Red rice is a major weed problem in rice production of the southern United States and other rice-producing countries. One hundred thirty red rice accessions from 26 rice-growing counties in Arkansas were tested for tolerance to imazethapyr in seed- and whole-plant response bioassays. The red rice accessions were compared with imazethapyr-resistant (ClearfieldTM) rice cultivars (‘CL121’, ‘CL161’, and ‘CL-XL8’) and conventional rice cultivars (‘Bengal’, ‘Dongjin’, ‘Drew’, and ‘Wells’). Red rice accessions 79, 84, and 118 showed 17-, 48-, and 37-fold more tolerance to imazethapyr, respectively, than the standard susceptible red rice accession (82) in whole-plant bioassays. The imazethapyr-resistant rice cultivars, CL121, CL161, and CL-XL8 were 41-, >177-, and 48-fold more resistant to imazethapyr, respectively than the susceptible standard. The imazethapyr-tolerant red rice and ClearfieldTM cultivars were generally cross tolerant to other acetolactate synthase (ALS; EC 4.1.3.18) inhibiting herbicides such as imazapyr, imazaquin, imazamox, and pyrithiobac. The tolerance level of red rice or rice to imidazolinone herbicides was highest with imazaquin and lowest with imazapyr. The imazethapyr-tolerant red rice accessions and ClearfieldTM rice were susceptible to glufosinate and glyphosate. The ALS enzyme of tolerant red rice accessions was less sensitive to imazethapyr than the susceptible standard, but tolerance at the enzyme level was less than at the whole-plant level. Therefore, tolerance of red rice to imazethapyr may involve other mechanisms besides an insensitive target site. We learned that a few imazethapyr-tolerant red rice populations existed probably before ClearfieldTM rice was introduced, supporting the hypothesis that evolution of herbicide-resistant red rice populations can happen with intensive herbicide selection pressure.

Growth room studies were conducted to determine the physiological basis of reduced glyphosate efficacy under low soil nitrogen using velvetleaf, common ragweed, and common lambsquarters as model species. Glyphosate dose–response experiments of weeds grown under low (1.5 mM) and high (15 mM) soil N were conducted. Velvetleaf and common lambsquarters grown under low N required 169 g ae ha−1 glyphosate for a significant reduction in biomass, but only 84 g ae ha−1 were required when grown under high N. However, when common ragweed was grown under low or high soil N there was no significant difference in response to glyphosate at all doses tested. The reduced glyphosate efficacy on velvetleaf and common lambsquarters under low N was primarily due to decreased herbicide translocation to the meristem. It appears that low N may decrease the net assimilation of carbon in plants, resulting in a decrease in the net export of sugars and hence glyphosate from mature leaves. Understanding the relationship between soil N and herbicide efficacy may help explain observed weed control failures with glyphosate and may contribute to our knowledge of the occurrence of weed patchiness in fields. This is the first report illustrating a physiological basis for decreased glyphosate efficacy under low soil N in selected weed species.

Greenhouse and laboratory experiments were conducted to evaluate the tolerance of four commercial sugarbeet varieties to s-metolachlor and dimethenamid-P, determine the principle site of absorption of these herbicides, and determine the physiological basis for differential tolerance among varieties to these herbicides. ‘Beta 5833R’ was the most tolerant sugarbeet variety, and ‘Hilleshog 7172RZ’ was the most susceptible sugarbeet variety to injury from s-metolachlor and dimethenamid-P. The primary site of s-metolachlor and dimethenamid-P absorption was through the sugarbeet roots; however, some absorption occurred through the sugarbeet hypocotyl. Sugarbeet injury was greater from dimethenamid-P than s-metolachlor when sugarbeet was grown in soil. However, when sugarbeet was grown hydroponically, injury from the herbicides was similar, indicating that the relative availability of these herbicides in the soil greatly influenced sugarbeet injury. Reduced translocation and slower metabolism of 14C-dimethenamid-P in both the roots and shoots of the sugarbeet plants most likely contributed to the greater susceptibility of sugarbeet to dimethenamid-P compared with s-metolachlor. Metabolism of 14C-herbicides in sugarbeet shoots was 0.7 to 2.1 h slower in the more susceptible sugarbeet varieties compared with the more tolerant variety, Beta 5833R. This was the most significant factor contributing to differences in sugarbeet variety tolerance to both s-metolachlor and dimethenamid-P.

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.

Gene flow between jointed goatgrass and winter wheat is a concern because transfer of herbicide-resistance genes from imidazolinone-resistant (IR) winter wheat cultivars to jointed goatgrass could restrict weed-management options for this serious weed of winter wheat cropping systems. The objectives of this study were (1) to investigate the frequency of interspecific hybridization between IR wheat and jointed goatgrass in eastern Colorado, and (2) to determine the gene action of the IR acetolactate synthase (ALS) allele in IR wheat by jointed goatgrass and in IR wheat by imidazolinone-susceptible (IS) wheat backgrounds. Jointed goatgrass was sampled side-by-side with IR wheat and at distances up to 53 m away in both experimental plots and at commercial field study sites in 2003, 2004, and 2005. A greenhouse-screening method was used to identify IR hybrids in collected jointed goatgrass seed. The average percentage of hybridization across sites and years when IR wheat and jointed goatgrass were grown side-by-side was 0.1%, and the maximum was 1.6%. The greatest distance over which hybridization was documented was 16 m. The IR ALS allele contributed 25% of untreated ALS activity in jointed goatgrass by IR wheat F1 plants, as measured by an in vitro ALS assay. The hybridization rate between wheat and jointed goatgrass and the expression of the IR wheat ALS allele in hybrid plants will both influence trait introgression into jointed goatgrass.

The predictability of the spring emergence period of kochia is a potentially exploitable attribute that may be applied to weed management. The emergence timing of kochia was monitored, during the spring and summer of 2005 and 2006, in 12 farmed fields in the northern region of the Northern Great Plains (NGP) of North America. Kochia emergence began at 50 cumulative growing-degree days (GDD Tbase 0 C, starting January 1) and continued into late summer. The onset of kochia emergence occurred well before the emergence onset of other weed species common to this region. Fall soil samples revealed few kochia seeds remained in the seedbank. In growth room studies, kochia seed placed at the soil surface had greater emergence compared with seed burial to a 10 mm or greater depth. Kochia did not emerge from seed planted 80 mm deep; however, radical emergence was evident indicating fatal germination. Kochia should be controlled early in the growing season, and burying seed will greatly reduce future kochia populations.

Croftonweed has established itself as a pernicious weed in China. Experiments were conducted to test for anemophily, self-pollination, hand crossing, and apomixis. Additionally, pollen viability, seed set, and seed viability were investigated. Pollen cultured in vitro with the use of 14 culture media did not germinate, nor did self- or cross-pollen germinate on the stigma, and no pollen tubes were seen in the style. Young embryos developed into globular- or heart-shaped embryos before the opening of the captimulum, although neither meiosis to form an embryo sac nor fertilization occurred. Furthermore, seeds developed normally after closed flowers were emasculated and bagged. The plump seed ratio (plump seeds/total seeds) from apomixis (emasculation and bagged treatment) was high (41 ± 17.0%), and plump seed germination ratio exceeded 50%. These results provided additional evidence for the previous conclusion that croftonweed reproduces by apomixis. Apomixis, high seed production, and high seed germination may explain why this weed spreads so rapidly.

African rue is an invasive herbaceous perennial that occurs in several states in the western United States. The ability of African rue seedlings to tolerate and recover from progressive drought was examined in greenhouse experiments. Water was withheld for 15 d, and a subset of plants were rewatered after 12 d of water deficit to examine recovery. Conductance rate decreased to 0.1 mol H2O m−2 s−1 and photosynthesis rate decreased to 2 µmol CO2 m−2 s−1 within 6 and 12 d, respectively. Leaf water potential decreased more slowly than gas exchange rates; after 15 d of water deficit plants maintained net carbon gain at −4.8 MPa. Photosynthesis and conductance rates of rewatered plants recovered to levels similar to well-watered controls within 9 and 12 d, respectively. After 9 d of water deficit, seedlings needed only 4 d to recover physiological function similar to well-watered controls. Reduced seedling biomass was observed after 6 d of water deficit, and biomass remained smaller than controls after 15 d of recovery. The rapid change in conductance rate and slower response in leaf water potential indicates that stomatal control is an important component of seedling response to water deficit. The success of African rue in arid environments is due in part to the ability of seedlings to tolerate and recover from water deficit.

Common groundsel is an important weed of strawberry and other horticultural crops. Few herbicides are registered for common groundsel control in such crops, and understanding and predicting the timing and extent of common groundsel emergence might facilitate its management. We developed simple emergence models on the basis of soil thermal time and soil hydrothermal time and validate them with the use of field-derived data from Minnesota and Ohio. Soil thermal time did not predict the timing and extent of seedling emergence as well as hydrothermal time. Soil hydrothermal time, adjusted for shading effects caused by straw mulch in strawberry, greatly improved the accuracy of seedling emergence predictions. Although common groundsel generally emerges from sites at or near the soil surface, the hydrothermal model better predicts emergence when using hydrothermal time at 5 cm rather than 0.005 cm, probably because of the volatility of soil temperature and water potential near the soil surface.

As atmospheric carbon dioxide concentration ([CO2]) increases, it is anticipated that the competitive ability of C3 crops could be enhanced relative to C4 weeds in agricultural systems. However, given the different nitrogen use efficiencies of C3 and C4 plants, it is unclear whether any effect of increasing [CO2] on C3/C4 competition is nitrogen dependent. To determine the interaction of [CO2] and N availability on species growth and competitive outcomes, the growth of rice (C3 photosynthetic pathway) was examined in both monoculture and in competition with a common weed, barnyardgrass (C4 photosynthetic pathway) at two levels of N supply (0.357 and 1.071 mmol N L−1) and two levels of [CO2] (ambient and ambient + 200 µmol mol−1) under field conditions in eastern China. In monoculture, the biomass response of rice to elevated [CO2] depended on N supply, whereas the response of barnyardgrass to elevated [CO2] was less dependent on nitrogen. Consequently, when grown in mixture, the proportion of rice biomass increased relative to that of barnyardgrass under elevated [CO2] if the supply of nitrogen was adequate. However, if N was low, elevated [CO2] significantly reduced the proportion of leaf area and root biomass relative to barnyardgrass biomass. Although data from this experiment confirm that competitiveness of rice could be enhanced relative to C4 weeds in response to rising [CO2] in situ, the data also indicate that such a response could be contingent on the supply of nitrogen. This suggests that, for rice cropping systems where N is in limited supply, rising atmospheric CO2 could still exacerbate competitive losses, even from C4 weeds.

Gene flow among herbicide-resistant (HR) canola varieties can lead to the development of multiple HR canola plants, creating volunteer canola management challenges for producers. In western Canada, escaped populations of HR canola are ubiquitous outside of cultivated fields, yet the extent of gene flow resulting in herbicide resistance trait stacking in individuals within these populations remains unknown. The objectives of this study were to document the presence of single and multiple herbicide resistance traits and assess the extent of gene flow within escaped canola populations. Seed was collected from 16 escaped canola populations along the verges of fields and roadways in four agricultural regions in southern Manitoba from 2004 to 2006. Glyphosate resistance was found in 14 (88%) of these populations, glufosinate resistance in 13 (81%) populations, and imidazolinone resistance in five (31%) populations. Multiple herbicide resistance was observed at levels consistent with previously published canola outcrossing rates in 10 (62%) of the tested populations. In 2005 and 2006, maternal plants from two escaped populations were tested using trait indicator test strips for glyphosate and glufosinate resistance to confirm outcrossing events. In 2005, two of 13 tested maternal plants with single herbicide resistance traits produced progeny with both glyphosate and glufosinate resistance. In 2006, of 21 tested plants, 10 single HR maternal plants produced multiple HR progeny, and five nonresistant maternal plants produced resistant offspring. This is the first report indicating that intraspecific gene flow results in stacking of herbicide resistance traits in individuals within escaped canola populations, confirming that multiple HR canola volunteers are not confined to agricultural fields. Results of this study suggest that escaped populations of crop plants can contribute to the spread of genetically engineered novel traits, which has important implications for containment, especially for highly controversial pharmaceutical and industrial traits in crop plants.

Weeds are often portrayed as growing in resource-rich environments. However, weeds growing within crops often deal with variable nitrogen (N) availability and reduced levels of light quantity and quality as a result of the crop canopy. In order to explore how weeds adapt to such stressful growing conditions, growth-cabinet studies were conducted using common lambsquarters as a model weed to determine how light, defined in terms of photosynthetic photon flux density (PPFD) and quality (red to far-red light ratio [R/FR]), and N stress influence the expression of adaptive traits that contribute to survival. Development rate of common lambsquarters was not influenced by low N; however, low N in addition to low R/FR delayed the rate of leaf appearance. Main-stem leaf number was reduced by low PPFD but was insensitive to N and R/FR. Neither doses of N had any influence on the shoot-to-root ratio. Plants also responded to the interaction of light and N. Under low PPFD and high N, plants adapted by growing taller, increasing biomass allocation to leaves, and producing more leaf area per mol of accumulated incident PPFD. Plants adapted to the most stressful treatment combination of low PPFD and low N by producing thinner leaves and increasing inflorescences per mol of accumulated incident PPFD. Seed production was reduced under low PPFD, but 1,000-seed weight and carbon concentration was unaffected. Although reduced in number, the total N concentration of the seed increased under low PPFD treatments, especially under low N. The adaptive traits identified in this study provide a greater understanding of the survival and persistence of common lambsquarters.

Crop tolerance (CT), the crop's ability to endure or avoid competitive stress from weeds, varies between old and modern dent corn hybrids; however, this hypothesis has not been tested in sweet corn. Three modern sweet corn hybrids, known to vary in canopy density, were subjected to a range of wild-proso millet densities under irrigated conditions in Washington and primarily rain-fed conditions in Illinois. A path analysis was used to identify relationships among CT and specific canopy properties important to competitive interactions. Crop tolerance varied among hybrids in three of four site–years. Sweet corn hybrid ‘Spirit’ suffered higher yield losses than hybrids ‘WHT2801’ and ‘GH2547’. Generally higher yield loss parameter estimates in Illinois, compared with Washington, suggests CT may have more to offer for weed management in the north-central than north-western United States. Path analysis indicated that wild-proso millet biomass and seedling population density were both important factors driving yield loss in canopy-sparse Spirit, whereas only early season wild-proso millet population density contributed to yield loss of canopy-dense WHT2801 and GH2547. Differential tolerance to weed interference exists among commercially available sweet corn hybrids.

Recent studies have shown major advantages of increased crop density and spatial uniformity for competition of wheat with weeds. Field experiments were performed over 3 yr to determine whether the effects of crop density and sowing pattern on weed suppression are influenced by nitrogen fertilization. The independent variables were crop sowing pattern (normal rows and a highly uniform pattern), seeding density (204, 449, and 721 seed m−2) and nitrogen fertilization (0 and 80 kg nitrogen ha−1) of spring wheat, grown under high weed pressure. Increased crop density had strong and consistent negative effects on weed biomass and positive effects on crop biomass and yield. At the highest crop density, weed biomass was less than half that at the lowest density. Weed biomass was generally lower, and yield higher, in the uniform pattern, except in one case in which a combination of factors gave one weed species an early size advantage over the crop. When weeds were controlled with herbicide, no effects of crop density or spatial uniformity on crop biomass or yield were observed. Nitrogen fertilization increased weed biomass in 2 of 3 yr, and it also increased crop biomass in 2 of 3 yr, but there was little evidence that the relative effects of crop density and spatial pattern on weed suppression were influenced by nitrogen fertilization. In the presence of weeds, the highest yields were obtained with high crop density, high spatial uniformity and nitrogen fertilization. The results indicate that increased weed suppression through increased crop density and spatial uniformity will occur over a wide range of nitrogen levels.

Certain winter annual weeds have been documented as alternative hosts to soybean cyst nematode (SCN), and infestations of such species have become common in no-till production fields in the Midwest. This research was conducted to determine the influence of herbicide- and cover-crop-based winter annual weed management systems and crop rotation on winter annual weed growth and seed production, SCN population density, and crop yield. Two crop rotations (continuous soybean and soybean-corn) and six winter annual weed management systems (a nontreated control, fall and spring herbicide applications, spring-applied herbicide, fall-applied herbicide, fall-seeded annual ryegrass, and fall-seeded winter wheat) were evaluated in no-tillage systems from fall 2003 to 2006 at West Lafayette, IN and Vincennes, IN. Fall or spring herbicide treatments generally resulted in lower winter annual weed densities than cover crops. Densities of henbit and purple deadnettle increased over years in the cover crop systems but remained constant in the herbicide systems. Averaged over sites and years, winter annual weed densities were nearly 45% lower in the spring than the fall due to winter mortality. Corn yield was reduced by the cover crops at West Lafayette but not Vincennes. Winter annual weed management system had no influence on soybean yield. SCN population density was reduced by including corn in the crop sequence but was not influenced by winter annual weed management. The density of weedy host species of SCN in the experimental area was relatively low (less than 75 plants m−2) compared to densities that can be observed in production fields. The results of these experiments suggest that inclusion of corn into a cropping sequence is a much more valuable SCN management tool than winter annual weed management. In addition, control of winter annual weeds, specifically for SCN management, may not be warranted in fields with low weed density.

Defoliation can affect plant competition in at least two ways: negative effects on the performance of defoliated plants and positive effects on nondefoliated plants due to neighbor defoliation. It has been suggested that these effects can alter competitive relations between plants, and this study was designed to test this in an experiment using Crofton weed as the invasive category, and its two neighbors Japanese brome and Dichrocephala as the native category. The plants were grown from seed and potted either singly or in pairs. The pairs consisted either of two conspecific plants (same pairs) or of one plant of each category (mixed pairs). Randomly preselected plants were defoliated twice in the growing season, at 9 wk and again at 5 wk before final biomass sampling. Competition reduced growth by 51 to 78%, with the plants of the native category consistently more affected than the invasive category. When grown singly, Crofton weed grew larger (average 67%) than the native species. As compared with plants grown singly, Crofton weed plants grown in mixed pairs were 40% smaller. The corresponding data for the native species were 78% smaller for Japanese brome and 73% smaller for Dichrocephala. The effects of neighbor defoliation differed among categories and competition. All the nondefoliated plants grown singly were larger than plants grown with a defoliated conspecific neighbor. For the corresponding relationship in mixed pairs, the native plants grown singly were significantly larger than the same native species grown together with a defoliated, invasive neighbor. Defoliation of Crofton weed reduced the growth of native neighbors and increased negative competitive effects on native plants. For the invasive species, however, Crofton weed plants grown in mixed pairs with neighbor defoliation compensated fully for the competitive effects, i.e., they were not significantly different from nondefoliated plants grown singly. Consequently, the effects of defoliation on competition between invasive Crofton weed and its native neighbors may depend on the ability to undertake compensatory growth, and probably on the allelopathic exudation of Crofton weed to herbivory. Our data suggest that herbivory by biocontrols can make an already superior competitor even stronger, especially if the biocontrol agent does not effectively damage or kill the target plant. Owing to complex interactions between competition and defoliation, the indirect effects of herbivory may be more complicated than currently conceived, and understanding the indirect effects of biocontrol agents before their release is crucial.

Dodder is a parasitic weed that is troublesome to the growth of many plants. Our study shows that this invasive species contains strong allelopathic potential, exerting strong inhibition against the growth of indicator plants and noxious paddy weeds in bioassay and pot trials. In a greenhouse, incorporation of 0.5 t ha−1 of dried dodder plants to paddy soil reduced spontaneous growth of paddy weeds by about 50%, whereas the 1.5 to 2 t ha−1 dose suppressed biomass of paddy weeds by more than 75% and completely controlled emergence of barnyardgrass and monochoria. By the use of a separation resin, 22 compounds were separated from dodder and identified by gas chromatography–mass spectrometry as belonging to terpenes, long-chain fatty acids, phenols, phenolic acids, and lactone. Among these compounds, 15 substances were quantified and tested for their herbicidal activity. Quantity of cinnamic acid was the highest (37.3 mg g−1), followed by dihydro-5,6-dehydrokavain (DDK; 6.0 mg g−1), myristic acid (3.2 mg g−1), and methyl cinnamate (2.1 mg g−1), whereas the amounts of other compounds were between 0.01 and 0.1 mg g−1. It is suggested that the content of the terpenes within dodder, which was rather high in amount (0.41–2.1 mg g−1), correlated to its strength of chemical cues to find host plants. Cinnamic acid, DDK, methyl cinnamate, and vanillin exerted the most potent herbicidal activities against radish growth. Findings of this study propose that cinnamic acid, DDK, and methyl cinnamate are responsible for its strong phytotoxic action of dodder plants. However, whether these plant growth inhibitors and other compounds detected from the dodder can suppress emergence of their hosts as well as contributing to its strong invasiveness needs further elucidation.

Experiments were initiated to determine if the tropical soda apple (TSA) biological control agent, Tobacco mild green mosaic tobamo virus (TMGMV), could be mixed with synthetic herbicides to provide effective broad-spectrum weed control. When TMGMV was mixed with 2,4-D ester or amine, metsulfuron, or hexazinone, TSA control ranged between 80 and 100%. On average, TMGMV increased TSA control by 81% as compared to these herbicides applied alone. Treatment applications were made by rubbing only three leaves, not as a broadcast application. Although this is not the optimum method for herbicide application, it does indicate the level of control the herbicide alone potentially provided relative to the herbicide/TMGMV mixture. Results indicate that the majority of TSA control was due to virus and that the herbicides mixed with TMCMV did not interfere with the virus's ability to infect TSA. Additions of organosilicone adjuvants or low rates of crop oil or nonionic adjuvants to TMGMV solutions resulted in greater infection of TSA. The finding that TMGMV remains infective when mixed with herbicides will allow greater flexibility for landowners attempting to control TSA and other troublesome weeds.

Understanding the spatial variability of herbicide sorption to soil is important in determining the bioavailability as well as leaching potential of the chemical across a field. Multiple methods have been used to estimate herbicide sorption variability at the macroscale, but it has been difficult to measure soil heterogeneity or herbicide sorption at the individual field level. One method to determine soil heterogeneity is to create zones within a field based on maps of the apparent bulk soil electrical conductivity (ECa). These zones can be used to direct soil sampling to determine the fraction of organic carbon (foc) of each zone. The foc, in turn, can be used to predict the variability of herbicide binding among zones. Surface (0 to 30 cm) bulk-soil electrical conductivity (ECs) maps were made for three sandy fields in eastern Colorado, and soil samples were taken from the ECs zones within each field. The foc, and the soil–water partition coefficient (Kd) for EPTC, metribuzin, and metolachlor were determined for each sample. There were significant correlations between ECs and foc (R = 0.75) and between foc and Kd for EPTC, metribuzin, and metolachlor (R = 0.66, 0.61, and 0.71, respectively) across all three fields. Additional soil samples taken from the ECs zones located in previously unsampled areas of the three fields showed that one could reasonably predict Kd values for metribuzin, metolachlor, and possibly, EPTC based on the foc zones derived from ECs maps.