Leaf tissue and leaf-tissue extracts from annual wormwood and pure artemisinin were evaluated for their effects on plant growth when incorporated into sandy loam soil. Dried leaf tissue was successively extracted with methylene chloride (MeCl2), ethanol (EtOH), and water, and the extracts and residue were reduced to dryness and stored at −20 C. Leaf tissue was incorporated in soil at rates equivalent to 0, 0.37, 0.73, or 1.1% (w/w) based on soil dry weight. Peat moss treated with extracts or artemisinin was incorporated into soil at a rate equivalent to the 0.73% (w/w) treatment. Inhibition of growth was species-specific; estimated reduction of dry weight by 0.73% (w/w) leaf tissue was 82, 49, 25, and 9% for redroot pigweed, common lambsquarters, soybean, and corn, respectively. The effects of the MeCl2 extract, which contained all of the extractable artemisinin, on germination and growth of redroot pigweed were similar to that of leaf tissue. Annual wormwood leaf tissue and MeCl2-extract treatments were the only treatments that resulted in a reduction in seedling survival. Artemisinin at levels equivalent to that contained in the MeCl2 extract and leaf-tissue treatments had significantly less effect on seedling survival, germination, and growth of redroot pigweed than the MeCl2 extract. Furthermore, the aqueous extract, which did not contain artemisinin, and the extract residue had activities similar to that of the artemisinin treatment. Thus, the allelopathic effects of annual wormwood can not be attributed to artemisinin alone.

Sorghum (Sorghum bicolor (L.) Moench) yields were correlated negatively with weed yields and positively with stover yields, sorghum heads per plant, and sorghum weight per 1000 seeds. Sorghum yields were not different when weeds were removed 1, 2, or 3 weeks after planting and each week thereafter, but a significant yield decrease occurred each week when weeds were not removed for 4, 5, 6, or 8 weeks. Sorghum kept weed-free the first 4 weeks after planting showed little yield loss from weeds emerging later. Weeds left after two cultivations reduced sorghum yields 38% below plots weeded weekly. As weed competition increased, sorghum grain yield, stover yield, sorghum heads per plant, seed weight per head, and weight per 1000 seeds decreased, whereas sorghum seed moisture at harvest increased.

Crop rows oriented at a right angle to sunlight direction (i.e., east–west within the winter cropping system in Western Australia) may suppress weed growth through greater shading of weeds in the interrow spaces. This was investigated in the districts of Merredin and Beverley, Western Australian (latitudes of 31° and 32°S) from 2002 to 2005 (four trials). Winter grain crops (wheat, barley, canola, lupines, and field peas) were sown in an east–west or north–south orientation. Within wheat and barley crops oriented east–west, weed biomass (averaged throughout all trials) was reduced by 51 and 37%, and grain yield increased by 24 and 26% (compared with crops oriented north–south). This reduction in weed biomass and increase in crop yield likely resulted from the increased light (photosynthetically active radiation) interception by crops oriented east–west (i.e., light interception by the crop canopy as opposed to the weed canopy was 28 and 18% greater in wheat and barley crops oriented east–west, compared with north–south crops). There was no consistent effect of crop row orientation in the canola, field pea, and lupine crops. It appears that manipulation of crop row orientation in wheat and barley is a useful weed-control technique that has few negative effects on the farming system (i.e., does not cost anything to implement and is more environmentally friendly than chemical weed control).

Studies to predict pesticide fate often lack measurements of model input parameters. Using independent data sets and understanding how soil properties affect herbicide retention and degradation may result in more accurate prediction of herbicide fate. We conducted laboratory studies to determine the influence of soil properties on atrazine adsorption and degradation. These data will be used in a separate study involving a pesticide fate model. Atrazine adsorption and desorption isotherms were constructed for six soil depths of a Hastings silty clay loam (fine, montmorillonitic, mesic Udic Argiustoll) using batch equilibration. The Freundlich adsorption constants (log Kf ) ranged from 0.38 (60 to 90 cm) to 2.91 (0 to 30 cm). Adsorption was higher in the low pH, high organic matter-containing surface soil compared to the lower soil depths. Multiple regression of the adsorption constants against selected soil properties indicated that organic matter content was the best single predictor of atrazine adsorption (R 2 = 0.98) followed by soil pH (R 2 = 0.82). Combining organic matter and cation exchange capacity in the model produced the lowest Cp statistic (2.33) and highest R 2 value (0.99). We observed hysteresis in atrazine adsorption–desorption isotherms by higher adsorption slopes (1/n)ads compared to desorption slopes (1/n)des. Soils that adsorbed more atrazine also desorbed less atrazine. Desorption correlated negatively with organic matter content and positively with soil pH. Atrazine degradation after 84 d of incubation generally decreased with increasing depth. The first-order degradation rate was highest 0 to 30 cm deep (0.0187 day−1) and lowest 270 to 300 cm deep (0.0031 day−1). Atrazine degradation was faster in soil treated annually for 12 yr than in soil with no previous atrazine history (p = 0.01).

California's interior grasslands have undergone dramatic changes during the last two centuries. Changes in land-use patterns and plant introductions after European contact and settlement resulted in the conversion of perennial-dominated grasslands to exotic annual grasses. More recently, the annual grasslands have been heavily invaded by the deeply rooted late-maturing forb yellow starthistle. This series of invasions and conversions has changed the community structure and phenology of the grasslands. We hypothesized that these changes have resulted in significant differences in soil water–use patterns in the grasslands. We studied soil water depletion and recharge patterns of three grassland community types dominated by perennial grasses, annual grasses, or yellow starthistle with contrasting phenology and rooting depths for 4 yr. Soil moisture measurements were taken every month from March to December in 1998, 1999, and 2000 and every other month in 2001. Measurements were taken with a neutron probe at depths of 30 to 150 cm at 30-cm intervals. The results indicate that the yellow starthistle community maintained a significantly drier soil profile than the annual grass community. The perennial grass community maintained an intermediate soil water content that was not significantly different from either of the other two communities. Significant time by community and depth by community interactions indicated that the yellow starthistle community continued depleting soil moisture later into the season and at deeper depths than the other grass communities. This study demonstrates the effect of plant invasion on soil water recharge and depletion patterns in California grasslands.

Competitive effects of cocklebur (Xanthium pensylvanicum Wallr.) and redroot pigweed (Amaranthus retroflexus L.) on cotton (Gossypium hirsutum L.) were evaluated in four experiments at two locations in Alabama during the period 1966 to 1968. Cocklebur was more competitive with cotton than was pigweed on Norfolk sandy loam soil. A density of eight cocklebur plants per 7.31 m of row reduced cotton yields 20 to 40%. Higher densities of cocklebur resulted in greater yield reductions. Forty-eight cocklebur plants per 7.31 m of row caused cotton yields to be reduced more than 80%. Pigweed at a density of 48 weeds per 7.31 m of row reduced cotton yields less than 50% on the Norfolk soil. On Lucedale sandy clay loam, cocklebur and pigweed were equally competitive and both species were more competitive than on Norfolk soil. A density of eight cocklebur or pigweed plants per 7.31 m of row resulted in 60 to 70% cotton yield reductions. The highest density of pigweed studied, 48 plants per 7.31 m of row, reduced cotton yields 90% on the Lucedale soil. Vegetative growth, boll, and seed size were not affected as much as cotton yields by either weed species. Percentage lint and fiber properties were unaffected by plant competition.

The inheritance of resistance to the auxinic herbicide dicamba was examined in a kochia population from Nebraska. An inbred, resistant line was developed by selection and selfing over seven generations to ensure any resistance alleles would be homozygous in the parents. An inbred, susceptible line was similarly developed, but without selection. Dose–response experiments with dicamba determined a glyphosate-resistant concentration required to inhibit dry weight accumulation by 50% (GR50) of 45 and 1,331 g ae ha−1 for the susceptible and resistant populations, respectively. F1 crosses were made between resistant and susceptible inbred individuals by hand-pollination, and the F1 plants were selfed to produce F2 plants. The F2 population was screened with 280 g ha−1 dicamba, a rate that could discriminate between susceptible and resistant plants. A total of eight F2 families were screened twice. In the first screen, seven F2 families segregated in a 3:1 ratio, consistent with a single dominant allele controlling resistance, and in the second screen six F2 families segregated in a 3:1 ratio. F2 individuals were selfed, the F3 progeny were tested with 280 g ha−1 dicamba, and the genotype of each F2 parent was determined based on F3 progeny segregation. F3 family segregation was consistent with the F2 parents having a 1:2:1 homozygous-susceptible:heterozygote:homozygous-resistant pattern, confirming that resistance to dicamba in kochia is likely conferred by a single allele with a high degree of dominance.

Genetic diversity of kochia was determined using 45 polymorphic intersimple sequence repeat markers. Comparisons of genetic diversity within and among populations were made after structuring 453 kochia plants into samples representing 13 populations on the basis of the collection site and into two sample groups on the basis of the cropping history and geographic region of the sites. Mean Nei's gene diversity (h) for the 13 populations was 0.35, and total diversity (H T) was 0.35, indicating that genetic diversity in kochia is very high. There was a greater proportion of diversity within (H S = 0.31) populations than among (G ST = 0.09) populations. A hierarchical analysis of molecular variance showed that 90% of the variation occurred within populations (P = 0.0001). Of the remaining variance, 4.5% was attributed to differences between the two groups (P = 0.0001) and 5.5% to differences among the 13 populations within the groups (P = 0.0001). Gene flow (Nm) among populations was 2.4. The high level of gene diversity and large proportion of within-population diversity in kochia suggest that, despite generations of herbicide selection, kochia maintains high genetic diversity through substantial levels of gene flow within and among populations.

The influence of environmental factors on seed germination and seedling emergence of American sloughgrass was studied in laboratory and greenhouse conditions. The optimum temperature for seed germination was 10 C and light was not necessary. Seed germination was sensitive to osmotic potential and completely inhibited at an osmotic potential of −0.6 MPa, but it was quite tolerant to salinity: germination occurred even at 160 mM NaCl (36%). More than 80% of seeds germinated at pH values ranging between 4 and 10. Seedling emergence was highest when seeds were placed on the soil surface (91%) but declined with burial depth. Few (3%) seedlings emerged when seeds were planted at a depth of 5 cm. Information gained in this study will lead to a better understanding of the requirements for American sloughgrass germination and emergence.

Controlling herbicide resistance (HR) and its associated environmental risks is impossible without integrating social and economic science with biophysical and technology aspects. Herbicide resistance is a dynamically complex and ill-structured problem involving coupled natural–human systems that defy management approaches based on simple scientific and technology applications. The existence of mobile herbicide resistance and/or herbicide tolerance traits add complexity because susceptibility to the herbicide is a resource open to all farmers, impacting the weed population. Weed scientists have extensively researched the biophysical aspects and grower perceptions of HR. They also recognize that the “tragedy of the commons” can appear when herbicide resistance is mobile across farms. However, the human structures and processes, especially private and public institutions that influence individual and group decisions about HR, have received little analysis. To start filling that gap, we discuss an integrative management approach to sustainable weed control that addresses the social complexity of farm heterogeneity. For example, the need for a private or public collective mechanism becomes apparent to address common-pool resource (CPR) aspects when one farmer's weed control actions influence their neighbors' situations. In such conditions, sole reliance on education, technical assistance, and other incentives aimed at changing individual grower behavior likely will fail to stem the advance of HR. Social science theories can be used to enrich the understanding of human interaction with the biophysical environment and identify key actors and social change processes influencing those interactions in the case of HR. The short-run economic advantages of herbicides such as glyphosate work against social change to address HR, including the development of collective actions when mobile HR conditions exist. We discuss seven design principles that can improve the efficacy and cost of such collective approaches and draw insights from CPR approaches outside of HR.

Cover crops are increasingly being used for weed management, and planting them as diverse mixtures has become an increasingly popular strategy for their implementation. While ecological theory suggests that cover crop mixtures should be more weed suppressive than cover crop monocultures, few experiments have explicitly tested this for more than a single temporal niche. We assessed the effects of cover crop mixtures (5- or 6-species and 14-species mixtures) and monocultures on weed abundance (weed biomass) and weed suppression at the time of cover crop termination. Separate experiments were conducted in Madbury, NH, from 2014 to 2017 for each of three temporal cover-cropping niches: summer (spring planting–summer termination), fall (summer planting–fall termination), and spring (fall planting–subsequent spring termination). Regardless of temporal niche, mixtures were never more weed suppressive than the most weed-suppressive cover crop grown as a monoculture, and the more diverse mixture (14 species) never outperformed the less diverse mixture. Mean weed-suppression levels of the best-performing monocultures in each temporal niche ranged from 97% to 98% for buckwheat (Fagopyrum esculentum Moench) in the summer niche and forage radish (Raphanus sativus L. var. niger J. Kern.) in the fall niche, and 83% to 100% for triticale (×Triticosecale Wittm. ex A. Camus [Secale × Triticum]) in the winter–spring niche. In comparison, weed-suppression levels for the mixtures ranged from 66% to 97%, 70% to 90%, and 67% to 99% in the summer, fall, and spring niches, respectively. Stability of weed suppression, measured as the coefficient of variation, was two to six times greater in the best-performing monoculture compared with the most stable mixture, depending on the temporal niche. Results of this study suggest that when weed suppression is the sole objective, farmers are more likely to achieve better results planting the most weed-suppressive cover crop as a monoculture than a mixture.

Research was conducted from 2015 to 2017 to investigate the potential for 2,4-D and multiple herbicide resistance in a waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] population from Missouri (designated MO-Ren). In the field, visual control of the MO-Ren population with 0.56 to 4.48 kg 2,4-D ha−1 ranged from 26% to 77% in 2015 and from 15% to 55% in 2016. The MO-Ren population was highly resistant to chlorimuron, with visual control never exceeding 7% either year. Estimates of the 2,4-D dose required to provide 50% visual control (I50) of the MO-Ren population were 1.44 kg ha−1 compared with only 0.47 kg 2,4-D ha−1 for the susceptible population. Based on comparisons to a susceptible population in dose–response experiments, the MO-Ren population was approximately 3-fold resistant to 2,4-D, and 7-, 7-, 22-, and 14-fold resistant to atrazine, fomesafen, glyphosate, and mesotrione, respectively. Dicamba and glufosinate were the only two herbicides that provided effective control of the MO-Ren population in these experiments. Examinations of multiple herbicide resistance at the individual plant level revealed that 16% of the plants of the MO-Ren population contained genes stacked for six-way herbicide resistance, and only 1% of plants were classified as resistant to a single herbicide (glyphosate). Results from these experiments confirm that the MO-Ren A. tuberculatus population is resistant to 2,4-D, atrazine, chlorimuron, fomesafen, glyphosate, and mesotrione, making this population the third 2,4-D–resistant A. tuberculatus population identified in the United States, and the first population resistant to six different herbicidal modes of action.

This article briefly reviews how long weed seeds can live in the soil and what happens to them during burial. Freshly matured seeds of many weeds are water-permeable, but those of others are water-impermeable. Water-permeable seeds may have morphological, physiological, or morphophysiological dormancy, with physiological dormancy (PD) being the one most commonly found in buried weed seeds. Further, nondeep PD is very common in buried weed seeds, and many of them exhibit annual dormancy cycles in response to seasonal temperature changes. The time of year when seeds are nondormant varies with the species, i.e., autumn, spring, or spring to summer. A light requirement for germination plays an important role in preventing nondormant seeds from germinating in the soil. To germinate, soil disturbance that exposes seeds to light must occur at a time of year when seeds are nondormant. Buried seeds of some species come out of dormancy and remain nondormant regardless of seasonal changes in environmental conditions; however, a light requirement for germination prevents them from germinating in the soil. Water-impermeable seeds have either physical dormancy (PY) or a combination of PY and PD, with PY being the most common, e.g., in members of the Fabaceae and Malvaceae. Seeds with PY have a water gap in the seed coat that opens in response to an environmental signal, thereby allowing water to enter. When disturbance brings seeds to the soil surface, temperatures that are higher than those in the soil can cause the water gap to open. Consequently, the water gap indirectly serves as a depth sensor. A challenge for the future is to use information about buried weed seeds to better manage weeds in crops, and modeling is an important step in that direction.

Field studies were conducted in 2007 and 2008 at Clinton and Faison, NC, to evaluate the influence of Palmer amaranth density on ‘Beauregard’ and ‘Covington’ sweetpotato yield and quality and to quantify the influence of Palmer amaranth on light interception. Palmer amaranth was established at 0, 0.5, 1.1, 1.6, 3.3, and 6.5 plants m−1 within the sweetpotato row and densities were maintained season-long. Jumbo, number (no.) 1, and marketable sweetpotato yield losses were fit to a rectangular hyperbola model, and predicted yield loss ranged from 56 to 94%, 30 to 85%, and 36 to 81%, respectively for Palmer amaranth densities of 0.5 to 6.5 plants m−1. Percentage of jumbo, no. 1, and marketable sweetpotato yield loss displayed a positive linear relationship with Palmer amaranth light interception as early as 6 to 7 wk after planting (R2 = 0.99, 0.86, and 0.93, respectively). Predicted Palmer amaranth light interception 6 to 7, 10, and 13 to 14 wk after planting ranged from 47 to 68%, 46 to 82%, and 42 to 71%, respectively for Palmer amaranth densities of 0.5 to 6.5 plants m−1. Palmer amaranth height increased from 177 to 197 cm at densities of 0.5 to 4.1 plants m−1 and decreased from 197 to 188 cm at densities of 4.1 to 6.5 plants m−1; plant width (69 to 145 cm) and shoot dry biomass plant−1 (0.2 to 1.1 kg) decreased linearly as density increased.

Studies were conducted at the Main Agricultural Experiment Station in Fayetteville and the Vegetable Substation in Kibler, Arkansas, in 1992 and 1993 on the same plots to evaluate weed suppression by winter cover crops alone or in combination with reduced herbicide rates in no-till sweet corn and to evaluate cover crop effects on growth and yield of sweet corn. Plots seeded to rye plus hairy vetch, rye, or wheat had at least 50% fewer early season weeds than hairy vetch alone or no cover crop. None of the cover crops reduced population of yellow nutsedge. Without herbicides, hairy vetch did not suppress weeds 8 wk after cover crop desiccation. Half rates of atrazine and metolachlor (1.1 + 1.1 kg ai ha−1) reduced total weed density more effectively in no cover crop than in hairy vetch. Half rates of atrazine and metolachlor controlled redroot pigweed, Palmer amaranth, and goosegrass regardless of cover crop. Full rates of atrazine and metolachlor (2.2 + 2.2 kg ai ha−1) were needed to control large crabgrass in hairy vetch. Control of yellow nutsedge in hairy vetch was marginal even with full herbicide rates. Yellow nutsedge population increased and control with herbicides declined the second year, particularly with half rates of atrazine and metolachlor. All cover crops except hairy vetch alone reduced emergence, height, and yield of sweet corn. Sweet corn yields from half rates of atrazine and metolachlor equalled the full rates regardless of cover crops.

Season-long interference of shattercane, giant foxtail, common cocklebur, and common lambsquarters in corn was evaluated in the field at weed densities from 0.4 to 13.1 plants or clumps/m of corn row during 1985, 1986, and 1987. Corn seed yields decreased linearly with increases in the density of clumps of 2 to 3 shattercane plants and 5 to 8 giant foxtail plants, reaching 22% yield loss at 6.6 shattercane clumps/m of row and 18% yield loss at 13.1 giant foxtail clumps/m of row. Increases in common cocklebur density caused corn yields to decrease curvilinearly in 1985 with a maximum predicted yield loss of 27% occurring at a density of 4.7 common cocklebur plants/m of row. In 1986 and 1987, yields decreased linearly as common cocklebur density increased to 6.6 plants/m of row, where a 10% yield loss was observed. Common lambsquarters reduced corn yields only in 1985. In this year, yields decreased curvilinearly with increasing weed density, resulting in a maximum yield loss of 12% at 4.9 common lambsquarters plants/m of row. Corn yields averaged 11600 kg/ha when grown without weed interference in these experiments.

The disappearance of EPTC (ethyl N,N-di-n-propylthiolcarbamate) from soils was studied by steam distilling the remaining EPTC from the soil samples and analyzing the distillates. The most important factor affecting the loss of EPTC from soil was the amount of moisture in the soil. During the first 15 minutes after spraying on the soil surface, 20% of the applied EPTC disappeared from dry soil, 27% from moist soil and 44% from wet soil. The loss was 23, 49, and 69% after one day and 44, 68, and 90% after six days on dry, moist, and wet soils, respectively. No significant loss of EPTC occurred the first few hours after applying granular EPTC to dry soil or soil containing 4 to 6% moisture, but on soil at 15% moisture the loss from granules was 60% in two hours. There appeared to be a critical moisture level for each soil, above which large losses of EPTC occurred after application to the surface or after shallow incorporation. Depth of incorporation was the second most important factor in EPTC loss from soils. Incorporation to at least 2 to 3 inches was necessary to prevent large losses of EPTC when light rain or sprinkler irrigation followed treatment. EPTC was lost at a faster rate on sunny days than cloudy days. Increasing the temperature from 32 to 60 F, increased the rate of EPTC vaporization from moist soil but had no effect on the loss from dry soil.

Imazethapyr-resistant biotypes of Palmer amaranth and common waterhemp were studied to determine the magnitude of resistance and cross-resistance to three acetolactate synthase (ALS)-inhibiting herbicides. Resistant biotypes of Palmer amaranth and common waterhemp demonstrated > 2,800- and > 130-fold resistance to phytotoxicity of imazethapyr compared to susceptible biotypes, respectively. Concentrations of imazethapyr required for 50% in vivo inhibition of ALS activity were at least > 13,100 and > 1,900 times greater for resistant biotypes of Palmer amaranth and common waterhemp, respectively, compared to susceptible plants. Resistant biotypes of both species demonstrated cross-resistance to the sulfonylurea herbicides thifensulfuron and chlorimuron at the whole plant and enzyme levels, indicating that a less sensitive ALS enzyme confers this resistance to these plants.

Herbicides are small molecules that inhibit specific molecular target sites within plant biochemical pathways and/or physiological processes. Inhibition of these sites often has catastrophic consequences that are lethal to plants. The affinity of these compounds for their respective target sites makes them useful tools to study and dissect the intricacies of plant biochemical and physiological processes. For instance, elucidation of the photosynthetic electron transport chain was achieved in part by the use of herbicides, such as terbutryn and paraquat, which act on photosystem II and I, respectively, as physiological probes. Work stemming from the discovery of the binding site of PS II–inhibiting herbicides was ultimately awarded the Nobel Prize in 1988. Although not as prestigious as the seminal work on photosynthesis, our knowledge of many other plant processes expanded significantly through the ingenious use of inhibitors as molecular probes. Examples highlight the critical role played by herbicides in expanding our understanding of the fundamental aspects of the synthesis of porphyrins and the nonmevalonate pathway, the evolution of acetyl-coenzyme A carboxylase, cell wall physiology, the functions of microtubules and the cell cycle, the role of auxin and cyanide, the importance of subcellular protein targeting, and the development of selectable markers.

Defatted seedmeals from 15 glucosinolate-containing plant species were analyzed for herbicidal activity by determining inhibition of seedling emergence when added to a sandy loam soil containing wheat and sicklepod seeds at concentrations of 0.1, 0.5, and 1% (w/w). In general, the seedmeals were more phytotoxic to wheat than sicklepod. For wheat, all of the seedmeals significantly inhibited seedling emergence at the 1.0% concentration. At the 0.1% concentration three of the seedmeals (Indian mustard, money plant, and field pennycress) completely inhibited wheat emergence. For sicklepod emergence, eight of the seedmeals were completely inhibitory at the 1% level (Indian mustard, field pennycress, garden rocket, Siberian wallflower, English wallflower, garden cress, sweet alyssum, and evening stock) and four were completely inhibitory at the 0.5% level (brown mustard, garden rocket, English wallflower, and sweet alyssum). Intact glucosinolates and their corresponding hydrolysis products varied among the seedmeals with the highest activity. Major hydrolysis products produced by the seedmeals with the most phytotoxicity, respectively, included 2-propenyl (allyl) isothiocyanate (AITC) by brown mustard seedmeal, allyl thiocyanate and AITC by field pennycress seedmeal, erucin (4-methylthiobutyl isothiocyanate) by arugula seedmeal, 3-butenyl isothiocyanate and lesquerellin (6-methylthiohexyl isothiocyanate) by sweet alyssum seedmeal, and isopropyl isothiocyanate by money plant seedmeal. From our data it appears that both the type and concentration of glucosinolates and their hydrolysis products present in the seedmeals affect seed-emergence inhibition.