea lespedeza, and trumpetcreeper. The addition of surfactant (0.5% v/v) to glyphosate had no effect on the control of the weeds studied. Glyphosate applied in mid-June to September at 1.1 or 2.2 kg/ha controlled blackberry. Mid-August glyphosate applications of 2.2 kg/ha controlled 83% of actively growing Japanese honeysuckle; there was less than 75% at one site due to moisture stress. Use of 2.2 kg/ha of glyphosate from mid-June through mid-August controlled 87% of poison ivy. Consistent commercially acceptable control of sericea lespedeza was obtained when glyphosate was applied at 1.1 or 2.2 kg/ha at the time of flowering. Applying glyphosate at 1.1 or 2.2 kg/ha from late July through early October controlled 50% or more of the trumpetcreeper.

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 control of three summer annual grass weeds with herbicides during establishment of forage grasses was studied near Concord and Mead, NE, in 1984, 1985, and 1986. Three cool-season forage grasses, intermediate wheatgrass, tall fescue, and smooth bromegrass, and two warm-season grasses, big bluestem and switchgrass, were included. The control of three major summer annual grasses, green foxtail, barnyardgrass, and large crabgrass, was excellent with fenoxaprop at 0.22 kg ai/ha. Slight to moderate injury to cool-season forage grasses and severe injury to warm-season grasses were evident. Sethoxydim at 0.22 kg ai/ha and haloxyfop at 0.11 kg ai/ha controlled green foxtail and large crabgrass, but not barnyardgrass. Sulfometuron-treated big bluestem and switchgrass plots had the best forage stand frequencies and yields and, at the rate used, sulfometuron satisfactorily controlled green foxtail but only marginally controlled barnyardgrass and large crabgrass.

Field experiments were conducted during 1984 and 1985 to determine the influence of 2,4-D and MCPA amine and ester formulations on control of foxtail millet with diclofop. Foxtail millet control increased as the diclofop rate increased from 0.6 to 1.7 kg ai/ha whether diclofop was applied alone or with 2,4-D or MCPA amine and ester formulations; however, foxtail millet control was lower when diclofop was applied with 2,4-D or MCPA amine and ester formulations compared to diclofop applied alone. MCPA ester, MCPA amine, 2,4-D ester, or 2,4-D amine at 0.14 kg ai/ha added to a diclofop spray mixture reduced foxtail millet control 8, 15, 20, and 30% compared to diclofop applied alone, averaged over diclofop rates of 0.6, 0.8, 1.1, 1.4, and 1.7 kg/ha. Adding a petroleum oil to diclofop increased foxtail millet control with diclofop at 0.6 kg/ha but not at 0.8 kg/ha. The petroleum oil additive did not overcome the antagonism of foxtail millet control when diclofop at 0.6 kg/ha was applied with 2,4-D or MCPA amine and ester formulations. The amount of 14C-diclofop absorbed by oats was greater when diclofop was applied with the ester formulations of 2,4-D or MCPA compared to the amine formulations. Either once-refined sunflower oil or sunflower oil methyl ester applied with 14C-diclofop increased the amount of 14C absorbed and translocated in oat plants compared to 14C-diclofop applied alone.

Postemergence treatments utilizing various combinations of fluazifop-P, paraquat, and 2,4-DB were compared to a preplant-incorporated (PPI) application of benefin followed by a ground-cracking application of alachlor and dinoseb plus naptalam and a postemergence application of 2,4-DB for weed control, peanut yield, and net economic return to land, overhead, and management. The greatest peanut yields (3-yr average of 4510 kg/ha) and net returns (3-yr average of $521/ha) were provided by a postemergence system that utilized one ground-cracking and one postemergence application of paraquat and one postemergence application of fluazifop-P and 2,4-DB. Seven postemergence systems provided equivalent or greater yield and net returns than the PPI and dinoseb plus naptalam system. Fresh weight reductions of Texas panicum, sicklepod, Florida beggarweed, and pitted morningglory from postemergence weed control systems were equivalent to reductions obtained from the PPI and dinoseb plus naptalam system. The addition of paraquat and 2,4-DB to the PPI and dinoseb plus naptalam system improved the 3-yr average peanut yield and net economic return by 510 kg/ha and $136/ha, respectively, compared to the same system without paraquat and 2,4-DB.

In 1985, when weed densities were low (169 plants/m2 in untreated control), imazaquin applied at 0.07 kg ai/ha early preplant controlled over 90% of all weeds before no-till planting of soybeans. In 1986 and 1987 when weed densities were higher (589 plants/m2 in untreated control), addition of 1.1 kg ai/ha or more of metolachlor to imazaquin (0.07 kg/ha) before soybean planting controlled 95% or more of the grass weeds and 83% or more of the broadleaf weeds. Imazaquin plus metolachlor applied less than 1 day after soybean planting controlled less than 70% of the emerged weeds in 1986 and 1987; common lambsquarters was most tolerant. Early preplant treatments controlled more weeds throughout the growing season than treatments applied after planting. Splitting herbicide treatments among application times generally did not increase weed control compared to single applications. Early preplant applications resulted in higher soybean densities and taller soybeans 30 days after planting in 1986 and 1987 than treatments applied after planting. Soybean yields increased as weed control increased. Weed control and soybean yields were greater with early preplant treatments than paraquat plus alachlor plus metribuzin applied preemergence in 1986 and 1987. No carryover of imazaquin residue was detected through corn bioassay in the field.

The antagonistic effect of bentazon on sethoxydim adsorption and activity was studied in quackgrass. The diffusion of 14C-sethoxydim into and through an isolated tomato fruit cuticle was inhibited in the presence of the sodium salt of bentazon. Bentazon also increased the partitioning of 14C-sethoxydim into CH2Cl2 and water; however, it decreased partitioning into ethyl acetate. Removal of epicuticular wax from quackgrass leaf surfaces did not prevent the antagonism. Addition of sodium acetate or sodium bicarbonate to the sethoxydim spray solution at 10 mM reduced uptake of 14C-sethoxydim by quackgrass similar to the effect of bentazon. Sodium ions in the bentazon formulation appeared responsible for the antagonism by exchanging with the H+ of the sethoxydim hydroxyl group to form a more polar sodium salt of sethoxydim. The addition of Li+, K+, Cs+, Ca++, and Mg++ cations associated with a weak acid also reduced 14C-sethoxydim absorption. Addition of organic acids to the spray solution overcame the antagonism by preventing the formation of sodium salt of sethoxydim. In the field, the addition of a 3000 ppm sodium acetate solution delivering 0.56 kg/ha produced the same antagonism as bentazon on quackgrass control with sethoxydim.