Medusahead (Elymus caput-medusae L.) matures later in the spring than most associated species, and has a seed head moisture content of above 30 per cent for approximately a month after leaves and stems begin to dry. High temperature is more injurious to seed viability when seed moisture content is high. Control burns of medusahead infested rangeland were most effective in late afternoon when burning slowly (into a mild wind) and at the soft dough stage of medusahead seed development.

A 4-year field study was conducted to determine losses and gains in alfalfa production and utilization due to direct competition of various densities of Canada thistle (Cirsium arvense (L.) Scop.) and their control. Regardless of initial Canada thistle density, within 4 years all stands under no control approached one plant/sq ft. Mowing after each grazing period for 4 years almost eliminated Canada thistle. Throughout the study, the rate of alfalfa stand reduction was the same under mowing treatment regardless of Canada thistle density. Mowing increased alfalfa production 6.2 T/A in 4 years over no control in plots having initial Canada thistle stands of two plants/sq ft. The production loss due to Canada thistle stands of two plants/sq ft was 7.4 T of alfalfa for a 4-year period. Alfalfa consumption was reduced 4.5 T/A.

A series of experiments was carried out on oat to test the efficacies of fluazifop and glyphosate against water-stressed plants, plants grown in low and high nitrogen (N), and plants treated with gibberellic acid (GA). In the laboratory, plants maintained at wilting point for 5 days before and 9 days after spraying with fluazifop (0.5 kg ae/ha) appeared healthy 32 days after herbicide application, while plants supplied with water throughout the experiment were completely chlorotic/necrotic and had main stem detachment from within the leaf sheaths. In the field, plants maintained unirrigated until 14 days after spraying with fluazifop (0.25 kg/ha) or glyphosate (0.18 kg ae/ha) showed greater tolerance of the herbicides than plants irrigated regularly. Under well-watered conditions in the laboratory and field, fluazifop (0.25 kg/ha) and glyphosate (0.18 kg/ha) were less toxic at low N than high N. Increased fluazifop activity at high N was associated with increased transport of herbicide to apical meristems. Addition of 200 μg GA into the leaf sheaths 2 days prior to spraying with fluazifop or glyphosate increased the efficacy of both herbicides at low N.

Penetration, translocation, and metabolism of acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} in soybean [Glycine max (L.) Merr. ‘Ransom’], common ragweed (Ambrosia artemisiifolia L.), and common cocklebur (Xanthium pensylvanicum Wallr.) were studied. Using liquid scintillation spectrometry and autoradiography, little movement of 14C-acifluorfen from the leaf surfaces of the two weed species could be detected in 24 h. After 48 h, less 14C was recovered from the leaf surface and more was found within the leaves of the two weed species. Autoradiographs of the weed showed limited acropetal movement of 14C from leaves 24 and 48 h after treatment. For soybean, most of the 14C still remained on the leaf surface after 48 h. Autoradiographs of soybean plants showed no movement from the treated leaflet. Studies using thin layer chromatography suggested that acifluorfen was metabolized within the plants. Rate of metabolism was inversely related to plant susceptibility (common ragweed and common cocklebur>soybean). The more rapid penetration and translocation, coupled with slower metabolism of acifluorfen by the weed species in comparison to soybean, may account for the difference in susceptibility of the weeds and soybean to acifluorfen.

Two separate field experiments were conducted to quantify the degree of plant-to-plant outcrossing and pollen-mediated gene flow (PMGF) in wild oat. The purpose of the study was to determine the extent to which pollen movement could contribute to the spread of herbicide resistance in this species. In both experiments, an acetyl-CoA carboxylase inhibitor–resistant (R) wild oat genotype (UM1) was used as the pollen donor and a susceptible (S) genotype (UM5) was used as the pollen receptor. Hybrid progeny resulting from a cross between UM1 and UM5 were identified using the herbicide resistance trait as a marker. In the plant-to-plant outcrossing experiment, single UM5 plants were closely surrounded by 20 homozygous R UM1 plants in hills. By screening seed from the S parent for resistance, outcrossing was determined to range from 0 to 12.3%, with a mean of 5.2% over 10 hills. In the PMGF experiment, single homozygous R UM1 plants were surrounded by UM5 plants arranged in a hexagonal pattern at low and high densities (total of 19 and 37 wild oat plants m−2), growing within spring wheat and flax crops. In the wheat crop, mean wild oat outcrossing was 0.08 and 0.05% at low and high densities, respectively. In the less competitive flax, corresponding outcrossing values were 0.07 and 0.16% at low and high densities, respectively. Distance from the pollen source was a significant factor only for the high-density planting arrangement in flax. Up to 77 R hybrid seeds were recovered from 6 m2 in the PMGF experiment, indicating that PMGF contributes to the evolution of resistance in wild oat populations. However, the contribution of pollen movement to resistance evolution and the spread of resistance in wild oat populations would be relatively small when compared with R seed production and dispersal from a resistant plant.

EDITOR'S NOTE: This manuscript was reviewed by six colleagues whose recommendations varied widely. Lack of repetition was a major concern. The authors address the problem in the last paragraph of the results section. Factors favoring publication included the worldwide importance of wild oats, the minimal data on gene flow in the species, and the fact that the results are consistent with those of other studies cited in this manuscript. The points raised by reviewers who did not favor publication, especially the role of the environment in pollen production and viability, are acknowledged.

R. L. Zimdahl, Editor

Scanning electron micrographs of spray droplet residual on wheat, sunflower, and kochia indicated that salts and various adjuvants applied with glyphosate affected deposit crystal content, thickness, and contact with the leaf surface. Spray deposits of glyphosate applied with diammonium sulfate contained distinct crystals, which related to enhanced toxicity of glyphosate applied alone, or to overcoming antagonism of glyphosate toxicity by calcium chloride, when applied to wheat. In general, glyphosate applied with antagonistic calcium chloride salt formed spray deposits that were amorphous, thick, and without crystals. Glyphosate sprayed with nonantagonistic diammonium sulfate and ammonium chloride salts had deposits with crystals and an evenly spread residue beneath the crystals. The various micrographs indicate that the antagonism of glyphosate phytotoxicity by salts may be in part from physical entrapment of glyphosate in the spray deposit. Glyphosate spray droplet residue contact with wheat, sunflower, and kochia surfaces related to observed differences in glyphosate toxicity to these species.

The ability to predict weed phenological development under field conditions is fundamental to the development of mechanistic weed–crop competition models. We studied how phenological development of common ragweed grown under field conditions could be explained using temperature and photoperiod responses derived from growth room experiments. We also determined the relationship between phenological development and common ragweed leaf area, dry matter production, and partitioning. Phenological development of common ragweed emerging at different times in the field was described by photothermal time based on temperature and photoperiod responses derived from growth room experiments. Estimated dates of phenological events of common ragweed were within 4 d of recorded values. Common ragweed seedling density did not influence phenological development. Common ragweed leaf area development, biomass partitioning, and total biomass were related to photothermal time accumulation. The results of this study are consistent with our hypothesis that phenological development is a major factor influencing the outcome of weed–crop competition. Results obtained from this study can be incorporated into a mechanistic model of weed–crop competition.

Studies were conducted to investigate the uniformity of epicuticular wax deposition on leaf blades of johnsongrass. Johnsongrass leaves grown under drought stress had greatly increased epicuticular wax weights compared to leaves from plants with adequate moisture, but relative humidity (95% vs. 40 ± 5%) had little effect on wax deposition. Wax weights decreased as leaves matured. Sections of lower leaf surfaces of young johnsongrass leaves tended to have more wax than sections of upper leaf surfaces, but weights were nearly equal on upper vs. lower leaf surfaces of older leaves. The narrow side of asymmetrical johnsongrass leaf blades often had more wax per unit area than the wide side. The area over the midvein contained more wax per unit area than either the narrow or wide side of the leaf blade. Greatest wax concentrations on individual leaves were over the midvein area near the leaf apex. Leaf blades of johnsongrass had more wax per unit area than leaves of corn or grain sorghum.

Intercropping with functionally diverse crops can reduce the availability of resources that could otherwise be used by weeds. An experiment was conducted across 6 site-years in New York and Maryland in 2013 and 2014 to examine the effects of functional diversity and crop species richness on weed suppression. We compared four annual crop species that differed in stature and nitrogen acquisition traits: (1) pearl millet, (2) sorghum sudangrass, (3) cowpea, and (4) sunn hemp. Crops were seeded in monoculture and in three- and four-species mixtures using a replacement design in which monoculture seeding rates were divided by the number of species in the intercrop. Crop and weed biomass were sampled at ~45 and 90 d after planting. At the first sampling date, intercrops produced more crop biomass than monocultures in all but 1 site-year; however, weed biomass in intercrops was lower than monocultures in only 1 site-year. By the second sampling date, crop biomass was consistently greater in the intercrops than in the monocultures, and weed biomass was lower in the intercrops than in monocultures in 2 site-years. Although we observed several negative relationships between crop species richness and weed biomass, crop biomass was a more important factor than species richness for suppressing weeds. Despite the weak weed suppression from the two legumes compared with the two grasses, legume crops can provide other benefits, including increased forage quality, soil nitrogen for subsequent crops, and resources for pollinators if allowed to flower. On the other hand, if weed suppression is the top priority, our results suggest that monocultures of high biomass–producing grasses will provide more effective suppression at a lower seed cost than functionally diverse intercrops that include low biomass–producing legumes in warm-season intercrops.

Greenhouse, laboratory, and field studies were conducted to evaluate the potential of nonionic surfactant (NIS), crop oil concentrate (COC), methylated seed oil (MSO), and 28% urea ammonium nitrate (UAN) to enhance whole plant efficacy, absorption, and spray retention of foliar applications of isoxaflutole to giant foxtail. In greenhouse studies, isoxaflutole at 10 g ai ha−1 reduced giant foxtail growth 5%, whereas the addition of a spray adjuvant reduced giant foxtail growth at least 75%. The addition of UAN improved giant foxtail growth reduction when used in combination with isoxaflutole plus NIS. Isoxaflutole spray retention on the leaf surface was increased with an adjuvant and a further increase was observed with the addition of UAN. Isoxaflutole applied with NIS, COC, and MSO resulted in 42, 60, and 91% 14C absorption, respectively, compared to 21% absorption from isoxaflutole applied alone 24 h after treatment (HAT). Increased 14C absorption and entry into the cuticle when an adjuvant was utilized with isoxaflutole resulted in greater translocation of 14C from isoxaflutole out of the treated leaf. Significant basipetal movement from foliar applications of 14C-isoxaflutole suggests phloem mobility. In field studies, isoxaflutole applied with MSO provided greater giant foxtail growth reduction compared to isoxaflutole applied with NIS and in some cases COC. The addition of UAN to isoxaflutole did not increase whole plant efficacy in field studies. These studies indicate isoxaflutole has excellent potential to be used for control of existing giant foxtail present at the time of corn planting if an adequate adjuvant is utilized.

Paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) was applied to peanuts (Arachis hypogaea L. ‘Florunner’) for Texas panicum (Panicum texanum Buckl. # PANTE) control at 0.07, 0.14, and 0.28 kg ai/ha under five application schedules; a single application at 0, 1, 3, or 5 weeks after ground cracking, or multiple applications at 1 + 5 weeks or 1 + 3 + 5 weeks after ground cracking. Peanuts were planted at two different times (normal and late). A herbicide application at the third week (normal planting), or the third or fifth week after ground cracking (late planting) provided acceptable Texas panicum control. Control generally increased with rate. However, increasing paraquat rates with single applications on late-planted peanuts or multiple applications to normal- and late-planted peanuts tended to reduce yield and grade. A single application of 0.07 or 0.14 kg/ha applied the third week after ground cracking consistently provided the best balance between Texas panicum control and crop safety.

There is much discussion as to why a plant becomes invasive in a new location but is not problematic in its native range. One example is yellow starthistle, which originates in Eurasia and is considered a noxious weed in the United States. We grew yellow starthistle originating from native and introduced regions in a common environment to test whether differences in growth would be observed. In growth chamber studies, seedlings originating from the invasive range were larger than seedlings from the native range after 2 wk. Seed starch content is an important component of initial seedling growth. The starch content of seeds from introduced populations was higher than that of seeds from native populations. Regression analysis showed a relationship between the amount of starch in the seeds and the weight of yellow starthistle seedlings after 2 wk growth. There was no difference in chromosome number, except in accessions originating from Sicily and Sardinia. Field studies conducted in France and Russia revealed that rosettes and mature plants grown under natural conditions were larger when grown from seeds originating from the invasive range than from seeds originating from the native range. The number of capitula per plant and stem diameters were not significant among all populations, but differences were noted. The F1 progeny of plants originating from U.S. seed, but grown and pollinated in France, showed no differences in seedling growth, mature plant characteristics, and seed starch content from the plants grown from field-collected U.S. seed. The changes in seed starch resource allocation and its relation to plant growth is useful in understanding factors that contribute to yellow starthistle's invasibility.

Because of its ability to induce contact dermatitis, the establishment and spread of poison ivy is recognized as a significant public health concern. In the current study, we quantified potential changes in the biomass and urushiol content of poison ivy as a function of incremental changes in global atmospheric carbon dioxide concentration (CO2). We also examined the rate of new leaf development following leaf removal to simulate responses to herbivory as functions of both CO2 and plant size. The experimental CO2 values (300, 400, 500. and 600 µmol mol−1) corresponded approximately to the concentration that existed during the middle of the 20th century, the current concentration and near and long-term projections for this century (2050 and 2090), respectively. Over 250 d, increasing CO2 resulted in significant increases in leaf area, leaf and stem weight, and rhizome length relative to the 300 µmol mol−1 baseline with the greatest relative increase occurring from 300 to 400 µmol mol−1. There was a nonsignificant (P = 0.18) increase in urushiol concentration in response to CO2; however, because of the stimulatory effect of CO2 on leaf biomass, the amount of urushiol produced per plant increased significantly for all CO2 above the 300 µmol mol−1 baseline. Significant increases in the rate of leaf development following leaf removal were also observed with increasing CO2. Overall, these data confirm earlier, field-based reports on the CO2 sensitivity of poison ivy but emphasize its ability to respond to even small (∼ 100 µmol mol−1) changes in CO2 above the mid-20th century carbon dioxide baseline and suggest that its rate of spread, its ability to recover from herbivory, and its production of urushiol, may be enhanced in a future, higher CO2 environment.

Managing weed resistance has become a major challenge for many agricultural producers. Resistance is growing in terms of the number of weeds exhibiting resistance and the number of herbicides to which weeds are becoming resistant. The susceptibility of weeds to herbicides in many regions is a diminishing common pool resource affected by local producer weed control actions and natural conditions. Given the growing number of weeds exhibiting resistance, and the recognition that weed resistance is not a private property issue, we argue that managing resistance must be viewed as a wicked problem with no standard template across regions. Finding farm management approaches that help farmers successfully address weed resistance requires a shared perspective that incorporates an improved understanding of the human dimensions of weed management. Through an analysis of wicked problem characteristics, we argue that a people-centered approach to weed management is necessary. We offer principles learned from tackling other wicked agriculture and resource conservation issues to guide such approaches. Education, technical assistance, incentive schemes and regulatory efforts, and other strategies can play roles in constructing management approaches for herbicide resistance, but will have to vary from current efforts to unravel the mysteries of more effective weed management. Building a more inclusive approach, in terms of stakeholders and disciplines, will be key to achieving progress.

An induced mutation of the common wheat (2n = 6x = 42, AABBDD genomes) cultivar ‘Fidel’ has been shown to provide resistance to the imidazolinone class of herbicides. This class of herbicide gives broad-spectrum weed control including the weedy relative of wheat, jointed goatgrass (2n = 4x = 28, CCDD genomes). Because wheat and jointed goatgrass share a common genome, genes present on the D genome may transfer between the two species as a result of natural hybridization and selective pressures. Our objectives were to determine which genome of common wheat contained the herbicide resistance gene in the mutated Fidel and to genetically map its position. We investigated the chromosomal location of this gene using both durum (2n = 4x = 28, AABB genomes) and common wheat (6x) backgrounds. From crosses of durum wheat genotypes as the recurrent parent with mutated Fidel (cv. 9804, resistant), only BC1 plants containing chromosome 6D (inherited from cv. 9804) were resistant to applications of labeled rates of imazamox, an imidazolinone herbicide. No other D-genome chromosome was absolutely associated with herbicide resistance. To confirm this chromosomal location and genetically map the position of this gene, two populations of F3 families from the cross of cv. 9804 to the common wheat cultivars ‘Cashup’ and ‘Madsen’ were screened for reaction to imazamox, followed by genetic mapping with microsatellite markers. Two linked microsatellite markers were associated with the resistance trait, and one of them, Xgdm127, was located to chromosome 6D using aneuploid stocks, confirming the location of this gene on 6D. These results indicate that this resistance gene is in the genome that common wheat shares with jointed goatgrass. Therefore, imidazolinone-resistant wheat will need to be carefully managed to minimize the occurrence and spread of resistant jointed goatgrass, whether such plants arise because of hybridization with resistant common wheat or by independent mutation, a frequent occurrence with this herbicide class.

Sulfentrazone persistence in soil requires many crop rotational restrictions. The sorption and mobility of sulfentrazone play an important role in its soil persistence. Thus, a series of laboratory experiments were conducted to mimic the soil properties of cation and anion exchange with different intermediates. The molecular characterization and ionization shift of sulfentrazone from a neutral molecule to an anion were determined using a three-dimensional graphing technique and titration curve, respectively. Sorption and mobility of 2.6 × 10−5 M 14C-sulfentrazone were evaluated using a soil solution technique with ion exchange resins and polyacrylamide gel electrophoresis, respectively. Solution pH ranged from 4.0 to 7.4. As pH increased, sulfentrazone sorption to an anion resin increased and its sorption to a cation resin decreased. Percent sulfentrazone in solution was pH-dependent and ranged between 0 to 18% and 54 to 88% for the anion and cation resins, respectively. Mobility of sulfentrazone on a 20% polyacryalmide gel resulted in Rf values of +0.02 and +0.39 for pH of 4.0 and 7.4, respectively. A double peak for sulfentrazone was detected in the polyacrylamide gel when the pH (6.0 and 6.8) was near the reported pKa of 6.56. There was no clear interaction for the sorption of sulfentrazone at 1.0 mg kg−1 to Congaree loamy sand or Decatur silty clay loam saturated with either calcium or potassium. Sulfentrazone behavior with the polyacrylamide electrophoresis gels and ion resins indicate the potential for this herbicide to occur as a polar or Zwitter ion. Sulfentrazone was adsorbed by potassium, calcium, and sodium saturated resins and subsequently desorbed using variable pH solutions. The level of sulfentrazone adsorption will vary among soil types and the amount of desorption into solution may be soil cation-dependent.

Formation and distribution of 14C-atrazine degradation products in the top 120 cm of soil were determined over 16 mo under field conditions in an Estherville sandy loam. After 16 mo, 78% of applied 14C was still present in the soil. By 2 mo after treatment (MAT), 14C had moved to the 30- to 40-cm depth; however, movement to depths greater than 40 cm was not observed. Greater than 98% of the 14C remaining in the soil profile after 16 mo was in the top 20 cm. Twenty-seven percent of the 14C applied was atrazine 16 MAT. Atrazine was the predominant 14C-compound in soil below 10 cm. Hydroxyatrazine (HA) was the major degradation product in the top 10 cm of soil. The proportion of 14C as HA in the top 10 cm increased from 15% 2 MAT to 37% 16 MAT. Deethylatrazine (DEA) was the predominant degradation product at the 10- to 30-cm depth and accounted for up to 23% of the 14C present in the 10- to 20-cm depth. Deisopropylatrazine (DIA) accounted for less than 6% of the radioactivity recovered at any soil depth. The proportion of DEA and DIA increased while the proportion of HA decreased as soil depth increased, indicating that DEA and DIA are more mobile in soil than HA. Detection of HA at depths greater than 10 cm appears to be due to in situ degradation of atrazine previously moved to that soil depth. The large amount of 14C remaining in the soil 16 MAT suggests that a large pool of atrazine and its degradation products are present in the soil for an extended period following application and have the potential to contaminate ground water.

Crop rotation has long been considered one of the simplest and most effective tools for managing weeds. In this paper, we demonstrate how crop rotations can be strategically arranged to harness a novel mechanism of weed suppression: weed-weed competition. Specifically, we consider how crop stacking, or increasing the number of consecutive plantings of a single crop within a rotation, can decrease the size of the weed seed bank, by forcing weeds to compete with each other in similar environments for longer periods of time, while still reaping the traditional benefits of crop rotation. Using an annual plant model, we investigate the theoretical effects of stacked crop rotations on weeds that have different life-history strategies and phenology. Our results show that when weeds compete within a season, stacking can reduce the weed seed bank compared to rotations without stacked crops. Although more research is needed to fully understand the effects of crop stacking on other aspects of the system, such as insect pests and diseases, our research suggests that crop stacking has the potential to improve weed suppression without additional inputs, and their associated costs and externalities. More generally, improving management by changing the temporal arrangement of disturbances is a novel, process-based approach that could likely be applied to other weed management practices, such as mowing and herbicide application, and which could involve mechanisms other than weed-weed competition. Leveraging this new application of existing ecological theory to improve weed management strategies holds great promise.

In western Canada, more money is spent on wild oat herbicides than on any other weed species, and wild oat resistance to herbicides is the most widespread resistance issue. A direct-seeded field experiment was conducted from 2010 to 2014 at eight Canadian sites to determine crop life cycle, crop species, crop seeding rate, crop usage, and herbicide rate combination effects on wild oat management and canola yield. Combining 2× seeding rates of early-cut barley silage with 2× seeding rates of winter cereals and excluding wild oat herbicides for 3 of 5 yr (2011 to 2013) often led to similar wild oat density, aboveground wild oat biomass, wild oat seed density in the soil, and canola yield as a repeated canola–wheat rotation under a full wild oat herbicide rate regime. Wild oat was similarly well managed after 3 yr of perennial alfalfa without wild oat herbicides. Forgoing wild oat herbicides in only 2 of 5 yr from exclusively summer annual crop rotations resulted in higher wild oat density, biomass, and seed banks. Management systems that effectively combine diverse and optimal cultural practices against weeds, and limit herbicide use, reduce selection pressure for weed resistance to herbicides and prolong the utility of threatened herbicide tools.

The herbicide 3’,4′-dichloropropionanilide (propanil) effectively controlled barnyardgrass [Echinochloa crusgalli (L.) Beauv.], brachiaria [Brachiaria platyphylla (Griseb.) Nash], crab-grass [Digitaria spp.], sesbania [Sesbania exaltata (Raf.) Cory], curly indigo [Aeschynomene virginica (L.) B.S.P.], spikerush [Eleocharis spp.], and umbrellasedge [Cyperus spp.] in the 1- to 4-leaf stages without significant injury to commercial varieties of rice in the 1- to 2-leaf stages. It was ineffective for control of ducksalad [Heteranthera spp.], redstem [Ammannia coccinea Rottb.], morningglory [Ipomoea spp.], sprangletop [Leptochloa panicoides (Presl.) Hitchc.], and red rice [Oryza sativa L.]. Mixtures of propanil and a blended surfactant, containing petroleum sulfonate, free and combined fatty acids, and petroleum oil, and mixtures of propanil and isopropyl N-(3-chlorophenyl)carbamate (CIPC) and methyl 2,4-dichlorocarbanilate (swep), controlled barnyardgrass better than propanil alone. Propanil sometimes injured the rice, but it recovered in 5 to 10 days after treatment and yield and quality were unaffected.