Experiments were conducted to examine the utility of a spectrophometric leaf disc assay for detecting shikimate accumulation after glyphosate application in sunflower, proso millet, and wheat. The assay was conducted on both greenhouse- and field-grown plants. Glyphosate was applied at five rates ranging from 840 to 53 g ae ha−1. Shikimate accumulation data were generated at 1, 4, 7, and 14 d after application (DAA). Sunflower accumulated shikimate more rapidly and at lower glyphosate rates than the other two species. At 14 DAA, glyphosate at the two highest rates remained detectable in all three species. Plants receiving lower glyphosate doses (210, 105, and 53 g ae ha−1) had begun to grow out of the injury, or at least the shikimate levels in the plants were no longer significantly different than that present in the untreated controls. This spectrophotometric assay is both rapid and simple, with respect to other means of detecting shikimate, and it can be used to detect glyphosate drift. For it to be used by crop managers, samples from potentially drift-affected crops should be taken as soon as possible after the suspected drift event or immediately after the appearance of glyphosate injury.

The effect of formulation and adjuvants on absorption and translocation of 14C-clethodim was determined at 1, 4, 12, 24, 48, and 72 h after treatment (HAT) in bermudagrass under greenhouse conditions. Absorption of 14C-clethodim with the 0.12 kg L−1 (15 to 85%) formulation was higher than with the 0.24 kg L−1 (5 to 40%) formulation, regardless of presence or absence of adjuvant. There was considerable variation in the effect of adjuvant on 14C-clethodim absorption. When either ammonium sulfate (AMS) or AMS plus crop oil concentrate (COC) was added to the 0.12 kg L−1 formulation, 14C-clethodim absorption increased significantly at all harvest times except at 12 HAT compared with 0.12 kg L−1 formulation alone, whereas, 14C-clethodim absorption after addition of COC to the 0.12 kg L−1 formulation was similar to the 0.12 kg L−1 formulation alone up to 24 HAT. Conversely, COC enhanced 14C-absorption at all harvest times when added to 0.24 kg L−1 formulation. Most of 14C-clethodim (79 to 100% of absorbed) remained in the treated leaf, independent of formulation or adjuvant. Formulation did not have an impact on distribution of absorbed 14C-clethodim; however, presence of an adjuvant increased movement of 14C-clethodim out of treated leaf. Of the absorbed 14C-label, most remained in the treated leaf. 14C-clethodim that translocated out of the treated leaf remained in the shoot, and negligible amount of 14C-clethodim translocated to roots. These results demonstrated improved absorption of clethodim with formulations containing half the active ingredient (0.12 kg L−1) and inclusion of both AMS and COC.

Hydrilla is one of the most serious aquatic weed problems in the United States, and fluridone is the only herbicide approved by the U.S. Environment Protection Agency that provides systemic control. Recently, hydrilla biotypes with varying levels of fluridone resistance have been documented in Florida. Hydrilla biotypes of varying fluridone resistance levels were maintained in 900-L tanks under natural atmospheric conditions from September 2004 to September 2005 in the absence of fluridone. Hydrilla shoot tips were collected from each biotype during September 2004 (at planting), December 2004 (3 mo after planting [MAP]), March 2005 (6 MAP), June 2005 (9 MAP), and September 2005 (12 MAP) and exposed to 5, 10, 15, 20, 30, and 50 µg L−1 fluridone to assess changes in susceptibility to this herbicide over time. Nonlinear regression analysis was used to calculate EC50 values for phytoene and β-carotene (effective fluridone concentration to increase/decrease the phytoene/β-carotene content in hydrilla plant tissue by 50% over the untreated control) at each time interval. EC50 values did not change in the susceptible hydrilla biotype over time. The EC50 values for phytoene and β-carotene for the susceptible biotype were 7.5 and 8.9 µg L−1, respectively, at planting and 7.6 and 9.4 µg L−1, respectively, at 12 MAP. Resistant hydrilla biotypes (R1–R5) also showed no change in EC50 phytoene values over time. Although, EC50 β-carotene values in resistant biotypes R1, R3, R4, and R5 did not change over time, R2 recorded a reduction in EC50 β-carotene at 12 MAP. Also, a 0.5-point decrease in resistance factor was observed for all resistant biotypes. Future long-term studies are needed to evaluate stability of resistant hydrilla biotypes in the absence of fluridone selection pressure.

Giant ragweed is a competitive, allergenic weed that persists in agricultural fields and early successional sites. Field experiments were conducted to determine the effects of seed size and seed burial depth on giant ragweed emergence and seed demise. In a seedling emergence experiment, small (< 4.8 mm in diameter) and large (> 6.6 mm in diameter) seeds were buried 0, 5, 10, and 20 cm in fall 1997, and weed emergence was monitored over the next seven growing seasons. A generalized linear mixed model fit to the cumulative emergence data showed that maximum emergence for both seed sizes occurred at the 5-cm burial depth, where probability of emergence was 19% for small seeds and 49% for large seeds. Emergence probability at the 10-cm burial depth was 9% for small seeds and 30% for large seeds, and no seedlings emerged from the 20-cm burial depth. The model predicted that ≥ 98% of total cumulative emergence was completed after four growing seasons for large seeds buried 5 cm, five growing seasons for small seeds buried 5 cm and large seeds buried 10 cm, and seven growing seasons for small seeds buried 10 cm. Seed size and burial treatment effects on seed demise were tested in a second experiment using seed packets. Rates of seed demise were inversely proportional to burial depth, and the percentage of viable seeds remaining after 4 yr ranged from 0% on the soil surface to 19% at the 20-cm burial depth. Some seeds recovered from the 20-cm burial depth were viable after 9 yr of burial. These results, coupled with previous research, suggest that seed size polymorphism facilitates giant ragweed adaptation across habitats and that a combination of no-tillage cropping practices, habitat modification, and timely weed control measures can reduce its active seed bank in agricultural fields by 90% or more after 4 yr.

Experiments were conducted in Sri Lanka to compare weed seedling emergence in three sugarcane plots of 0.1 ha planted in October 1995, January 1996, and April 1996. In each plot, weed seedling emergence was monitored for 20 wk in five permanent quadrats on each of three microsites: on ridges, in furrows, and on adjacent fallow land. Soil moisture (0 to 5 cm) and soil temperature (at 2.5 cm) were also recorded. Only crowfootgrass, swamp millet, and guineagrass (all grasses) occurred in all nine planting time-by-microsite combinations. About half of all seedlings emerging over the three planting times were swamp millet, and the next most frequent species was tropic ageratum. The composition of the emerged flora was similar on ridges and in furrows, but more seedlings emerged in the furrows than on the ridges. The highest number of emerged seedlings and of species occurred on adjacent fallow land. The major factor influencing seedling emergence appeared to be soil moisture.

Fatal germination of weed seeds occurs when a weed seed germinates, but the seedling dies before reaching the soil surface. Controlled-environment bioassays of velvetleaf and giant foxtail seed fate in Michigan field soil (Kalamazoo silt loam, 1.1% soil organic matter) were used to determine the role of pathogenic fungi and seed burial depth in fatal germination of these species. Fatal germination at 2 cm seed depth was nonexistent for giant foxtail, and rare (< 10% of seeds studied) for velvetleaf. At greater depths, fatal germination remained close to zero for giant foxtail, whereas it increased to as high as 40% for velvetleaf at a 10-cm burial depth. Cultures taken from fatally germinated velvetleaf seedlings were found to contain Pythium ultimum, a soilborne pathogen known as the causal agent for pea root rot. When samples of infected media taken from these cultures were used to inoculate field soil in pots, fatal germination of velvetleaf from depths of 4 to 6 cm increased, compared with field soil inoculated with sterile media. At seed burial depths of 8 and 10 cm, fatal germination of velvetleaf increased to 20 and 40%, respectively, and was the same for unsterilized soil and P. ultimum–inoculated soil. Given that maximal fatal germination of velvetleaf occurred in the unsterilized soil treatment at the 10 cm depth, burial of newly shed velvetleaf seeds to a 10 cm, or possibly greater, depth with tillage equipment may be a practical way of reducing velvetleaf seed banks through fatal germination.

In the Grande Ronde Valley of eastern Oregon, two perennial grass species in the genus Puccinellia, weeping alkaligrass and Nuttall's alkaligrass, are weeds of Kentucky bluegrass grass-seed production fields. Weeping alkaligrass is introduced from Eurasia, whereas Nuttall's alkaligrass is native to the region. These two species were studied to determine dormancy attributes and optimal temperature conditions for seed germination. Results from the current studies indicate that both species have a high level of embryonic dormancy immediately following seed harvest, which is primarily eliminated through dry storage (afterripening) and an incubation temperature of 20 C. Following adequate afterripening, a prechill treatment of 5 d at 5 C had an inconsistent effect on germination of weeping alkaligrass (P = 0.012 in 2002, 0.156 in 2003) and improved germination of Nuttall's alkaligrass over both years (P < 0.0001). The afterripening requirement for weeping alkaligrass was more than 90 d, whereas Nuttall's alkaligrass required more than 180 d. Following adequate afterripening, both species had rapid and well-synchronized germination at fluctuating day/night temperatures of 30/10 C given unlimited moisture conditions. Given these results, it is unlikely that seeds of either species would germinate in eastern Oregon during the summer months. The data predict a long viability period under dry storage for both species. Weeping alkaligrass and Nuttall's alkaligrass should exhibit a rapid, well-synchronized germination in the spring as observed in the field.

Siamweed is an asteraceous shrub native to the Neotropics that ranks among the world's most widespread and troublesome invasive species. It was introduced in several regions of Africa, Southeast Asia, and the Pacific Islands, where it severely infests natural habitats and plantation crops. Although extensive data document the weed's abundance and distribution throughout the invaded continents, the details of its current range are not fully known, especially within its native region. In this study, we used point-occurrence data and digital maps summarizing relevant environmental parameters to generate predictions for the species' geographic distributional potential—specifically, we modeled the native range of siamweed in the Neotropics using the genetic algorithm for rule-set prediction, an evolutionary computing approach. The native range occurrence data set contained 239 published and herbarium records. Models were trained on a random subset of half the points and tested using the other half. The rule sets of the native-range models were projected onto the invaded continents to predict the weed's potential for invasion, blind to its known occurrences in such regions. Native-range models predicted a wide potential distribution of siamweed throughout tropical America, from southern United States to northern Argentina and southern Brazil. The weed's occurrence has been confirmed on the northern Pacific coast, in southeast Brazil, and in other South American areas, where it was supposed to be absent. Independent model projections to Africa, Asia, and Oceania are supported by known occurrence records. Four regions are predicted to be susceptible to siamweed spread: (1) Central Africa, currently being invaded from Western Africa; (2) Infestations spreading northward from South Africa, which have already reached Swaziland and Mozambique and may extend to East Africa and Madagascar; and (3) northern New Zealand and (4) Australia, which are at risk from uncontrolled infestations on several western Pacific islands.

Understanding the factors regulating recruitment in diverse tillage systems will improve the efficacy of weed-management strategies. Experiments measured the effect of hairy nightshade winter seed position (burial depth) on seedling recruitment and seed germination, dormancy, and mortality. Hairy nightshade seeds were placed in soil tubes at 1 cm below the soil line and buried in the fall so that seeds were positioned in the soil at 1, 6, 13, and 25 cm below the soil surface. Tubes with undisturbed soil and seeds were removed from the field in March, April, and May of 2004 and 2005 and placed in wells in a heated, aluminum block with a linear temperature gradient from 22.7 to 36.0 C at 2.7 C intervals, and recruitment was measured. Seeds were also recovered from the soil tubes to determine effects of seed position on germination rate, mortality, and seed dormancy. Hairy nightshade seedling recruitment was greater for seeds positioned at 6, 13, and 25 cm during the winter than at 1 cm. Seed dormancy and mortality were greatest for seeds positioned at 1 cm, but that did not adequately account for the significant decrease in recruitment at 1 cm compared to seeds buried at 6, 13, or 25 cm. Protecting the seeds buried at 1 cm from rainfall during the winter increased seedling recruitment from 0 to 20% of buried seeds but had a negligible effect on seed mortality and dormancy. Soil density was negatively correlated with recruitment.

With the recent interest in genetically engineered (GE) wheat and the commercialization of novel-trait imidazolinone herbicide-resistant wheat in North America, volunteer wheat as a weed has also been the subject of renewed interest, specifically, its recruitment and persistence in annual cropping systems. The recruitment of seed from a wheat seedbank established the previous autumn was monitored in a flax crop at two field sites in southern Manitoba, Canada, in 2003 and 2004. Seeds of eight Canadian Western Hard Red spring wheat cultivars, which exhibit a range of preharvest sprouting-resistance characteristics, were broadcast and incorporated into the soil in the autumn at 500 seeds m−2. Tillage treatments consisted of autumn tillage only, and autumn and spring tillage. Recruitment the following spring occurred very early in terms of accumulated growing–degree days (base temperature of 0 C) but expressed as a proportion of total seeds broadcast was low and variable. Total cumulative emergence of wheat over all 4 site yr ranged from 0.9 to 13.1%, with an overall average of 4.3%. There was no relationship between preharvest sprouting-resistance characteristics and recruitment proportion, and no significant influence of tillage treatment on wheat recruitment. Wheat seed that did not recruit was rapidly degraded in the soil and did not persist for more than 12 mo. However, some emerged volunteer wheat plants escaped all control measures normally used in establishing and growing a typical flax crop, and these escaped volunteer wheat plants set viable seed. Therefore, results of this study indicate that efforts and attention should be directed toward achieving very high levels of volunteer wheat control in subsequent rotational crops and that reseeding by escaped volunteer wheat plants may be a more important persistence mechanism for spring wheat than multiyear soil seedbank persistence.

There is no published information on the impact of volunteer barley on wheat yield loss or on the economics of controlling barley with a herbicide. With the registration of imazamox-resistant wheat, it is now possible to control volunteer barley in wheat. Thus, the likelihood of growing wheat in rotation with barley may increase. Field experiments were conducted in 2003 and 2004 at Beaverlodge, Lacombe, and Edmonton, AB, Canada, and Saskatoon, SK, Canada, to determine the impact of volunteer barley on yield of imazamox-resistant spring wheat seeded at relatively low (100 kg ha−1) and high (175 kg ha−1) rates. Barley was seeded at different densities to simulate volunteer barley infestations. Regression analysis indicated that wheat-plant density influenced the effects of volunteer barley interference on wheat yield loss, economic threshold values, and volunteer barley fecundity among locations and years. Economic thresholds varied from as few volunteer barley plants as 3 m−2 at Beaverlodge in 2003 and 2004 to 48 m−2 at Lacombe in 2003. In most cases, wheat yield loss and volunteer barley fecundity were lower and economic thresholds were higher when wheat was seeded at the higher rate. For example, averaged over both years at Beaverlodge initial slope values (percentage of wheat yield loss at low barley density) were 4.5 and 1.7%, and economic threshold values of volunteer barley plants were 3 m−2 and 8 m−2 at low and high wheat seeding rates, respectively. Results indicate that volunteer barley can be highly competitive in wheat, but yield losses and wheat seed contamination due to volunteer barley can be alleviated by seeding wheat at a relatively high rate.

In several European nations, including France and Germany, atrazine has been banned because of environmental concerns. However, in Canada, atrazine remains an important component of modern weed control in corn. The objectives of this study were to determine the value of atrazine to corn producers by examining weed control efficacy, yield of corn, adjusted gross return, and the variability associated with PRE and POST herbicides applied alone or in combination with atrazine. A randomized complete-block design experiment was conducted at two locations for 3 yr to evaluate the performance of selected PRE and POST herbicides with and without atrazine. The addition of atrazine to PRE herbicides increased weed control (25%), improved herbicide performance consistency, increased corn yields (8%), increased adjusted gross return (Can$59 ha−1), and reduced risk ($30 ha−1) over sites and years. Although improving weed control, the addition of atrazine to POST herbicides increased the risk of return compared with treatments without atrazine by about $20 ha−1 because the increased cost of atrazine was not always offset by higher corn yields. Our results clearly demonstrate a value of atrazine for broadleaf weed control in corn, both in terms of efficacy and economic return. From our findings, we estimated that the economic benefit of atrazine to Ontario, Canada, corn producers to be at least $26.1 million in 2004. Under current economic pressures facing agricultural producers, our findings show that a balance between the environmental effects and the benefits of atrazine to corn producers must be found because no alternative herbicide with equal economic and agronomic attributes is available at this time. To meet this balance, research on further reducing atrazine use rates while maintaining effective weed control in corn and on developing a sustainable stewardship plan is warranted.