Field experiments were conducted in 1986 and 1987 to determine the effects of shoot and root interference of common cocklebur and soybean. Plants were grown in porous membrane envelopes. Common cocklebur plants had a longer vegetative growth period than soybean and were twice as tall as soybean at maturity. Soybean root and shoot dry weight and seed yield were reduced by shoot and whole plant (root and shoot) interference of common cocklebur, with whole plant interference giving the greatest reduction. Common cocklebur growth was affected little by soybean interference. Common cocklebur shoot interference alone reduced total soybean seed weight by 48%, the amount caused by common cocklebur whole plant interference.

Cotton (Gossypium hirsutum L. ‘Stoneville 7A’) grown for 3 yr in competition with various weed densities and nitrogen levels showed annual weeds could reduce cotton heights as much as 75% and cotton yields as much as 88%. Cotton yields were increased by 18 to 30% by adding nitrogen to plots with no weeds. In 1973, cotton supplied with 224 kg/ha of nitrogen could tolerate up to 50% weeds without loss of yield. Weeds left to grow and reseed with no control resulted in increasingly greater competition with each succeeding year.

In field experiments at Prosser, WA, black nightshade (Solanum nigrum L. # SOLNI), eastern black nightshade (S. ptycanthum Dun. # SOLPT), American black nightshade (S. americanum Mill. # SOLAM), hairy nightshade (S. sarrachoides Sendtner # SOLSA), and sometimes accessions within a species responded differentially to herbicides applied preplant incorporated, preemergence, or postemergence. Also, the response of the nightshades to some of the herbicides varied significantly between years. Generally, black nightshade was more tolerant of herbicides than were the other three species. An accession of black nightshade from England and an accession of eastern black nightshade from Minnesota were the most tolerant of herbicides of eight accessions evaluated. Of the 25 herbicides evaluated, preplant-incorporated applications of ethalfluralin [N-ethyl-N-(2-methyl-2-propenyl)-2,6-dinitro-4-(trifluoromethyl) benzenamine] and preemergence applications of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], chloramben (3-amino-2,5-dichlorobenzoic acid), lactofen [1′-(carboethoxy)ethyl 5-[2-chloro-4-(trifluoromethyl) phenoxy]-2-nitrobenzoate], and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl] acetamide] controlled all species best. These herbicides reduced the oven-dry weights of both accessions of all four species of nightshades by 93% or more. Nightshades varied widely in their response to the thiocarbamates.

The residual effect of three cotton (Gossypium hirsutum L.) herbicide programs, including the use of no herbicides, a minimum program consisting of fluometuron {N,N-dimethyl-N′-[3-(trifluoromethyl)phenyl] urea} and MSMA (monosodium salt of methylarsonic acid), and an intensive program consisting of trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine], fluometuron, MSMA, and linuron [N′-(3,4-dichlorophenyl)-N-methoxy-N-methylurea], were evaluated from 1976 to 1982. Herbicide injury to wheat (Triticum aestivum L.) and hairy vetch (Vicia villosa Roth.) on three soils showed carryover effects in the following sequence: Sharkey silty clay > Dundee silt loam > Loring silt loam. The intensive program was the most injurious on the Sharkey silty clay. The effects of the two herbicide programs were nearly equal on the Dundee and Loring silt loams. Possible replacement crops for cotton, such as grain sorghum [Sorghum bicolor (L.) Moench.] and corn (Zea mays L.) suffered the least damage from carryover; rice (Oryza sativa L.), soybeans [Glycine max (L.) Merr.] and cucumber (Cucumis sativis L.) suffered severe damage. Greenhouse bioassays generally confirmed field results, and fluometuron appeared to be the major component of carryover.

Three applications/yr of glyphosate [N-(phosphonomethyl)glycine] with a ropewick applicator (RWA), a roller applicator (RA), and a recirculating sprayer (RCS) were compared to hoeing and cultivation for the control of johnsongrass [Sorghum halepense (L.) Pers. ♯3 SORHA] in cotton (Gossypium hirsutum L. ‘Acala SJ-2′) in six field experiments from 1979 to 1981. Averaged over the 3 yr and dates of application, plots treated with glyphosate contained 78% fewer live johnsongrass shoots (15/m2) than cultivated-control plots (68/m2). Although applications of glyphosate with the RWA and RA improved or tended to improve yields of cotton, only plots treated with the RWA yielded significantly more seed cotton than cultivated control plots in all years. However, yields of plots treated with the RWA still averaged 38% less than hoed plots. Plots treated with the RA produced significantly more cotton than cultivated-control plots only in 1981.

Crownbeard [Verbesina encelioides (Cav.) Benth. & Hook. f. ex. A. Gray # VEEEN (Compositae)] exhibited rapid seedling, vegetative, and reproductive growth. Seed germination under ideal conditions was high and occurred in varying soils except in gravel. It was significantly suppressed under drought and waterlogged conditions and was maximum at 21% moisture regimes. Larger and heavier seeds which exhibited higher germination were produced by the plants growing in open and sand dune habitats. Seedling emergence was highest from surface-sown seeds and least in those sown at 2.5-cm soil depth, beyond which no emergence occurred. High phenotypic plasticity, ecological variability, phenological diversity, seed germination in varied soils and under diverse moisture regimes, quick seedling growth and subsequent establishment, coupled with versatile breeding system, contribute to the successful growth, propagation, and spread of this species in nature in India.

Three musk thistle (Carduus thoermeri Weinm.) sites that were infested with a weevil (Rhinocyllus conicus Froel.) and a fourth non-infested site were evaluated for numbers of insects and seeds per seed head. At four successive weekly intervals, seed heads were sampled just after flowering. Initial sampling was in 1978 and was repeated in 1979. Weevil incidence in heads blooming during the first week was 6.7/head in 1978 and 28.0 in 1979. At infested sites, production of fully developed seeds averaged 28% less than at the non-infested site in 1978 and 78% less in 1979. Heads that were in bloom during the fourth sampling week had an infestation rate less than one weevil/head in both years. Fully developed seeds germinated well (96 to 99%) and were not affected by date of bloom. Because many heads bloomed during the third and fourth weeks, a large number of viable seeds were still produced at infested sites.

Failure to control Xanthium strumarium with acetolactate synthase (ALS) inhibitor herbicides has been reported in Iowa and surrounding states. Single-seed descent techniques were used to isolate three X. strumarium biotypes: CAM-10 from near Cambridge, Iowa; Colo-25 from near Colo, Iowa; and Ohio-1 from Fulton County, Ohio. Ohio-1 and Colo-25 were selected because of apparent resistance to imazethapyr, whereas CAM-10 was selected for observed sensitivity to imazethapyr. The biotypes were assayed in vitro with three different ALS inhibitor herbicides, and ALS activity was measured. The 50% inhibition values (I50) of ALS for imazethapyr were determined to be ninefold or higher for the Ohio-1 and Colo-25 biotypes compared to the CAM-10 biotype. The I50 for imazaquin was determined to be about ninefold higher for the Colo-25 biotype and sixfold higher for the Ohio-1 biotype when compared to the CAM-10 biotype. All biotypes were equally sensitive to chlorimuron ethyl. The resistance was due to a single dominant nuclear gene.

Hemp dogbane (Apocynum cannabinum L.) was investigated relative to seed germination, depth of seedling emergence, response to clipping, influence of soil type and fertility on vegetative growth, and herbicidal control. Seed germination in this study was influenced by light and scarification. Seedling emergence was influenced by the amount of soil cover. Soil type and fertility affected vegetative growth. In a greenhouse clipping study, hemp dogbane became perennial 41 days after emergence and by 65 days was producing more than one shoot per plant following clipping. Hemp dogbane was more completely controlled following September application of herbicide than from application made in June.

The development of problematic weed populations is a concern in western Canadian fields where canola and pea are grown in a 4-yr sequence with spring cereal grains. Weed densities were examined at a site near Melfort, Saskatchewan, Canada, from 1994 to 1997 in seven zero-till managed crop rotations. Four rotations that included canola, pea, or flax in at least 3 of 4 yr (HBF: high broadleaf–crop frequency) were compared with three rotations that included broadleaf crops grown in 2 of 4 yr (LBF: low broadleaf–crop frequency). Spring wheat and barley were the cereal crops in rotation. Residual (postherbicide application) weed density for each weed species in a given year was summed across all phases for each rotation to reflect the overall weed infestation. Four annual broadleaf weed species were most abundant in 1996 and a second group of three species, having a variety of reproductive strategies, became progressively less abundant as the study progressed. The difference between the HBF and LBF rotations for the density of these species varied and was most prominent in years when environmental conditions were conducive for their growth. More frequent applications of ethafluralin, with its residual weed control, best explained why wild oat and catchweed bedstraw generally were less abundant in the HBF rotations. Of particular interest was the 8 plants m−-2 greater density of dandelion and perennial sowthistle in the HBF vs. LBF rotations in the last year of the study. It is thought that the limited herbicide options for the control of these species could present a future problem if they continued to develop in the HBF rotations. Differences in herbicide use between the HBF and LBF rotations were considered the primary factor controlling the rotation effects on weed density.

Sequential applications of preemergence (PRE) and postemergence (POST) herbicides were applied over a two-year period for large crabgrass and goosegrass control in common bermudagrass. Large crabgrass control was better when prodiamine at 0.3 kg ai ha−1 or oryzalin at 0.8 kg ai ha−1 were applied in late February, and followed by MSMA at 1.1 kg ai ha−1 in June than when prodiamine (0.8 kg ha−1) or oryzalin (2.4 kg ha−1) were applied at recommended rates. Goosegrass control was better when prodiamine was applied at 0.3 kg ha−1 followed by MSMA plus metribuzin at 2.2 + 0.1 kg ai ha−1, or when oryzalin was applied at 1.7 kg ha−1followed by MSMA plus metribuzin at 1.1 + 0.1 kg ha−1 than when prodiamine was applied at 0.8 kg ha−1 or oryzalin was applied at 3.3 kg ha−1. There was no increase in goosegrass control when diclofop was applied in sequence after prodiamine or oryzalin compared to diclofop alone. POST applications of diclofop, MSMA, and MSMA plus metribuzin did not injure common bermudagrass more severely when applied to plots previously treated with prodiamine and oryzalin than with POST herbicides alone.

Exposure of pea plants (Pisum sativum L. ‘Little Marvel’) to vapors of S-(2,3-dichloroallyl)diisopropylthiocarbamate (diallate) increased the subsequent foliar uptake of 14C-labeled herbicides. Increased amounts of 14C-activity were found in the tissues of diallate-treated plants as compared to the controls when herbicides were applied to the leaves as aqueous solutions or embedded in agar blocks. The herbicides were (2,4-dichlorophenoxy)acetic acid-1-14C (2,4-D), uniformly ring-14C 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine),trichloroacetic acid-2-14C (TCA), and ethylene-14C 6,7-dihydrodipyrido[1,2-a:2′,1′-c]pyrazinediium ion (diquat). The increased movement of herbicides into diallate-treated peas was primarily attributed to a reduction of epicuticular lipids on leaves of the treated plants.

Yellow starthistle is a problematic invasive plant in the western United States. Attempts to control it often include the use of herbicides although herbicides can have detrimental effects on desired native species. We studied the effect of clopyralid on a native bunchgrass and vernal pool community in the Central Valley of California. Areas invaded by yellow starthistle were treated with clopyralid, and the presence and cover of all species was monitored for 2 yr. There were significant differences in species cover between treated and untreated plots 2 yr after treatment (P = 0.0001). These included a decline in cover of yellow starthistle and other native and exotic members of the Asteraceae and Apiaceae families in both years and a decline of exotic members of the Fabaceae family in the second year. There also were decreases in the frequency of the Asteraceae (P < 0.0001), Apiaceae (P < 0.05), Fabaceae (P < 0.05), and Polygonaceae (P < 0.05) families 1 yr after treatment, and the Asteraceae (P < 0.05) and Fabaceae (P < 0.05) families 2 yr after treatment. We also observed a significant negative effect of clopyralid on fecundity measures of a native Viola species during the first year of application (P = 0.0074). This effect did not persist into the second year, when violets were more common and produced more flowers in treated plots (0.02 < P < 0.05).

Dispersal mechanisms of the alien plant species buffalobur during its invasion of cold desert areas in Xinjiang, northwestern China, were investigated. Seeds and fruits were readily moved by water in irrigation canals in the transition zone between natural desert and a farmed oasis. Maximum flotation time in moving canal water was ∼ 4 h for seeds and > 48 h for fruits, and water moved fruits 279 m in 10 min. Also, 100% of the seeds remained viable during 8 wk of flooding in the laboratory. Mean dispersal distance was 3.4 m by wind-driven rolling of detached plants and 0.5 m by ants. Retention time for 50% of fruits on wool of live sheep was ∼ 4 h. Seeds and fruits that fall into the canals (which are without irrigation water from mid-October to April) are cold-stratified during winter, and then during canal cleaning in spring soil and germinable seeds are deposited along the sides of the canals. The disturbed soil is a highly favorable site for plants to grow. The local spread of buffalobur away from the sides of canals is facilitated by sheep, wind, and ants. We conclude that water in the irrigation canals is the primary dispersal agent for seeds of this invasive species and that the best way to control its spread is to prevent plants growing beside the canals from setting seed.

Patterns of absorption and distribution of dalapon (2,2–dichloropropionic acid) were determined by radioautography and counting. Dalapon was absorbed through the cuticle and through the large open pores of hypostomatous leaves. Stomata were clearly the most expeditious routes of entry, but results were erratic when no surfactant was included. An anionic surfactant, sodium dioctylsulfosuccinate, enhanced both (a) cuticular and (b) stomatal plus cuticular penetration 3- to 4-fold during a 2-hr. absorption period. Studies confirmed the view that dalapon is translocated to regions of high metabolic activity and/or storage in association with and dependent upon the movement of food materials. Dalapon did not move appreciably out of albino leaves or green leaves previously depleted of food reserves, but was transported readily out of normal leaves in the light when a surfactant was used to enhance cuticular penetration.

The effect of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] and alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] combinations on various physiological processes was studied in order to establish a basis for the synergistic effect of this mixture on Japanese millet [Echinochloa crus-galli (L.) Beauv. var. frumentacea L.]. In contrast to atrazine, alachlor had no effect on the Hill reaction activity of isolated Japanese millet chloroplasts. When used in combination with atrazine, alachlor had no influence on the inhibitory effect of atrazine on the Hill reaction. Atrazine and alachlor combinations reduced chloroplast protein and severely inhibited chloroplast protein synthesis relative to protein synthesis by other particulate fractions. This inhibition appears to be the basis for the synergistic effect of this herbicide mixture on Japanese millet. Although the mechanism of action of the herbicide mixture is not understood, it does not seem to involve inhibition of atrazine detoxification.

Flixweed [Descurainia sophia (L.) Webb ex Prantl] is a notorious broadleaf weed that is widely distributed in winter wheat–growing areas of China and has evolved resistance to tribenuron-methyl mainly due to target-site resistance (TSR) mutations in acetolactate synthase (ALS). In the current research, two ALS genes were identified in tribenuron-methyl–susceptible (TS) or tribenuron-methyl–resistant (TR) D. sophia. Resistance mutations of Asp-376-Glu and Pro-197-Ala were identified on ALS1 and ALS2 isozymes in TR D. sophia, respectively. The TR D. sophia evolved 10,836.3-fold resistance to tribenuron-methyl and displayed cross-resistance to multiple ALS-inhibiting herbicides with different chemical structures. Dose response experiments and ALS activity assay indicated that two mutated ALS isozymes contributed differentially in resistance to tribenuron-methyl, flucetosulfuron, and pyribenzoxim. In addition, the relative expression level of the ALS1 gene was 2.2- and 1.6-fold higher than ALS2 genes in TR D. sophia at 1 and 7 d after tribenuron-methyl treatment, respectively. In contrast, the relative expression level of ALS1 and ALS2 in TS D. sophia is similar. This is the first research that explored different roles of ALS isozymes in resistance to ALS-inhibiting herbicides, which might provide a new perspective for the weed resistance management.

The response of four growth stages of Setaria glauca (L.) Beauv. (yellow foxtail), Eleusine indica (L.) Gaertn. (goosegrass), and Dactyloctenium aegyptium (L.) Richt (crowfootgrass) to dimethylarsinic acid (cacodylic acid) applied at rates of 0.25, 0.5, and 1.0 lb/A was studied.

The four growth stages for chemical treatments were generally: (a) early tillering, (b) late tillering, (c) boot, and (d) post-heading. Seed production was reduced by as much as 96% in yellow foxtail, 80% in goosegrass, and 37% in crowfootgrass. Application of the chemical at the “boot” stage was most effective although earlier applications also were effective. Inflorescences often were malformed. Other growth effects suggested a growth regulator type activity.

Mixed stands of big sagebrush (Artemisia tridentata Nutt.) and green rabbitbrush (Chrysothamnus viscidiflorus (Hook.) Nutt.) were sprayed with butyl ester of 2,4-dichlorophenoxyacetic acid (2,4-D) at 1, 2, 3, and 4 lb/A on six phenological dates in 1956, 1957, and 1958 at the Squaw Butte Experiment Station, Burns, Oregon. Data on soil moisture; soil temperature; growth development of herbaceous species; twig elongation of big sagebrush and green rabbitbrush; dry-matter, crude-protein, and total-carbohydrate contents of green rabbitbrush herbage; and crude-protein and total-carbohydrate contents of big sagebrush and green rabbitbrush roots were obtained at weekly to biweekly intervals in all 3 years to evaluate the ecological and physiological conditions that might indicate seasonal patterns of susceptibility to 2,4-D. Big sagebrush, a non-deciduous, non-sprouting plant, was easily killed with 2,4-D at 2 lb/A when soil temperatures and moisture contents were satisfactory for vigorous growth. The development of abundant leaf area on green rabbitbrush, a deciduous, sprouting plant, was important for spray interception and a photosynthetic rate sufficient to promote carbohydrate accumulation. Green rabbitbrush was controlled about 80% with a single application of 2,4-D butyl ester at 3 lb/A after the new twigs were 3 inches long and while soil moisture contents were sufficient for active growth. Differences in susceptibility between the two species are compared with morphological, phenological, and physiological differences, and seasonal patterns of susceptibility are further compared with environmental conditions.

The research worker who attempts the application of herbicides to a small area in such a way as to simulate general practice, is faced with a unique problem. The ultimate objective is to apply the chemical to as small an area as is feasible but in such a way that it will correctly represent what the activity on both weeds and crop plants would have been had the application been made on an acreage basis. Small plots are desirable if land or labor are limited, if expensive crops in farmers’ fields are being used, and particularly when the chemicals may be highly toxic, or the activity is unknown.