Five biotypes of common cocklebur that were not controlled with acetolactate synthase (ALS)-inhibiting herbicides were tested in greenhouse and laboratory studies to determine the magnitude of resistance and cross-resistance to four ALS-inhibiting herbicides. In vivo inhibition of ALS was also evaluated. Based on phytotoxicity, all five ALS-resistant biotypes of common cocklebur were > 390 times more resistant than the susceptible biotype to imazethapyr. However, only four of these biotypes were also resistant to another imidazolinone, imazaquin. Two biotypes were cross-resistant to the sulfonylurea, chlorimuron, and the triazolopyrimidine sulfonanilide, NAF-75. One biotype demonstrated intermediate susceptibility to imazaquin, chlorimuron, and NAF-75. In all cases, the resistance exhibited at the whole plant level was associated with an insensitive ALS.

Uptake and translocation of glyphosate in three commercial formulations were examined in velvetleaf, a dicotyledonous weed that is commonly treated with glyphosate. The formulations included Roundup® (MON 35085), Roundup Ultra, and Touchdown® as sold in Canada. A minimal amount of 14C-glyphosate was spiked into a lethal rate of each formulation, and the short-term (3 to 72 h) uptake into the treated leaf and subsequent translocation into the plant were measured. Time-course studies showed very rapid uptake and translocation of glyphosate in the Ultra formulation. In comparison, the uptake and translocation of glyphosate in Touchdown was much slower but continued throughout the 72-h period. Glyphosate in the Roundup formulation showed intermediate uptake and translocation. Tissue necrosis at the application sites of Ultra and Roundup was visible within 24 h after treatment. Examinations using stereo and fluorescence microscopy revealed extensive cell death and tissue disruption. Tissue necrosis from Ultra and Roundup was also observed in blank formulations containing no glyphosate and therefore was likely caused by the surfactants. In contrast, the application sites of Touchdown produced little to no leaf damage. Our results demonstrated a direct correlation between tissue necrosis and rapid rates of glyphosate uptake and translocation.

Soil moisture, relative humidity (RH), and the addition of bentazon were examined for effects on absorption and translocation of 14C-imazethapyr in common ragweed. When common ragweed was growing under conditions of low soil moisture (-300 kPa), absorption of 14C-imazethapyr was reduced from 51 to 42%. Translocation of 14C-imazethapyr was not affected by soil moisture. In the absence of bentazon, absorption and translocation of 14C-imazethapyr was similar at 65 and 85% RH 48 and 168 h after treatment. Bentazon decreased absorption of 14C-imazethapyr from 61 to 26% at 65% RH and from 65 to 57% at 85% RH 168 h after treatment. Bentazon decreased translocation of 14C-imazethapyr from 35 to 9% at 65% RH and from 30 to 21% at 85% RH when averaged over time. This reduction in the absorption and translocation of 14C-imazethapyr when bentazon was added may result in a reduction in common ragweed control compared to imazethapyr alone when RH is 65% or lower.

Clopyralid applied to Canada thistle rosettes has provided better control in the following growing season than applications to bolted plants. The objectives of this research were to determine if using cultivation to prevent plants from bolting prior to herbicide application (the rosette technique) could be successfully incorporated into a row crop production system and to evaluate the effect of Canada thistle growth stage on the absorption and translocation of 14C-clopyralid. Canada thistle control 8 mo after postharvest herbicide treatment (MAFT) using the rosette technique was similar to control when using conventional in-crop plus postharvest herbicide treatments in corn and soybean. Glyphosate and clopyralid plus 2,4-D were the most consistent postharvest herbicide treatments for Canada thistle control 8 MAFT in corn and soybean. Corn yields were similar, but soybean yields were slightly lower when Canada thistle was controlled using cultivation compared to conventional herbicide treatments. 14C-clopyralid translocation to Canada thistle roots and lower shoot parts was greater when clopyralid was applied to the rosette stage than when applied to bolted Canada thistle plants. The increased translocation probably accounts for the increased Canada thistle control observed in the field. Incorporating the rosette technique into a weed management program should allow growers to control Canada thistle with less herbicide input than do standard practices.

Field, greenhouse, and laboratory studies were conducted to examine the potential for antagonism of postemergence graminicides when tank-mixed with cloransulam and to determine the role of herbicide absorption and translocation in observed antagonistic responses. Cloransulam antagonized annual grass control with aryloxyphenoxypropionate herbicides fluazifop-P, quizalofop, and the prepackaged formulation of fluazifop-P plus fenoxaprop. Cloransulam did not affect annual grass control with the cyclohexandiones clethodim and sethoxydim. In the greenhouse, increasing the rate of the graminicides was a more effective strategy for overcoming antagonism for quizalofop than for fluazifop-P or fluazifop-P plus fenoxaprop, and success was species dependent. Annual grass control with clethodim, sethoxydim, and glyphosate was not adversely affected by tank mixtures with cloransulam. Control of large rhizome johnsongrass was initially reduced when cloransulam was mixed with sethoxydim, fluazifop-P plus fenoxaprop, or quizalofop. By 6 wk after treatment, control of rhizome johnsongrass was antagonized only when cloransulam was mixed with sethoxydim. Rainfall within 1.5 h of application reduced johnsongrass control with glyphosate and sethoxydim but did not affect activity of the other herbicides. Absorption of 14C-fluazifop-P and 14C-quizalofop into broadleaf signalgrass was not affected by cloransulam 6 or 24 h after treatment. Translocation of 14C-fluazifop-P to broadleaf signalgrass shoot tissue above and below the treated leaf was decreased when fluazifop-P was combined with cloransulam. Translocation of quizalofop was not affected by cloransulam.

Studies were conducted to compare foliar- and root-based absorption and translocation of SC-0051 in corn and soybean using radio-tracer techniques. Visual injury symptoms expressed in soybean by SC-0051 were bleaching in newly formed (meristematic) tissue followed by necrosis. Corn expressed little to no visual injury. SC-0051 was absorbed more rapidly by foliage of corn and soybean than by their roots. Although soybean absorbed more SC-0051 through roots than did corn, there was no difference between species in foliar absorption of SC-0051. SC-0051 was rapidly translocated throughout corn plants but was concentrated mainly in the meristematic tissue of soybean. Even though there were slight differences in absorption and translocation of SC-0051 in corn and soybean, it is likely that differential metabolism of the herbicide explains selectivity.

Field studies have shown that rimsulfuron can move laterally with mower tires and injure neighboring cool-season grasses, indicating that persistent chemical can dislodge from turfgrass foliage. Laboratory studies were conducted to evaluate persistence and stability of 14C rimsulfuron on perennial ryegrass and annual bluegrass foliage. Rimsulfuron was absorbed by annual bluegrass and perennial ryegrass equivalently, and persisted equally on foliage of each species. When extracted with a water rinse, 57% of applied rimsulfuron was recovered after 10 min, and 42% of applied rimsulfuron was recovered after 96 h. Rimsulfuron was stable 4 d after application based on comparison of rinse water chromatograms to stock solution chromatograms. These data indicate that appreciable rimsulfuron persists on turf foliage for 4 d. Thus, limiting traffic on treated areas for several hours to allow drying is not a viable method to prevent lateral relocation of rimsulfuron, and subsequent injury to cool-season turfgrasses.

The objective of this study was to evaluate the feasibility of using 2-aminobutyric acid (2-aba) as a chemical marker for the detection of sulfonylurea (SU) herbicides in crop and soil environments. A bioassay, on the basis of the injury index to vegetable seedlings, showed that both field mustard and Chinese mustard were more sensitive to bensulfuron, imazosulfuron, and pyrazosulfuron than lettuce and cabbage in winter. In a similar study, field mustard was found to be the most sensitive species among six summer vegetables. After foliar application of bensulfuron at 0.487 μM, 2-aba accumulation reached the maximum in field mustard within 6 to 12 h, before any visible symptoms appeared, and this high level was maintained up to 72 h. Among all vegetables tested for differential SU sensitivities, maximum accumulation of 2-aba occurred, in coincidence with the onset of injury, in field mustard. The accumulation of 2-aba in field mustard showed a linear regression to the log-transformed concentrations, ranging from 10−2 to 102 μM, of each of the three SUs. However, no such relationship between the applied rate, especially at the lower rates, and the residue content of SUs in field mustard plant was found. In addition, with the extract of SU-treated paddy soil sprayed on field mustard, the accumulation of 2-aba in this plant was found to reflect indirectly the amount of SU residues in the soil.

Research was conducted to evaluate root uptake, translocation, and metabolism of 14C-sulfentrazone alone or in a mixture with clomazone in solution in flue-cured tobacco transplants. Uptake and translocation of sulfentrazone was rapid and was not affected by the addition of clomazone. Fifty-nine and 65% of the 14C absorbed by the plant was translocated to the leaves within 24 h with sulfentrazone alone and in the clomazone plus sulfentrazone mixture, respectively. Differences in plant metabolism were observed between sulfentrazone alone and sulfentrazone plus clomazone. After 3 h, 66% of the 14C recovered from the leaves was metabolized when sulfentrazone was applied alone, compared to 91% when sulfentrazone was applied with clomazone. The difference could indicate that metabolism of sulfentrazone by tobacco transplants was enhanced by the presence of clomazone.

Response of creeping bentgrass, annual bluegrass, and Kentucky bluegrass to foliar, soil, or foliar plus soil applications of bispyribac-sodium was evaluated in greenhouse studies. Soil-alone and foliar plus soil applications of bispyribac-sodium at 148 or 296 g ai/ha resulted in greater injury and shoot dry weight reduction of all species 28 d after treatment (DAT) compared to foliar-alone treatments. Creeping bentgrass was less injured than annual or Kentucky bluegrass regardless of application placement. Further studies evaluated foliar and root absorption and translocation of 14C-bispyribac-sodium in creeping bentgrass, annual bluegrass, Kentucky bluegrass, and roughtstalk bluegrass. Foliar absorption into creeping bentgrass was less than that of other species at most harvest timings from 4 to 72 h after treatment. Annual and roughstalk bluegrass translocated greater amounts of foliar-absorbed 14C to the crown and shoots compared to creeping bentgrass. Annual and roughstalk bluegrass accumulated approximately 47% more 14C per dry weight of plant tissue than creeping bentgrass and 74% more than Kentucky bluegrass after 72 h in nutrient solution containing 14C-bispyribac-sodium. Annual and roughstalk bluegrass translocated approximately 80% of root-absorbed 14C to shoots, whereas creeping bentgrass and Kentucky bluegrass translocated slightly less (66% of absorbed for both species). These studies suggest that bispyribac-sodium is readily absorbed by roots and translocated to shoots which may contribute to its total activity within a plant. In addition, creeping bentgrass displayed lower amounts of foliar and root absorption and subsequent translocation than annual and roughstalk bluegrass which may contribute to greater bispyribac-sodium tolerance displayed by creeping bentgrass.

The allelopathic potential of wild radish was evaluated in controlled environments by determining if an aqueous extract from oven-dried wild radish shoots suppressed germination and radicle growth of some crops and weeds common to the southeastern United States. In addition, phytotoxicity from topical applications of the aqueous extract was assessed, along with crop and weed suppression by soil incorporated, air-dried wild radish residues. Germination and radicle growth of all species were reduced by the extract compared with distilled water. However, topical applications of the aqueous extract failed to induce injury on any species by 7 d after treatment. Emergence and shoot fresh weight of the bioassay plants were reduced by wild radish residue incorporated into soil, with the level of suppression dependent on the quantity of residue incorporated. Sicklepod and prickly sida were extremely sensitive to incorporated wild radish residues, with > 95% fresh weight reduction at 0.5% (wt/wt) residue, compared with an untreated control. Conversely, yellow nutsedge showed a high degree of tolerance in all trials. Of the crops evaluated, cotton emergence and growth were most sensitive to incorporated wild radish residues. These data indicate that wild radish aqueous extract or incorporated residues (or both) suppress seed germination, radicle growth, seedling emergence, and seedling growth of certain crops and weeds and these responses are attributed to an allelochemical effect.

Greenhouse studies were conducted to determine the effect of 1-aminomethanamide dihydrogen tetraoxosulfate (AMADS) as a spray adjuvant on the efficacy of three different glyphosate formulations, the isopropylamine salt (glyphosate-IPA), potassium salt (glyphosate-K), and the acid of glyphosate dissolved in AMADS (glyphosate-A). All formulations were tested at multiple rates with and without AMADS (2% v/v) on greenhouse-grown corn, and growth inhibition was determined by measuring the elongation of the newest emerging leaf between 1 and 7 d after treatment. AMADS increased the efficacy of all three glyphosate formulations by threefold to fourfold. The IC50 values for glyphosate-IPA, glyphosate-K, and glyphosate-A without AMADS on corn were 77, 54, and 53 g ae/ha, respectively; and with AMADS the values were 20, 18, and 21 g/ha, respectively. AMADS was more effective than ammonium sulfate (2% w/v) in overcoming the antagonism of hard water (200 parts per million Ca+2) on glyphosate-K efficacy on corn. The rainfastness of glyphosate-IPA, glyphosate-A, and glyphosate-K was improved with AMADS.

Ground ivy is a stoloniferous, perennial weed that persists in lawn turf. With the widespread use of 2,4-D on turf sites, the development of 2,4-D–tolerant ground ivy is a possibility. Ground ivy populations showed a highly variable response to foliar 2,4-D application. Ground ivy from Nebraska (NE) was tolerant to 2,4-D, whereas Ohio (OH) ground ivy was susceptible. The 2,4-D–susceptible OH population absorbed 37% more foliar-applied 14C–2,4-D than the 2,4-D–tolerant NE population. Although OH and NE populations total translocation of applied 14C was similar and averaged 5%, the OH population translocated 42% more toward the apical meristem of the primary stolon than the NE population, primarily because of the OH population's higher 14C–2,4-D absorption. The variation in response to 2,4-D found between these two populations occurred after exposure of roots to 2,4-D, but the effect was less pronounced. These results suggest that the difference in foliar uptake may partially contribute to differences in response to 2,4-D between these two populations. Likewise, differences in acropetal translocation may contribute to the differential sensitivity of 2,4-D–tolerant and –susceptible ground ivy populations.

Three commercial formulations of glyphosate were spiked with 14C-glyphosate and applied via a spray nozzle to velvetleaf plants. The use of 14C-glyphosate as a marker caused minimal alteration to formulation properties, and the use of spray application simulated field practices. Formulation retention, calculated based on maximum plant-leaf area, showed that only 27 to 33% of the available area retained the spray. The small differences in retention among the formulations suggest that they contribute little to differences in efficacy. Following spray application, plants were harvested at various times to measure the levels of glyphosate uptake into the plant and translocation into roots. Significant differences were observed among the formulations in the rate of glyphosate uptake. The most efficient formulation absorbed about one third of the dose by 24 hr after treatment. Root translocation of glyphosate was approximately proportional to uptake and accounted for less than one third of the absorbed dose. The relationship between uptake and rainfastness was examined in greenhouse studies with simulated rainfall at various times after glyphosate application. A direct correlation was observed between rainfastness with the speed and quantity of glyphosate uptake.

The cause of differential susceptibility of barnyardgrass, hemp sesbania, pitted morningglory, and prickly sida to glyphosate was examined by measuring the absorption of 14C-glyphosate, quantifying the amount of epicuticular wax, and observing the wettability of leaf surfaces. In greenhouse experiments, the biomass of barnyardgrass and prickly sida was reduced by 95% by Roundup Ultra®. Hemp sesbania and pitted morningglory showed more tolerance, with 66 and 51% average biomass reduction, respectively. Absorption of 14C-glyphosate in a controlled environment did not follow the trend in species susceptibility with barnyardgrass, 30%; prickly sida, 18%; hemp sesbania, 52%; and pitted morningglory, 6%; absorption. The high tolerance of pitted morningglory to glyphosate can be attributed mostly to limited absorption, but the tolerance of hemp sesbania is due to other mechanisms. The addition of nonionic surfactant (NIS) to a low rate of Roundup Ultra® reduced absorption of 14C-glyphosate by barnyardgrass and hemp sesbania, but had no effect on the herbicidal activity. Glyphosate absorption in the four weed species was not correlated with quantity of chloroform-extracted wax or leaf wettability. Pitted morningglory and prickly sida, which contained the least leaf wax, also had smaller contact angles or higher leaf wettability than the species with more waxy leaves. The adjuvant in Roundup Ultra® reduced contact angles of the four species compared to contact angles obtained using deionized water alone. The addition of 0.25% v/v NIS alone to water reduced contact angles more than did the adjuvant in Roundup Ultra® solution.

Greenhouse and laboratory experiments were conducted to investigate the response of common lambsquarters to POST applications of halosulfuron–methyl plus 2,4-D admixtures and to investigate the effects of 2,4-D on the absorption, translocation, and metabolism of halosulfuron. In the greenhouse, halosulfuron at 0, 4.5, 9, 18, and 36 g ai/ha was applied alone and mixed with 2,4-D at 0, 17, 35, and 70 g ai/ha POST to 7.5- to 9-cm seedlings, and plant fresh weights were determined 4 wk after treatment (WAT). Halosulfuron alone did not control this weed, while fresh weights of common lambsquarters treated with 2,4-D declined hyperbolically as rates increased. A synergistic response for mixtures of these herbicides occurred, as observed fresh weights for all combinations were less than expected based on independent action and the calibrated marginal responses. In the laboratory, 7.5- to 9-cm seedlings were treated POST with commercially formulated halosulfuron at 9 and 18 g/ha and 2,4-D at 0, 70, and 140 g/ha, respectively, followed by foliar-applied 14C-halosulfuron. Absorption of 14C-halosulfuron increased with time, and absorption and translocation were not influenced by the addition of 2,4-D. Results from these studies inferred that halosulfuron and 2,4-D were generally synergistic on common lambsquarters and that mechanisms other than absorption, translocation, and metabolism may explain this response.