Saflufenacil is a PRE herbicide for the control of broadleaf weeds. Field and growth room studies were conducted to explore the tolerance of corn to POST treatments of saflufenacil and BAS 781. Additionally, the potential use of sodium as a safener for saflufenacil was evaluated. Crop injury caused by saflufenacil or BAS 781 was 8 and 38%, respectively, when applied at twice the recommended dose at the spike to two-leaf stage of crop growth. This injury increased to 28 and 65%, respectively, when applied at the three- to four-leaf stage. This level of crop injury resulted in yield loss, particularly when applied at the three- to four-leaf stage. The addition of Na-bentazon to saflufenacil reduced this injury and increased crop dry weight under both field and laboratory conditions. In the field, Na-bentazon also increased corn collar height and yield compared with saflufenacil applied alone. Na-bentazon reduced injury through a reduction in foliar uptake of saflufenacil. Sodium derived from baking soda also provided a safening effect, but only at the lowest dose of saflufenacil tested.

Controlled environment experiments showed that velvetleaf plants grown under drought stress or low temperature (LT) treatments had greater leaf epicuticular wax (ECW) deposition compared to plants grown in soil with moisture at field capacity (FC) or a high temperature (HT) regime. Light intensity did not affect ECW deposition; however, increasing light intensity decreased the leaf ECW ester content and increased the secondary alcohol content. Plants grown at an LT regime or under FC had leaf ECW with fewer hydrocarbons and more esters than those grown at an HT or drought stress regime. Velvetleaf absorption of acifluorfen increased as light intensity decreased for plants grown in adequate soil water content, while the opposite was true for drought-stressed plants. Velvetleaf absorption of acifluorfen was approximately 3 and 10 times greater, respectively, with the addition of 28% urea ammonium nitrate (UAN) in comparison to crop oil concentrate (COC) or no adjuvant, regardless of the environmental treatments. Plants absorbed more acifluorfen when subjected to the LT regime in comparison to the HT regime when UAN was the adjuvant, while the opposite was true when COC was the adjuvant. Velvetleaf absorption of acifluorfen was not affected by drought stress when COC or no adjuvant was used and varied between studies when UAN was used. Velvetleaf absorption of bentazon was greatest for plants grown under HT/FC or high light/FC treatments and least with plants grown under HT/drought stress or low light/drought stress treatments, regardless of the adjuvant. However, bentazon absorption was higher with the addition of an adjuvant and for plants grown at a high light intensity or FC condition compared with medium to low light intensity or drought stress treatments.

Resistance in waterhemp to herbicides that inhibit protoporphyrinogen oxidase (PPO) previously was shown to result from the deletion of a glycine codon at position 210 (ΔG210) in the PPO-encoding gene, PPX2. Research was conducted to determine if this same mechanism accounted for resistance in geographically separated populations—from Illinois, Kansas, and Missouri—and, if so, to determine if the mutation conferring resistance was independently selected. A dose–response study with lactofen indicated that the resistant populations had different levels of resistance. These differences, however, could be accounted for by different frequencies of resistant individuals within populations and, therefore, the dose–response data were consistent with the hypothesis that the populations contained the same resistance mechanism. Direct evidence in support of this hypothesis was provided by DNA sequencing, which showed that nearly all resistant plants evaluated contained the ΔG210 mutation. A variable region of the PPX2 gene was sequenced and resulting sequences were aligned and organized into a phylogenetic tree. The phylogenetic tree did not reveal clear clustering by either geography or phenotype (resistant vs. sensitive). Possibly recombination within the PPX2 gene has masked its evolutionary history.

The increase in atmospheric CO2 levels can influence the growth of many invasive exotic plant species. However, it is not well-documented, especially for C4 plants, how these growth responses will alter the effectiveness of the world's most widely used herbicide for weed control, glyphosate. We aimed to address this question by carrying out a series of glasshouse experiments to determine if tolerance to glyphosate is increased in four C4 invasive exotic grasses grown under elevated CO2 in nonlimiting water conditions. In addition, traits including specific leaf area, leaf weight ratio, leaf area ratio, root : shoot ratio, total leaf area, and total biomass were measured in order to assess their contribution to glyphosate response under ambient and elevated CO2 levels. Three of the four mature grass species that were treated with the recommended concentration of glyphosate displayed increased tolerance to glyphosate under elevated CO2. This was due to increased biomass production resulting in a dilution effect on the glyphosate within the plant. From this study, we can conclude that as atmospheric CO2 levels increase, application rates of glyphosate might need to be increased to counteract the growth stimulation of invasive exotic plants.

Greenhouse and field trials were conducted at the Aberdeen Research and Extension Center to determine the effect of hairy nightshade competition on two potato varieties with different growth habits. Greenhouse replacement trials included treatments of three plants total in each pot with potato : hairy nightshade ratios of 3 : 0, 2 : 1, 1 : 2, or 0 : 3. Varieties tested were ‘Russet Burbank’ and ‘Russet Norkotah’. Greenhouse-grown hairy nightshade (cotyledon to one-leaf stage) were transplanted into pots earlier than or at the same time as potato plant emergence. As the number of Russet Burbank plants per pot decreased, potato plant biomass dry weight (average per plant) increased, regardless of hairy nightshade number or transplant time. When hairy nightshade were transplanted before Russet Norkotah emergence, potato plant biomass dry weight per plant was similar, regardless of potato : nightshade ratio. Field trials were conducted with Russet Burbank and Russet Norkotah in 2004 and 2005. At potato emergence, greenhouse-grown hairy nightshade plants (one- to two-leaf) were transplanted in between potato rows at densities of 0, 1, 2, or 3 plants m−1 row, and solid-seeded at approximately 100 plants m−1 row. Hairy nightshade biomass, stem and berry number, and seeds per berry were reduced by competition from Russet Burbank due to the amount and duration of shading, as compared with Russet Norkotah. Russet Burbank U.S. No. 1 and total tuber yields in plots with 1 hairy nightshade plant m−1 row were similar to weed-free control yields, whereas yields in plots with 2, 3, or 100 m−1 row were reduced in comparison. In contrast, Russet Norkotah yields were reduced when only 1 hairy nightshade plant m−1 row was present. Overall, Russet Norkotah were less competitive with hairy nightshade than Russet Burbank in both the greenhouse replacement and field trials.

Recent studies have suggested that soybeans express shade avoidance in response to low red : far-red (R : FR) light reflected from neighboring plants and that this response may determine the onset and outcome of crop–weed competition. We tested the hypothesis that the low R : FR ratio would trigger characteristic shade avoidance responses in soybean and that the subsequent phenotype would experience reproductive costs under non–resource-limiting conditions. Soybeans were grown in a fertigation system in field trials conducted in 2007 and 2008 under two light quality treatments: (1) high R : FR ratio (i.e., weed-free) i.e., upward reflected light from a baked clay medium (Turface MVP®), or (2) low R : FR ratio (i.e., weedy) of upward reflected light, from commercial turfgrass. Results of this study indicated that a reduction in the R : FR ratio of light reflected from the surface of turfgrass increased soybean internode elongation, reduced branching, and decreased yield per plant. Shade avoidance also increased the plant-to-plant variability in biomass and yield per plant. Per plant yield losses were, however, more closely associated with reductions in biomass accumulation than population variability as the expression of a shade avoidance response did not influence harvest index. While these results suggest that weed induced shade avoidance decreases soybean per plant yield by reducing branching, it is possible the productivity of a soybean stand as a whole may be buffered against these reduction by a similar, but opposite, expression of plasticity in branching.

Soil water availability is the most important factor limiting crop yield worldwide. Understanding crop and weed transpiration in response to water supply may provide valuable insight into the mechanisms of crop yield loss in water-limited environments. A greenhouse experiment was conducted to quantify corn and velvetleaf transpiration in response to drying soil. Five plants of each species were well watered by adding back the equivalent water loss each day to reach field capacity, and five plants were subjected to drought stress (dry-down) by not replacing lost water. Normalized daily transpiration of dry-down plants was regressed on soil water content expressed as the fraction of transpirable soil water (FTSW). The critical soil water content below which plants begin to close their stomates occurred at FTSWcr = 0.36 ± 0.015 for corn and 0.41 ± 0.018 for velvetleaf. Total water transpired did not differ among species. Velvetleaf also responded to drought by senescing its oldest leaves, whereas corn mainly maintained its leaf area but with rolled leaves during peak drought stress. During a short-term drought, corn is expected to perform better than velvetleaf because it maintains full transpiration to a lower FTSW and does not senesce its leaves. Under severe long-term drought, the species that closes its stomates at greater FTSWcr will conserve water and increase its chances of survival. Moreover, senescing all but the youngest leaves may ensure at least some seed production. Research is needed to evaluate the effects of soil water supply on corn–velvetleaf interference in the field.

Horseweed has become increasingly common and difficult to control in San Joaquin Valley vineyards, due in part, to the evolution of glyphosate resistance. The development of weed-suppressive vineyard designs in which the trellis design, spacing, and row orientation combine to cast dense shade on the weed canopy zone (WCZ) may reduce weed growth. The relevance of such a system to horseweed, which can grow to be as tall, or taller, than a typical grapevine trellis, is uncertain. Also unknown is whether a glyphosate-resistant (GR) biotype and glyphosate-susceptible (GS) biotype would perform similarly under such conditions. Therefore, we compared the growth and development of two potted horseweed biotypes (GR and GS) in vinerows oriented east–west (EW) and north–south (NS). Rows oriented EW allowed less light penetration to the WCZ than NS rows throughout the study, and horseweed biotypes responded to low light levels by producing leaves with larger specific leaf area and leaf area ratios than those in the NS rows. Also, the leaf, stem, and root dry weight of the horseweed plants in the EW rows was reduced by 30% compared to the horseweed plants in NS rows. Leaf number was also reduced in the horseweed plants in the EW rows, but only for the GS biotype. Row orientation did not affect phenological development or the number of seeds produced by the GR or GS biotypes, but the GR biotype budded, flowered, and set seed approximately 1 wk earlier than the GS biotype. Thus, shade associated with the EW vinerows reduced horseweed growth, but not fecundity, and the GR biotype reached reproductive maturity earlier than the GS biotype.

Flax is a minor oilseed crop in Canada largely exported to the European Union for use as a source of industrial oil and feed ingredient. While flax could be genetically engineered (GE) to enhance nutritional value, the adoption of transgenic technologies threatens conventional flax market acceptability. Harvest seed loss of GE crops and the persistence of GE crop volunteers in the seed bank are major factors influencing transgene persistence. Ten commercial fields in Alberta, Canada, were sampled after harvesting conventional flax in 2006 and 2007, and flax seed density and viability were determined. Additionally, artificial seed banks were established at two locations in Alberta in 2005 and 2006 to quantify persistence of five conventional flax cultivars with variability in seed coat color (yellow or brown) and α-linolenic acid (ALA, 18:3cisΔ9,13,15) content (3 to 55%) at three soil depths (0, 3, or 10 cm). Harvest methods influenced seed loss and distribution, > 10-fold more seed was distributed beneath windrows than between them. Direct harvested fields had more uniform seed distribution but generally higher seed losses. The maximum yield loss was 44 kg ha−1 or 2.3% of the estimated crop yield. Seed loss and the viability of flax seed were significantly influenced by year, presumably because weather conditions prior to harvest influenced the timing and type of harvest operations. In artificial seed bank studies, seed coat color or ALA content did not influence persistence. Flax seed viability rapidly declined in the year following burial with < 1% remaining midsummer in the year following burial but there were significant differences between years. In three of four locations, there was a trend of longer seed persistence at the deepest burial depth (10 cm). The current study predicts that seed-mediated gene flow may be a significant factor in transgene persistence and a source of adventitious presence.

The critical period for weed control for many crops has been well investigated and the results have been used to develop better weed management recommendations. However, research is limited on the critical period for weed control for alfalfa, a perennial crop with multiple harvests. With the advent of new, more effective POST herbicides for alfalfa, an understanding of the critical period may further enhance forage yield. The objective of this study was to determine the critical period for weed control in spring-seeded alfalfa. Alfalfa was seeded conventionally at two locations in Pennsylvania in the spring of 2004 and 2005; glyphosate and glyphosate-resistant alfalfa were used for weed management. A surrogate weed, Japanese millet, was seeded at three planting rates to simulate varied levels of weed density. In the analysis, the critical period for weed control was identified for the 2004 seedings, but not for 2005. In 2004, the critical period varied by location, but began at the 0.5 trifoliate alfalfa growth stage and ended at the 7 trifoliate growth stage. When identified, the critical period was similar for both first-harvest yield and first-year cumulative yield. Higher weed density caused the critical period to begin earlier than under lower weed densities, but weed density had no effect on when the critical period ended. Under moderate to heavy weed populations, the critical period for weed control can increase herbicide use efficiency and maximize alfalfa returns, although under low weed severity, the critical period may not exist.

The activity-density of Amara aenea (DeGeer) and Harpalus pensylvanicus (DeGeer) (Coleoptera: Carabidae) was monitored in an experiment that compared five management treatments representing a range of disturbance frequencies, crops, and aboveground biomass production. In 2004 and 2005, three treatments comprised of multiple summer cover crops were compared to bare fallow and soybean, the latter of which used mechanical cultivation to manage weeds. In 2005 weed seed predation was assessed from June to September in two of the treatments (bare fallow and oat–pea/rye–hairy vetch). Beetle activity-density varied with treatment, time of sampling, and year. In 2004 peak activity-density of A. aenea was highest in the mustard/buckwheat/canola, but there was no difference in H. pensylvanicus activity-density. In 2005 activity-density of H. pensylvanicus was higher in oat–pea/rye–hairy vetch than in soybean treatment. Seed predation rates were relatively consistent across treatments, averaging between 38 and 63%. In fallow and oat–pea/rye–hairy vetch, H. pensylvanicus activity-density accounted for 29 and 33% of the variation in seed predation, respectively. Our findings suggest cover crops have a positive effect on the activity-density of A. aenea and H. pensylvanicus and that disturbance negatively influences their activity-density in the absence of cover crops.

This study describes the distribution patterns of Johnsongrass populations present in 38 commercial corn fields located in three major corn growing regions of Spain. A total of 232 ha were visually assessed from the cabin of a combine during harvesting using a three-category ranking (high density, low density, no presence) and recording the georeferenced data in a tablet personal computer. On average, 10.3 and 3.9% of the surveyed area were infested with high and low density of Johnsongrass, respectively. Most of the infested area was concentrated in a few large patches with irregular shape. Small patches (less than 1,000 m2) represented only 27% of the infested area. Management factors could explain much of the spatial distribution of this weed in the studied fields. Tillage direction was the main factor explaining patch shape: the length width−1 ratio of the patches was greater than two in the tillage direction. In sprinkler irrigated fields, higher levels of infestation were generally observed close to the sprinkler lines. Areas close to the edges of the field had a higher risk of infestation than the areas in the middle of the fields: a negative relationship between distance from the edge and weed abundance was established. Because a few patches, located in some predictable parts of the field, such as field edges, represent most of the seriously infested area, site-specific treatments of these areas could reduce herbicide inputs, until more reliable, spatially precise and practical detection, mapping, and spraying systems are developed.

Common lambsquarters, a summer annual weed, has occupied comparatively different ecogeographical regions around the globe. To investigate the extent to which germination and emergence traits have differentiated in two natural populations of common lambsquarters from different environments (Denmark as mesic and Iran as xeric population), experiments were conducted in the laboratory. Germination of both populations was stimulated by light. The greatest germination percentage of xeric and mesic populations occurred at 25/15 and 20/10 C, respectively. The xeric population showed significantly greater germinability at the lowest (15/5 C) and the highest (35/25 C) fluctuating temperature regimes. Germination of the xeric population was only slightly affected, and the mesic population was severely retarded at a salinity level of 20 desi siemens (DS) m−1. Greater than 42% of the xeric seeds germinated at 30 dS m−1 salinity, while the mesic germination almost ceased at this salinity level. Germination of seeds placed in distilled water after the 14-d salinity exposure treatments (recovery rates) was also greater for the xeric vs. mesic populations. Xeric populations showed more than 65% germination up to the osmotic potential of −0.4 MPa, while decreasing osmotic potential from 0 to −0.4 MPa caused an 80% reduction in mesic population germinability (9% germination). The greatest emergence of xeric (77%) and mesic (70%) populations occurred for seeds placed on the soil surface and no seedlings emerged from burial depth 3 cm. These study results suggest that common lambsquarters populations have differentiated for heat, salinity, and drought tolerance at germination stage. These contrasting germination patterns are appeared to be due to either adaptation via natural selection or maternal effects, or some combination of both. Though this study has certain limitations, it, through its findings and their propositions, adds to the existing knowledge about interpopulation differences in germination requirements of common lambsquarters, as a globally distributed species.

A conservation tillage study provided the opportunity to test whether tillage effects on the germinable weed seedbank would be consistent across different crop rotations and to investigate the potential residual effects of herbicide treatments terminated 12 yr earlier. Our objective was to measure the effects of tillage (moldboard plow [MP] vs. chisel plow [CP] vs. no-till [NT]), crop rotation (2-yr barley–red clover followed by 4-yr barley–canola–wheat–soybean rotation, compared to a cereal monoculture), and of a prior weed management factor (three intensity levels of herbicide use) on the density, diversity, and community structure of weed seedbanks. Species richness, evenness (Shannon's E), and diversity (Shannon's H′) of spring seedbanks varied little across treatments and over time. Total seedbank density generally increased as tillage was reduced, with some variations due to weed management in 1993 and crop rotation in 2006. Crop rotations generally had smaller seedbanks with fewer species than the monoculture. In 1993, seedbanks with minimum weed management were twice as dense as those with intensive or moderate weed management (approximately 6,000 vs. 3,000 seed m−2). By 2006, seed density averaged 6,838 seed m−2 across intensive and moderate weed management regardless of tillage, but was nearly twice as large in NT (12,188 seed m−2) compared to MP (4,770 seed m−2) and CP (7,117 seed m−2) with minimum weed management (LSD0.005 = 4488). Species with abundant seedbanks responded differently to treatments. Barnyardgrass and green foxtail had larger seedbanks in the monoculture than in the rotation. Common lambsquarters and pigweed species had large seedbanks in tilled treatments in the rotation, whereas yellow foxtail and field pennycress contributed to the large seedbanks observed in NT treatments. The latter two species were also associated with residual effects of weed management treatments (terminated 12 yr earlier) in NT. The differential seedbank response of weed species, attributed in part to contrasting weed emergence patterns and agronomic practice effects on seed rain, explained some of the weak treatment effects observed for total seedbank density and diversity. The large weed seedbanks observed in NT plots after 18 yr confirms the importance of seed rain and seedbank management for the sustainability of NT systems.

Ripgut brome has become a problematic weed in Spain both as a consequence of the continuous cropping of winter wheat through minimal tillage systems and its difficult control with selective herbicides. Ripgut brome populations collected in the regions of Castilla-León and Cataluña, two main cereal-growing areas in Spain, were screened in the greenhouse for response to sulfosulfuron, a selective herbicide for the control of brome grasses in wheat, and to glyphosate, often used as a pre-plant knockdown to control bromes in no-till systems. The fresh weight percentage relative to the untreated controls was calculated for each ripgut brome population and herbicide and was used as a measure of the herbicide response. Results showed variation in fresh weight response to both herbicides among populations. Fresh weight of the populations after sulfosulfuron was applied at the two-leaf stage at a rate of 20 g ai ha−1 varied from 3% in the most susceptible population to 35% in the most resistant; the response was similar (6 to 38%) when the herbicide dose was reduced to half. For glyphosate at 800 g ae ha−1, fresh weight varied from 2 to 25% among populations, but the range of variation in fresh weight response increased as herbicide dose decreased to one half, with rates of from 4% to 90% among populations. The location of the collection site (inside the field or in-margin) showed no differences in response to both herbicides, but there was a statistically significant, geographically correlated differentiation for glyphosate response, with a greater resistance in the populations from Castilla-León. Undamaged plants were found after treatments with both herbicides, showing differences in resistance among plants. The study shows inter- and intrapopulation variability for the response of ripgut brome to sulfosulfuron and glyphosate. The implications for resistance development are discussed within the framework of relationships of the structure of the populations relative to their herbicide response.

The analysis of allelopathic bioassay data commonly encounters two problems: one is the small number of biological replicates and the other is that the parameters used to infer allelopathic effects such as percentage and average time of seed germination are analyzed individually, and consequently, information on the global effects resulting from the cumulative effects of the tested agent (e.g., leaves extract) may be missed. Therefore, we propose an index to analyze several parameters altogether so as to have a better view of the global influence of a donor plant on a receptor, whose interference is more likely to be detected. The global effect index can help to detect allelopathic interferences of one plant on another, allowing for a more accurate interpretation of the data in the actual biological setting.

Studies were conducted to determine the growth responses of rice to PRE application of oxadiazon and POST application of bispyribac-sodium. Oxadiazon at 1.0 and 1.5 kg ha−1 and bispyribac-sodium at 0.030 and 0.045 kg ha−1 were applied to four rice varieties (‘IR64’, ‘IR72’, ‘RC09’, and ‘RC18’), which were grown in saturated and aerobic (30% of saturation) soils. Control treatments, where no herbicides were applied, were also included in the study. Shoot and root biomass, and height of rice plants were measured 14 d after application. Phytotoxic effects for both herbicides, including reduced shoot and root biomass, were consistent in all varieties. Rice phytotoxicity symptoms were greater when herbicides were applied to saturated than to aerobic soils. Oxadiazon at 1.0 kg ha−1 reduced rice shoot biomass by 22 to 36% in aerobic condition, and 43 to 56% in saturated condition when compared with the control. Bispyribac-sodium reduced rice shoot biomass by 9 to 17% at 0.030 kg ha−1 in aerobic soil and 23 to 37% in saturated soil. The results of this study suggest that soil water content is an important factor influencing herbicide phytotoxicity in rice, and its influence warrants further research to improve understanding of physiology of phytotoxicity to minimize the effects of these herbicides on crop production.

Bittercress (Brassicaceae) is one of the most prolific and costly weeds of the container nursery industry. Bittercress accessions from container nurseries throughout the major production zones in the United States were examined and compared with herbarium specimens. The identity of these weedy bittercress species were further explored using sequences of the nuclear ribosomal DNA (nrDNA) internal transcribed spacer (ITS) region and the nrDNA region for the COP1-interacting protein 7 (CIP7). Four species of bittercress were detected in the nursery industry of the United States, including New Zealand bittercress, hairy bittercress, flexuous bittercress, and little bittercress. The taxon referred to here as Cardamine flexuosa With. (flexuous bittercress) likely contains two genotypes previously reported as European C. flexuosa and Asian C. flexuosa. Phylogenetic relationships between the four species we examined, particularly in relationship to flexuous bittercress, were not fully resolved by the molecular evidence generated for this study. New Zealand bittercress is nonnative and does not appear in current keys to the species for the United States. Flexuous bittercress is also an alien species, which appears in some U.S. keys but not in all. To aid nurserymen and botanists in identification of these four closely related bittercress species, a key was developed and is accompanied by detailed descriptions and illustrations.