Alfalfa (Medicago sativa L.) was killed or severely injured by parasitism from dodder (Cuscuta campestris Yunck. and C. indecora Choisy) when seeded August 1 or earlier in soil containing dodder seed. Alfalfa seeded August 15 or September 2 escaped injury from dodder, and became well-established before winter. Isopropyl m-chlorocarbanilate (chlorpropham) applied at 6 lb/A the following April controlled dodder effectively and did not injure alfalfa. A preemergence application of dimethyl tetrachloroterephthalate (DCPA) at 10 lb/A, followed by sprinkler irrigation, controlled 98 to 100% of the dodder at each of six dates of seeding in each of 2 years. DCPA often injured alfalfa seedlings severely, but most of them recovered and grew satisfactorily.

Interference between redstem filaree and round-leaved mallow was studied under controlled environmental conditions. Each species was grown in monoculture at densities of 2, 4, 8, and 12 plants per 20-cm-diam pot and in mixtures at all possible combinations of these densities. Leaf area per plant was similar for both species but round-leaved mallow grew taller and produced more shoot biomass than redstem filaree when each was grown in monoculture. Mixed culture responses varied with the proportion density of each species. A reciprocal yield model was tested and modified to account for this significant density interaction. When grown in mixture, round-leaved mallow usually gained in leaf area and shoot biomass at the expense of redstem filaree indicating that it was the superior competitor. Calculated competition ratios indicate that round-leaved mallow was about twice as competitive as redstem filaree under the growing conditions of this study.

Metabolism of the herbicide 2-(α-naphthoxy)-N,N-diethyl propionamide (R-7465) by tomato plants (Lycopersicon esculentum Mill. ‘Mechanical Harvester’) was investigated. Ring-labeled R-7465-14C was taken up rapidly by the roots and distributed throughout the leaves within 8 hr. R-7465 was converted primarily into water soluble metabolites. The principal metabolite was identified as a hexose conjugate of 2-(α-naphthoxy-4-hydroxy)-N,N-diethyl propionamide. This metabolite represented 47% of the soluble radioactivity in the plant. A different hexose conjugate of 2-(α-naphthoxy-4-hydroxy)-N,N-diethyl propionamide together with a hexose conjugate of 2-(α-naphthoxy-4-hydroxy)-N-ethyl propionamide accounted for another 22% of the radioactivity. Nonmetabolized R-7465 was found to represent only 5% of the total soluble radioactivity. Other identified metabolites included 2-(α-napthoxy)-N-ethyl propionamide, 2-(α-naphthoxy-5-hydroxy)-N,N-diethyl propionamide, and 1,4-naphthoquinone. None of these metabolites individually represented more than 3% of the soluble radioactivity in the plants. Ring hydroxylation and N-dealkylation appeared to be the initial steps in detoxification of R-7465 followed by conjugation with sugars.

When applied in early February, simazine [2-chloro-4,6-bis(ethylamino)-s-triazine], pronamide [3,5-dichloro-(N-1,1-dimethyl-2-propynyl)benzamide], secbumeton [N-ethyl-6-methoxy-N′-(1-methylpropyl)-1,3,5-triazine-2,4-diamine], and metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] effectively removed weedy Bromus species from established alfalfa (Medicago sativa L.) and increased yields of alfalfa forage. Diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], terbacil (3-tert-butyl-5-chloro-6-methyluracil), and hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine- 2,4(1H,3H)-dione] had no adverse effects on yields of alfalfa when applied in late March. Paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) was somewhat less effective for controlling weeds when applied in late March than when applied in February. When applied to weed-free stands of alfalfa after dormancy was broken, metribuzin, terbacil, hexazinone, and simazine, but not pronamide, significantly reduced yields of alfalfa. Terbacil applied immediately after the first cutting did not significantly reduce yields of the second cutting. Terbacil and bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] applied when alfalfa had 5 cm of regrowth significantly reduced yields.

Combinations of herbicides as single or separate applications to the soil were compared in furrow-irrigated cotton (Gossypium hirsutum L.). In seven tests, preplanting applications of four herbicides, primarily effective for grass control, were combined with postemergence applications of 3-(3,4-dichlorophenyl)-l,l-dimethylurea (diuron). Preplanting applications of 4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline (nitralin) and α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) before furrowing retarded growth of cotton and sometimes reduced stands. Herbicides applied after furrowing did not appear to affect cotton development. Combinations of the four herbicides with diuron gave satisfactory control of annual weeds until harvest. In 15 tests, preplanting applications of O,O-diisopropyl phosphorodithioate S-ester with N-(2-mercaptoethyl) benzenesulfonamide (bensulide) were combined with preplanting or postemergence applications of four herbicides primarily effective for control of broadleaf weeds. Preplanting applications of 3-(p-chlorophenyl)-l,l-dimethylurea (monuron) usually caused temporary chlorosis and stunting of cotton seedlings. Postemergence applications of monuron and 2,4-bis(isopropylamino)-6-(methylthio)-s-triazine (prometryne) sometimes caused temporary chlorosis of cotton foliage. Combinations of herbicides controlled weeds until harvest in 14 of the 15 tests. There was no difference among the herbicide combinations in yield of cotton in 21 of the 22 tests. In the 15 tests sampled there were no differences in fiber properties and boll components among herbicide combinations.

WSSA members were surveyed in 1993–1994 to determine their perceptions of the contribution of weed biology to weed management. A questionnaire was included in the society newsletter, from which 152 responses were returned by mail or collected at the 1994 annual meeting. Over half the respondents felt that the overall contribution of weed biology to weed management had been moderate to high. Aspects of population dynamics and competition emerged as the areas that respondents felt should have the greatest impact on weed management in the future. The areas of computer modeling, interactions between weeds and other pests, and seedbank dynamics were predicted to show the greatest increases in importance in the future. The relative importance of taxonomy and weed identification was expected to decrease. Allelopathy, morphology and anatomy, and genetics and evolution were considered least likely to be important to weed management.

The plant pathogenic fungus Albugo tragopogi (Pers.) S. F. Gray (“white rust”), isolated from a local population of common ragweed (Ambrosia artemisiifolia L.) and inoculated into ragweed seedlings at the two-leaf stage, reduced pollen production by 99%, seed production by 98%, and top weight by 79% of plants that developed systemic disease symptoms while growing in a controlled environment. However, only 14% of the inoculated plants developed systemic symptoms. A convenient method of inoculating leaves of seedling plants with zoospores of a fungus is described.

Field studies were conducted in Colorado and New Mexico to examine growth and development of spurred anoda accessions from Colorado, Mississippi, New Mexico, and North Carolina. Accessions were grown in a common garden in both locations. Leaf and stem weight ratios, leaf area ratio, and leaf area per plant were determined. North Carolina and Mississippi accessions were similar in morphology and appearance. Leaf shape, degree of pubescence, and coloration were different in the Colorado and New Mexico accessions. At both sites, the Colorado accession was taller and the canopy wider than other accessions. In Colorado in 1993, North Carolina and New Mexico accessions had the highest total weight followed by Colorado, then Mississippi. In 1994, final total weights of North Carolina, Mississippi, and New Mexico accessions were similar and greater than the Colorado accession. All accessions had similar final total weights in New Mexico in 1993. In 1994, Mississippi, North Carolina, and New Mexico accessions had similar total weights. The Colorado accession flowered earlier than other accessions. Seeds from Mississippi and North Carolina accessions grown in Colorado were heavier than the New Mexico or Colorado accession. This research demonstrates why making inferences of weed biology for variable species that occur over diverse geographic and climatic conditions is difficult and why regionally specific data are needed for variable species.

PALWEED:WHEAT is a bioeconomic decision model for determining profit-maximizing postemergence herbicide treatments for winter wheat in the Washington–Idaho Palouse region. PALWEED:WHEAT performed relatively well economically in 2 yr of on-farm field tests. However, the model was less sensitive than desired in prescribing postemergence broadleaved herbicides in the presence of high densities of broadleaved weed seedlings. Therefore, PALWEED:WHEAT was revised in response to the field testing. This paper compares the revised model's agronomic and economic performance to the original model in computer simulations. The revised model, PALWEED:WHEAT II, differs from the original model in several respects: (1) exponential functions replace linear functions in predicting weed survival, (2) preplant application of a nonselective herbicide is entered as an exogenous binary variable, (3) separate indices of broadleaved and grass competition are substituted for an aggregate weed competition index in the wheat yield function, (4) hyperbolic replaces logistic functional representation of weed damage to wheat yield, and (5) separate models are estimated for winter wheat after spring dry pea and for winter wheat in all examined crop rotation positions. In simulations including a variety of agronomic and economic conditions, PALWEED:WHEAT II recommended postemergence herbicide types and rates that consistently complied with agronomic and economic theory. Furthermore, the revised model, especially when estimated from the relevant wheat after pea data set, was markedly more balanced in recommending both broadleaved and grass herbicides in response to observed densities of both weed groups. The substantial change in herbicide recommendations in response to changes in model functional specifications following field testing confirms the importance of field testing and revision of bioeconomic decision models.

Research was conducted on the leachability and dissipation of sodium azide applied to soil as a solution or as granules with various coatings. The sodium azide leached readily from all formulations, leaving sections of the soil profile with insufficient concentrations to stimulate germination of dormant wild oat (Avena fatua L.) seed. Sodium azide-impregnated clay granules coated with 5% (w/w) polyethylene glycol increased germination of wild oat seed two-fold over uncoated granules at a rate of 5.6 kg sodium azide/ha in field experiments. The polyethylene glycol coating reduced the rate of sodium azide dissipation from the soil in growth chamber experiments. Formulation of sodium azide has potential for increasing wild oat germination stimulation under field conditions.

Dormancy of spurred anoda [Anoda cristata (L.) Schlecht] seeds was not broken 60 days after harvest by stratification, presoaking in tap water, or by treatment with thiourea, potassium nitrate, or kinetin. Dormancy was broken by scarification or treatment with concentrated sulfuric acid. A high percentage of seedling emergence was obtained when scarified seeds were planted in soil at depths of 7.5 cm or less. Emergence decreased as planting depth increased. No seedlings emerged at depths of 12.5 cm or greater. Even at depths of 15.0 cm germination occurred, but no seedlings emerged.

Two clones of field bindweed (Convolvulus arvensis L.) which differed in their susceptibility to (2,4-dichlorophenoxy)acetic acid (2,4-D) under field and greenhouse conditions also exhibited similar differences when stem cells were cultured in liquid and agar media. Amino acids added to the culture media altered the response to 2,4-D. Glutamic acid increased the tolerance of the susceptible (S) clone, but reduced the tolerance of the resistant (R) clone. Glutamine increased the susceptibility of the S clone to a much greater degree than it did the R clone. No significant differences were noted in the rates of absorption of metabolism of 2,4-D by the two clones. Glutamine increased and glutamic acid decreased 2,4-D absorption by both clones. Levels of nitrate reductase activity (NRA), soluble protein (SP), and gross RNA (GR) increased in the S tissues but decreased or remained constant in the R tissues exposed to 4.5 × 10−5M 2,4-D. Correlations between 2,4-D susceptibility and NRA demonstrated a relationship between the effects of 2,4-D and nitrogen metabolism. Differential binding of 2,4-D within the cells appears to be the most likely explanation for the differences in response to 2,4-D.

Understanding the factors regulating recruitment in diverse tillage systems will improve the efficacy of weed-management strategies. Experiments measured the effect of hairy nightshade winter seed position (burial depth) on seedling recruitment and seed germination, dormancy, and mortality. Hairy nightshade seeds were placed in soil tubes at 1 cm below the soil line and buried in the fall so that seeds were positioned in the soil at 1, 6, 13, and 25 cm below the soil surface. Tubes with undisturbed soil and seeds were removed from the field in March, April, and May of 2004 and 2005 and placed in wells in a heated, aluminum block with a linear temperature gradient from 22.7 to 36.0 C at 2.7 C intervals, and recruitment was measured. Seeds were also recovered from the soil tubes to determine effects of seed position on germination rate, mortality, and seed dormancy. Hairy nightshade seedling recruitment was greater for seeds positioned at 6, 13, and 25 cm during the winter than at 1 cm. Seed dormancy and mortality were greatest for seeds positioned at 1 cm, but that did not adequately account for the significant decrease in recruitment at 1 cm compared to seeds buried at 6, 13, or 25 cm. Protecting the seeds buried at 1 cm from rainfall during the winter increased seedling recruitment from 0 to 20% of buried seeds but had a negligible effect on seed mortality and dormancy. Soil density was negatively correlated with recruitment.

The effect of soil temperature, soil moisture content, and soil bulk density on the diffusion of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], propachlor (2-chloro-N-isopropylacetanilide), and diazinon [O,O-diethyl O-(2-isopropyl-6-methyl-4-pyrimidinyl) phosphorothioate] in Ida silt loam soil was studied in the laboratory. Diffusion coefficients of 1.90, 1.36, and 0.63 mm2/day at 27 C for propachlor, atrazine, and diazinon, respectively, show in a relative way the expected movement of these chemicals in Ida silt loam soil. The greatest amount of movement occurred with high temperatures and high moisture contents. Soil moisture had more effect on atrazine movement than on propachlor movement, and very little effect on diazinon movement. Movement for all chemicals decreased with an increase in bulk density.

Monuron, 3–(p–chlorophenyl)–1,1–dimethylurea, is used extensively for weed control in asparagus. Growers apply approximately 1.5 lb/A of monuron in the spring before harvest begins, and the same amount in late June at the end of the harvest season. Each application usually controls weeds for 6 to 8 weeks. Since this chemical controls weeds for such a long period, growers are concerned as to whether, after continued use on the same soil for a period of years, this material might accumulate in the soil to the point where asparagus is injured, or the soil might be made unfit for crops which might follow asparagus. Therefore, a study was made of the persistence and penetration of monuron in soils of asparagus experiments where this chemical had been applied each year for a number of years.

The absorption of chemicals by the leaf of a plant is a problem of major importance in chemical weed control. The presence of a waxy cuticle on the surface presents a barrier to absorption. Interspersed in the cuticle are the stomata which may serve as a portal of entry for chemicals but even the interior chambers of the stoma are lined with a cuticle. It may be presumed that the more intimate the contact between a chemical and the leaf surface the greater the possibility for absorption. The water carrier commonly used in application of these chemicals makes but a poor contact with the cuticle; however, this deficiency may be overcome by the addition of agents to lower the surface tension of the solution. While intimacy of contact of the chemical with the leaf surface is important in absorption, the state, i.e. liquid or solid, of the chemical may also influence the ease of absorption. Consideration of the physical processes involved shows that a chemical will be absorbed with relative ease from a liquid phase in contrast to the same chemical deposited as a crystal on the surface of the leaf. Either humid conditions following application or the addition of hygroscopic agents may help to maintain a liquid state thus facilitating absorption. Many surface active agents used in sprays are also fair hygroscopic agents.

Shortawn foxtail (Alopecurus aequalis Sobol.) is an obligate wetland plant that is widely distributed throughout Europe, temperate Asia, and North America. In China, it is widespread in the middle and lower reaches of the Yangtze River as a noxious weed in winter cropping fields with a rice (Oryza sativa L.) rotation. The acetolactate synthase (ALS)-inhibiting herbicide mesosulfuron-methyl has been widely used to control annual grass and broadleaf weeds, including A. aequalis, in wheat (Triticum aestivum L.) fields, leading to the selection of herbicide-resistant weeds. In this study, an A. aequalis population, AHFT-4, that survived mesosulfuron-methyl at the field-recommended rate (9 g ai ha−1) was collected in Anhui Province. Single-dose testing confirmed that the suspected resistant AHFT-4 had evolved resistance to mesosulfuron-methyl. Target gene sequencing revealed a resistance mutation of Pro-197-Ala in ALS1 of the resistant plants, and a derived cleaved amplified polymorphic sequence marker was developed to specifically detect the mutation. A relative expression assay showed no significant difference in ALS expression between AHFT-4 and a susceptible population without or with mesosulfuron-methyl treatment. Whole-plant dose–response bioassays indicated that AHFT-4 had evolved broad-spectrum cross-resistance to ALS-inhibiting herbicides of all five chemical families tested, with GR50 resistance index (RI) values ranging from 21 to 206. However, it remained susceptible to the photosystem II inhibitor isoproturon. Pretreatment with the cytochrome P450 inhibitor malathion or the glutathione S-transferase inhibitor 4-chloro-7-nitrobenzoxadiazole had no significant effects on the resistance of AHFT-4 to mesosulfuron-methyl. To our knowledge, this study reports for the first time the ALS gene Pro-197-Ala substitution conferring broad-spectrum cross-resistance to ALS-inhibiting herbicides in A. aequalis.

The environment in which a plant grows (maternal environment) can affect seed viability, germination, and dormancy. We assessed the effects of maternal environment on wild oat seed viability, germination, dormancy, and pathogen infection by collecting and analyzing wild oat seed from above and below a barley canopy at three field sites in Montana. The viability of wild oat seed collected below a crop canopy was consistently less than it was for seed from the overstory but varied among sites and years. Reductions in viability because of relative canopy position ranged from 10% to 30%. Effects of position relative to crop canopy on weed seed germination/dormancy rates varied by site and suggest that the direction and magnitude of the effects of maternal environment on dormancy depend on environmental conditions. These effects may be driven by crop competition or by changes in seed pathogen pressure or both. Seven species each of fungi and bacteria were isolated from wild oat seeds. The only fungi causing reductions in seed viability (15%) was isolated from understory seeds, and several bacteria from both overstory and understory sources reduced seed germination. Results suggest that, in addition to the known weed-suppressive effects of using taller or earlier emerging varieties of crops, such crops can reduce weed spread through effects on weed seed demography because weeds growing beneath the crop canopy produce a reduced amount of viable seed that is less likely to germinate in the following year.

Hydrilla is one of the most serious aquatic weed problems in the United States, and fluridone is the United States Environment Protection Agency (USEPA)-approved herbicide that provides relatively long-term systemic control. Mature (6-wk-old) and young (freshly planted from 10-cm apical shoot apices) hydrilla were grown in 540 L fiberglass vaults under short- (natural 8 to 10 hr light/14 to 16 hr dark photoperiod) or long- (artificially extended 16 h light/8 h dark photoperiod) day greenhouse conditions. Fluridone treatments of 0, 1, 5, and 10 µg L−1 were applied after 2 wk and maintained within each population and photoperiodic regime throughout the study. Short days promoted subterranean turion formation, but this effect was reduced by long days and 5 and 10 µg L−1 fluridone. Fluridone caused a reduction in chlorophyll and carotenoid levels but the effect on anthocyanin content was variable. Short days caused elevated anthocyanin, and this effect was diminished by fluridone. Fluridone reduced the abscisic acid content of mature apical stems and was higher under short days in younger plants. These studies provide further evidence that fluridone can be used as a fall herbicide application to reduce turion production.

Recent innovations in 3D imaging technology have created unprecedented potential for better understanding weed responses to management tactics. Although traditional 2D imaging methods for mapping weed populations can be limited in the field by factors such as shadows and tissue overlap, 3D imaging mitigates these challenges by using depth data to create accurate plant models. Three-dimensional imaging can be used to generate spatiotemporal maps of weed populations in the field and target weeds for site-specific weed management, including automated precision weed control. This technology will also help growers monitor cover crop performance for weed suppression and detect late-season weed escapes for timely control, thereby reducing seedbank persistence and slowing the evolution of herbicide resistance. In addition to its many applications in weed management, 3D imaging offers weed researchers new tools for understanding spatial and temporal heterogeneity in weed responses to integrated weed management tactics, including weed–crop competition and weed community dynamics. This technology will provide simple and low-cost tools for growers and researchers alike to better understand weed responses in diverse agronomic contexts, which will aid in reducing herbicide use, mitigating herbicide-resistance evolution, and improving environmental health.