Field studies were conducted to determine hazelnut tolerance to quinclorac and clopyralid and control efficacy of Canada thistle and field bindweed at three commercial orchards in western Oregon. Hazelnut cultivars evaluated included ‘Jefferson’, ‘Wepster’, and ‘McDonald’. Clopyralid at 278, 547, and 1,090 g ae ha−1, and quinclorac at 420, 840, and 1,680 g ai ha−1 were applied once a year as basal-directed applications to trees that were 1, 2, and 5 yr old. Treatments were imposed in the early spring of 2019 and reapplied in 2020. In both years, treatments covered hazelnut suckers. Hazelnut injury from clopyralid and quinclorac was consistently between 0% and 13% and not different from nontreated control plants (P > 0.05) between 14 d and 455 d after initial treatment. Similarly, there was no treatment effect on plant canopy index, leaf chlorophyll content, trunk cross-sectional area, internode length, or yield among treatments, even at the highest rates of clopyralid and quinclorac. In separate efficacy studies, clopyralid (278 g ae ha−1) resulted in 68% Canada thistle control and did not differ when clopyralid was mixed with carfentrazone (278 + 35 g ai ha−1) or glufosinate (278 + 1,148 g ai ha−1). Clopyralid-containing herbicide treatments suppressed field bindweed growth but did not kill plants even when mixed with carfentrazone or glufosinate. Quinclorac (420 g ha−1) alone provided 80% control of field bindweed and 93% and 98% control when combined with rimsulfuron (35 g ai ha−1) or carfentrazone (35 g ai ha−1), respectively. Still, all herbicide treatments resulted in similar field bindweed biomass. Results indicate that clopyralid and quinclorac are effective tools to help manage Canada thistle and field bindweed and that hazelnut can tolerate clopyralid and quinclorac at rates equivalent to 4-fold commercial-use rates not affecting plant growth and yield.

Field bindweed is a perennial vining weed with vigorous growth, and is commonly found in highbush blueberry fields of Oregon. It requires and integrated strategy using multiple applications of postemergence herbicides and hand weeding for adequate control. Quinclorac is a herbicide that has been shown to control field bindweed, but no information is available indicating the tolerance of blueberry to quinclorac. The objective of this study was to evaluate the response of blueberry to quinclorac and to evaluate field bindweed control with quinclorac in different mixtures. Three groups of field studies were designed to assess 1) single application control of field bindweed, 2) use of sequential treatments to control field bindweed, and 3) long-term impact of quinclorac on field bindweed. In the single application control studies, a single application of quinclorac at 210 or 420 g ai ha−1 alone or in a mixture with rimsulfuron (35 g ai ha−1) or carfentrazone (35 g ai ha−1), controlled field bindweed by 69% to 76% while reducing its biomass between 22% and 44% compared to the nontreated control (61 g m−2). In a sequential treatment study, a single application of quinclorac (420 g ai ha−1) provided 83% to 100% control of field bindweed, outperforming three sequential applications of carfentrazone. In the long-term study, a single application of quinclorac reduced field bindweed biomass by 50% to 82% in 2019 and 62% to 87% in 2020. These results indicate that quinclorac can be safely applied to highbush blueberry plants. Early spring applications of quinclorac to field bindweed will reduce or eliminate the need for subsequent applications later in the season.

The objective of this field research was to compare relative effectiveness and profitability of alternative herbicides applied to the same plots for four consecutive years for controlling and reducing dense Canada thistle infestations in no-till spring wheat. Chlorsulfuron at 30 g ai ha−1 plus nonionic surfactant or clopyralid plus 2,4-D at 70 plus 280 g ae ha−1, respectively, applied annually for 4 yr controlled Canada thistle and was more effective for gradually reducing Canada thistle stands than 2,4-D at 560 g ae ha−1, MCPA plus bromoxynil at 280 plus 280 g ae ha−1, or tribenuron at 20 g ai ha−1. Chlorsulfuron and clopyralid plus 2,4-D also controlled Canada thistle ≥ 90% earlier (by 2 yr) than other treatments. Stochastic dominance analysis, a form of economic analysis, predicted that either chlorsulfuron or clopyralid plus 2,4-D would be preferred by farmers to the untreated check, MCPA plus bromoxynil, or 2,4-D treatments. Chlorsulfuron also would be preferred to clopyralid plus 2,4-D by risk-neutral farmers, whereas clopyralid plus 2,4-D would be preferred to chlorsulfuron by highly risk-averse farmers, those who are most likely to pick only consistently effective herbicides.

Control of common milkweed (Asclepias syriaca L.) with postemergence herbicides was investigated in greenhouse and field experiments. In greenhouse experiments, amitrole (3-amino-s-triazole), dicamba (3,6-dichloro-o-anisic acid), glyphosate [N-(phosphonomethyl)glycine], and combinations of amitrole + dicamba, dicamba + 2,4-D [(2,4-dichlorophenoxy) acetic acid], glyphosate + 2,4-D, and glyphosate + dicamba reduced regrowth of 6-week-old common milkweed as compared with the untreated check. Postemergence herbicides commonly used in corn (Zea mays L.), sorghum [Sorghum bicolor (L.) Moench], and soybeans [Glycine max (L.) Merr.] had little effect on emerged common milkweed. Removal of common milkweed top-growth as early as 1 day after treatment did not effect subsequent growth reduction from glyphosate at 0.3 kg/ha or higher. In the field, satisfactory control (70% or better) of common milkweed was obtained with glyphosate at 2.2 kg/ha applied at the early or late bud growth stage. Plots plowed 1 day after application of glyphosate at 2.2 kg/ha still retained satisfactory control of common milkweed 1 yr after treatment. Herbicides applied with a herbicide glove or a roller applicator resulted in common milkweed control similar to that obtained from broadcast spray applications.

Several sequences of POST herbicides applied each year in fall alone, in spring alone, or both in fall and again in spring controlled Canada thistle stands in spring wheat by severely decreasing root biomass and the numbers of adventitious root buds to a depth of 50 cm over four years. These treatments included dicamba applied at 1.7 or 2.2 kg ae ha–1 for the first two successive falls followed in wheat by either chlorsulfuron at 30 g ai ha–1 plus nonionic surfactant, MCPA plus bromoxynil at 280 plus 280 g ha–1, or 2,4-D amine at 560 g ha–1 applied annually for each of four consecutive years from the start. Chlorsulfuron at 30 g ha–1 applied alone in spring for each of four years also reduced and prevented Canada thistle root growth as effectively as a sequence of fall-applied dicamba followed by spring-applied chlorsulfuron in spring wheat.

A rhizotron containing 10 1.5-by 1.2-by 2.4-m chambers and two 1.5-by 1.2-by 1.2-m chambers for plant growth was constructed for teaching purposes at Montana State University. The rhizotron, constructed of pressure-treated plywood and redwood lumber, has a life expectancy of 40 yr and a prorated cost of approximately $100/yr.

The effect of reduced water availability on glyphosate efficacy and adventitious shoot growth in male and female clones of Canada thistle was studied. Water availability was regulated with polyethylene glycol (PEG) in hydroponically grown plants or by withholding water from soil-grown plants. In hydroponic media, there was 50% more adventitious shoot growth from male plants than from female plants. Both PEG and foliarly applied glyphosate reduced the number of adventitious shoots, regardless of gender. In soil, the same number of adventitious shoots formed on untreated male and female plants. However, when glyphosate was applied to plants growing under dry conditions, there was 75% more adventitious shoot growth from males than females. Distribution of glyphosate was similar in male and female plants. Approximately 23% of applied 14C-glyphosate was transported throughout Canada thistle plants within 10 d of treatment regardless of gender or moisture condition. Gender differences in capacity for vegetative reproduction may alter the sex ratio of Canada thistle populations in the field following glyphosate application.

In glasshouse experiments, Panicum repens rhizomes with more than two nodes emerged from soil depths of 1 to 20 cm. Emergence from one-node rhizomes was greatly reduced at depths more than 5 cm. In contrast, culm emergence from ginger like rhizomes decreased when burial depth (BD) was greater than 20 to 30 cm. P. repens emerged from deeper soil levels with an increasing number of nodes per cutting. Culms emerged from 36% of total underground regenerative organs. About 75 and 95% of the emerged culms were recorded 30 and 60 d after burial, respectively, and 91% emergence was recorded from 20-cm BD. In field studies, only 9% of nodes produced culms following cross-plowing. About 61 and 67% of the emerged culms were recorded 50 and 60 d after land preparation, respectively. Results indicate that mechanical and chemical control methods could be most effective for P. repens when applied approximately 50 d after land preparation. Burying deeper than 30 cm by deep plowing and reducing rhizome length by cross-plowing might effectively reduce emergence of P. repens.

The translocation pattern and distribution in rhizomes of 14C-glyphosate, 14C-quizalofop, and 14C-sucrose was examined in five quackgrass biotypes. Translocation of radioactivity in the different plant parts varied among biotypes. Translocation in the whole plant after treatment with the three 14C-chemicals varied among biotypes but was not correlated with their tolerance to herbicides. Detailed analysis of distribution of radioactivity in the primary rhizome showed that more 14C was found in the apical sections after treatment with 14C-sucrose. A similar pattern was observed after 14C-glyphosate application with all biotypes except one. Very low radioactivity was found in rhizomes after 14C-quizalofop application, but a preferential accumulation in apical sections of the primary rhizome was detected in two biotypes. The tolerance of one biotype to glyphosate was explained by the absence of radioactivity accumulation in the apical sections of the rhizome.

An experiment was conducted to evaluate the translocation pattern of 14C-glyphosate and 14C-quizalofop in five quackgrass biotypes at two stages of growth. Whole plant translocation differed between stages for each herbicide. The percentage of 14C-glyphosate and 14C-quizalofop in the leafwash and in the treated leaf did not differ among biotypes, but differences occurred for the translocation to the underground parts. The partitioning of the 14C-glyphosate and 14C-quizalofop in the treated crown, other crowns, propagating rhizomes, and other rhizomes differed among biotypes. On a disintegration per minute (DPM) mg-1 basis, differences among biotypes were found with 14C-glyphosate but much less with 14C-quizalofop. Metabolism of 14C-quizalofop in the treated leaf was similar in the two biotypes tested except for the percentage of polar metabolites 1 d after treatment. The percentage of 14C-quizalofop available for translocation was the same in both biotypes.

Field studies were conducted to characterize the predominant nonstructural carbohydrates in roots and crowns of purple loosestrife and to determine the seasonal fluctuation of root and crown carbohydrates in three Minnesota wetland habitats. Starch was the primary nonstructural carbohydrate present, and concentrations were consistently higher in roots than in crowns. Starch content decreased following shoot emergence and declined until flower bud formation, at which time levels reached seasonal lows. Following bud initiation, levels of starch increased during flowering, and this trend continued through to plant senescence in late September or October. Sucrose was the predominant soluble sugar in roots and crowns of purple loosestrife, but fructose and glucose were detected. Levels of sucrose in roots and crowns followed the same seasonal trends as starch.

Field studies were conducted from 1994 to 1996 in Michigan to evaluate postemergence (POST) herbicides for hemp dogbane control in corn. Studies were initiated at no-tillage and chisel-plowed sites each of the three years. Nicosulfuron and primisulfuron were evaluated alone and in combination with 2,4-D amine or dicamba. In 1995 and 1996, CGA-152005 plus primisulfuron was also applied alone and in combination with 2,4-D or dicamba. Control varied among years and sites. Nicosulfuron, primisulfuron, and CGA-152005 plus primisulfuron applied alone controlled 30% of the hemp dogbane, and dicamba or 2,4-D alone controlled 42 and 66%, respectively. Tank mixtures of nicosulfuron, primisulfuron, or CGA-152005 plus primisulfuron with dicamba were more effective and more consistent than dicamba alone. Combinations of nicosulfuron, primisulfuron, or CGA-152005 plus primisulfuron with 2,4-D gave the most effective and consistent control across sites, with an average of 93% control. In general, treatments controlled only shoots that had emerged at the time of application. New shoots emerged following herbicide application at the chisel-tillage sites in 1994 and 1995 but not at the no-tillage sites. However, in 1996, shoot emergence following treatment occurred in both no-tillage and chisel-tillage sites.

Field experiments were conducted in 1985 and 1986 under eight environments to evaluate the population dynamics of a range of silverleaf nightshade densities and to measure the effects of those populations on cotton lint yield. Dry weed weights of silverleaf nightshade stands were influenced by growing conditions among years, but were positively related to initial densities as long as 2 yr after establishment. Stem numbers increased as initial densities and stand age increased. A negative linear relationship existed between cotton lint yield and weed biomass and between cotton lint yield and stem number from both 1- and 2-yr-old weed stands. For each 1 kg/10 m of row increase in dry weed weight from 1- and 2-yr-old stands, a 9 and 21% lint yield loss/ha was predicted, respectively. For each stem/10 m of row, a 0.35 and 0.31% yield loss was predicted, respectively. Late-planted cotton was less vulnerable to yield reductions by silverleaf nightshade; however, its yield potential was also less.

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.

Absorption, translocation, root release, and metabolism of imazethapyr by leafy spurge were determined under growth chamber conditions. 14C-imazethapyr was applied to vegetatively propagated leafy spurge plants in a 1% solution of 28% urea ammonium nitrate containing 0.25% by vol nonionic surfactant Plants were harvested 2 and 8 d after herbicide application. Imazethapyr absorption increased from 9% at 2d to 20% at 8d. Acropetal and basipetal translocation out of the treated leaf was observed, with 3.4 to 4.2% of the applied radioactivity accumulating in the root by the end of the 8-d time course. Eight days after herbicide application, radioactivity in dormant and elongated adventitious shoot buds was twofold higher than in root tissue (compared on a dry wt basis). Two days after herbicide application, 93% of the radioactivity remained as intact imazethapyr in the treated leaf, crown, root, and shoot buds. Eight days after application, crown, roots, and adventitious shoot buds had metabolized an average of 61, 36, and 47% of the imazethapyr, respectively, while only 14% was metabolized in the treated leaf. The primary metabolite cochromatographed with 5-hydroxyethyl-imazethapyr standard.

A field study was conducted in 1994 and 1995 to determine the effect of MON 12037, linuron, and ametryn on purple nutsedge density in field corn in a 2-yr rotation with cotton. Each corn treatment included cultivation and noncultivation. A standard treatment for purple nutsedge control was applied to all cotton plots in the second season of the corn–cotton rotation. Purple nutsedge shoot densities were reduced in cultivated MON 12037, noncultivated MON 12037, and cultivated ametryn in the corn–corn and corn–cotton rotations. Purple nutsedge tuber population was reduced in the corn–corn rotation by MON 12037 with and without cultivation, cultivated linuron, and cultivated ametryn treatments relative to the noncultivated check. In the corn–cotton rotation, purple nutsedge tuber population was reduced only by the cultivated MON 12037 and cultivated ametryn treatments relative to the noncultivated check.

The effect of photoperiod and growth stage on translocation of 14C-glyphosate was compared in Canada thistle plants at the bud and rosette stage of growth. Canada thistle plants grown under controlled environment conditions with a 10 h photoperiod remained as low growing rosettes and developed a mature root system. When the photoperiod for half of these plants was increased to 16 h, stem elongation occurred and flowering was initiated. Growth stage at the time of application affected the distribution of 14C-glyphosate within the elongated shoot and between the shoot and root. The shoot tissue of the bud stage plants contained 25.9% of the 14C-glyphosate recovered, while the rosette plants contained only 3.6%; a seven-fold difference. 14C-glyphosate was applied to leaves 19 and 20, which corresponded to the mid-point of the elongated stem of the bud-stage plants. In the bud-stage plants, 14C-glyphosate moved preferentially into the apical portion of the stem, with three to four times as much in the apical portion of the elongated stem as in the basal portion. In the roots, the effect of growth stage on distribution of 14C-glyphosate was reversed, application at the rosette stage resulted in a four-fold increase in the amount of 14C-glyphosate in the root. When applied in the rosette stage, 19.1% of the 14C-glyphosate detected was in the root compared to only 4.9% when applied at the bud stage. Although the root of the rosette plants was larger than in bud-stage plants, the concentration of 14C-glyphosate in the root tissue of the rosette plants was three times greater. Photoperiod indirectly affected the distribution of 14C-glyphosate in Canada thistle by its effect on growth.

Canada thistle control in no-tillage corn was studied in Western Maryland from 1990 to 1992. The best treatment at planting for controlling Canada thistle was a tank mixture of 2240 g/ha glyphosate plus 560 g/ha 2,4-D. Clopyralid applied at 210 and 280 g/ha alone and 106 g/ha tank mixed with 560 g/ha 2,4-D effectively controlled Canada thistle in no-tillage corn (85 to 96%). Nicosulfuron applied at 35 g/ha and 20 or 40 g/ha primisulfuron suppressed Canada thistle (59 to 75%). Tank mixtures of nicosulfuron or primisulfuron with 2,4-D or dicamba generally increased Canada thistle control (75 to 87%) compared with control by these herbicides applied alone. Treatments that controlled Canada thistle generally increased corn yields compared with the weedy controls. Yield increases were most dramatic in 1991 when precipitation was low.

Greenhouse and field experiments were conducted to determine effects of light, water, and chlorsulfuron on growth of Canada thistle. In the greenhouse, shoot and root dry weight, leaf area, and number of inflorescences decreased as light and water levels decreased. In the field, shoot and root dry weight, leaf area, and number of Canada thistle inflorescences were positively correlated with light intensity from 1832 to 30 μmol m−2 s−1 photosynthetic photon flux density (PPFD). The greatest effect of light was on inflorescence production which was eliminated at 30 μmol m−2 s−1 (PPFD). The combined effect of water stress and chlorsulfuron decreased root and shoot growth but did not eliminate it.

Absorption and translocation of 14C-fluroxypyr were determined for leafy spurge at various growth stages, under differing relative humidity and temperature conditions, and when applied with picloram or 2,4-D. Absorption of 14C-fluroxypyr was greater in vegetative plants (39%) than in flowering or postflowering plants (25%), while total translocated to roots averaged 2% of applied herbicide regardless of growth stage. Greater 14C-fluroxypyr absorption and translocation occurred in plants exposed to high relative humidity (>90%) compared to low humidity (<30%) for 6 h or longer following treatment. Absorption and translocation to roots was independent of temperature (18 or 24 C). Absorption and translocation declined by 50% when applied with picloram or 2,4-D. However, absorption and translocation of 14C-picloram or 14C-2,4-D were not affected by fluroxypyr. Fluroxypyr would likely be used in a leafy spurge control program only during environmental conditions adverse to control with picloram such as unseasonably warm or cold temperatures or in areas with a high water table.