A synergistic interaction occurred when sugarbeets (Beta vulgaris L. ‘Mono-Hy Al’) were treated with mixtures of ethofumesate (2-ethoxy-2,3-dihydro-3,3-dimethyl-5-benzofuranyl methanesulphonate) and desmedipham [ethyl m-hydroxycarbanilate carbanilate (ester)]. Depending on the stage of weed growth synergistic interactions were also observed on two weed species: wild mustard [Brassica kaber (DC.) L.C. Wheeler ‘pinnatifida’ (Stokes) L.C. Wheeler] and wild oat (Avena fatua L.). Desmedipham penetrated the foliage more slowly than did ethofumesate. The rate of desmedipham penetration was positively correlated with the concentration of its formulants (solvents and adjuvants) in the spraying emulsion, and to a lesser extent with the formulants of ethofumesate. Increasing the spray volume also increased desmedipham penetration. None of these factors affected penetration by ethofumesate. 14C-labeled ethofumesate and desmedipham did not translocate out of treated leaves regardless of the concentration of formulants or active ingredients. These data suggest that the synergistic interaction is mainly due to the increased penetration by desmedipham when applied with ethofumesate.

Wild oat (Avena fatua L.) was removed at several stages of growth from flax (Linam usitatissimum L.) plots in 1964, 1965, and 1966. The flaxseed yield was reduced as the period of competition increased during each year. In 1965, competition from wild oat until the 4 to 5-leaf stage reduced the flaxseed yield 15.1% while competition until maturity reduced the yield 74.9%. The number of bolls/sq ft also was reduced as the period of competition increased; but the number of seed/boll, plants/sq ft, and the weight/1000 seed were not affected consistently by wild oat competition. Both the percent oil of flaxseed and the iodine number of the oil were reduced by wild oat competition, but not until the wild oat was in the heading stage and the flax was in the boll stage.

Puccinia obtegens (Link) Tul., an autoecious rust pathogen, is a potential biological control agent of Canada thistle [Cirsium arvense (L.) Scop.]. Ten ecotypes of Canada thistle were inoculated with uredospores of P. obtegens and sporulation was observed on all ecotypes. Infection types varied among and within ecotypes, indicating that host-resistance is one factor limiting rust infection. No correlation was found between Canada thistle susceptibility to the rust and host plant ecotype classification, stomatal density, amount of leaf pubescence, or spore germinability on leaf surfaces.

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.

Because all agricultural chemicals applied to plants eventually reach the soil, and because many herbicides are applied directly to the soil for absorption through plant roots, it is desirable to test the persistence of these reagents in soils of varied types. Many such tests have already been conducted. Additional studies will be necessary as new phytotoxic chemicals are discovered. In this paper tests on some of the herbicides which have been studied during the past decade are described. Most of these herbicides are already in extensive use and much information has already been obtained relative to their reaction in soils. The data included here are submitted in an effort to round out the available information, particularly as related to long time effects of the herbicides in soils.

In the greenhouse, picloram (4-amino-3,5,6-trichloropicolinic acid) prevented germination of side-oats grama [Bouteloua curtipendula (Michx.) Torr.,] big bluestem (Andropogon gerardi Vitman), switchgrass (Panicum virgatum L.) and blue grama [Bouteloua gracillis (HBK) Lag.] when applied preemergence. When applied at 1.5 lb/A or more at the two-leaf stage in the greenhouse and field, picloram significantly reduced plant numbers of all species. Of the four species big bluestem appeared to be the most tolerant to picloram in the seedling stages. Germination was reduced when 2,4-D (2,4-dichlorophenoxyacetic acid) was applied preemergence to side-oats grama and the plant numbers of this species and switchgrass were reduced when treated at the two-leaf stage.

The application of picloram to established native range did not reduce forage production or desirable plant frequency. All treatments reduced forb production. Picloram gave very good control of western yarrow (Achillea lanulosa nutt.), white heath aster (Aster ericoides L. nutt.), and western ironweed (Vernonia baldwini Torr.). White sage (Artemisia ludoviciana nutt.) was resistant to 4 lb/A of picloram.

Five rates of 1-(2 methylcyclohexyl)-3-phenylurea (siduron) were applied to Cohansey and IaGreen cultivars of creeping bent-grass (Agrostis palustris). Siduron had no effect upon leaf respiration or photosynthesis of either cultivar. Cohansey was tolerant to 100 lb/A of siduron while top growth of IaGreen was reduced 90% at 1.5 lb/A. Siduron inhibited new root growth of the IaGreen cultivar and decreased root respiration.

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.

Genetic engineering is a powerful new technology that makes possible rapid development of herbicide tolerances in superior crop varieties. Use of the technology will have a major impact on future agricultural practices and the agrichemical industry. Possible risks as well as significant likely benefits – to agricultural users, corporate suppliers, and the environment – are discussed. Increased product lifetimes, decreased costs, and increased usefulness of broad-spectrum, high-potency, environmentally safe herbicides and herbicide combinations are predicted.

Under certain conditions, application of glyphosate to glyphosate-resistant (GR) cotton can lead to fruit shedding and yield reductions. Field studies were conducted at the Texas Agricultural Experiment Station using GR cotton, cv. ‘DeltaPine 5690RR’, to determine if application method and timing affect cotton fruit retention. Glyphosate at 1.12 kg ai ha−1 was precisely postdirected (PD), postdirected with 25% foliage coverage (PDFC), or applied over the top (OT) at the 8- or 18-leaf stage after an initial topical application of 1.12 kg ha−1 glyphosate at the four-leaf stage. In one of the years of this study, 8 PD, 18 PDFC, and 18 OT reduced yield. In 1999 and 2000, 8 PDFC and 8 OT applications of glyphosate caused yield loss, mainly due to lower mean boll weight. Glyphosate applied topically at the eight-leaf stage also affected the Position 1 boll retention throughout the plant in both years. Glyphosate contact with leaves and stems should be avoided when applying glyphosate after the four-leaf stage to prevent possible yield loss.

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.

Several investigators have now reported that phenylurea herbicides inhibit the photochemical activity (Hill reaction) of isolated chloroplasts. Many of the phytotoxic symptoms in plants treated with phenylurea herbicides can be explained on the basis that the herbicides have blocked the photosynthetic mechanism. This action has been suggested as the primary cause of toxicity under field conditions. The metabolic product of biochemical pathways containing the primary site of action of a herbicide can be applied to barley plants in sufficient quantity to eliminate completely some of the toxic symptoms produced by the herbicide. Geoghegan has reported that glucose applications caused an increase in the concentration required for three phenylurea compounds to inhibit growth of Chlorella vulgaris. Therefore, a determination of the effect of carbohydrates on the inhibition of growth of barley seedlings in the presence of several phenylurea herbicides seemed of interest.

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.

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.

Growth of an agricultural weed population and a coastal nonweed population of poorjoe was compared in a competition experiment with soybean and when grown alone. Mean aboveground biomass production of the weed population was roughly twice that of the nonweed population, whether grown alone or with soybean. A path-coefficient model of growth was fitted to the data in order to compare the populations with respect to five independent measurements of weed growth: establishment rate, early and later growth rates, growth form, and final growth rate. These measurements indicated plant growth over successive intervals of the growing season by statistically equating the size of all plants at the beginning of an interval of growth using analysis of covariance. In general, growth measurements were positively associated with aboveground biomass production. The weed population of poorjoe had a greater establishment rate and a greater early aboveground growth rate compared to the nonweed population; however, the final growth rates of the two populations were similar whether soybean was present or absent. Genetic changes leading to earlier establishment and faster early growth, but not increasing tolerance of soybean and intraspecific competition, may have occurred in the adaptation of poorjoe to an agricultural ecosystem.

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.