Weed resistance surveys that monitor the spread of resistant weeds have mainly been conducted through time-consuming, labor-intensive, and destructive greenhouse herbicide screens. As an alternative, we introduce here a nondestructive leaf-disk assay based on chlorophyll fluorescence (Fv/Fm values that measure photosynthetic efficiency) that allows the detection of resistance to both systemic and contact herbicides within ∼48 h. The current study validated the assay for detecting resistance to fomesafen, glyphosate, and dicamba in Palmer amaranth (Amaranthus palmeri S. Watson), waterhemp [Amaranthus tuberculatus (Moq.) Sauer], kochia [Bassia scoparia (L.) A.J. Scott], and goosegrass [Eleusine indica (L.) Gaertn.]. Negative correlation between Fv/Fm values and spray injury levels was observed in all herbicide–weed combinations at the discriminating doses, except for glyphosate in Amaranthus. The correlation coefficients were −0.41 for fomesafen (10 µM, P < 0.0001) in Amaranthus, −0.92 for glyphosate in E. indica (250 µM, P < 0.0001), and −0.44 for dicamba in B. scoparia (800 µM, P = 0.0023). At the population level, the assay clearly separated susceptible from highly resistant populations. However, the assay showed lower sensitivity in distinguishing populations of different resistance levels or separating populations with low resistance from susceptible populations. At the individual plant level, results from the leaf-disk assay and whole-plant spray tests were concordant in 85.5%, 92.3%, and 71.7% of the plants tested for fomesafen–Amaranthus, glyphosate–Eleusine, and dicamba–Bassia, respectively. The assay yielded 1% to 15% false-positive and 6% to 13% false-negative results across herbicides. The current study demonstrated that the leaf-disk assay is a useful tool to identify weed resistance. Optimization is needed to improve its sensitivities and expand its usage to more diverse herbicide–weed species combinations.

Weedy rice (WR) (Oryza spp.) is the most troublesome weed infesting rice paddies in Brazil. Several changes have occurred in this region regarding crop management, especially WR control based on the Clearfield® (CL) rice production system launched in 2003. This survey’s objective was to evaluate the WR infestation status by assessing the producers’ perception and the management practices used in southern Brazil after 18 yr of CL use in Brazil. Rice consultants and extension agents distributed a questionnaire to 213 producers in the Rio Grande do Sul (RS) and Santa Catarina (SC) states in the 2018 to 2019 growing season. In RS, most farms are larger than 150 ha, and farmers have adopted the CL system for more than 2 yr and use minimal or conventional tillage, permanent flooding, clomazone PRE tank-mixed with glyphosate at the rice spiking stage, and crop rotation with soybean [Glycine max (L.) Merr.] or pasture. In SC, rice farms are small, averaging from 20 to 30 ha, farmers predominantly plant pre-germinated rice and do not rotate rice with other crops, and roguing is practiced. Comparing both states, the CL system is used in 99.5% and 69.3% of the total surveyed rice areas in RS and SC, respectively. Imidazolinone-resistant WR is present in 68.4% and 26.6% of rice farms in RS and SC, respectively. Rice cultivation in Brazil is currently coexisting with WR with minimal integration of control methods. However, integrated practices can control this weed and are fundamental to the sustainability of systems based on herbicide-resistant rice cultivars.

Red sorrel is a common herbaceous creeping perennial weed in wild blueberry fields in Nova Scotia that spreads by seeds and an extensive creeping root system. Experiments were established to determine temperature thresholds for ramet sprouting from creeping root fragments and to develop growing-degree-day (GDD) models for predicting ramet emergence and flowering under field conditions in wild blueberry fields in Nova Scotia. Ramets sprouted from root fragments at temperatures as low as 1 C, with an optimum temperature for ramet sprouting around 22 C. Ramet sprouting was completely inhibited at temperatures above 35 C. Cumulative ramet emergence and flowering under field conditions were adequately explained as functions of GDD by a three-parameter power equation (R2 = 0.98) and a four-parameter logistic equation (R2 = 0.87), respectively. Ramet emergence began between 110 and 265 GDD and continued throughout the season at each site. Model prediction for the initiation of emergence was 92 GDD, and 50 and 95% emergence were predicted to occur at 1,322 and 2,696 GDD, respectively. Red sorrel ramets began to flower in the field between 308 and 515 GDD. Model prediction for the initiation of flowering was 289 GDD, and 50 and 95% flowering were predicted to occur at 545 and 1,336 GDD, respectively. Model validation was conducted with the use of two additional independent data sets for emergence and flowering and indicated good performance of the proposed models (R2 and root-mean-square error values ranging from 0.96 to 0.99 and 4.0 to 13.8, respectively). The models allow for direct comparison of red sorrel phenology to that of the wild blueberry and will aid in the development of new management strategies.

Millions of acres in the United States are being chemically treated each year for the control of weeds. Weed investigators have generally accepted the fact that certain herbicides affect certain plants to a greater degree than others. Because of this, numerous chemicals are being used. Except for 2,4-D, there has been little work done on the bactericidal and bacteriostatic properties of the recent herbicidal compounds. It is the purpose of this paper to report a study of the influence that several recommended and experimental herbicides have on the microbial activity of the soil population and to establish the usefulness of a simple procedure outlined by Nelson in making preliminary determinations of this type.

Astragalus miser Dougl., var. oblongifolius (Rydb.) Cronq. was analyzed for seasonal and physical distribution of its contained poison. Small chickens (Gallus domesticus, Arbor Acre X Vantress hybrids) and cattle (Bos taurus, Hereford) were used as test animals. The poisonous compound was extracted from dried plants with 95% ethanol, evaporated and redissolved in water, then semi-purified by several extractions with chloroform for toxicological studies. The poison was found primarily in the green leaves. The plants were very poisonous until they began drying in August. Once leaves became bleached at senescence, toxicity disappeared. Plants treated with the triethylamine salt of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and the alkyl amine salts of 2-(2,4,5-trichlorophenoxy)-propionic acid (silvex) were less poisonous to cattle than untreated plants. Acute poisoning in chickens was characterized by depression, accelerated heart action, followed by congestion of lungs, cardiac insufficiency, paralysis, and heart failure. Chronically poisoned birds exhibited the same syndrome but sustained more severe liver damage.

We examined the relationship between temperature and emergence of yellow nutsedge tubers to generate predictive models for the arid southwestern United States. Field experiments were conducted in California and Arizona to obtain phenological and temperature data needed to generate degree-day models. The effect of air temperature on emergence was tested with available programs using four methods to calculate degree-days (single sine, double sine, single triangle, and double triangle). Separate models were tested for each genotype examined (Arizona source and California source) as no one model was a good predictor of emergence when data were pooled. Results indicate that there is year to year variation in model accuracy but predictions of date of emergence can be made to within 2 d of actual emergence. This information can be used to schedule cultivations to reduce early yellow nutsedge competition in the field.

Organic crop production is often limited by the inability to control weeds. An 18-yr data set of weed cover in organic crop rotations at the long-term Farming Systems Project at Beltsville, MD, was analyzed to identify meteorological and management factors influencing weed abundance. A path analysis using structural equation models was employed to distinguish between the direct effect of factors on weed cover and the indirect effect on weed cover through effects on crop competitiveness. Grain yield of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] served as a surrogate for crop competitiveness and was found to be the most important factor influencing weed cover. Precipitation during late vegetative and early reproductive crop growth had a strong positive effect on crop yield, and thereby a negative indirect effect on weed cover, but this effect was partially offset by a positive direct effect on weed cover. Delayed crop planting date and crop rotational diversification including crops other than summer row crops had a moderate negative effect on weed cover, while having minimal effect on crop performance. Rotary hoeing also had a direct negative effect on weed cover, but a corresponding negative effect on crop performance resulted in a diminished total effect on weeds. Results demonstrate the complex interactions that define the relative abundance of weeds faced by organic growers, but, generally, factors that enhanced crop competitiveness provided the most effective weed management.

Field and laboratory studies were conducted to examine herbicide dissipation when applied to low density polyethylene (LDPE) mulch for dry scenarios vs. irrigation. Analytical chemical analysis was used for quantification. In field studies, halosulfuron, paraquat, carfentrazone, glyphosate, and flumioxazin were surface applied to black 32-μm-thick (1.25 mil) LDPE mulch. LDPE mulch harvest began 1 h after treatment (HAT) then continued every 24 h for five consecutive rain-free days after treatment (DAT) to determine the level of herbicide dissipation from the LDPE mulch surface. In a related study, treated LDPE mulch was harvested 1 HAT, then sprinkler irrigation was applied, followed by a sampling five HAT, then the same irrigation and sampling procedure was repeated every 24 h for five consecutive DAT. The order for half-life, as defined as time for 50% dissipation (DT50), varied by herbicide and method of dissipation for dry and irrigated studies. Data indicated that glyphosate and paraquat dissipation was rapid following irrigation. Glyphosate and paraquat DT50 were both 1 h in the irrigated study, but 84 and 32 h for the dry scenario, respectively. This indicated that glyphosate and paraquat could be removed from LDPE mulch with rainfall or irrigation, primarily due to their high water solubility. Halosulfuron and flumioxazin DT50 were 3 and 6 h in the irrigated study, and 18 and 57 h for the dry study, respectively. Carfentrazone DT50 was similar at 28 and 30 h for the irrigated and dry studies, respectively. This indicated that carfentrazone was adsorbed to the LDPE mulch, and irrigation water did not remove it from the LDPE mulch. Results from 14C-herbicide laboratory studies were similar to those from field studies for halosulfuron, glyphosate, paraquat, and flumioxazin.

Herbicide-resistant (R biotype) and -sensitive (S biotype) individuals were identified from the same population, and seed was increased for each biotype for three generations. We conducted laboratory experiments to determine the effects of temperature, light, salt stress, osmotic stress, pH, and burial depth on the germination and emergence of resistant and sensitive biotypes of Japanese foxtail. The results revealed that there was no difference in the final germination rate between the two biotypes under different temperature conditions, but time to obtain 50% germination or emergence (tE50) and mean germination time of the R biotype were higher than that of the S biotype at 10 C and 15/10 C 12-h day/night regime. In dark conditions, the final germination rate of the S biotype was higher and lower than that of the R biotype at 10 and 25 C, respectively. The overall germination rate of the R biotype was lower than that in the S biotype, and extended germination time was required in extreme conditions, such as 250 mM NaCl and −0.4 MPa osmotic potential. The change in environmental pH had no effect on the germination of the two biotypes. Emergence of the R biotype was lower than the S biotype when seed was buried at least 8 cm deep in an organic matter substrate. This study demonstrated the pleiotropic effects of a resistance allele on seed germination and emergence under different environmental conditions. Deep tillage could be used to reduce the growth and spread of resistant Japanese foxtail individuals.

Toxicity of soil-applied 2-(tert-butylamino)-4-(ethylamino)-6-(methylthio)-s-triazine (terbutryn) to cucumbers (Cucumis sativus L. ‘Gemni’) and oats (Avena sativus L. ‘Nodaway’) was increased 2.2 times when the light intensity was raised from 11,000 to 22,000 lumens/m2 and 1.5 times when the moisture level was raised from low (25 to 50% field capacity) to high (75 to 100% field capacity). When the temperature was raised from the low level (19 C day, 14 C night) to the high level (29 C day, 24 C night) toxicity of terbutryn to oats was increased 1.6 times and toxicity to cucumber was increased 1.3 times. Analysis of interactions of the three variables on toxicity of terbutryn showed that environmental conditions favoring rapid growth resulted in an increase in phytotoxicity.

Of 15 pesticides evaluated in a screening test with respect to their effects on growth and nitrogen assimilation of the Azolla mexicana (Presl) – Anabaena azollae (Strasburger) symbiosis, the bipyridilium and phenolic herbicides at 0.1 ppmw were the most detrimental, causing up to a 75% reduction in nitrogen fixation and nitrate reduction with little or no effect on growth. Chloramben [3-amino-2,5-dichlorobenzoic acid] at 1.0 ppmw, and dicamba [3,6-dichloro-o-anisic acid], and benomyl [methyl-1-(butylcarbamoyl)-2-benzimidazolyl carbamate] at 10.0 ppmw caused an 84 to 99% reduction in nitrogen fixation without affecting nitrate reduction or growth. Simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] at 10.0 ppmw stimulated nitrate reduction 20 fold, causing a 99% reduction in nitrogen fixation. Growth and nitrogen assimilation were reduced at similar concentrations between 0.1 and 10 ppmw for each of the other benzoic, triazine, dinitroanaline, and urea herbicides tested. Naptalam [N-1-naphthylphthalamic acid] was the only pesticide tested that had no effect on growth or nitrogen assimilation at 10.0 ppmw.

Oxyfluorfen [2 - chloro -1 - (3 - ethoxy - 4 - nitrophenoxy) -4 - (trifluoromethyl)benzene] and RH-8817 {ethyl 5 - [2-chloro - 4 -{trifluoromethyl) phenoxy] - 2 - nitrobenzoate} require light for activation. Removal of carotenoids by treating corn (Zea mays L.) seedlings with norflurazon [4 - chloro - 5 - (methylamino) - 2 - (α, α, α - trifluoro -m - tolyl) - 3 (2H) - pyridazinone] rendered them partially tolerant of oxyfluorfen and RH-8817 even when grown in the light. The activation of substituted diphenylether herbicides may occur as a result of the absorption of light energy by carotenoids.

We investigated the forces involved in the adsorption and desorption of 4-amino-3,5-6-trichloropicolinic acid (picloram), 1,1-dimethyl-4,4′-bipyridinium ion (paraquat), and α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) using cationic, anionic, and nonionic exchange resins. The anionic resin (Cl-form) adsorbed 375, 0.08, and 0.67 mg of picloram, paraquat, and trifluralin, respectively, per gram of oven-dry resin. The nonionic resin adsorbed 4.0, 0.34, and 10.0 mg of picloram, paraquat, and trifluralin, respectively, per gram of oven-dry resin, while the cationic resin (H-form) adsorbed 2.3, 226, and 0.17 mg of picloram, paraquat, and trifluralin, respectively. Other cationic resins (Na-form and Ca-form) performed similarly to the hydrogen form. Desorption studies indicated that picloram was adsorbed mainly in the anionic form by coulombic forces (electrostatic) and to a lesser degree by weak physical bonding (van der Waal's forces). Paraquat was adsorbed as a cation through coulombic forces. Trifluralin was mainly absorbed by physical bonds at sites on the resins where there were no coulombic forces.

Interspecific hybridization of Palmer amaranth and common waterhemp produce hybrids with unique DNA fragments not found in either parent. The objective of this research was to investigate the mechanisms involved in the formation of the polymorphic fragments. Six novel fragments were cloned and sequenced. Five of the six were significantly similar to plant transposons, the sixth was similar to squamosa promoter–binding proteins from other plant species. Southern blot analysis using one of the novel fragments as probe revealed a consistent pattern of repetitive DNA that was species and biotype specific. These results indicate that transposon-like elements may play an important role in the formation of new fragments in Amaranthus hybrids derived from interspecific hybridization, suggesting that considerable instability of the hybrid genome may occur.

Sicklepod (Cassia obtusifolia L.) leaves and leaflets changed their orientation throughout 24-h periods. These endogenously controlled rhythmic movements resulted in dramatic changes in the percent projected leaf area (PPLA), the leaf area observed from directly above and expressed as a percentage of the total actual leaf area. The PPLA of sicklepod varied more than seven-fold throughout a 24-h period, being greatest during the day and least at night. Control of sicklepod with linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] applied postemergence at intervals throughout 24-h periods was greater from daytime applications than from late evening, night, and early morning applications. Percent control was highly correlated with PPLA, suggesting that rhythmic leaf movements and the resulting change in PPLA could be an important factor in determining the efficacy of postemergence herbicide applications on sicklepod by altering the area of the leaf surface most likely to intercept and retain herbicide spray from an over-the-top application. In addition to endogenously controlled leaf movements, we observed leaf movements caused by simulated cloud cover (80% shading). Shading increased the PPLA of sicklepod, suggesting that more leaf surface might be exposed to an over-the-top spray during cloudy periods than during sunny periods.

Research was conducted from 1983 through 1986 to compare the competitiveness of three cotton cultivars of different height with johnsongrass. Cultivars evaluated were: ‘1209-619-7’, ‘TAMCOT SP37-H’, and 'Stoneville 213’, which had an average height at maturity of 66, 122, and 168 cm, respectively. Each cultivar was grown with johnsongrass densities of 0, 4, and 6 plants/6 m of row with a 1-m row spacing. Johnsongrass biomass data confirmed that all three cultivars did compete with johnsongrass, but regression analyses using cultivar as a covariate revealed no difference in the competitiveness of the three cultivars.

Tobacco (Nicotiana tabacum L. ‘X-73’) callus tissue cultures were used in a bioassay system for determining the effect of the following substituted 2,6-dinitroaniline herbicides on growth: trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine); oryzalin (3,5-dinitro-N4,N4-dipropylsulfanilamide); benefin (N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine); AC 92390 (N-sec-butyl-2,6-dinitro-3,4-xylidine); penoxalin [N-(1-ethylpropyl)-2,6-dinitro-3,4-xylidine]; GS-38946 (N-ethyl-N-tetrahydrofurfuryl-4-trifluoromethyl-2,6-dinitroaniline); chlornidine [N,N-di(2-chloroethyl)-4-methyl-2,6-dinitroaniline]; nitralin [4-(methylsulfonyl)2,6-dinitro-N,N-dipropylaniline]; dinitramine (N4,N4-diethyl-α,α,α-trifluoro-3,5-dinitrotoluene-2,4-diamine); isopropalin (2,6-dinitro-N,N-dipropylcumidine), and profluralin [N(cyclopropylmethyl)-α,α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine]. The molar concentration required to inhibit fresh weight gain by 50% (I50) was determined by using linear regression analysis on data from a range of five concentrations of each chemical. I50 values did not correlate with structural variations or available physical constants. Herbicides listed in order of increasing activity are AC 92390< GD-38946<profluralin = isopropalin<benefin = chlornidine = trifluralin = nitralin<oryzalin = penoxalin < dinitramine. Exogenously applied D-α-tocopherol acetate at 100 times the I50 concentrations decreased the inhibition of tissue growth by the herbicides.

Texas panicum was grown in controlled-environment chambers in 16 day/night temperature regimes ranging from 19/11 to 34/26 C. After 33 days, maximum plant height, leaf area, and dry weight occurred at 34/26 C. The greatest number of tillers and leaves occurred at 34/16 C. Partitioning of plant biomass into stems increased with increasing night temperature, while partitioning into leaves tended to decrease. Partitioning of biomass into roots decreased with increasing night temperature at day temperatures of 19 or 24 C but not at 34 C. Plant dry matter production (DMP) was highly correlated with both leaf area duration (LAD) and net assimilation rate (NAR) during the first 2 weeks of growth. During the subsequent 3 weeks, DMP was highly correlated with LAD but not with NAR. The low-temperature threshold for growth of Texas panicum was approximately 11 C. Texas panicum attained 25% of its maximum growth at an average daily temperature of 21 C, 50% at 24 C, and 75% at 26 C. Texas panicum is unlikely to become a highly competitive weed outside the southern and southwestern United States.

Single over-the-top and directed applications of dicamba (3,6-dichloro-o-anisic acid) were made to cotton (Gossypium hirsutum L. ‘Deltapine 16′). Dicamba applied over-the-top of cotton caused foliage malformation for about 6 weeks. Dicamba (32 g/ha) applied to cotton foliage 3 to 15 weeks after emergence reduced average cotton yields. Applications of 16 to 64 g/ha of dicamba over-the-top of cotton before bloom had less effect on cotton foliage, yield, boll components, and fiber properties than application at bloom. Directed applications of dicamba to the soil or to the base of cotton and soil caused foliage malformation but did not affect cotton yield, boll components, and fiber properties.

Several populations of smooth pigweed with resistance to imidazolinone (IMI) herbicides have been identified in recent years. One IMI-resistant population (R2) was 10-fold more sensitive to cloransulam-methyl in the greenhouse when compared with a susceptible (S) population. Laboratory studies were conducted to determine if differences in the absorption, translocation, and metabolism of cloransulam-methyl existed between S and R2 populations and to determine if these differences could account for the whole-plant responses observed in the greenhouse. Enzyme assays were also conducted to determine if differences in acetolactate synthase (ALS) sensitivity to cloransulam-methyl existed between S and R2 populations. Absorption of foliar-applied cloransulam-methyl was rapid and similar in both populations of smooth pigweed. Translocation of the absorbed radioactivity out of the treated leaf was symplastic and generally similar in both populations. Translocated radioactivity was detected primarily in shoots above and below the treated leaf with little movement to the roots. 14C-cloransulam-methyl metabolism was also similar in S and R2 populations. Differential tolerance to cloransulam-methyl in S and R2 populations in the greenhouse cannot be explained by differences in the absorption, translocation, and metabolism of this herbicide. However, ALS from R2 was 25-fold more sensitive to inhibition by cloransulam-methyl than ALS from S.