Optimum temperature for germination and emergence of redvine seed in either petri dishes or soil was 35 C. Reductions in germination and emergence occurred at constant temperatures above or below this temperature. No germination or emergence occurred at temperatures above 40 or below 25 C. Germination in petri dishes first occurred on day 8, and emergence was first observed 12 days after planting in a soil medium. Maximum redvine seedling emergence of 74% occurred in soil when seed were planted 0.5 cm deep, with decreases in emergence noted at depths below 5 cm. No emergence occurred over the 35-day period evalμated when redvine seed were planted on the soil surface or at a depth of 10 cm in soil. Redvine germination was not affected by solution pH, whereas germination did not occur when osmotic stress of as little as −200 kPa was applied to seed.

Mixtures of annual weeds emerging 1 week after and competing all season with potatoes (Solanum tuberosum L.) reduced tuber yields by an average of 54% compared to 16% in comparable treatments in which weeds emerged 3 weeks after potatoes. Weeds reduced tuber yields 19% 8 weeks after potatoes emerged and 25 to 45% 10 weeks after emergence. After full-season competition, for each additional 10% of the total dry biomass represented by weeds, there was a decrease of 12% in fresh tuber yield. This relationship was not valid if potatoes were harvested early, which reduced the deleterious effect of weeds on potato yields. Weed competition reduced both size and number of tubers, but had little effect on specific gravity of tubers. In some instances, the reduction in tuber size by weeds reduced the percentage of tubers that were marketable.

In a 2-year study, kochia [Kochia scoparia (L.) Schrad.] competed with sugarbeets (Beta vulgaris L.) for various periods after emergence. Kochia then was removed and plants that emerged subsequently were controlled by hand-weeding. Yield of sugarbeet roots was reduced when kochia competed for more than 5 or 6 weeks. Kochia that competed with sugarbeets for the entire season reduced yields more than 95%. In another study, kochia was controlled by frequent hand-weeding for different periods after sugarbeet emergence. When kochia was controlled initially for 3 to 4 weeks, yield of sugarbeets was not reduced. Some kochia emerged following the 3 to 4 weeks of hand-weeding. However, competition from sugarbeets reduced the weight of this kochia at harvest by 59 and 92%.

Fatal germination of weed seeds occurs when a weed seed germinates, but the seedling dies before reaching the soil surface. Controlled-environment bioassays of velvetleaf and giant foxtail seed fate in Michigan field soil (Kalamazoo silt loam, 1.1% soil organic matter) were used to determine the role of pathogenic fungi and seed burial depth in fatal germination of these species. Fatal germination at 2 cm seed depth was nonexistent for giant foxtail, and rare (< 10% of seeds studied) for velvetleaf. At greater depths, fatal germination remained close to zero for giant foxtail, whereas it increased to as high as 40% for velvetleaf at a 10-cm burial depth. Cultures taken from fatally germinated velvetleaf seedlings were found to contain Pythium ultimum, a soilborne pathogen known as the causal agent for pea root rot. When samples of infected media taken from these cultures were used to inoculate field soil in pots, fatal germination of velvetleaf from depths of 4 to 6 cm increased, compared with field soil inoculated with sterile media. At seed burial depths of 8 and 10 cm, fatal germination of velvetleaf increased to 20 and 40%, respectively, and was the same for unsterilized soil and P. ultimum–inoculated soil. Given that maximal fatal germination of velvetleaf occurred in the unsterilized soil treatment at the 10 cm depth, burial of newly shed velvetleaf seeds to a 10 cm, or possibly greater, depth with tillage equipment may be a practical way of reducing velvetleaf seed banks through fatal germination.

Tillage is a foundational management practice in many cropping systems. Although effective at reducing weed populations and preparing a crop seedbed, tillage and cultivation can also dramatically alter weed community composition. We examined the impact of soil tillage timing on weed community structure at four sites across the northeastern United States. Soil was tilled every 2 wk throughout the growing season (late April to late September 2013), and weed seedling density was quantified by species 6 wk after each tillage event. We used a randomized complete block design with four replicates for each tillage-timing treatment; a total of 196 plots were sampled. The timing of tillage was an important factor in shaping weed community composition and structure at all sites. We identified three main periods of tillage timing that resulted in similar communities. Across all sites, total weed density tended to be greatest and weed evenness tended to be lowest when soils were tilled early in the growing season. From the earliest to latest group of timings, total abundance decreased on average from 428±393 to 159±189 plants m−2, and evenness increased from 0.53±0.25 to 0.72±0.20. The effect of tillage timing on weed species richness varied by site. Our results show that tillage timing affects weed community structure, suggesting that farmers can manage weed communities and the potential for weed interference by adjusting the timing of their tillage and cropping practices.

In greenhouse studies, spurred anoda [Anoda cristata (L.) Schlecht.] control by Alternaria macrospora Zimm. was influenced by stage of growth, inoculum concentration, and the temperature and duration of the dew periods following inoculation. Seedlings inoculated at the cotyledon to first-leaf stage were most easily controlled. Optimum dew periods for killing seedlings were ≥ 24 h at 20 to 30 C. Spray mixtures containing 2.5 to 10 × 105 spores/ml gave the best control. In field studies, A. macrospora significantly reduced the number and the dry weight of spurred anoda plants.

Cropping system characteristics affect weed management by altering key demographic rates of weeds, including recruitment, seedling survival, fecundity, and seed survival. To facilitate the design and improvement of cropping systems that limit weed population growth, analytical methods are needed to identify weed management “choke points” (weed life stages that vary in response to management and whose variation strongly affects weed population growth rate). The objectives of this study were to (1) determine whether wheat–red clover green manure can limit giant foxtail population growth rate (λ) in a wheat–corn–soybean crop sequence and (2) identify choke points in the giant foxtail life cycle with respect to the green manure treatment. Demographic data were used to construct a periodic matrix model of giant foxtail population growth in a wheat–corn–soybean crop sequence, with either a wheat sole crop (W) or a wheat–red clover intercrop (R) in the wheat phase. Identification of choke points was accomplished by adapting the life-table response experiment (LTRE) design for retrospective perturbation analysis of the periodic matrix model. The difference in λ (Δλ) between the two treatments was decomposed into contributions from each parameter in each rotation phase of the periodic model. Each LTRE contribution was equal to the product of the sensitivity of λ to changes in a given parameter by the treatment difference in that parameter. Those parameters making large contributions to Δλ represented weed management choke points. Giant foxtail population growth rate in the simulation was more than twice as great in the W treatment (λ = 2.54) than in the R treatment (λ = 1.16). Retrospective perturbation analysis indicated that the proportion of seeds surviving predation in the wheat phase made the largest LTRE contribution (0.55) to Δλ, followed by seedling recruitment in the soybean (0.41) and corn (0.20) phases. By identifying weed management choke points within a given system, retrospective perturbation analysis can target research and management efforts for greater reductions in weed population growth.

The effect of FMC 57020 [2-(2-chlorophenyl) methyl-4,4-dimethyl-3-isoxalidinone] on chloroplast development was examined in the cotyledons of 5-day-old, etiolated pitted morningglory (Ipomoea lacunosa L. ♯ IPOLA) seedlings grown from seeds inbibed for 24 h in water or 0.5 mM FMC 57020. In etiolated tissues, protochlorophyllide content was unaffected by FMC 57020; however, the herbicide eliminated carotenoid accumulation. There was no effect of FMC 57020 on phytoene or phytofluene content, although norflurazon [4-chloro-5-(methylamino)-2-(3-trifluoromethyl) phenyl)-3(2H)-pyridazinone] increased phytoene content in these tissues. The Shibata shift was greatly retarded in FMC 57020-treated cotyledons, suggesting that phytol levels are also reduced by the herbicide. There were no ultrastructural effects on etioplasts; however, under low white light (150 μE·m-2·s-1 PAR), plastids of FMC 57020-treated seedlings did not develop into chloroplasts but rapidly developed ultrastructural symptoms of photobleaching. Starch was not mobilized in herbicide-treated plastids and sugar levels were higher in these plastids than in control plastids. Etiolated hypocotyl growth was inhibited by FMC 57020, whereas norflurazon had no effect upon it. Our results suggest that FMC 57020 blocks both diterpene and tetraterpene synthesis.

Application of chlorimuron and imazaquin at 0.28 kg ai/ha to field-grown sicklepod at early bloom and early fruit stages in 1984 and 1985 almost eliminated seed production. In addition, none of the seed produced following these treatments were capable of emergence during a 4-week period following acid scarification. Glyphosate applied at 0.28 kg ai/ha at early bloom decreased seed production 84% but did not affect seedling emergence in 1984, and precluded production of seed capable of emergence in 1985. Glyphosate applications at the early fruit stage reduced the number of seed that emerged 93 and 90% in 1984 and 1985, respectively. Application of 2,4-DB at 0.28 kg ai/ha and 2,4-D at 0.56 kg ai/ha at early bloom did not affect seed production or emergence in 1984 but almost eliminated production of seed capable of emergence in 1985. Applications of 2,4-DB and 2,4-D at the early fruit stage decreased the number of seed that emerged 99 and 52% in 1984 and 46 and 57% in 1985, respectively. Herbicide applications at the late fruit stage were generally less effective than earlier applications in reducing seed production and emergence.

Concentrations of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] in contaminated groundwater recovered from 41 monitoring wells in Hall and Buffalo counties, Nebraska range from 0.01 to 8.29 μg/L. Over a 1-year period concentrations fluctuated sufficiently in a seasonal pattern to indicate that atrazine dissipation occurs. This reduction in concentration can be attributed to adsorption, dispersion, and degradation. A limited amount of atrazine degraded chemically to hydroxyatrazine [2-hydroxy-4-(ethylamino)-6-(isopropylamino)-s-triazine] under simulated aquifer conditions; microbial degradation could not be detected. A geometrical progression model for predicting long term residue accumulation indicates that current contamination levels probably reflect a steady-state situation between the amount that yearly enters into, and the partial degradation that occurs within, the aquifer. Oat (Avena sativa L. ‘Neal’) bioassays indicate that present levels of groundwater contamination remain well below the threshold necessary for phytotoxicity.

Absorption, translocation, and distribution of 14C-glyphosate [N-(phosphonomethyl)glycine] were examined in quackgrass [Agropyron repens (L.) Beauv.] plants growing at 10/8, 15/12, and 21/18 C day/night temperatures at 300 μE·m-2·s-1. Absorption of 14C-glyphosate followed similar trends in all environments, apart from an initial delay at the highest temperature. Approximately 67% of the applied 14C-glyphosate was absorbed after 120 h. Glyphosate translocation to the rhizomes was initially slower in plants growing at 10/8 C than at the higher temperatures, but after 24 h continued at a rate similar to that observed at the higher temperatures. CO2 exchange rates (CER) were only slightly influenced by temperature between 5 and 25 C, which may explain the lack of temperature effect on the rate of glyphosate translocation. Approximately 47% of the applied 14C-glyphosate was recovered in the rhizomes and associated roots 120 h after application in all environments. Glyphosate accumulated predominantly in new rhizomes under all growing conditions.

Studies were conducted to determine the relative fitness and competitive ability of an acetolactate synthase (ALS) inhibitor–resistant (R) downy brome biotype compared with a susceptible (S) biotype. In previous research, the mechanism of resistance was determined to be an altered ALS enzyme. Seed germination of the R biotype was compared with that of the S biotype at 5, 15, and 25 C. There were no different germination characteristics between R and S biotypes at 15 and 25 C. However, the R biotype germinated 27 h earlier than the S biotype and had reached over 60% germination when the S biotype initially germinated at 5 C. Under noncompetitive greenhouse conditions, growth of the R biotype was similar to that of the S biotype on the basis of shoot dry weight, leaf area, and plant height. Seed production of the R biotype was 83%, when compared with the S biotype, but seeds of the R biotype were larger than those of the S biotype. Replacement series experiments were conducted in the greenhouse to determine the relative competitive ability of R and S biotypes. No difference in competitive ability was observed between R and S biotypes on the basis of shoot dry weight, leaf area, or plant height. Thus, it appears that ALS-resistance trait is not associated with growth penalty in either noncompetitive or competitive conditions. In the absence of ALS inhibitors, these results suggest that the R biotype would remain at a similar frequency in a population of R and S biotypes.

In controlled-environment chambers at 29/33 C day/night and 1000 μE·m-2·s-1 photosynthetic photon flux density (PPFD), increasing the CO2 concentration from 350 to 675 ppm (v/v) did not affect leaf area or total dry weight of well-watered plants of barnyardgrass [Echinochloa crus-galli (L.) Beauv. # ECHCG], goosegrass [Eleusine indica (L.) Gaertn. # ELEIN], or southern crabgrass [Digitaria ciliaris (Retz.) Koel. # DIGSP] after 30 days. However, the whole plant transpiration rate per unit leaf area decreased, and the water use efficiency increased, in response to CO2 enrichment. In a subsequent experiment, with water availability limited by an imposed drought, CO2 enrichment reduced the effects of water stress and significantly increased leaf area and total dry weight of the three C4 grasses and soybean [Glycine max (L.) Merr. ‘Ransom’]. Growth enhancement in response to CO2 enrichment was greater in soybean than in the C4 grasses. By improving their water economy, CO2 enrichment can increase the growth of both C3 and C4 plants under water stress. However, growth stimulation can be expected to be greater in C3 plants.

In the first of 2 field studies, weed biomass and soybean seed yield were used to evaluate 16 soybean genotypes for competitive ability against 12 weed species at Rosemount, MN, in 1992 and 1993. The yield and ranking of soybean genotypes often varied with the weed species. Grass weed species reduced yields the most, and small-seeded broadleaf weeds reduced yields the least across years. ‘Parker’ was highly competitive, as it suppressed weed biomass and produced high soybean yield. ‘Kato,’ ‘Kasota,’ ‘Dawson,’ and ‘Glenwood’ minimized weed biomass and maintained soybean yield while in competition with grass weeds but yielded poorly relative to other soybean genotypes in weed-free conditions. ‘Lambert’ produced high soybean yield in weed-free conditions, but yield dropped markedly when in competition with grass weeds. ‘Grande,’ ‘Heifeng 25,’ and ‘Norman’ soybeans were poor competitive genotypes in weedy situations and low yielding in weed-free conditions. A 2nd field study conducted at Rosemount and St. Paul, MN, during 1993 evaluated 16 soybean genotypes under 4 levels and durations of weed pressure for weed competitiveness. Parker, ‘Sturdy,’ and M89-794 were most competitive in suppressing weed biomass and producing high yields. Lambert yielded fairly well but allowed high weed biomass. M89-1743, M89-1006, ‘Archer,’ and ‘Ozzie’ yielded poorly and did not sup press weed biomass production. No relationship was found between weed competitiveness and soybean canopy area, height, and volume measured 30–45 d after planting (DAP).

Twelve locations in the United States and Puerto Rico were the sites for determining the loss of 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) and 2,3,6-trichlorophenylacetic acid (fenac) from soil. The herbicides were contained in tubes (1.88 by 6 inches) placed at depths of 3, 9, and 15 inches in the field. The samples were placed horizontally to minimize losses due to vertical movement of water from the tubes. After at least 3 months in the soil, the samples were returned to Beltsville and analyzed. Average recoveries showed 61% more atrazine and 41% more fenac from the 15-inch depth than from the 3-inch depth. Five northern samples contained more than twice as much atrazine and fenac residue as four southern samples. A positive correlation existed between fenac retention and soil organic matter content. Increasing soil organic matter and depth of placement, and decreasing temperature, tended to make the herbicides more persistent. However, the data were quite variable and the variations were often unexplainable.

Invasive plants impose threats to both natural and managed ecosystems. Downy brome is among the most aggressive invasive weeds that has infested the shrub-steppe ecoregion of eastern Washington. Hyperspectral remote sensing has potential for early detection and for monitoring the spread of downy brome—information that is essential for developing effective management strategies. Two airborne hyperspectral Advanced Visible Infrared Imaging Spectrometer (AVIRIS) images (electromagnetic spectrum ranging from 400 to 2,500 nm) were acquired at a nominal 4-m ground resolution over a study area in south-central Washington on July 27, 2000 and May 5, 2003. We used a mixture-tuned matched filtering (MTMF) algorithm to classify downy brome and predict its percent cover in each dataset plus a merged multiseasonal dataset using the transformed bands from a minimum noise fraction (MNF) output. The correlation coefficient was 0.79, calculated for the multidate MTMF predicted downy brome abundance, compared to 0.41 and 0.51 derived from the July 2000 and May 2003 data, respectively. Although this study used high spatial resolution (∼3 to 4 m) hyperspectral imagery, this result shows that data acquired in different seasons is more effective for detection of downy brome invasion, compared to single-date datasets. These results support expanded use of multitemporal data for weed mapping to capitalize on spectral differences between seasons for weeds, in this case downy brome, and the surrounding environment.

Chemical ripening of sugarcane is an important component to profitable sugar production in the United States as well as other sugarcane industries throughout the world. Harvesting of sugarcane often begins before the sugarcane reaches the desirable maturity level. This is especially true in the Louisiana sugarcane industry where the window for harvesting is limited because of the risk of freezing temperatures encountered in a temperate climate. Research on the application of chemicals, mostly of herbicide origin, to enhance sucrose accumulation (ripening) or limit flowering to conserve stored sucrose has been conducted for more than 60 yr. The only sugarcane ripener currently registered for use in the United States is glyphosate applied before harvest. The herbicide fluazifop is used as the primary ripener of sugarcane in South Africa. The herbicides glyphosate, fluazifop, and sulfometuron-methyl and the growth regulators ethephon and trinexapac-ethyl are registered for use in Brazil. There is a continuing need to evaluate sugarcane ripeners to increase the utility of currently registered ripeners and to find additional ripeners for use by sugarcane industries. The need for alternatives to glyphosate is especially critical before a glyphosate-tolerant sugarcane can be utilized to improve control of problematic weeds.

Three wash techniques, each with 1, 10, or 95% (v/v) ethanol:water were used to measure foliar absorption of 14C-glyphosate [N-(phosphonomethyl)glycine], 14C-3,6-dichloropicolinic acid, and 14C-chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} in Tartary buckwheat [Fagopyrum tataricum (L.) Gaertn. ♯3 FAGTA], Canada thistle [Cirsium arvense (L.) Scop. ♯ CIRAR], and barley (Hordeum vulgare L. ‘Galt’). For the herbicides and species tested, the most suitable common procedure for determining absorption consisted of a double or triple rinse with or immersion in 10% ethanol. Wiping the treated leaves with cotton balls moistened with the solvent was much less effective. Efficiency of herbicide removal by a given solvent was not related consistently to solubility of the herbicide in the solvent.

Removal and plant-back studies were conducted in North Carolina in 1991 and 1992 to determine the critical period of large crabgrass competition in transplanted triploid (seedless) watermelon. For every week that large crabgrass remained in watermelon, medium (3.6 to 7.3 kg) melon yield decreased 3,996 kg and 716 fruit ha−1. For every week that large crabgrass emergence was delayed, yield increased by 814 kg and 142 fruit ha−1. Likewise, for every week that large crabgrass remained in watermelon, marketable (3.6 kg and over) yield decreased 5,582 kg and 911 fruit ha−1. For every week that large crabgrass emergence was delayed, yield was increased 881 kg and 151 fruit ha−1. Large crabgrass emerging after 6 wk had no effect on marketable fruit or number of watermelon. To achieve the greatest quality or quantity of medium or marketable fruit, a large crabgrass-free period between 0 and 6 wk after transplanting was necessary.

Both 14C-clopyralid (3,6-dichloropicolinic acid) and 14C-chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzensulfonamide} were readily absorbed by Canada thistle [Cirsium arvense (L.) Scop. ♯ CIRAR] leaves, with 99 and 75%, respectively, of the applied doses absorbed 144 h after application. Absorbed 14C-clopyralid was rapidly exported from the treated leaves, whereas 14C-chlorsulfuron was translocated much more slowly. After 144 h, 29% of the applied 14C-clopyralid and 5% of the applied 14C-chlorsulfuron were recovered in the roots and developing root buds of Canada thistle plants. Smaller amounts of the two herbicides were absorbed and translocated in perennial sowthistle (Sonchus arvensis L. ♯ SONAR) than in Canada thistle. More 14C-clopyralid than 14C-chlorsulfuron was absorbed and translocated out of treated leaves of perennial sowthistle, but equal amounts, 3 to 4% of the applied doses, were recovered in the roots and root buds 144 h after application. Foliar applications of clopyralid, followed by removal of the treated shoot 24, 72, or 144 h after application, markedly reduced shoot regrowth in both Canada thistle and perennial sowthistle. Similar treatment with chlorsulfuron did not prevent shoot regrowth in either species.