Glyphosate [N-(phosphonomethyl)glycine] (30, 125, 250 mg/L) in petri dishes had no effect on germination of prickly sida (Sida spinosa L.), velvetleaf (Abutilon theophrasti Medic), barnyardgrass [Echinocloa crus-galli (L.) Beauv.] and johnsongrass [Sorghum halepense (L.) Pers.] seeds, but additional experimentation indicated that glyphosate stimulated germination of redroot pigweed (Amaranthus retroflexus L.) seeds. Paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) (30, 125, 250 mg/L) did not affect germination of the three broadleaf species, but inhibited johnsongrass and barnyardgrass germination. In the greenhouse, soil surface applications of glyphosate (1.1, 2.2, 9.0 kg/ha) did not significantly affect emergence of these five weed species when they were on or beneath the soil surface at time of treatment. Paraquat (same rates) did not affect broadleaf weed emergence but some rates inhibited grass weed emergence when the seeds were treated while on the soil surface. It is unlikely that normal field use rates of glyphosate will influence weed emergence; whereas paraquat may inhibit the emergence of some grass weeds if the herbicide contacts seeds on the soil surface.

The main site of uptake of soil-applied 2-chloro-2′, 6′-diethyl-N-(methoxymethyl) acetanilide (alachlor) by yellow nutsedge (Cyperus esculentus L.) plants is the portion of the plant above the tuber. Primarily acropetal and some basipetal 14C translocation occurred from 14C-alachlor applications to seedlings with shoots 2 to 4 cm in length. Alachlor is metabolized rapidly in yellow nutsedge to at least one water-soluble metabolite. Applications of 14C-alachlor to shoots 15 cm in length resulted in limited acropetal 14C translocation.

We have shown by means of laboratory experiments with a wide range of soil types that the decomposition rate of picloram (4-amino-3,5,6-trichloropicolinic acid) in soil is dependent on soil temperature and moisture content and to no significant extent on organic matter, sand, silt, clay, pH, or initial concentration. A fractional order rate law (0.8) describes the disappearance rate best. It was not possible to develop a suitably precise equation for prediction of loss rate as affected by the above soil and climatic factors. Application of the Arrhenius equation to the data indicates an activation energy of 5.4 kcal/mol for the decomposition of picloram in soil, thus suggesting that the reaction is biological rather than chemical.

We characterized soil and vegetation assemblages, many of which are infested with medusahead (Taeniatherum asperum (Sim.) Nevski), on the margin of the Great Basin. Interpretations of these assemblages provide an index of the validity of the basic environmental unit of this ecosystem which can be manipulated through weed control and revegetation techniques. Vertisol (churning clay soils) sites with sparse native plant communities are more susceptible to medusahead invasion than more complex low sagebrush (Artemisia arbuscula Nutt.) or low sagebrush-woodland communities on related clay soils. If the more complex communities are degraded to a low seral state, medusahead can invade and occupy the site. Wet meadows and burned coniferous forest sites at high elevations were the only sites where medusahead occurred on soils with textures other than clay. Big sagebrush (Artemisia tridentata Nutt.) communities on medium to coarse textured soils were very resistant to medusahead invasion. The restriction of medusahead to certain sites controls the mechanism of invasion and interacts with the breeding system of the species to influence its evolution.

Greenhouse and field studies were conducted with a population of common waterhemp resistant to POST protoporphyrinogen oxidase (protox)-inhibiting herbicides to compare its response to PRE and POST applications of selected herbicides. In the greenhouse, a dose–response study of PRE applications of acifluorfen, fomesafen, or lactofen was conducted on protox-susceptible and -resistant common waterhemp. The protox-resistant biotype was approximately 6.3, 2.5, and 2.6 times more resistant than the susceptible biotype to acifluorfen, fomesafen, and lactofen, respectively. In a separate study under field conditions, protox-resistant common waterhemp were treated with PRE and POST applications of acifluorfen, azafenidin, flumioxazin, fomesafen, lactofen, oxyfluorfen, or sulfentrazone. At 14 and 28 d after POST treatment (DAPT) in 2002 and 2004, all PRE applications of herbicides gave greater control than did POST applications. At 14 DAPT, oxyfluorfen had the greatest difference in PRE and POST control, with 85 and 10% control in 2002, respectively. An additional field study was conducted to determine the stage of growth at which resistance to protox-inhibiting herbicides becomes most prevalent. Protox-resistant common waterhemp were treated with herbicides at the 2-leaf, 4- to 6-leaf, and 8- to 10-leaf growth stage. Acifluorfen and fomesafen at 420 g ha−1 gave greater than 90% control at the 2-leaf stage and 4- to 6-leaf stage, except in 2003 when control was 85% with acifluorfen. In 2003 and 2004, common waterhemp control at the 8- to 10-leaf stage ranged between 54 and 75% with acifluorfen or fomesafen. Results indicate that common waterhemp resistance to customary rates of POST protox-inhibiting herbicides becomes prevalent after the 4- to 6-leaf growth stage.

Control of rattail fescue, a winter annual grass, can be difficult in spring or winter wheat. Although rattail fescue is not a new weed species in the Pacific Northwest, occurrences have been increasing in circumstances where soil disturbances are minimized, such as in direct-seed cropping systems. To develop integrated management strategies for rattail fescue, information is needed on the longevity of seed viability in the soil, the presence of seed dormancy, vernalization requirements, and optimal environmental conditions for seed germination and establishment under field conditions. Controlled experiments on the biology of rattail fescue indicated that newly mature seed required an afterripening period of 1 to 12 mo to obtain high levels of seed germination, depending on germination temperature. Maximum seed germination was observed at constant day/night temperatures of approximately 20 C from thermogradient plate studies. Germination tests from seed burial studies indicated that a majority of buried seed was not viable after 2 to 3 yr. Field-grown rattail fescue plants required vernalization to produce panicles and germinable seed. A short afterripening period, cool germination temperature, and vernalization requirements support the classification of rattail fescue as a winter annual. This information will facilitate development of rattail fescue management systems, including crop rotations and various control tactics such as tillage or herbicide application timing during fallow periods.

EPTC (S-ethyl dipropylthiocarbamate), 1,8-naphthalic anhydride (NA), N,N-dialryl-2,2-dichloroacetamide (R-25788), CDAA (N,N-diallyl-2-chloroacetamide) and CDEC (2-chloroallyl diethyldithiocarbamate) at concentrations of 10-5 M inhibited the incorporation of acetate-1-14C (Ac∗) into fatty acids of isolated spinach (Spinacia oleracea L.) chloroplasts. Lower concentrations of these chemicals did not affect acetate incorporation into fatty acids. However, the inhibition by EPTC at 10-5 M was counteracted by NA and R-25788 [agriculturally important antidotes for EPTC injury to corn (Zea mays L.)] at 10-7 M.

Metabolism and phytotoxicity of bentazon by suspension-cultured cells of soybean and velvetleaf were compared. Growth of suspension cells of both species was reduced when the cells were exposed to increasing concentrations of bentazon. However, soybean plants were tolerant and velvetleaf plants were susceptible to postemergence applications of bentazon. After incubation with 1 μM 14C-bentazon for 6 h, soybean and velvetleaf cells in the log phase of the culture growth cycle contained similar levels of 14C (6 nmol/g fresh weight). Of the total 14C in the soybean cells, 57 to 92% was present as the glucosyl conjugates of 6-OH- and 8-OH-bentazon with the remainder present as bentazon; the percentage depended on the phase of the culture growth cycle. Bentazon metabolism was greatest in the stationary phase of growth. Thin, transverse sections of soybean hypocotyl metabolized bentazon to the same two metabolites as soybean suspension cells did. The ratio of 6-O-glucosyl-bentazon to 8-O-glucosyl-bentazon was always greater than 1:1 for both the hypocotyl sections and the suspension cells. Bentazon metabolites were not detected in the velvetleaf cells, the velvetleaf hypocotyl sections, or the media of either species. Soybean suspension-cultured cells appear to be a valid and advantageous system for studying the hydroxylation and glucosylation of bentazon, the primary reactions believed to be responsible for detoxication of the herbicide in tolerant plants.

A pine kraft lignin (PC940C) controlled the release of 14C-chloramben (3-amino-2,5-dichlorobenzoic acid), 14C-metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one], and 14C-alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] as measured by water leaching in soil columns. As more PC940C was used, a concentration effect was found with more of the three herbicides being retained in the top portion of the soil columns. Combinations of alachlor and metribuzin applied with PC940C did not alter the retention in the top portion of the soil columns of either 14C-metribuzin or 14C-alachlor compared to each herbicide applied with PC940C alone. This would suggest that, at the rate of PC940C used, there was no competition between either herbicide for the controlled-release function of PC940C. The percentage of 14C-chloramben, 14C-metribuzin, and 14C-alachlor retained by PC940C in the top portion of the columns was very similar. Combinations of metribuzin and alachlor without PC940C reduced the mobility of both 14C-metribuzin and 14C-alachlor. Finally, 3H-PC940C was found to be immobile in soil columns leached with water.

Quantitative measurements were made by cucumber root bioassay of 2,4-dichlorophenoxyacetic acid (2,4-D) contained in washoff (water-soil mixture) from cultivated fallow Cecil sandy loam soil. Formulations of 2,4-D used included iso-octyl and propylene glycol butyl ether esters and an alkanolamine salt of the ethanol and isopropanol series. Simulated rainfall intensities and storm durations used represented storm frequencies of 1, 10, 80, and greater than 100 years. The 2,4-D applied at rates of 2.2 or 4.4 lb/A was assayed in washoff and as residue in the soil. Concentrations of 2,4-D in washoff were positively correlated with the rate applied, were greatest early in each storm, and decreased with duration of the storm. The iso-octyl and butyl ether ester formulations of 2,4-D were far more susceptible to removal in washoff than the amine salt. When the amine salt was used, 2,4-D concentrations in washoff were less than 1 ppm, even in the early stages of runoff, whereas concentrations as high as 4.2 ppm were measured using an iso-octyl ester formulation of 2,4-D. When a direct comparison was made between the butyl ether ester and amine salt forms of 2,4-D, ester or amine losses were respectively, 13 and 4% following a 1-year frequency storm and 26 and 5% following a 100-year frequency storm. Soil bioassays showed that most of the 2,4-D remained in the surface 3 in of soil.

Chain- and ring-labeled 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) were supplied to corn by way of the nutrient solution. Autoradiographs of the paper chromatograms revealed that two identical C14-labeled metabolites were present in the extracts whether the plants were supplied with ring-or chain-labeled simazine. Corn, cotton, and soybeans all yielded appreciable amounts of C14O2 when grown in solutions containing chain-labeled simazine. One lb/A 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) (preemergence) caused a reduction in respiration as well as the activity of catalase, peroxidase, phenol oxidase, ascorbic acid oxidase, and glycolic acid oxidase in oats, soybeans, beans, cotton, peanuts, Johnsongrass, and corn, 7 and 11 days after treatment. Attempts to correlate enzyme activity with sensitivity to atrazine were unsuccessful, except that increasing phenol oxidase activity appeared to be roughly correlated with increasing resistance.

Although the prolonged emergence pattern of common waterhemp is well known, its effect on flowering phenology and success is less well understood. The ecological significance of later common waterhemp cohorts could have been underestimated. Ecological knowledge of common waterhemp, such as reproductive phenology and success, would help us better understand the invasiveness of this weed and thus facilitate the development of more targeted control methods. Field studies were conducted at Ames, IA, to evaluate temporal variation in flowering phenology (e.g., date of flower initiation, number of flowering plants per day) and reproductive success (e.g., seed production) of natural common waterhemp cohorts. Later-emerging common waterhemp cohorts flowered quicker and had a relatively shorter flowering period than early cohorts. Common waterhemp cohorts showed very large variations for the duration of their vegetative phases, indicating different photoperiod sensitivity among cohorts. Furthermore, common waterhemp cohorts exhibited a pulsed flowering pattern, which was probably influenced by temporal distribution of rain events; up to seven distinct flowering pulses within 40 d were observed in 2009, and about eight flowering pulses scattered over a 60 d period were observed in 2010. Common waterhemp maintained high seed production throughout the growing season. Seed production for the entire cohort was influenced more by emergence timing, whereas individual plant fecundity was more affected by plant population densities. Common waterhemp demonstrates strong phenological plasticity by evolving a pulsed flowering pattern, which rendered it plastic enough to tailor flowering to variable environmental conditions thus facilitating effective pollination. Common waterhemp later cohorts have a strong seed production potential, which may be ecologically significant with regard to herbicide resistance evolution.

A methodological approach to determine the optimum time to control weeds that integrates aspects of weed biology, weed-crop competition, and economics is presented. The approach is based on the concept of Time Density Equivalent: this is defined as the density of weed plants that germinate with the crop and compete until harvest that causes the same yield loss caused by a group of weeds with a given density, time of emergence, and time of removal. A model was developed that accounts for pattern of weed emergence and permits determination of timing of weed control that minimizes economic loss due to weeds emerging both before and after treatments. The outcomes of the model are presented with two examples: corn in competition with velvetleaf and soybean in competition with Amaranthus cruentus. For both crops, six different weed control strategies involving preemergence, chemical, and mechanical postemergence treatments are considered. The results obtained with the model are compared with the calculation of net margin based on assumptions of simultaneous emergence of crop and weeds and no effect of different times of control. Different control strategies are compared considering not only maximum net margin but also its dependence on time of control, because a strategy with a lower value of maximum net margin, but a flatter net margin curve, allows more flexibility of time of control.

Development was followed in Connecticut and Maryland. Germination of yellow foxtail (Setaria lutescens (Weigel) F. T. Hubb. occurred over a long time period. Tillering of the developing plants was affected by spacing between plants and location. The average number of tillers in Maryland was 75 for plants spaced 12 in. apart and 26 for plants spaced 2 in. apart. In the greenhouse under short day conditions seed heads were readily produced. Under continuous light no seed heads formed. Giant foxtail (S. Faberii Herrm.) followed a similar growth pattern but the plants were larger and formed fewer tillers. Growth of both species was severely affected by shading. Both species rooted from cuttings.

The toxicity, adsorption, and leaching of four herbicides in different soils were determined by a method based on a sensitive root bio-assay which appeared suitable for herbicides affecting root growth. The rate of leaching of the herbicides was determined by measuring the herbicidal effect of the leachate obtained from three heights of soil columns. The same bio-assay was utilized for testing the extent of inactivation of herbicides due to adsorption by soil. In this case, mixtures of various rates of soil with silica sand were used as cultures for the bio-assay.

Winter annual weeds are becoming prolific in agricultural fields in the midwestern United States. The objectives of this research were to understand the roles of soil temperature (daily average and fluctuation) and moisture on the emergence of nine winter annual weed species and dandelion and to develop predictive models for weed emergence based on the accumulation of modified thermal/hydrothermal time (mHTT). Experiments were established at Lincoln, NE; Mead, NE; and at two sites (irrigated and rainfed) near Clay Center, NE, in 2010 and 2011. In July of each year, 1,000 seeds of each species were planted in 15 by 20 by 6-cm mesh baskets installed between soybean rows. Soil temperature and water content were recorded at the 2-cm depth. Emerged seedlings were counted and removed from the baskets on a weekly basis until no additional emergence was observed in the fall, resumed in late winter, and continued until emergence ceased in late spring. Weather data were used to accumulate mHTT beginning on August 1. A Weibull function was selected to fit cumulative emergence (%) on cumulative mHTT (seven base temperature [Tbase ] by six base water potential [Ψbase ] by three base temperature fluctuation [Fbase ] candidate threshold values = 126 models); it was also fit to days after August 1 (DAA1), for a total of 127 candidate models per species. The search for optimal base thresholds was based on the theoretic-model comparison approach (Akaike information criterion [AIC]). All three components (Tbase , Ψbase , and Fbase ) were only important for Virginia pepperweed. For downy brome and purslane speedwell, including Tbase and Ψbase resulted in the best fit, whereas for dandelion including Tbase and Fbase resulted in the best fit. A model including only Tbase resulted in the best fit for most species included in this study (Carolina foxtail, shepherd's-purse, pinnate tansymustard, henbit, and field pansy). For field pennycress, the model based on DAA1 resulted in the best fit. Threshold values were species specific. Soil temperature was the major environmental factor influencing winter annual weed emergence. Even though soil moisture and often temperature fluctuation are essential for seed germination, Ψbase and Fbase were not as critical in the predictive models as initially expected. Most seedlings (> 90%) of downy brome, pinnate tansymustard, Carolina foxtail, henbit, and field pansy emerged during the fall. Virginia pepperweed, purslane speedwell, dandelion, shepherd's-purse, and field pennycress seedlings emerged during both fall and spring. The results of this research provide robust information on the prediction of the time of winter annual weed emergence, which can help growers make better management decisions.

Greenhouse studies were conducted to evaluate the growth response of itchgrass to water stress. Itchgrass plants produced the greatest aboveground biomass and seeds at 75% of field capacity and these parameters at 50 and 100% of field capacity were similar. With further increase in water stress, seed production was sharply reduced, but itchgrass was still able to produce an average of 63 and 9 seeds plant−1 at 25 and 12.5% of field capacity, respectively. Itchgrass plants responded to increasing water stress with increased leaf weight ratio; it was 2.5 times greater at 12.5% of field capacity than at 100% of field capacity. In another study, compared with daily irrigation, intervals of 9 d between irrigations reduced aboveground biomass of itchgrass by 27% and 12-d intervals reduced aboveground biomass by 67%. Compared with the daily irrigation regime, itchgrass seed production was reduced by 61% at intervals of 12 d between irrigations; however, the weed plants produced a considerable number of seeds (153 seeds plant−1) at the 12-d intervals. The ability of itchgrass to produce biomass and seeds under water stressed conditions necessitates strategies that minimize weed survival while maximizing irrigation efficiency for the crop at the same time.

The effect of black and eastern black nightshade on the amount of photosynthetically active radiation (PAR) intercepted by a processing tomato canopy was studied along with the correlation between PAR and tomato growth and yield. During 1989 and 1990, black and eastern black nightshade were established at densities of 0 to 4.8 m−2 within rows of transplanted, irrigated processing tomatoes. Increasing the density of either nightshade species decreased the number of tomato fruit; however, eastern black nightshade reduced tomato yield more than black nightshade. Eastern black nightshade was taller than the tomatoes, reducing PAR reaching the top of the tomato canopy. PAR reaching the top of the tomato canopy was positively correlated with yield and negatively correlated with eastern black nightshade density. Eastern black nightshade intraspecific competition decreased both stem and berry weight. Black nightshade was never taller than tomatoes and did not affect PAR reaching the top of the crop canopy. Increasing the density of black nightshade decreased berry dry weight but increased the weight of stems and leaves.

Tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl)oxirane] is a potent synergist of atrazine for postemergence control of weeds such as giant foxtail (Setaria faberi Herrm. ♯ SETFA) and large crabgrass [Digitaria sanguinalis (L.) Scop. ♯ DIGSA]. We determined whether: a) tridiphane could syngergize the activity of other herbicides known to be detoxified via enzymatic conjugation to glutathione, and b) differential toxicities of the synergized herbicides could provide control of proso millet (Panicum miliaceum L. ♯ PANMI) in corn (Zea mays L. “PAG SXIII”). Four herbicides were used in this study: EPTC (S-ethyl dipropylthiocarbamate), atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], CDAA (N-N-diallyl-2-chloroacetamide), and alachlor [2-chloro-2’,6’-diethyl-N-(methoxymethyl) acetanilide]. Herbicides and herbicide-synergist combinations were applied postemergence. In corn, tridiphane synergized the activity of EPTC at 2 and 4 kg ai/ha, causing 22 to 25% growth reduction. Atrazine was ineffective and CDAA only slightly effective in combination with tridiphane in causing growth reduction. However, alachlor was markedly synergized by tridiphane at both 2 and 4 kg ai/ha, resulting in 36 to 54% growth reduction compared with plants treated with herbicides alone. Proso millet followed a similar trend to corn: EPTC and alachlor were effectively synergized by tridiphane, while atrazine and CDAA were ineffective. Importantly, however, the rates of herbicide used for proso millet control were much lower than those used for corn. Our data suggest that tridiphane used in combination with EPTC or alachlor could provide improved control of proso millet in corn.

Blackgrass is a common winter annual grass weed in autumn-sown crop rotations in Atlantic European countries. Control with a minimum amount of herbicides in integrated cropping systems would be facilitated by modeling the effect of cropping systems on its demography. To develop the submodel relating weed seed bank to emerged seedlings, laboratory experiments were conducted to analyze and quantify seed germination and shoot elongation. These processes were studied as a function of environmental conditions during seed production (nitrogen availability, water deficit, plant density, and crop) and of seed characteristics (seed weight, harvest date, storage length, and dry-stored/soil-buried). Nonlinear equations relating germination and shoot elongation to time calculated as cumulated degree-days were fitted to the observed germination and shoot elongation data. These were used to estimate parameters for germination proportion and rate, as well as final shoot length and elongation rate. Recently harvested seeds germinated best and fastest when they were collected in spring crops compared with winter crops. Germination proportion and rate increased with seed storage length, especially for seeds collected in winter crops. Midgermination time decreased with seed weight and water deficit during seed production; it increased with nitrogen amounts available to the mother plants. Maximum shoot length increased with seed weight and plant density during seed production. It decreased with nitrogen availability to mother plants and with storage length, irrespective of whether the seeds were dry-stored or buried in soil. Elongation rate was highest for early- and fast-germinating seeds. Time to midelongation increased with maximum shoot length. These germination and preemergence growth models can now be combined with other submodels to develop a blackgrass emergence model.