The ability of the pesticide root zone model (PRZM) and the groundwater-loading effects of agricultural management systems (GLEAMS) model to predict movement of two herbicides in soil was evaluated using site-specific environmental data from sites in three states. Predictions of herbicide movement with site-specific data were compared to predictions using more generalized database soil and pesticide data within each model. Field experiments examined fluometuron and norflurazon movement in three soils representative of the cotton-growing regions of the southeastern United States. In comparing the use of site-specific vs. database values, the small increase in accuracy using site-specific inputs would not justify the large cost to obtain the data. The databases for each model gave predictions similar to those using the site-specific numbers. Both the PRZM and the GLEAMS model had similar accuracy levels in predicting the presence of fluometuron or norflurazon present in three surface soils, although each model tended to overpredict movement and total herbicide concentration, especially at lower herbicide concentrations. At higher herbicide concentrations, prediction accuracy was less than that probably needed to predict agronomically relevant herbicide concentrations in surface soils.

Field experiments were conducted in 1989 and 1990 to evaluate seasonal effects of herbicide application, smutgrass (Sporobolus indicus) growth stage, and herbicide rate on smutgrass control. The experimental design was a split–split plot, with season (late spring, midsummer, and fall) as the whole plot, physiological stage at application (uncut, 15-, and 30-cm regrowth) the subplot, and hexazinone rate (0.0, 0.56, 0.84, and 1.12 kg ai/ha) as the sub-subplot treatment. Whole plots were arranged in three randomized complete blocks. Smutgrass control for the mean of the hexazinone treatments (1989 and 1990) was 95%. Application of 0.56 kg/ha hexazinone during late spring, midsummer, and fall seasons provided 77, 92, and 92% smutgrass control, respectively. Generally, the influence of plant stage at time of hexazinone application had no effect (P > 0.05) on smutgrass control. At 20 d after treatment (DAT), hexazinone had injured bahiagrass (Paspalum notatum) plants by yellowing them, but the plants reverted to their usual dark green color by 40 DAT. Total nonstructural carbohydrates were lower for the 30-cm regrowth than for the uncut plants, regardless of season; however, 30-cm regrowth had little effect on smutgrass control.

Observations in a citrus grove indicated that hairy beggarticks numbers were higher in plots receiving thiazopyr alone or thiazopyr with oxyfluorfen than in nontreated plots. At 120 days after treatment (DAT), approximately 90% more hairy beggarticks plants had emerged in plots receiving thiazopyr at 0.56 kg ai/ha than in nontreated plots. A tank mix of thiazopyr plus oxyfluorfen, each at 0.56 kg/ha, resulted in a 55% increase in hairy beggarticks number at 120 DAT. Greenhouse studies using seeds collected from sites with or without herbicide history produced similar results. Oxyfluorfen at 0.28 kg/ha stimulated seed germination but injured seedlings by 47% or more at 36 DAT when applied at higher rates. Thiazopyr applied at 0.56 kg/ha antagonized the preemergence activity of oxyfluorfen at 0.28 or 0.56 kg/ha. Thiazopyr at 1.12 kg/ha stimulated hairy beggarticks germination by 37% compared with nontreated containers.

Currently, there are no effective herbicides registered for the control of hoary cress (Cardaria draba) in alfalfa (Medicago sativa) or alfalfa/grass mixtures. Due to this lack of control options, field experiments were conducted near St. Ignatius, MT, from 1995 through 1997 to determine the optimum imazethapyr rate and adjuvant combination for hoary cress control in alfalfa/orchardgrass (Dactylis glomerata) pastures. Imazethapyr was applied in the spring to rosette stage hoary cress plants at 33, 67, and 100 g ai/ha with either a nonionic surfactant or methylated seed oil alone or combined with 28% urea ammonium nitrate (UAN) liquid fertilizer. Hoary cress control was poorest when imazethapyr was applied at 33 g/ha, yet provided a minimum of 80% control. Imazethapyr applied at 67 and 100 g/ha provided greater than 90% control during the year of application regardless of surfactant or UAN additions. None of the treatments controlled hoary cress the year following application. Alfalfa yield was unaffected by imazethapyr applications. First-cutting orchardgrass yield decreased as imazethapyr rate increased, but total seasonal forage yield was unaffected.

A study was conducted in 1996 and 1997 near Rohwer, AR, to evaluate weed control with standard herbicide programs in nontransgenic and glyphosate-resistant soybean (Glycine max) compared with glyphosate-based herbicide programs in glyphosate-resistant soybean. Total postemergence (POST) programs containing 0.84 kg ae/ha glyphosate 14 d after emergence (DAE) followed by 0.42 kg/ha at 42 DAE controlled pitted and entireleaf morningglory (Ipomoea lacunosa and I. hederacea var. integriuscula) 80 to 83%. This control was lower than with any other weed control program in the study. All glyphosate containing herbicide programs controlled barnyardgrass (Echinochloa crus-galli) at least 94%. Soybean injury was 8% or less for all herbicide treatments except for ‘Hartz 5088 Roundup Ready’ treated with pendimethalin plus chlorimuron and metribuzin preplant incorporated (PPI) followed by glyphosate or acifluorfen plus bentazon 42 DAE. All weed control and soybean injury ratings were similar at 24 d after the 42 DAE application (DAA) when compared with the 10 DAA rating. Two weed control programs with ‘Northrup King S-59-60’ soybean using standard herbicide programs yielded 3,015 and 3,070 kg/ha, and net returns were $92.20/ha and $122.82/ha, respectively. Two production systems containing the Hartz 5088 Roundup Ready cultivar had positive net returns of $35.11/ha and $39.49/ha.

Flumioxazin plus metolachlor mixtures preemergence (PRE) were evaluated with or without postemergence (POST) herbicides for weed control and peanut (Arachis hypogaea) response in three North Carolina studies. Metolachlor PRE at 2.24 kg ai/ha controlled goosegrass (Eleusine indica) and yellow nutsedge (Cyperus esculentus) 93 and 80%, respectively, and control was not improved with flumioxazin or norflurazon. Metolachlor plus flumioxazin PRE at 0.07 or 0.11 kg ai/ha controlled common lambsquarters (Chenopodium album); entireleaf (Ipomoea hederacea var. integriuscula), ivyleaf (I. hederacea), and pitted morningglory (I. lacunosa); and prickly sida (Sida spinosa) better than metolachlor plus norflurazon PRE at 1.34 kg ai/ha. Morningglories (Ipomoea spp.) were controlled 77 and 86% with flumioxazin PRE at 0.07 and 0.11 kg/ha, respectively, and control was increased to nearly 100% with acifluorfen plus 2,4-DB or lactofen plus 2,4-DB POST. Peanut injury by flumioxazin and norflurazon was observed at one location in 1997; however, yields were not reduced. Peanut treated with metolachlor plus flumioxazin PRE at either rate yielded at least 3,750 kg/ha compared to 3,120 kg/ha with metolachlor plus norflurazon PRE or 1,320 kg/ha with metolachlor PRE.

Greenhouse and field experiments were conducted to confirm and quantify linuron resistance in common purslane (Portulaca oleracea) collected from a carrot (Daucus carota) field in Imlay City, MI. Preliminary evaluation was made using a flotation test kit to identify resistance to linuron and atrazine. Subsequent greenhouse experiments indicated that this common purslane was resistant to 11.2 kg/ha linuron and 179 kg/ha atrazine. The resistance ratio for linuron was > 300 and was > 400 for atrazine. The resistant common purslane was also highly resistant to diuron, cyanazine, and prometryn but had a low level of negative cross-resistance to bromoxynil. Both resistant and susceptible biotypes of common purslane were sensitive to hexazinone and bentazon.

Resistance of weeds to herbicides that were initially controlled by that herbicide has increased in all major crops and forages. An internet web site, http//weedscience.com, logs all confirmed resistant weed species and their location by country. Management of these species and the cost of those management techniques was the focus of this symposium. The group of scientists assembled for this symposium were chosen to discuss the systems currently utilized in the management of species that are resistant to a single or to multiple groups of herbicides. It is hoped that information presented on the cost of managing these resistant species will further encourage the judicious use of herbicides, the implementation of agronomic practices, and crop rotations to delay the development of resistance to other areas.

A method of calculating the cost to farmers of herbicide resistance in weeds is described along with examples to demonstrate the inherent difficulties and benefits. The main problem is defining cropping systems/practices and herbicide strategies that prevent the development of resistance. The benefits include focusing advisory and research efforts and helping to persuade farmers to adopt practices that prevent the development of resistance.

Surveys were conducted across the northern Great Plains of Canada in 1996 and 1997 to determine the nature and occurrence of herbicide-resistant (HR) biotypes of wild oat (Avena fatua). The surveys indicated that resistance to acetyl-CoA carboxylase (ACCase) inhibitors (Group 1) occurred most frequently relative to other herbicide groups. Group 1-HR wild oat occurred in over one-half of fields surveyed in each of the three prairie provinces. Of particular concern was the relatively high incidence of multiple-group resistance in wild oat in Saskatchewan and Manitoba. In Saskatchewan, 18% of Group 1-HR populations were also resistant to acetolactate synthase inhibitors (imidazolinones), even though these herbicides were not frequently used. In Manitoba, 27% of fields surveyed had wild oat resistant to herbicides from more than one group. Four populations were resistant to all herbicides registered for use in wheat (Triticum aestivum). Depending on the nature of resistance in wild oat, alternative herbicides available for their control may substantially increase costs to the grower. The cost to growers of managing HR wild oat in Saskatchewan and Manitoba using alternative herbicides is estimated at over $4 million annually. For some HR biotypes, alternative herbicides either are not available or all have the same site of action, which restricts crop or herbicide rotation options and threatens the future sustainability of small-grain annual cropping systems where these infestations occur.

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.

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.

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.

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.

The arguments for and against placing a symbol containing a letter to denote the mode of action on herbicide labels in Europe are reviewed. There is no clear view within and between the organizations that may influence the decision. However, it is generally accepted that such an initiative only should be part of an integrated approach to prevent the development of herbicide resistance in weeds.

Industry, public-sector researchers and extension agents, and growers were surveyed in 1998 to determine their perspectives on how labeling herbicides with their site of action (group number) would affect the herbicide use practices of growers. The crop protection industry in Canada represented by the Crop Protection Institute (CPI) generally supports herbicide resistance labeling but has some concerns regarding the wording of the labels, including the identification symbol. Most researchers and extension agents believe that labeling herbicides with their site of action will facilitate herbicide group rotation by growers who frequently use herbicides from the same group. Of the two-thirds of the 126 surveyed growers who were familiar with herbicide groupings, 58% practiced herbicide group rotation. Those who did not tended to lack understanding of the basis and purpose of herbicide classification. Grower responses were similar to those from the research and extension community, although only 29% of the growers who currently do not rotate herbicides from different groups believed that resistance management labeling would influence them.

One of the most consistent recommendations, from both the private and public sectors, for the prevention and management of herbicide-resistant weeds is to rotate herbicides with different sites of action. Therefore, it is imperative that those who make herbicide recommendations and those who apply herbicides know which herbicides have the same or different sites of action. An herbicide's site of action is not obvious from its trade, common, or chemical name, and it is not possible to determine from the label if it has the same site of action as another herbicide.

The North American Herbicide Resistance Working Group (NAHRWG), an organization to facilitate the effective management of herbicide resistance by fostering understanding among manufacturers and distributors, academics, consultants, government, and end users requested I speak with you on the legal ramifications of placing resistance information on the herbicide label. As an Assistant General Counsel for Regulatory Affairs with Monsanto Company, my practice is devoted to agricultural biotechnology, crop chemical regulatory matters, and to a large extent, the proper stewardship of Monsanto's agricultural products.

The genus Bromus in the Poaceae includes 100 species of annuals, biennials, and perennials. Several species are important as forage crops while others are grown as ornamentals. It is distributed in the temperate areas and in tropical mountainous regions (Hyam and Parkhurst 1995). Bromus tectorum L., downy brome, an introduced annual, occurs throughout most of the U.S., Canada, and northern Mexico (Sheley and Petroff 1999). It was introduced to North America from Europe before 1861. Downy brome has a dual role as a serious weed and as an important forage (Upadhyaya et al. 1986).