Metabolites of diphenamid (N,N-dimethyl-2,2-diphenyl-acetamide) were purified from extracts of pepper plants (Capsicum frutescens L. ‘Early Calwonder’) treated via nutrient solution with the herbicide or several of its analogs. The major metabolites were characterized. Diphenamid was metabolized partially via a previously unreported pathway to N,N-dimethyl-2-phenyl-2-[(hydroxyphenyl)-β-0-D-glucosyl] acetamide and its monomethyl analog, and to N-hydroxymethyl glycosides previously reported in other species. Ozone fumigation stimulated the production of both types of glycoside-conjugates. Leaves of plants that had been treated with 30 μM diphenamid and fumigated with ozone for 146 to 149 h contained 304 and 560 nmoles per gram of fresh weight of the hydroxyphenyl and N-hydroxymethyl conjugates, respectively.

Laboratory studies were conducted to evaluate possible interactions of atrazine and glyphosate on soybeans at the molecular level. Because these compounds alter the extractable activities of nitrate reductase (NR)3 and phenylalanine ammonia-lyase (PAL), these enzymes were chosen as biochemical markers. Levels of anthocyanin, hydroxyphenolic compounds, and chlorophyll were also determined. Three-day-old dark-grown soybean seedlings, susceptible to both herbicides, were transferred to 2 mM CaSO4 or CaSO4 containing high-purity 10–4 M atrazine, 10–4M glyphosate, or a combination of both herbicides at 10–4 M each. Plants were placed in a growth chamber under continuous light (200 μE·m–2·s–1) at 25 C and harvested at 24-h intervals over a 96-h time course. Interactions of atrazine and glyphosate on hypocotyl elongation were detected after 96 h when the compounds were supplied simultaneously. Intermediate PAL levels were found in the atrazine + glyphosate treatment compared to the levels for each herbicide alone. At 96 h, anthocyanin content in hypocotyls in the atrazine + glyphosate treatment was twice that of the atrazine treatment, and hydroxyphenolic content was increased by 25% (per axis). Interactions of these herbicides were also apparent on NR activity; i.e., on a per organ basis the combination treatment resulted in levels 20% higher than glyphosate alone in roots and 16 times greater than glyphosate alone in leaves after 96 h. Total chlorophyll content in hypocotyls (μg/organ) was decreased by atrazine but not significantly by glyphosate after 96 h. Atrazine + glyphosate resulted in chlorophyll content equal to that of atrazine alone. Mathematical analyses showed that interactions on NR, PAL, anthocyanin levels, and hypocotyl elongation were antagonistic at 96 h. These data demonstrate interaction between atrazine and glyphosate in plant tissues that is related to some biochemical parameters associated with the secondary molecular mode of action of these herbicides.

Multiple herbicide-resistant populations of horseweed [Conyza canadensis (L.) Cronquist] continue to spread rapidly throughout Ontario, notably in areas where no-till soybean [Glycine max (L.) Merr.] is grown. The occurrence of multiple herbicide resistance within these populations suggests that the future role of herbicide tank mixtures as a means of control will be limited. An integrated weed management strategy utilizing complementary selection pressures is needed to reduce the selection intensity of relying solely on herbicides for control. Field studies were conducted in 2018 and 2019 to test the hypothesis: if fall-seeded cereal rye (Secale cereale L.) can reduce C. canadensis seedling density and suppress seedling growth, then the interaction(s) of complementary selection pressures of tillage, cereal rye, and herbicides would improve the level of C. canadensis control. Laboratory studies were conducted to determine whether the allelopathic compound 2-benzoxazolinone (BOA) affected the root development of C. canadensis seedlings. The interactions observed among multiple selection pressures of tillage, cereal rye, and herbicides were inconsistent between the 2 yr of study. A monoculture of cereal rye seeded in the fall, however, did reduce seedling height and biomass of C. canadensis consistently, but not density. This reduction in seedling height and biomass was likely caused by the allelopathic compound BOA, which reduced seedling root development. Control of C. canadensis seedlings in the spring required the higher registered rates of dicamba or saflufenacil. The addition of shallow fall tillage and the presence of cereal rye did not improve the variability in control observed notably with 2,4-D or the lower rates of saflufenacil or dicamba. With the implementation of complementary weed management strategies, environmental variables in any given year will likely have a direct influence on whether these interactions are additive or synergistic.

Twelve sequential herbicide treatments were compared to cycloate (S-ethyl N-ethylthiocyclohexanecarbamate), a standard treatment, for control of annual weeds in sugarbeets (Beta vulgaris L.) in three field experiments conducted from 1971 through 1977. At harvest, seven sequential treatments had less than 10 annual broadleaf weeds per 30 m of row, whereas there were 40 broadleaf weeds per 30 m of row for the cycloate treatment. Four of these sequential treatments had significantly higher root yields and net returns than the cycloate treatment. Dependent on the sequential treatment and year, tonnage was increased 7.3 to 20.3 t/ha, and net returns $150 to $515/ha above those with cycloate. The most effective sequential treatment for control of weeds was a preplanting mixture of 2.2 kg/ha of ethofumesate [(±)-2-ethoxy-2,3-dihydro-3,3-dimethyl-5-benzofuranyl methanesulfonate] plus 1.7 kg/ha of diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid} followed by a postemergence mixture of 0,6 kg/ha each of desmedipham [ethyl m-hydroxycarbanilate carbanilate (ester)] plus phenmedipham (methyl m-hydroxycarbanilate m-methylcarbanilate). This sequential herbicide treatment increased root yields by an average of 20.3 t/ha and net returns by $515/ha above those with cycloate.

In a test of laboratory precision and accuracy, streamwater samples fortified with 0, 10, or 50 μg ae/L 2,4-D [(2,4-dichlorophenoxy)acetic acid] and picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid) were submitted in quadruplicate for residue analysis to 10 laboratories that analyze samples for a fee. Varying amounts of the herbicides were detected by eight laboratories, and two reported no residues. Analyses from most laboratories had a negative bias (ranging from −2 to −92%), although a positive bias appeared in a few instances. Precision was highly variable, the coefficient of variation ranging from 3.4 to 200%. The accuracy range in 19 of 20 cases was ±100 to ±78% for 10 and 50 μg/L 2,4-D and ±100 to ±114% for 10 and 50 μg/L picloram, respectively. Results show that careful selection of contract laboratories and a quality assurance program should be part of monitoring for water contamination with herbicides.

A rapid cotyledon disc bioassay developed for photosynthetic inhibitors was tested on several herbicides of different modes of action. In addition to being an excellent bioassay method for photosynthetic inhibitors it was found to be useful for the detection of diquat (6,7-dihydrodipyrido[1,2-α:2′,1′-c] pyrazine-diium ion), paraquat (1,1′-dimethyl-4,4′-dipyridinium ion), and dinoseb (2-sec-butyl-4,6-dinitrophenol). Herbicides representing several other modes of action could not be readily detected by this method.

This field research was designed to compare the long-term effectiveness of late-September applications of several herbicides for reducing Canada thistle shoot density on noncropped, untilled abandoned farmland when reapplied annually for 3 yr. Clopyralid at 560 and 840 g ae ha−1 or picloram at 280 and 560 g ae ha−1 reduced Canada thistle shoot density as well as either glyphosate3 at 0.8 to 2.8 kg ae ha−1 or dicamba at 1.1 and 2.2 kg ae ha−1. These treatments were much more effective than 2,4-D at 1.1 and 2.2 kg ae ha−1, chlorsulfuron at 34 and 67 g ai ha−1, and metsulfuron at 34 and 67 g ha−1 for progressively reducing Canada thistle shoot density over three annual fall applications. Picloram and clopyralid greatly reduced and delayed shoot emergence from adventitious root buds in spring after two fall-applied treatments compared with nontreated checks.

Ground applications of pelleted tebuthiuron {N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N′-dimethylurea} at 0.6 kg ai/ha (20% formulation) reduced big sagebrush (Artemisia tridentata Nutt. # ARTTR) density by an average of 84 and 92% after 28 months, whereas 1.1 kg/ha killed 94 and 97% of the plants when applied in January or May, respectively. Total standing crop of grasses was unaffected 4 months after tebuthiuron applications. Significant increases in grass production occurred during the second and third growing seasons following tebuthiuron treatment. Desirable forage grasses such as galleta [Hilaria jamesii (Torr.) Benth.], blue grama [Bouteloua gracilis (Wild. ex H.B.K.) Lag. ex Griffiths], and crested wheatgrass [Agropyron cristatum (L.) Gaertn.] generally increased in tebuthiuron-treated plots, whereas sand dropseed [Sporobolus cryptandrus (Torr.) Gray # SPECR] and ring muhly [Mulenbergia torreyii (Kunth.) Hitchc. & Chase] production remained unchanged in comparison with untreated areas.

A field study was conducted on an s-triazine–adapted soil to determine the effects of s-triazine exclusion interval (1, 2, 3, or 4 yr), crop production system (continuous corn or continuous soybean), and rhizosphere proximity (bulk or rhizosphere soil) on atrazine degrader populations and activity. Atrazine degrader populations were quantified by a radiological Most Probable Number technique, while degrader activity was assessed via mineralization of ring-labeled 14C-atrazine. As the s-triazine exclusion interval increased, atrazine degrader populations declined exponentially, regardless of crop or rhizosphere proximity. Crop and exclusion interval interacted to affect degrader populations (P = 0.0043). Pooled over rhizosphere and bulk soil, degrader populations were 1.5-fold higher and declined 2.8-fold faster in soybean than corn. An interaction between rhizosphere proximity and exclusion interval was also noted (P = 0.0021), whereby degrader populations were 1.9-fold higher and declined 2.8-fold slower in rhizosphere compared with bulk soil, regardless of crop. The time required for 50% mineralization of ring-labeled 14C-atrazine (DT50) following exclusion of s-triazine herbicides increased linearly at a rate of 2.2 d yr−1. In contrast, the DT50 for this site prior to a known s-triazine application was 85 d and declined exponentially over 5 yr of successive atrazine applications: 24.5 d after 1 yr, 10.8 d after two successive years, and 3.8 d after five successive atrazine applications. Omitting s-triazines can reduce degrader populations and activity in adapted soils, but more than 4 yr is required to return mineralization kinetics to nonadapted levels, regardless of crop or rhizosphere proximity.

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.

An increase in Ca and SO4 uptake from nutrient solution in juvenile corn (Zea mays L. var. N. J. 9), soybean (Glycine max (L.) Merr., var. Kent) and large crabgrass (Digitaria sanguinalis (L.) Scop.) was obtained with 3-(3,4-dichloroyhenyl)-l-methoxy-l-methyl-urea (linuron) treatment. The uptake of K, Mg, NO3, and PO4 also was followed and varied with the ion and species.

Adventitious root bud development and assimilate translocation were studied in Canada thistle [Cirsium arvense (L.) Scop. ♯ CIRAR] grown in nutrient solution in controlled environments using combinations of two photoperiods (PP) (13 and 15 h), three day/night shoot temperatures (ST) (15/5, 25/15, and 30/22 C), and three root temperatures (RT) (10, 20, and 30 C). Total root bud elongation increased with RT and length of PP and was greatest (65 cm/plant) at 25/15 C ST, 15-h PP, and 30 C RT. The number of root buds produced was greatest at 20 C RT (7.3 to 10.3 buds/plant), whereas variations in PP and ST had little effect. Total dry-matter production was greatest (7.2 g/plant) at 15-h PP, 30/22 C ST, and 20 C RT. To study phloem translocation, photoassimilates were labeled in Canada thistle plants by exposing mature leaves to 14CO2. Net assimilate translocation from a source leaf following 24-h temperature acclimation was affected little by RT and ST, but was greater under the 13-h PP than under the 15-h PP. After 7 days of temperature preconditioning, net translocation of 14C-assimilates increased with both RT and ST, but no effects due to PP were noted. With 24-h temperature acclimation, net assimilate accumulation in roots was enhanced by 13-h PP and low ST (15/5 C), whereas RT itself had no effect. In temperature-preconditioned plants, 10 C RT enhanced assimilate accumulation in roots, but ST and PP had no effect.

Cell cultures of soybean (Glycine max (L.) Merrill ‘Acme’) were exposed to media containing 4-amino-3,5,6-trichloropicolinic acid (picloram) for 15 days. Picloram also was supplied once in droplets (water) to cotyledons of 10 to 13-day-old seedlings of cotton (Gossypium hirsutum L. ‘Champion’). The amounts of picloram necessary to reach and exceed the 50% tolerance limit (TL50) of the cell cultures (inhibition) and of the primary leaf buds (toxicity) were established, and internal picloram concentrations then were determined. Internal concentrations at the TL50 were 0.17 nM/g fresh weight and 14.7 nM/g fresh weight for cell cultures and leaf buds, respectively. These values are approximately 10−7 and 10−5 molar. In leaf buds, concentrations increased rapidly for 36 hr after treatment and declined slowly thereafter. Primary leaf buds accumulated up to several times the lethal internal concentration of picloram when the dosage to the cotyledons was increased by one order of magnitude.

When discs of red beet (Beta vulgaris L.) root were immersed in various concentrations of glyphosate [N-(phosphonomethyl)glycine] either in phosphate (pH 6.8) or citrate (pH 6.2) buffer, no efflux of betacyanin occurred even after 1 week. This suggests that glyphosate had little or no effect on membrane permeability. When beet discs were immersed in distilled water containing glyphosate, the betacyanin efflux increased with time and concentration. Measurement of pH indicated that the solutions were strongly acidic at first, but tended to approach a pH of 6.0 over 8 h, after which time betacyanin efflux slowed down or stopped. The tissue was able to buffer the solutions sufficiently to prevent further efflux. When tissue discs were immersed in a series of citrate buffer ranging from pH 3 to 6.2, efflux occurred at pH 3 and slightly at pH 6.2. Efflux was not altered significantly with the addition of glyphosate.

Navy bean (Phaseolus vulgaris L.) treated with the combination of EPTC (S-ethyl dipropylthiocarbamate) and atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] developed more chlorosis of the leaves than beans treated with atrazine alone. Navy bean yields in field studies in 1972, were reduced 39% when grown in the presence of 0.55 kg/ha of atrazine and 3.4 kg/ha of EPTC, whereas the single application of either herbicide reduced yields 0 to 14%, respectively. In 1973, atrazine at 0.28 kg/ha did not significantly reduce yields, but combinations of 0.28 kg/ha of atrazine plus 2.2, 3.4, and 4.5 kg/ha of EPTC, reduced yields 40, 30, and 62% respectively. The combination of atrazine and EPTC also reduced plant weight and delayed flower and pod development. EPTC was found to increase wind blast damage under conditions of low relative humidity, high winds, and limited soil moisture. EPTC at 2.2, 3.4, and 4.5 kg/ha reduced the chloroform-soluble leaf extract from the surface of leaves and scanning electron micrographs confirmed that EPTC at 10-6 M altered leaf surface waxes. EPTC at 10-5 and 10-6 M increased the transpiration rate of plants. Increased uptake of 14C-atrazine from solution was associated with increased transpiration.

The life cycle of Notaris bimaculatus Fab. and the influence of this weevil on the control of quackgrass [Agropyron repens (L.) Beauv.] with glyphosate [N-(phosphonomethyl) glycine] were studied. Observations on this insect in 1978 and 1979 showed that its life cycle occurs in close association with quackgrass. The adult weevils feed on quackgrass culms and caryopses and use the inside of the culms for ovipositing. Adult populations, measured in quackgrass infestations during the summer months, ranged from 3 to 44/25 sweeps of an insect net. Larvae emerge from the eggs after 2 weeks, feed down the inside of the culms, chew an exit hole, and move into the soil where they attack the rhizomes. Larval numbers ranged from two to six/28 dm3 of soil during the summer months. Larvae feed on the rhizome surface or enter the rhizomes where they devour the vascular and cortical tissue. In dense quackgrass sods treated with glyphosate at 1.7 kg/ha, feeding damage on quackgrass rhizomes caused by the larvae reduced the control of quackgrass by disrupting the translocation of glyphosate in the rhizomes. Although soil-borne larvae of several insects were found in quackgrassinfested soil, larvae of N. bimaculatus were always present. Controlling these soil-borne insect larvae with a soil-applied insecticide for 2 months before applying glyphosate resulted in significantly increased quackgrass control. Shoot regrowth several months after the application of glyphosate at 1.4 kg/ha to quackgrass grown in cages infested with 400 adult weevils was 298 and 611 shoots/m2 in the 1979 and 1980 experiments, respectively. Similar glyphosate applications to weevil-free quackgrass resulted in only 26 and 15 shoots/m2 in the 1979 and 1980 experiments.

Effectiveness of cyclical in vitro selection for imazethapyr tolerance in tomato was studied. Cotyledons and leaf explants from shoots regenerated on media containing imazethapyr were used to initiate three successive cycles of selection. Increases in tissue viability, callus initiation, and callus proliferation were observed following three cycles of selection on 10−7 M imazethapyr. Dry matter accumulation of unselected tomato seedlings was reduced by 50% when imazethapyr was applied postemergence at a rate of 28 g ae ha−1. Progeny of eight tomato lines selected after one cycle of selection had greater tolerance to imazethapyr than control plants (60 to 70% dry matter accumulation versus 50%, respectively). Variability was also generated for imazethapyr tolerance among progeny of the same tomato line.

The maximum concentrations of 3-amino-s-triazole (amitrole) found in two flowing-water canals (1.13 and 0.65 cu m/sec) following treatment of a single ditchbank of each canal with 4.5 and 3.4 kg/ha of amitrole-ammonium thiocyanate (amitrole-T) were 31 and 43 ppb. Conjunctive treatment of both banks of another canal, prior to entry of water, resulted in a maximum of 98 ppb of amitrole upon turning 1.42 cu m/sec of water into the canal. Two hours after passage of the main body of amitrole-bearing water at the last sampling station on each canal, amitrole levels were down to approximately 1 ppb or less. The dissipation of amitrole in canals, for distances up to about 14.5 km, was a linear function of amitrole concentration and downstream flow. The rate of dissipation was fastest on a canal in which the herbicide was placed directly into the water (slope of 3.8), modest for a single bank treatment (slope of 2.7), and slowest where opposite banks of a canal were treated (slope of 1.8).

Crop rotations, particularly those that include legumes, often result in improved soil quality and crop yields. A study was conducted to confirm the presence and persistence of the residual effects of crop rotation and weed management on a test crop and weeds in three tillage systems (moldboard plow [MP]; chisel plow [CP]; no tillage [NT]). Rotation (spring barley monoculture and spring barley–red clover rotation) and weed management (intensive, moderate, minimum) treatments, initiated in 1987, were terminated, and a test crop of spring wheat was grown in 1995 and 1996. Tillage treatments were maintained throughout. Multivariate analysis showed that weed communities were more affected by treatment termination in the rotation NT treatment with minimum weed management than in all other treatments. The former treatment was dominated by perennial broadleaf weeds but sustained adequate wheat yields (3.3 Mg ha−1) compared with the monoculture (1.0 Mg ha−1) one year after termination. Weed communities in CP and MP plots were less affected by treatment termination. Yet, changes in herbicide use at termination caused the virtual elimination of quackgrass from tilled plots and allowed field pennycress to become ubiquitous across treatments. Residual effects from crop rotation were more important than those from weed management in increasing wheat yields in tilled systems. Differences in wheat yield in NT systems 2 yr after treatment termination were attributed to residual effects from previous weed management rather than from crop rotation. Beneficial effects of crop rotation and weed management may persist for 2 yr but will vary according to tillage system.

The objectives of this study were to use a computer simulation model to predict the influence of herbicides and mechanical treatments on giant foxtail population dynamics, annualized net return (ANR), and the giant foxtail economic optimum threshold (EOT) in a corn–soybean rotation over 20 yr. Mechanical treatments were interrow cultivation in corn and rotary hoe in soybean. Herbicides at full (1 ×) and half (½ ×) rates applied alone reduced giant foxtail seedbank 95% within 4 and 8 yr, respectively. Predicted seedbank dynamics had more variability when managed with herbicides at ½ × than at 1 × rates applied alone. Mechanical treatments integrated with herbicide at ½ × rates resulted in giant foxtail seedbank and variability similar to herbicides at 1 × rates applied alone. ANR was maximized when herbicides were applied between ⅜ × and 9/16 × rates applied alone. As initial giant foxtail density increased from 100 to 10,000 seeds m−2, the herbicide rate that maximized ANR increased. Economic optimum thresholds (EOTs) did not vary when herbicides were applied at different rates, but integrating mechanical treatment with herbicides increased the EOT from 0.1 to 0.7 seedlings m−2. Sensitivity analysis determined that giant foxtail seedbank demographics, seedling survival, and seed production per plant had the most influence on model predictions. Model sensitivity varied little between 1 × and ½ × rates. Integrating herbicides and mechanical treatment decreased the sensitivity of the model to perturbations in parameter estimates. Herbicides at reduced rates were more profitable over the long term than 1 × rates, but risk of herbicide failure increased as rate decreased. Integration of herbicides applied at reduced rates with mechanical treatments increased ANR and minimized the risk of herbicide failure compared to herbicides applied at 1 × rates alone.