Field experiments were conducted in 2001 and 2002 to study the effect of date of planting, herbicide, and straw mulch on menthol mint yield and oil quality in northwest India. Menthol mint was planted at three dates (December 10, December 30, and January 20). Diuron was applied prior to emergence at rates of 0.0, 0.4, and 0.6 kg ai/ha, with and without rice straw mulch (6 t/ha). Menthol mint planted on December 30 and January 20 produced more plantlets and dry matter than the December 10 planting. Menthol mint planted on December 10 had higher weed density and weed biomass, and lower menthol mint herbage and oil yield than the later planting dates. Straw mulch application reduced weed density and weed biomass, and increased plantlet population, menthol mint dry matter accumulation, fresh herbage, and menthol mint oil yield. Preemergence (PRE) applications of diuron at 0.4 and 0.6 kg ai/ha increased menthol mint population and crop dry matter accumulation, and decreased weed density and weed biomass compared to the weedy check. Diuron at 0.6 kg/ha increased fresh herbage and menthol mint oil yield over the untreated weedy check, but there were no other differences between the two rates of diuron. Physicochemical properties of menthol mint oil were not affected by any of the three tested factors.

Sugarbeet varieties vary in their response to herbicides. s-Metolachlor and dimethenamid-P were recently registered for use in sugarbeet. Field trials were conducted in Michigan in 2004, 2005, and 2006 to evaluate the response of 12 sugarbeet varieties to s-metolachlor and dimethenamid-P applied PRE and POST to two-leaf and four-leaf stage sugarbeet. s-Metolachlor and dimethenamid-P reduced sugarbeet density when rainfall occurred within 7 d of the PRE applications. Dimethenamid-P PRE caused the most injury across all varieties followed by s-metolachlor PRE. Applying dimethenamid-P POST to two-leaf sugarbeet injured plants more than s-metolachlor applied POST to two- and four-leaf stage sugarbeet. The least amount of sugarbeet injury from dimethenamid-P was from POST applications at the four-leaf stage. Sugarbeet varietal differences were most pronounced from PRE applications of both herbicides and from the POST two-leaf application of dimethenamid-P. Of the 12 sugarbeet varieties evaluated, Hilleshog 2771RZ and Beta 5833R were the most tolerant, whereas Hilleshog 7172RZ was typically the most sensitive variety to these herbicides. Growers will probably not choose varieties based on herbicide tolerance alone, but instead base variety selection on sugar yield and disease resistance. However, if a grower has chosen a particular variety, this information could assist in assessing the risk of using s-metolachlor or dimethenamid-P for weed control.

Currently there are no herbicides registered for direct application to buckwheat for broadleaf weed control. This 4-yr Lithuanian study examined weed control using several rates of clopyralid alone or combined with a single rate of desmedipham. Most applications were applied at the 1-leaf stage of crop growth, however, one rate of clopyralid was applied pre-emergence (PRE) in 2 of the 4 yr. Buckwheat injury was evident within a few days after application (or emergence) with all treatments, but by harvest, no symptoms were evident. In the 2 yr with greatest weed densities, densities were reduced with increasing clopyralid rates alone or with desmediphan. However, weed biomass and density were similar to the nontreated control at harvest. Common lambsquarters, scentless mayweed, wild buckwheat, and narrowleaf hawksbeard were species that were best (> 40% density reduction) controlled with clopyralid. With few exceptions, buckwheat yield in all herbicide-treated plots was similar to the nontreated control. Because yield was not increased with these herbicides, other weed control benefits, such as reduced interference with harvesting equipment or less dockage due to weed seed contamination, must be carefully weighed against the costs of herbicide and application and crop injury that reduced early-season vigor.

An experiment was conducted to determine the utility of multispectral imagery for identifying soybean, bare soil, and six weed species commonly found in Mississippi. Weed species evaluated were hemp sesbania, palmleaf morningglory, pitted morningglory, prickly sida, sicklepod, and smallflower morningglory. Multispectral imagery was analyzed using supervised classification techniques based upon 2-class, 3-class, and 8-class systems. The 2-class system was designed to differentiate bare soil and vegetation. The 3-class system was used to differentiate bare soil, soybean, and weed species. Finally, the 8-class system was designed to differentiate bare soil, soybean, and all weed species independently. Soybean classification accuracies classified as vegetation for the 2-class system were greater than 95%, and bare soil classification accuracies were greater than 90%. In the 3-class system, soybean classification accuracies were 70% or greater. Classification of soybean decreased slightly in the 3-class system when compared to the 2-class system because of the 3-class system separating soybean plots from the weed plots, which was not done in the 2-class system. Weed classification accuracies increased as weed density or weeks after emergence (WAE) increased. The greatest weed classification accuracies were obtained once weed species were allowed to grow for 10 wk. Palmleaf morningglory and pitted morningglory classification accuracies were greater than 90% for 10 WAE using the 3-class system. Palmleaf morningglory and pitted morningglory at the highest densities of 6 plants/m2 produced the highest classification accuracies for the 8-class system once allowed to grow for 10 wk. All other weed species generally produced classification accuracies less than 50%, regardless of planting density. Thus, multispectral imagery has the potential for weed detection, especially when being used in a management system when individual weed species differentiation is not essential, as in the 2-class or 3-class system. However, weed detection was not obtained until 8 to 10 WAE, which is unacceptable in production agriculture. Therefore, more refined imagery acquisition with higher spatial and/or spectral resolution and more sophisticated analyses need to be further explored for this technology to be used early-season when it would be most valuable.

Thermal weed control methods have been incorporated into weed control programs in organic and conventional production systems. Flaming is commonly used, but steaming has been proposed to increase efficiency of heat transfer to weeds and reduce the risk of fire. The objective of this research was to measure injury to leaves of plant species that differ in leaf morphology and to measure injury to plants at different stages of plant development. The study was conducted in a glasshouse and plants were exposed to steaming at 400 C for 0.36 s—equivalent to a steaming speed of 2 km/h. Overall, leaf thickness was the best morphological characteristic to predict injury ( = 0.51), with greater thickness resulting in less injury. For broadleaf species only, species with wider leaves were injured more than species with narrower leaves ( = 0.64). Injury was greatest when plants had fewer than six true leaves and when their shoots were less than 10 cm long. There was a wide range of injury across species, and the grass species bermudagrass and perennial ryegrass were injured (68 to 81%) more than other species such as common purslane and English daisy (23 to 34%). Biomass of all species tested was reduced by approximately 40%, indicating that leaf injury was not the sole effect of steaming on plant growth. These results indicated that considering both visual estimates of injury and morphological characteristics is important to properly assess thermal weed control effectiveness.

The extended production of allelochemicals from a living mulch cover crop may potentially enhance weed management and crop productivity compared with traditional zucchini squash production systems. However, the efficacy of a living mulch cover crop in no-tillage (NT) systems with herbicides has not been determined for many vegetable crops. The objective of this research was to evaluate weed management and zucchini squash production using the combination of NT and a herbicide-suppressed winter rye cover crop with or without PRE herbicides. When used in combination with herbicides, the winter rye living mulch reduced the biomass of redroot pigweed compared to the herbicides applied alone, but did not influence the biomass of smooth crabgrass for any herbicide treatment. Visual estimates of weed control were similar for each herbicide treatment with and without the winter rye living mulch. The herbicide-suppressed winter rye resulted in excessive stunting of zucchini squash at 56 d after treatment which correlated with 20 and 50% squash yield reductions for the 2004 and 2005 growing seasons, respectively. Although some squash yield loss may have resulted from direct competition with the winter rye living mulch, yield reductions most likely resulted from allelopathy. The winter rye living mulch was the primary contributing factor for reduced squash yield and did not interact with herbicide treatment. All treatments containing herbicides resulted in yields similar to the weed-free treatment. Because of excessive zucchini injury, this research suggests that the use of herbicide-suppressed winter rye living mulch does not appear to be a feasible option for increased weed control in zucchini squash production regardless of the herbicide treatment.

Weed control strategies based on conserving crop yields rather than preventing weed seed production may result in increased weed densities and management costs over the long-term, particularly in less competitive crops such as tomatoes. The effect of crop, tillage, and duration of weed control on weed seed bank size and composition was examined from spring 2001 to spring 2003 near Lafayette, IN. Main plots in 2001 and 2002 contained soybean or tomato planted in rotation (soybean-tomato, tomato-soybean). Subplots were managed with either conventional or no-till practices. Sub-subplots contrasted threshold strategies in which weeds were either controlled for four to six weeks (period threshold, PT) or throughout the growing season (no-seed-threshold, NST). Seed banks were sampled annually in the spring. Emergent weeds were counted` at four and twelve weeks after planting (WAP) in 2001 and 2002. Weed seed banks did not significantly change in the NST plots in any year. However, seed bank densities increased substantially following tomatoes in PT plots. In contrast, weed seed bank densities decreased following soybeans in PT plots. The difference in seed banks and emergent weeds between soybean and tomatoes could be attributed primarily to greater suppression of giant foxtail by the soybean canopy. Giant foxtail control was greater in PT soybeans than in PT tomatoes in both years and giant foxtail comprised most of the PT tomato seed bank in 2002 and 2003. Tillage did not affect weed seed banks in any year. This study highlights the need to control later emerging weeds in tomatoes to prevent large increases in the weed seed bank.

This study measured impact of cool-season crops on seedling emergence, survival, and seed production of weeds common in corn and soybean. Weed dynamics were monitored in permanently marked quadrats in winter wheat, spring wheat, and canola. Three species, green foxtail, yellow foxtail, and common lambsquarters, comprised more than 80% of the weeds observed in the study. Seedling emergence was reduced by winter wheat, but not by spring wheat or canola, when compared with adjacent quadrats without a crop canopy. Approximately 10% of seedlings in spring wheat and canola developed into seed-bearing plants, but no seed-bearing plants were present in winter wheat at harvest. Common lambsquarters produced more than 1,100 seeds/plant, whereas a foxtail plant produced 85 seeds, averaged across spring wheat and canola. At harvest, new seedlings were present in all crops; thus, control after harvest will be required to prevent seed production in the fall. Winter wheat may provide an opportunity to disrupt population dynamics of weeds common in corn and soybean without requiring herbicides.

Agricultural practices, other than herbicide use, can affect the rate of evolution of herbicide resistance in weeds. This study examined associations of farm management practices with the occurrence of herbicide (acetyl-CoA carboxylase or acetolactate synthase inhibitor)-resistant weeds, based upon a multi-year (2001 to 2003) random survey of 370 fields/growers from the Canadian Prairies. Herbicide-resistant weeds occurred in one-quarter of the surveyed fields. The primary herbicide-resistant weed species was wild oat, with lesser occurrence of green foxtail, kochia, common chickweed, spiny sowthistle, and redroot pigweed. The risk of weed resistance was greatest in fields with cereal-based rotations and least in fields with forage crops, fallow, or where three or more crop types were grown. Weed resistance risk also was greatest in conservation-tillage systems and particularly low soil disturbance no-tillage, possibly due to greater herbicide use or weed seed bank turnover. Large farms (> 400 ha) had a greater risk of weed resistance than smaller farms, although the reason for this association was unclear. The results of this study identify cropping system diversity as the foundation of proactive weed resistance management.

Field-scale experiments were conducted at several western Canada locations to determine the importance of early weed removal over variable landscapes. In eight of 10 cases, imidazolinone-resistant (IR) canola yield decreased linearly as herbicide application (15/15 g/ha imazamox/imazethapyr or 15/15 g/ha imazamox/imazethapyr plus 150 g/ha clopyralid) was delayed beyond the one- to two-leaf stage. In two of 10 cases, canola oil content also decreased as herbicide treatment was delayed. Canola yields at all environments (location by year combinations) averaged 2,073, 1,872, or 1,650 kg/ha when treated at the one- to two-, three- to five-, or six- to seven-leaf stage, respectively. Assuming canola prices from a low of $250/t to a high of $650/t, growers could lose $50 to $131/ha, respectively, by delaying herbicide application from the one- to two- to the three- to five-leaf stage, or $106 to $275/ha, respectively, by delaying herbicide application from the one- to two- to the six- to seven-leaf stage.