Dry bean producers have limited postemergence broadleaf weed control options. The purpose of this research was to determine whether bentazon increased dry edible bean tolerance to postemergence imazethapyr applications. Imazethapyr and bentazon were applied with a petroleum oil adjuvant to ‘Olathe’ pinto bean in the field and greenhouse. Imazethapyr injured pinto bean 7 DAT in the field and greenhouse. Chlorophyll a content, a quantitative measure of bean chlorosis, decreased compared to the untreated control following imazethapyr application. When 840 g ha−1 of bentazon was tank-mixed with 53 g ha−1 of imazethapyr, bean injury decreased and chlorophyll a increased compared to imazethapyr alone. Fifty three g ha−1 of imazethapyr delayed physiological maturity by 8 and 15 d compared to the untreated control in 1991 and 1992, respectively. Pinto bean seed yields were not reduced compared to the untreated control. When 840 g ha−1 of bentazon was tank-mixed with 53 g ha−1 of imazethapyr, maturity was not delayed. 14C-Imazethapyr absorption decreased by more than 40% and translocation of 14C from the treated leaf decreased by more than 50% when 14C-imazethapyr was tank-mixed with bentazon compared to 14C-imazethapyr alone. The addition of 20 mM Na-acetate inhibited absorption of 14C-imazethapyr, but did not inhibit translocation of 14C. The decreased absorption and translocation of imazethapyr when tank-mixed with bentazon likely accounts for the safening effect observed in greenhouse and field studies.

Field trials were carried out in 1989 and 1990 at St-Augustin, near Québec City, Canada, to determine the critical periods of quackgrass control in potato submitted to three levels of infestation. Potato yield losses due to quackgrass interference increased with quackgrass infestation and length of duration of interference. Quackgrass interference influenced marketable tuber yields more than total tuber yields. Duration of the critical period varied depending on the level of quackgrass infestation and year. Based on an arbitrary 5% level of marketable yield loss, the critical period started at ca. 15 days after emergence (DAE) of potato at low level of infestation, and at ca. 3 DAE at medium level of infestation. At high level of infestation, the critical period began prior to the emergence of potato. The end of the critical period of quackgrass removal was extremely variable across quackgrass infestation level and year and ranged from 23 to 68 DAE of potato at a 5 % yield loss level. It appears that onset of interference varied less than the end of it, indicating that early quackgrass control is necessary to prevent yield loss.

Gypsophila paniculata L., a dicotyledonous, herbaceous member of the Caryophyllaceae, was introduced into North America from Eurasia as a garden ornamental and has escaped to become troublesome in some areas. A life history study has revealed that this species is spread entirely by seed, its tumbleweed character being an important factor. Germination occurs mainly in the spring in Saskatchewan stands, perhaps as a result of no dormancy in the seeds. Although seedling development is characterized by a rapid penetration of the root system, mortality in natural stands is high. Under favorable conditions, at least 2 yr are required for the production of floral primordia. The mature plant is characterized by a root system which penetrates to depths of 4 meters or more. Shoots arise each year from a thick woody caudex.

Since the discovery of the growth substance activity of the phenoxy acids by Zimmerman and Hitchcock (14) in 1942 and the announcement of 2,4–dichlorophenoxyacetic acid (2,4–D) and 2,4,5–trichlorophenoxyacetic acid (2,4,5–T) as herbicides by Hamner and Tukey (9) in 1944, a new era in chemical weed control has developed. In 1949 workers at the Texas Agricultural Experiment Station at Spur, and the Weed Investigations Section of the U.S.D.A. (6) reported that aerial application of 2,4,5–T offered much promise of effectively and economically controlling mesquite on rangeland. Since that time, approximately 1,500,000 acres of mesquite infested grassland have been treated with this compound by aerial application. Though widespread use of 2,4,5–T has been made, the factors that influence its effectiveness in killing mesquite are not well understood. This paper attempts to summarize the information that is available on some of these factors.

Oat (Avena sativa L. ‘Elgin′) and soybean (Glycine max L. ‘Evans′) were treated with 14C-diclofop-methyl {methyl ester of 2-[4-(2,4-dichlorophenoxy)phenoxy] propionic acid]} or 14C-diclofop alone or in combination with 2,4-D [(2,4-dichlorophenoxy)acetic acid] or bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] and 14C-2,4-D alone or in combination with diclofop-methyl or diclofop. More radioactivity was recovered in the treatment zone after 14C-diclofop-methyl applications, alone or in combination, than after similar 14C-diclofop treatments in oat and soybean. Basipetal movement of radioactivity was 4 and 1% and acropetal movement was 1 and 4% in oat and soybean, respectively, regardless of the diphenyl ether treatment or time. Addition of 2,4-D or bentazon did not reduce translocation of radioactivity from 14C-diclofop-methyl treatments in either plant species. Basipetal movement of radioactivity from 14C-diclofop-methyl was greater than from 14C-diclofop in both oat and soybean. The addition of diclofop-methyl or diclofop did not affect the pattern or amount of 14C-2,4-D radioactivity translocated. In oats, radioactivity appeared to accumulate within the intercalary meristematic region with 14C-2,4-D and 14C-diphenyl-ether applications. Diclofop-methyl at 1 kg/ha applied either to an entire oat plant at the three-leaf stage or the apical meristemic region resulted in plant mortality. The extent to which transport contributes to diclofop-methyl efficacy is questioned.

Metriflufen {2-[4-(4-trifluoromethylphenoxy)phenoxy] propanoic acid} was applied as the methyl ester at 0.28 and 0.56 kg/ha over-the-top to johnsongrass [Sorghum halepense (L.) Pers.] growing from rhizomes and to soybeans [Glycine max (L.) Merr. ‘Lee 68′]. After herbicide treatment, plants were grown in the growth chamber for 14 days at 16, 24, or 32 C with relative humidity (RH) at 40 or 100% at each air temperature. Johnsongrass was not controlled at 16 C regardless of metriflufen rate, RH, or the addition of nonoxynol [α-(p-nonylphenyl)-ω-hydroxypoly (oxyethylene)] (with 9.5 moles of polyoxyethylene) surfactant at 0.25 (g/100 ml) to spray solutions. Johnsongrass control at 24 C varied from 5 to 98%, with significantly better control at 100% than at 40% RH. The presence of surfactant increased johnsongrass control at 24 C and 40% RH but not at 24 C and 100% RH. Johnsongrass control at 32 C varied from 48 to 98%, and it was not increased by the presence of the surfactant, regardless of metriflufen rate or RH level. At 16 C metriflufen was more injurious to soybeans than to johnsongrass, but at 24 and 32 C johnsongrass control was significantly greater than soybean injury. The presence of surfactant in spray solutions generally did not increase soybean injury, regardless of temperature or RH level. These results suggest that metriflufen is most selective in controlling johnsongrass in soybeans at 24 C, especially under high RH.

Experiments were conducted to evaluate effects of various densities of bermudagrass [Cynodon dactylon (L.) Pers. ♯ CYNDA] in cotton (Gossypium hirsutum L. ‘Stoneville 213’) during the initial and second seasons following grass establishment and to evaluate bermudagrass competition in three cotton row spacings. During the season of grass establishment, densities of 1 to 16 plugs/7.5 m of cotton row had minimal effects on yield. Established, perennated bermudagrass was much more competitive. During second-season competition, 1 plug/7.5 m had formed a 76% ground cover and reduced yields by 600 kg/ha (25%) or more. The highest densities reduced yields 60 to 80%. In separate experiments, a cotton row spacing of 0.5 m reduced bermudagrass ground cover compared to 1.5 m row spacing.

Field experiments were conducted in Pendleton, SC, in 2004 and 2005, to determine the influence of tillage with or without soybean on common cocklebur emergence. Treatments included no-till/no soybean (NTNS), no-till plus soybean (NTS), tillage/no soybean (TNS), and tillage plus soybean (TS). Emergence was monitored from an artificial seed bank in 2004 and a natural seed bank in 2005. Overall, common cocklebur emerged from early May through late October and presented multiple emergence. In no-till plots with or without soybean, initial emergence was delayed 7 d in both years. In TNS plots, major emergence (daily emergence > mean emergence plus standard deviation) of common cocklebur occurred from early May to late July. In NTNS plots, major emergence occurred from late May through late August. No-till reduced total common cocklebur emergence by 59 to 69% compared with tillage. At the V5 to V6 soybean growth stage, the daily soil thermal fluctuation at 2.5 cm soil depth diminished from approximately 15 to 5 C and reduced common cocklebur emergence by 84 to 91% for the rest of the growing season. Common cocklebur emergence was higher when the mean soil temperature was > 15 C, and the daily thermal fluctuation was > 7.5 C. This study suggests that strategies that promote early crop canopy development and minimum tillage should reduce common cocklebur emergence.

Seed dormancy of common milkweed (Asclepias syriaca L.) is broken by a few days of moist low-temperature after-ripening. The duration of low-temperature after-ripening of the seeds is temperature dependent. Seedling emergence is best when the seeds are planted 1 to 2 cm deep and extremely limited when planted 6 cm deep. Seedlings have the capacity to produce new shoots if clipped in the 1 to 1¾-leaf pair stage and multiple shoots if clipped in the 2 to 2½-leaf pair stage. All seedlings reaching the 4 to 4½-leaf pair stage before clipping produced new shoots.

The comparative performances of several herbicides incorporated 2.5 cm deep and unincorporated in furrow-irrigated soil were studied in four field experiments. Among eight herbicides evaluated, a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) was the most effective herbicide and controlled redroot pigweed (Amaranthus retroflexus L.), Palmer amaranth (Amaranthus palmeri S. Wats.), common purslane (Portulaca oleracea L.), and barnyardgrass (Echinochloa crusgalli (L.) Beauv.) when incorporated, without reduction of yield in carrots (Daucus carota L., var. saliva, cv. Long Imperator). Trifluralin and O,O-diisopropyl phosphorodithioate S ester with N-(2-mercaptoethyl)benzenesulfonamide (bensulide) gave consistently good results in all years although soil temperatures and evaporative losses of water were variable. Bioassays indicated and gas-liquid chromatographic (hereinafter referred to as GLC) assays confirmed that rainfall moved bensulide and trifluralin 2.5 cm downward in soil, and that dimethyl tetrachloroterephthalate (DCPA) was not moved below the original depth of incorporation in soil. GLC assays were most sensitive but generally agreed with bioassays of trifluralin-treated soils. Under high temperatures, infrequent rainfall, and furrow irrigation, the half-life of DCPA and trifluralin activities was 3 weeks, with no biological activity after 7 months. The half-life of bensulide activity was 6 months, with no biological activity after 22 months.

Field and laboratory studies were conducted to evaluate the efficacy of several herbicides applied after small-grain harvest on Russian thistle (Salsola iberica Sennen and Pau # SASKR) control and subsequent seed germination. Best postharvest control of Russian thistle was with chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} treatments, paraquat (1,1′-dimethyl-4,4′-bipyridinium ion), and bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) plus metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one], where control ranged from 73 to 96%. During the summer-fallow year, chlorsulfuron and paraquat decreased Russian thistle population and biomass compared to the untreated control. Germination of large seeds from plants treated with paraquat and both rates of chlorsulfuron was reduced at least 64% compared to seeds of similar size from untreated plants.

A study was conducted at two locations to evaluate the effects of cheat (Bromus secalinus L. ♯3 BROSE) competition on four seeding rates (4.5, 9.0, 13.5, and 22 kg/ha) of alfalfa (Medicago sativa L. ‘Riley’). Three stages of first harvest (early bud, early bloom, and late bloom) were imposed on fall-planted alfalfa, with and without cheat overseeding, to determine the effects of harvest stage on alfalfa yield and quality. Cheat infestations significantly decreased alfalfa production and forage quality. Increasing the seeding rate of alfalfa only partially offset these losses. Harvesting early helped minimize the effect of cheat competition. First-harvest protein yield of forage was significantly decreased by, cheat overseeding. Total protein yield at first harvest was not affected by cutting stage but did increase with the two higher seeding rates of alfalfa.

Landscape-induced differences in soil variability and other parameters have potential effects on herbicide sorption, persistence, degradation, and, ultimately, efficacy. This study examined the spatial variability of herbicide efficacy across an eastern South Dakota field in continuous corn. Atrazine and alachlor had been applied for the previous 10 yr. The spatial variability observed in weed control was compared with herbicide sorption (Kd), mineralization rate, and first-order half-life (t 1/2), and field herbicide dissipation rates (DT50). Spatial structure was present in atrazine mineralization, weed biomass, and corn biomass data. The amount of atrazine and alachlor sorbed to soil collected from the summit position of the field was 10 and 20% less, respectively, than the amounts sorbed to backslope or toeslope soils. Generally, both herbicides had faster mineralization rates and shorter t 1/2 in summit than in backslope or toeslope soils. Weed biomass was correlated positively with elevation and total amount of atrazine mineralized, whereas corn biomass was correlated negatively with these parameters. These findings suggest that weed control can be improved by accounting for the landscape positional effects on differential herbicide mineralization and dissipation in fields.

Prevalence of wild oat (Avena fatua L.) and sterile oat [Avena sterilis ssp. ludoviciana (Durieu) Gillet & Magne; referred to as A. sterilis hereafter], winter-season weeds, is increasing in the eastern grain region of Australia. Biological attributes of these weeds enable them to survive in a wide range of environments and under different weed infestation levels. The interference of A. fatua and A. sterilis in a wheat (Triticum aestivum L.) crop was examined in southeast Queensland, Australia, through field studies in 2019 and 2020. Different infestation levels (0, 3, 6, 12, 24, and 48 plants m−2) of A. fatua and A. sterilis were evaluated for their potential to cause yield losses in wheat. Based on a three-parameter logarithmic model, the A. fatua and A. sterilis infestation levels corresponding to 50% wheat yield loss were 15 and 16 plants m−2, respectively. The yield reduction was due to a reduced spike number per unit area because of an increased weed infestation level. At the highest weed infestation level (48 plants m−2), A. fatua and A. sterilis produced a maximum of 4,800 and 3,970 seeds m−2, respectively. Avena fatua exhibited lower seed retention (17% to 39%) than A. sterilis (64% to 80%) at wheat harvest, as most of the seeds of A. fatua had shattered at crop maturity. Our results implied that there is a good opportunity for harvest weed seed control if the paddock is infested with A. sterilis. This study suggests that in the absence of an integrated weed management strategy (using both chemical and nonchemical options), a high infestation of these weeds could cause a severe crop yield loss, increase weed seed production, and replenish the weed seedbank in the soil.

Effects of several inorganic ions on short-term glyphosate absorption by cultured plant cells were determined by measuring the glyphosate content of cells exposed to inorganic salts in a 0.01-mM glyphosate solution buffered at pH 5.7. Within 10 min, glyphosate absorption by potato and velvetleaf cells was stimulated about equally by 1.5 mM CaCl2 or 1.5 mM MgCl2 but was not affected by 1.5 mM KCl. Glyphosate absorption by both species increased linearly with log10 CaCl2 concentration; for velvetleaf cells, the response was greatest for CaCl2 concentrations greater than 0.2 mM. Over time of absorption, glyphosate concentration within velvetleaf cells approached an asymptote. Addition of 0.1 mM KH2PO4 inhibited glyphosate absorption by potato cells in the presence or absence of Ca2+ and Mg2+ salts; all other K+ salts tested had no effect. Glyphosate absorption by potato, velvetleaf, soybean, corn, and rice cells was stimulated by 5.0 mM CaCl2 but was inhibited by 0.1 mM KH2PO4 in the presence of 0.05 or 5.0 mM CaCl2.

Sorghum-almum (Sorghum almum Parod. # SORAL) is a weakly perennial, rhizomatous grass that in northern climates reproduces mainly by seeds. Environmental factors influencing seed germination and the longevity of buried seeds were studied in growth chamber and field experiments. Alternating temperatures of 20 C for 16 h and 30 C for 8 h were optimum for germination. Light was not critical to germination. Germination varied only slightly across a pH range from 5.0 to 8.8 but decreased sharply below pH 5. Osmotic potentials greater than −0.9 MPa reduced germination substantially, and no germination occurred at −1.5 MPa. Limited germination occurred at NaCl concentrations of 340 mM, and no germination occurred at 510 mM. Seeds that were either mechanically scarified or afterripened for 6 months had a higher percent germination and germination rate than untreated seeds. Sorghum-almum seedling emergence was 11% from seeding depths of 20 cm, but emergence was 67% or higher from seeding depths of 15 cm or less. In field studies, viability of seeds placed on the soil surface or buried 3 or 8 cm was 2% or less at 12 months and 0% at 18 months after burial. Viability of seeds buried 15 and 23 cm was 0.3 and 26%, respectively, at 24 months after burial. Thus, tillage that buried sorghum-almum seeds 20 cm or more could reduce emergence but would also prolong seed persistence. Shallow tillage should favor depletion of sorghum-almum seed reserves.

Studies were conducted to calibrate and validate a mathematical model previously developed to predict common lambsquarters seedling emergence at different corn seedbed preparation times. The model was calibrated for different types of soil by adjusting the base temperature of common lambsquarters seedling emergence to the soil texture. A relationship was established with the sand mineral fraction of the soil and was integrated into the model. The calibrated model provided a good fit of the field data and was accurate in predicting cumulative weed emergence in different soil types. The validation was done using data collected independently at a site located 80 km from the original experimental area. There were no differences between observed and predicted values. The accuracy of the model is very satisfactory because the emergence of common lambsquarters populations was accurately predicted at the 95% probability level. This model is one of the first to take into consideration seedbed preparation time and soil texture. This common lambsquarters emergence model could be adapted to model other weed species whose emergence is limited by low spring temperature.

Phorate {O,O-diethyl S-[(ethylthio)methyl] phosphorodithioate}, disulfoton {O,O-diethyl S-[2-(ethylthio)ethyl] phosphorodithioate}, linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea], or metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] when used alone did not reduce soybean [Glycine max (L.) Merr. ‘Calland’] grain yield or plant population. However, combinations of metribuzin with either phorate or disulfoton interacted synergistically to reduce both soybean grain yield and plant population significantly even at recommended rates of both chemicals. Combinations of linuron with either phorate or disulfoton at recommended rates did not reduce soybean grain yield or plant population. Greenhouse studies further substantiated the synergistic interaction of phorate or disulfoton with metribuzin.

Investigations were undertaken to ascertain whether residues of soil-applied carbaryl (1-naphthyl N-methyl carbamate) and diazinon [O,O-diethyl-O-(2-isopropyl-4-methyl-6-pyrimidyl) phosphorothioate] insecticides may be absorbed from the soil and translocated to foliage of rice (Oryza sativa L. ‘Nahda’) and to determine the phytotoxic interaction between them and foliar-applied propanil (3′,4′-dichloropropionanilide). Results from greenhouse studies showed that carbaryl and diazinon were translocated and affected hydrolytic activity in rice shoots at all rates of application. Carbaryl was more inhibitory but diazinon persisted longer in the soil. When propanil was applied in the presence of diazinon, no additive phytotoxicity occurred on rice plants over that occurring from propanil alone. Synergistic effects in rice, resulting in dry weight loss, were marked when soil was treated with 5 mg of diazinon per kilogram of soil and subsequently sprayed with propanil. Synergistic phytotoxicity was apparent when propanil was applied 1 day after carbaryl treatment and most plants were killed. Slight injury was noticed on plants treated with propanil 2 weeks after carbaryl treatment at 1 and 5 mg/kg.

The plant growth regulating properties of 2,4–D (2,4–dichlorophenoxyacetic acid) (26) and the resistance of most monocotyledonous plants to the toxic effect of this chemical (17) have been known for many years. This difference in sensitivity of various species to the toxic effects of 2,4–D has had wide practical use in the field of weed control. However, there is no generally accepted mechanism for the selectivity between sensitive and resistant plants or for the actual mechanism of 2,4–D action. The four areas of research which have been investigated to elucidate the mechanism of action are absorption, translocation, and two distinct metabolic approaches. Absorption and translocation are often studied simultaneously. Metabolism is studied from two approaches, either the fate of 2,4–D in the plant or the effect of 2,4–D on plant metabolism. All of these processes are operating simultaneously in the plant and the ultimate explanation as to the mechanism of 2,4–D action must incorporate all of the contributions from these areas of research.