Mugwort is a noxious perennial weed that interferes with the growth and establishment of crop species. The present study was designed to understand the role of allelopathy as a potential mechanism of interference by mugwort. Soils amended with mugwort plant material and leachates were analyzed for their chemical characteristics. The effect of amended soils on seedling growth of red clover was studied. The influence of NP fertilization and charcoal on modification of allelopathic potential of amended soils, in terms of their effect on soil characteristics and red clover seedling growth, was also investigated. In addition, red clover seedling growth was compared in sterilized and nonsterilized soils amended with mugwort leachate and NP fertilization. In general, addition of mugwort leachates and plant matter (amended soil) resulted in chemical changes in soil, including changes in available phenolics. Red clover seedling growth was reduced in amended soils, when compared to that in nonamended soils. Although the different amounts of NP fertilization in nonsterilized soil amended with mugwort leachate could not counteract its interference to red clover growth (root reduction, 67–79%; shoot reduction, 34–44%), the addition of charcoal did eliminate leachate effects on red clover growth. This indicates the probable allelopathic interference of mugwort to red clover growth. Addition of NP fertilization might have resulted in higher microbial activity, which is likely to influence qualitative and/or quantitative availability of phenolic compounds. Data on phenolic levels and red clover growth in sterilized and nonsterilized soil amended with mugwort leachate and NP fertilization indirectly indicate the significance of soil microbes in mugwort interference to red clover.

In Saskatchewan, a field survey was conducted in 1996 and a grain elevator survey in 1997 to determine the nature and occurrence of herbicide-resistant (HR) biotypes of green foxtail (Setaria viridis). Based on results of the field survey, one in every 20 fields (1 million ha) in Saskatchewan is estimated to have acetyl-CoA carboxylase (ACCase) inhibitor (Group 1)-HR green foxtail. Of the samples received from 30 grain elevators, 83% had Group 1 resistance. Similar to the field survey, incidence of aryloxyphenoxy propionate (AOPP) resistance was much higher than cyclohexanedione (CHD) resistance. Both surveys indicated relatively low incidence of dinitroaniline (Group 3) resistance or cross-resistance (Groups 1 and 3). Most alternative herbicides to control Group 1-HR green foxtail in broadleaf crops and cross-HR biotypes in cereal and broadleaf crops increase costs to the grower.

Clomazone was applied preplant incorporated, preemergence, delayed preemergence, or postemergence and compared to pendimethalin, quinclorac, and thiobencarb for control of barnyardgrass in rice. Seven days after the clomazone postemergence application, all clomazone treatments except those applied postemergence controlled barnyardgrass ≥ 86%. At 49 d after the postemergence application, control of barnyardgrass with 0.56 and 0.67 kg/ha clomazone postemergence improved to 92 and 93%, respectively. Clomazone and quinclorac applied delayed preemergence generally controlled barnyardgrass less than 70%. Visible injury to rice was ≤ 18% 7 d after emergence and had declined to less than 10% when evaluated 7 d after postemergence treatments. Rice yields were higher with all treatments compared to the nontreated control.

Herbicides are important components of weed management programs for most Kansas farmers. Monocropping systems and repeated use of the same or similar herbicides in some areas of the state have resulted in the development of herbicide-resistant weeds. The development of herbicide-resistant weed populations can have an immediate and a long-term effect on the cost, implementation, and effectiveness of weed control programs. In Kansas, resistance to triazine herbicides has been confirmed in kochia (Kochia scoparia), redroot pigweed, common waterhemp (Amaranthus rudis), Palmer amaranth (Amaranthus palmeri), and downy brome (Bromus tectorum) populations, and resistance to acetolactate synthase (ALS)-inhibiting herbicides has been confirmed in kochia, Russian thistle (Salsola kali), common waterhemp, Palmer amaranth, common cocklebur (Xanthium strumarium), shattercane (Sorghum bicolor), and common sunflower (Helianthus annum). The frequency and distribution of herbicide resistance varies among species. Producers who experience herbicide resistance problems adjust their weed control program accordingly. Producers that have not encountered an herbicide resistance problem tend to continue with a successful herbicide program until it fails. The recommended management strategies for herbicide-resistant weed populations include an integrated system of crop rotation, rotation of herbicide modes of action, tank-mixes of herbicides with different modes of action, and cultivation. The greatest direct cost to the producer occurs during the first year of poor weed control. The first response to an herbicide failure often is to reapply the same herbicide that has worked well previously. By the time the producer realizes that the treatment is not going to work, it usually is too late for any other remedial action. Consequently, the farmer experiences reduced crop production from weed competition, high herbicide costs, and a tremendous increase in the seed bank. The increase in seed bank may cost the farmer the most in the long run because the increased weed pressure often requires an intensified control program for several years.

A 1984–1985 and 1988–1989 field study near Pullman, WA, evaluated the effect of two winter wheat planting geometries on the growth of wheat and competition against jointed goatgrass. Treatments included paired and constant row planting geometries of wheat and locations of jointed goatgrass within each geometry. No planting geometry by weed location interactions occurred at any harvest date for plant height, shoot number, leaf area, plant dry weight, or N uptake for either wheat or jointed goatgrass. During the 1984–1985 growing season, N uptake data indicated that by heading, jointed goatgrass had taken up N that had been deep-banded between wheat rows located 25 cm from the weed. Winter wheat yields were not different in the paired-row and the constant row spacing geometry in a weed-free environment. Within years, for both planting geometries, winter wheat yield reduction from weed competition was similar for the jointed goatgrass locations. In contrast, based on jointed goatgrass spikelets produced, wheat grown in paired-rows was more competitive against jointed goatgrass compared to constant row spacing.

Control of natural infestations of common lambsquarters and giant foxtail in 1993, 1994, and 1995 and of velvetleaf in 1994 and 1995 by postemergence application of glyphosate to glyphosate-resistant soybean planted in narrow (20 cm) and wide (76 cm) rows was evaluated. Planting glyphosate-resistant soybean in narrow rows and applying reduced rates of glyphosate when common lambsquarters, giant foxtail, and velvetleaf were at their actively growing stage 3 to 18 cm, 5 to 28 cm, and 3 to 20 cm tall, respectively, resulted in > 90% control. The effect of time of herbicide application was greater than the rate of herbicide application, especially within the wide-row soybean plantings. Applying imazethapyr in combination with glyphosate did not improve weed control or soybean yield compared with glyphosate alone.

Paraquat, cultivation, or cultivation plus oat interseeding (56 or 84 kg/ha) into established alfalfa were evaluated for their effect on weeds, forage yield, and forage composition at Santa Ynez, Lancaster, and Stockton, CA. Treatments were applied during the winter to dormant alfalfa and evaluated in the subsequent harvest season. Paraquat reduced first harvest total forage yields 20 to 33% compared to untreated alfalfa, whereas oat interseeding increased total forage yields 16 to 36%. Cultivation reduced total forage yield in the first cutting by 26% at Lancaster but did not affect forage yield at the other sites. Total or component forage yields were not affected by any treatment in the second and subsequent harvests, with the exception of weeds. Paraquat reduced weed biomass in the first harvest by 45 and 49% at Lancaster and Stockton. Oat interseeding reduced first harvest weed biomass by 80% at Lancaster but did not affect weed biomass at Stockton or Santa Ynez. Oat interseeding or paraquat treatment reduced weed biomass in the second harvest at Stockton, but did not affect weeds in any other harvest at any of the sites. Oat interseeding was comparable to paraquat treatment for weed control, with the advantage of also increasing first harvest forage yield.

The fire-weed glows in the centre of the drive ways.

Time (weekly ed.) Rudyard Kipling, 1892

Fireweed is in the evening primrose family, Onagraceae, which includes 17 genera (Brummitt (1992). The genus Epilobium encompasses 200 especially western North America species of annuals, biennials, and herbaceous and semievergreen subshrubs. Epilobium is widely distributed in the temperate regions on waste and disturbed ground, stony slopes, river gravels, and subalpine screes and meadows. While some species are cultivated for their showy flowers, windborne seeds and spreading rhizomes make many others invasive (Brickhill and Zuh 1997).

Herbicide-resistant weeds are becoming a major problem in the Midwest, and strategies must be adopted to delay further selection. Strategies of rotating and tank-mixing herbicides with different modes of actions should be effective, but adoption may be limited and certain limitations may exist. Therefore, integrating nonchemical practices that indirectly lower selection pressure or restrict the growth of resistant populations is desirable. Appropriate integration of mechanical weeding, crop rotation, increased crop competition, and decision aids may further delay the development of resistance. Understanding the effect of these practices on weed population dynamics is required to more accurately predict their contributions toward resistance management. This knowledge will aid in justifying the adoption of improved management systems.

Cultural practices, such as delayed crop seeding, tillage, black fallow, crop rotation, hand weeding, and competitive crops, when used to replace herbicides, provide an opportunity to reduce the selection pressure that causes weeds to become resistant to herbicides. Herbicides on the other hand reduce the selection pressure that causes weeds to resist cultural practices. Rotation of the two systems should then delay resistance to both systems. Growers consider many factors in addition to weed resistance in selecting herbicide or cultural weed control, from associated convenience to economic potential. Rotation of different types of weed control practices would delay resistance, compared to a continuous single practice. The extent of the delay depends upon genetics of resistance, weed reproduction characteristics, weed seed survival, and fitness of resistant weed plants. An understanding of the basic aspects of weeds and herbicides, as well as their interaction with the environment, would help in predicting the delay in resistance to an herbicide from use of cultural practices in the rotation. A grower's final choice of a weed control practice will involve available equipment, time, markets, and soil erosion in addition to potential weed resistance. Weeds that develop resistance to a control practice still allow for reversion to preresistance practices, an important component of a grower's decision. The rate of resistance development is dependent upon the removal of susceptible genes from the population and fitness of the resistant plants. Resistance might be delayed for many years or be manageable when the resistance is genetically recessive or resistant weeds are poorly fit. Growers may not want to accept alternative cultural practices as long as there is the potential for development of another herbicide or reversion to cultural control after resistance occurs.

Field experiments were conducted over 3 yr at two locations in northwest Kansas to evaluate the efficacy and crop tolerance of EXP 31130A alone and with other herbicides in corn. EXP 31130A alone and in combination with acetochlor, atrazine, or metolachlor controlled kochia and redroot pigweed 93% or greater regardless of location, year, or tillage system. Green foxtail control in 1997 and 1998 was > 90% with EXP 31130A in combination with chloroacetamide herbicides but varied among experiments with EXP 31130A alone. Control of puncturevine with EXP 31130A alone or in tank mixtures was 75% or greater at five of six sites. EXP 31130A alone and in combinations caused minor, early-season, corn leaf bleaching and/or stunting under conventional tillage. However, no injury was detected in no-till systems. In 1996, no-till corn receiving EXP 31130A treatments yielded similarly or up to 29% more than hand-weeded corn. Yields did not differ among treated, untreated, and hand-weeded corn in 1996 under conventional tillage, where weed interference was not as intense as under no-till conditions. Conventional-tillage corn receiving EXP 31130A alone or in tank mixtures in 1997 yielded similarly to hand-weeded corn. Averaged over experiments, corn yield in 1998 increased by 10 to 18% with applications of EXP 31130A alone or in combinations compared to untreated corn.

Weed populations develop herbicide resistance when they evolve due to selection pressure. Mutations and gene flow contribute to genetic variability and provide resistant alleles. The speed of resistance gene frequency increase is determined by the inheritance of resistance alleles relative to wild-type susceptibility and is influenced by the interaction between gene expression and selection. The goal of herbicide resistance management is to minimize selection pressure while maintaining adequate weed control. However, the specific nature of each herbicide, weed, and resistance combination determines the practices that optimize undesirable selection pressure. Therefore, generalized management strategies should be recommended with caution and must not be mandated without thorough evaluation on a case-by-case basis.

Weed management systems were compared in bromoxynil-resistant, glyphosate-resistant, and nontransgenic cotton. A standard system of pendimethalin preplant incorporated (PPI), fluometuron preemergence (PRE), fluometuron plus MSMA early postemergence-directed (POST-DIR), and cyanazine plus MSMA late POST-DIR in combination with cultivation controlled broadleaf signalgrass, large crabgrass, common lambsquarters, jimsonweed, pitted morningglory, prickly sida, sicklepod, and smooth pigweed 98 to 100% late season. Weed control, cotton yield, and net returns were similar when pyrithiobac or bromoxynil plus MSMA postemergence (POST) replaced fluometuron plus MSMA POST-DIR. Fluometuron PRE had little to no effect in bromoxynil systems. Glyphosate POST to three- to four-leaf-stage cotton followed by cyanazine plus MSMA late POST-DIR and cultivation controlled weeds 96 to 100%. Glyphosate POST followed by glyphosate POST-DIR and cultivation controlled pitted morningglory and large crabgrass 89 to 90% and other species at least 94%. Yields and net returns at one location were similar for glyphosate applied twice or glyphosate POST followed by cyanazine plus MSMA POST-DIR and the standard system. Pendimethalin plus fluometuron in glyphosate systems did not increase yield or net returns. At a location severely infested with large crabgrass, pendimethalin plus fluometuron in glyphosate systems increased yield 37 to 44% and net returns 85 to 108%, respectively, when glyphosate was applied to cotton at the three-to four-leaf stage, but not if glyphosate was applied to cotton at the one-leaf stage. Yield and net returns were similar when bromoxynil-resistant, glyphosate-resistant, and nontransgenic cotton were treated using the standard system.