The acceptance of no-till crop production systems has been limited due to expected problems with weed management. Field experiments were established at two locations in Ontario in 1988 and one location in 1989. Band or broadcast applications of preemergence (PRE) combinations of high or low label rates of atrazine with or without metolachlor or inter-row cultivation, were evaluated for their effectiveness in controlling annual weeds in no-till corn. At each location, different herbicide and cultivation combinations were required to achieve adequate weed control. Corn grain yield was equivalent regardless of whether herbicides were applied as a band or broadcast treatment at all three sites. At two of the three sites, one cultivation combined with herbicides applied as a band was adequate to maintain weed control and corn grain yields. Selective application of herbicides in bands represented an approximate 60% reduction in total herbicide applied into the environment. The integration of a shallow post-plant inter-row cultivation combined with the soil conservation attributes of no-till, would enhance the sustainability of a modified no-till corn production system.

Field experiment were conducted to determine interactions of chlorimuron or imazaquin with fomesafen, lactofen, or acifluorfen on three-leaf and eight-leaf common cocklebur, hemp sesbania, pitted morningglory, and prickly sida. Antagonism was the most common interaction with common cocklebur, and was most severe with chlorimuron combined with fomesafen or acifluorfen, whereas lactofen did not antagonize common cocklebur control. Reductions in control were greater when low rates of chlorimuron were used. On three-leaf prickly sida, control synergistically increased when imazaquin was combined with fomesafen or acifluorfen, but the majority of these combinations were additive on eight-leaf prickly sida. Three-leaf pitted morningglory control synergistically increased when 36 g ai ha–1 imazaquin was combined with 210 g ai ha–1 fomesafen or 110 or 220 g ai ha–1 lactofen. With eight-leaf pitted morningglory, synergism occurred when 2 g ai ha–1 chlorimuron was combined with the high rate of any diphenylether herbicide tested, and when 36 g ha–1 imazaquin was combined with 110 g ha–1 lactofen or 210 g ai ha–1 acifluorfen; however, at higher rates of chlorimuron or imazaquin, several antagonistic interactions occurred. Hemp sesbania was controlled over 90% by all combinations, and no interactions occurred.

Herbicide resistance is ‘wicked’ in nature; therefore, results of the many educational efforts to encourage diversification of weed control practices in the United States have been mixed. It is clear that we do not sufficiently understand the totality of the grassroots obstacles, concerns, challenges, and specific solutions needed for varied crop production systems. Weed management issues and solutions vary with such variables as management styles, regions, cropping systems, and available or affordable technologies. Therefore, to help the weed science community better understand the needs and ideas of those directly dealing with herbicide resistance, seven half-day regional listening sessions were held across the United States between December 2016 and April 2017 with groups of diverse stakeholders on the issues and potential solutions for herbicide resistance management. The major goals of the sessions were to gain an understanding of stakeholders and their goals and concerns related to herbicide resistance management, to become familiar with regional differences, and to identify decision maker needs to address herbicide resistance. The messages shared by listening-session participants could be summarized by six themes: we need new herbicides; there is no need for more regulation; there is a need for more education, especially for others who were not present; diversity is hard; the agricultural economy makes it difficult to make changes; and we are aware of herbicide resistance but are managing it. The authors concluded that more work is needed to bring a community-wide, interdisciplinary approach to understanding the complexity of managing weeds within the context of the whole farm operation and for communicating the need to address herbicide resistance.

Downy brome, feral rye, and jointed goatgrass are problematic winter annual grasses in central Great Plains winter wheat production. Integrated control strategies are needed to manage winter annual grasses and reduce selection pressure exerted on these weed populations by the limited herbicide options currently available. Harvest weed-seed control (HWSC) methods aim to remove or destroy weed seeds, thereby reducing seed-bank enrichment at crop harvest. An added advantage is the potential to reduce herbicide-resistant weed seeds that are more likely to be present at harvest, thereby providing a nonchemical resistance-management strategy. Our objective was to assess the potential for HWSC of winter annual grass weeds in winter wheat by measuring seed retention at harvest and destruction percentage in an impact mill. During 2015 and 2016, 40 wheat fields in eastern Colorado were sampled. Seed retention was quantified and compared per weed species by counting seed retained above the harvested fraction of the wheat upper canopy (15 cm and above), seed retained below 15 cm, and shattered seed on the soil surface at wheat harvest. A stand-mounted impact mill device was used to determine the percent seed destruction of grass weed species in processed wheat chaff. Averaged across both years, seed retention (±SE) was 75% ± 2.9%, 90% ± 1.7%, and 76% ± 4.3% for downy brome, feral rye, and jointed goatgrass, respectively. Seed retention was most variable for downy brome, because 59% of the samples had at least 75% seed retention, whereas the proportions for feral rye and jointed goatgrass samples with at least 75% seed retention were 93% and 70%, respectively. Weed seed destruction percentages were at least 98% for all three species. These results suggest HWSC could be implemented as an integrated strategy for winter annual grass management in central Great Plains winter wheat cropping systems.

The first significant cases of herbicide-resistant weed populations were to the triazines in the 1970s. In the last 10 years there has been an increase in the number of weed populations that have become resistant to an array of herbicides. In some of these cases, like rigid ryegrass in Australia, a multitude of resistant biotypes has evolved with different mechanisms of resistance. If the present trend continues, the number of herbicides effective on certain weed species may diminish rapidly. To counteract this trend, industry has organized a number of intercompany working groups to specifically address the development of resistance and to implement plans to manage resistance. University and extension along with industry personnel across the world have begun educating growers on resistance management. However, this effort needs to be intensified to find new solutions for controlling weeds through the use of integrated weed management practices that incorporate new and established herbicides with cultural, mechanical, and biological control methods. The challenge is to develop cost effective, environmentally sustainable programs for weed control while maintaining the present efficiency in food and fiber production so that needs of an ever expanding human population can be met.

Large crabgrass is a problem weed in horticultural crops, particularly in turfgrass in the southeastern United States. If growth of large crabgrass could be suppressed via soil pH or calcium levels, control of this weed in turfgrass might be improved while minimizing herbicide usage. To determine the effect of soil calcium and pH on germination and growth of large crabgrass, seeds were sown in a loamy sand soil amended with calcium carbonate (CaCO3) or magnesium carbonate (MgCO3) that established a range of soil pH from 4.8 to 7.8. Seeds were also sown in soil amended similarly with calcium sulfate (CaSO4), which does not affect pH, that established a range of exchangeable Ca levels corresponding to the Ca range in CaCO3 from pH 4.8 to 7.8. Seed germination of large crabgrass was unaffected by pH when soil was amended with CaCO3, whereas seed germination decreased with increasing pH when soil was amended with MgCO3. Crabgrass germination was not affected by Ca (CaSO4) independent of pH changes. Increasing soil pH reduced shoot and root dry weights of seedlings regardless of material used to raise pH. Maximum shoot dry weights occurred at pH 4.8 in the unamended soil, whereas maximum root dry weights occurred at ranges from pH 5.8 to 6.3 for CaCO3 and pH 5.3 to 5.8 for MgCO3. Shoot and root dry weights were not affected by Ca when soil was amended with CaSO4. By raising soil pH levels, the growth of large crabgrass and its ability to compete with turfgrass may be reduced. Raising exchangeable Ca does not appear to be an effective management tool for control of this weed species.

Field studies were conducted in Arkansas in 1999, 2000, and 2001 to evaluate mesotrione applied preemergence (PRE) and postemergence (POST) for weed control in corn grown in the Mississippi Delta region of the United States. Mesotrione was applied PRE (140, 210, and 280 g/ha) alone and POST (70, 105, and 140 g/ha), alone or in tank mixtures with atrazine (280 g/ha). Standard treatments for comparison were S-metolachlor/atrazine PRE and S-metolachlor plus atrazine PRE followed by atrazine POST. All PRE treatments controlled velvetleaf, pitted morningglory, entireleaf morningglory, prickly sida, and broadleaf signalgrass 95% 2 wk after emergence (WAE). Mesotrione controlled velvetleaf 89% or more 4 and 6 WAE. Control of morningglory species by mesotrione POST averaged 92% 6 WAE. Prickly sida was controlled at least 90% by all treatments 4 WAE. Mesotrione applied alone PRE and POST controlled broadleaf signalgrass 83 to 91% 4 WAE. All treatments controlled broadleaf signalgrass less than 90% 6 WAE, except treatments that contained S-metolachlor, which gave 94% or greater control. Corn yield ranged from 10.5 to 12.4 Mg/ha and did not differ among treatments. Mesotrione PRE and POST provided excellent control of broadleaf weeds, but S-metolachlor was needed for broadleaf signalgrass control.

Experiments were conducted from 1991 through 1993 to evaluate AC 263,222 and imazethapyr for yellow and purple nutsedge control in peanut. AC 263,222 at 0.05 to 0.07 kg/ha controlled purple nutsedge (88 to 99% late season) whether applied preplant incorporated (PPI), preemergence (PRE), early postemergence (EPOST) at peanut emergence, or postemergence (POST). Yellow nutsedge control with AC 263,222 was inconsistent at the 0.04 kg/ha rate; however, the 0.05 and 0.07 rate gave 88% early season yellow nutsedge control, but late season control was variable. Yellow nutsedge control with soil-applied treatments of AC 263,222 and imazethapyr were similar but AC 263,222 provided better postemergence control than did imazethapyr. Purple nutsedge control was similar for both herbicides across all application methods.

The Chemical Movement through Layered Soils (CMLS) model was modified and combined with the USDA-SCS State Soil Geographic Data Base (STATSGO) and Montana Agricultural Potentials System (MAPS) digital databases to assess the likelihood of groundwater contamination from selected herbicides in Teton County, MT. The STATSGO and MAPS databases were overlaid to produce polygons with unique soil and climate characteristics and attribute tables containing only those data needed by the CMLS model. The Weather Generator (WGEN) computer simulation model was modified and used to generate daily precipitation and evapotranspiration values. A new algorithm was developed to estimate soil carbon as a function of soil depth. The depth of movement of the applied chemicals at the end of the growing season was estimated with CMLS for each of the soil series in the STATSGO soil mapping units and the results were entered into ARC/INFO to produce the final hazard maps showing best, weighted average, and worst case results for every unique combination (polygon) of soil mapping unit and climate. County weed infestation maps for leafy spurge and spotted knapweed were digitized and overlaid in ARC/INFO with the CMLS model results for picloram to illustrate how the results might be used to evaluate the threat to groundwater posed by current herbicide applications.

Soybean with resistance to dicamba (DR soybean) and glyphosate and cotton with resistance to glyphosate, glufosinate, and dicamba were recently commercialized in the United States and have been readily adopted. To evaluate results of over-the-top application of dicamba in DR crops, field studies were designed to examine off-target movement using proposed sprayer setup recommendations. Association analysis and nonlinear regression techniques were used to examine the effects of 26 field-scale drift trials conducted in 2014 and 2015 during soybean reproductive development (R1 through R6). The greatest predictors (injury, height reduction) of soybean yield reduction generally occurred and had steeper relationships after drift events at the R1 growth stage than at later stages. Using non-DR soybean as an indicator, dicamba was documented to move as much as 152 m from the application area (distance to 5% injury). Instances of height reduction (5%) differed among growth stages, with the greatest distance occurring at R1 (83.4 m). Soybean yield reduction was erratic, with the greatest distance to 5% loss in yield occurring at 42.8 m after an R1 drift event. Overall, the data suggest flowering-stage soybean is more sensitive than later reproductive soybean to injury, height reductions, and yield loss. Average and maximum wind speeds did not account for the injury documented from dicamba, and it is hypothesized that other meteorological variables also play a notable role in dicamba off-target movement as well as growing conditions following exposure. With concerns surrounding off-target movement of dicamba, proper stewardship of this new technology will be key to its longevity.

Germinability and viability of leafy spurge seeds following ingestion by sheep and goats were evaluated. Four sheep and four goats were separated into eight pens and fed individually a known number of leafy spurge seeds. Following ingestion, goats and sheep passed leafy spurge seeds for 4 and 9 d, respectively. Eighteen percent of the seeds fed were recovered in the fecal material of both species of livestock. Number of seeds recovered, expressed as percent of total seeds fed, decreased with time (P < 0.01). Germinability and viability of the recovered seeds were less than the control (P < 0.01). Sheep were more effective than goats in reducing germinability and viability of leafy spurge seeds (P < 0.01). Results indicate that sheep and goats are effective biological tools for reducing spread of leafy spurge seeds. However, animals grazing mature leafy spurge should be confined for 5 d, until all viable seeds have passed through the digestive system, before they are rotated onto pastures that are devoid of leafy spurge.

Greenhouse and field studies were conducted to examine weed control with, and potato tolerance to, rimsulfuron or rimsulfuron + metribuzin applied POST. In greenhouse trials, hairy nightshade control with rimsulfuron applied POST at 9 to 35 g/ha was ≥ 90%, but cutleaf nightshade control was < 30%. In field studies, rimsulfuron at 18 to 35 g/ha provided ≥ 85% control of hairy nightshade, redroot pigweed, tumble pigweed, and shepherd's-purse; 75 to 82% control of volunteer oats; and 25 to 87% control of common lambsquarters. Common lambsquarters control was increased to 92 to 100% when rimsulfuron at 18 to 35 g/ha was applied in combination with metribuzin at 280 g/ha. Hairy nightshade and redroot pigweed control usually were not improved by adding metribuzin to the rimsulfuron spray mixture and tumble pigweed and shepherd's-purse control were improved only slightly. Volunteer oat control with rimsulfuron sometimes was antagonized by the addition of metribuzin to the spray mixture. In weed-free trials, yield of potatoes grown for the fresh market or to produce seed tubers was not reduced by rimsulfuron at rates as high as 70 g/ha.

Herbicide-resistant cultivars may improve some aspects of weed management in wheat. However, negative consequences such as gene transfer among species, increased development of resistant weeds, or less effective volunteer wheat control may result from their use. Therefore, we reviewed literature on volunteer (self-sown) wheat seedling emergence and seed longevity in soil for insight in managing herbicide-resistant wheat. Data from classical burial studies suggested that wheat seeds were short-lived in soil, persisting less than 1 yr. Yet, in field studies, volunteer wheat seedlings were still emerging 16 mo after harvest; occasionally, seedlings have been observed 2 yr after harvest. Volunteer wheat emergence was extremely variable; causes of the variability are numerous and include genotypic, environmental, and production factors. This variability makes it difficult to predict volunteer wheat infestations in future years. Diverse cropping systems will enable producers to accrue the benefits of herbicide-resistant cultivars and yet still manage wheat volunteers, minimize gene flow by pollen, and avoid transfer of herbicide resistance. In regions where alternative crops are not viable, a key concern will be controlling volunteers and gene transfer in the next wheat crop.

Over a decade ago, the Weed Science Society of America sponsored a symposium on the then emerging technology of herbicide-tolerant crops (HTCs). The symposium and subsequent proceedings addressed potential benefits and concerns about that new technology to control weeds. Technological, biological, and ethical questions were addressed that were likely to emerge from the widespread adoption of HTCs. It was suggested at that time that if such questions were answered, HTC development would proceed on a more solid foundation, whereas continued uncertainty and criticism would probably result if the questions were not answered. We now review developments in HTC technology. Questions and concerns posed one decade ago are still pertinent, but current knowledge is still insufficient to address them. Adoption of HTC has risen dramatically since their commercial introduction, but there is still no evidence of associated production cost reductions or enhanced yields

Sulfonylurea resistance has been found in Japan, and four of the nine resistant weeds are in the genus Lindernia (Scrophulariaceae). To develop appropriate control methods for these resistant weeds, a rapid method to diagnose sulfonylurea-resistant weeds was improved and applied to common falsepimpernel (L. procumbens), two varieties of low falsepimpernel (L. dubia var. dubia and var. major), and azetogarashi (L. micrantha). Illumination is crucial for diagnosis in Lindernia spp., and treatment with 7.5 or 75 ng ai/ml thifensulfuron-methyl produced clear results.

Control of Texas panicum and southern crabgrass with sethoxydim was reduced under field conditions when bentazon, acifluorfen, bentazon + acifluorfen, pyridate, or naptalam plus 2,4-DB was added. The addition of lactofen to sethoxydim did not result in antagonism. Complete control of annual grasses did not appear to be a necessity in order to improve peanut yields. Peanut yields in plots with 70 to 80% control of annual grasses were comparable to those where annual grass control was 99%.

Nonsynthetic herbicides offer a potentially useful addition to the suite of weed management tools available to organic growers, but limited information is available to guide the optimal use of these products. The objectives of this research were to (1) evaluate the efficacy of clove oil– and vinegar-based herbicides on weeds across multiple states, and (2) assess the potential role of temperature, relative humidity (RH), and cloud cover in explaining inter-state variations in results. From 2006 to 2008, a total of 20 field trials were conducted in seven states using an identical protocol. Seeds of brown mustard were sown and herbicides applied to both mustard and emerged weeds when mustard reached the three- to four-leaf stage. Treatments included clove oil at 2.5, 5, 7.5, and 10% v/v concentrations at 54 L ha−1, and vinegar at 5, 10, 15, and 20% v/v concentrations at 107 L ha−1. Results varied widely across trials. In general, concentrations of at least 7.5% for clove oil and 15% for vinegar were needed for adequate control of mustard. Both products were more effective at suppressing mustard than Amaranthus spp. or common lambsquarters. Poor control was observed for annual grasses. No significant effects of cloud cover on the efficacy of either product were detected. In contrast, RH was positively correlated with control of brown mustard by both clove oil and vinegar with improved control at higher RH. Temperature had no detectable effect on the efficacy of clove oil, but higher temperatures improved control of brown mustard by vinegar.

Knowledge of weed community structure in vegetable crops of the north central region (NCR) is poor. To characterize weed species composition present at harvest (hereafter called residual weeds) in processing sweet corn, 175 fields were surveyed in Illinois, Minnesota, and Wisconsin from 2005 to 2007. Weed density was enumerated by species in thirty 1-m2 quadrats placed randomly along a 300- to 500-m loop through the field, and additional species observed outside quadrats were also recorded. Based on weed community composition, population density, and mean plant size, overall weed interference level was rated. A total of 56 residual weed species were observed and no single species dominated the community of NCR processing sweet corn. Several of the most abundant species, such as common lambsquarters and velvetleaf, have been problems for many years, while other species, like wild-proso millet, have become problematic in only the last 20 yr. Compared to a survey of weeds in sweet corn more than 40 yr ago, greater use of herbicides is associated with reductions in weed density by approximately an order of magnitude; however, 57% of fields appeared to suffer yield loss due to weeds. Sweet corn harvest in the NCR ranges from July into early October. Earlier harvests were characterized by some of the highest weed densities, while late-emerging weeds such as eastern black nightshade occurred in fields harvested after August. Fall panicum, giant foxtail, wild-proso millet, common lambsquarters, and velvetleaf were the most abundant species across the NCR, yet each state had some unique dominant weeds.

Volunteer potato plants are difficult to control in rotational crops, and they harbor harmful diseases, nematodes, and insects. Five herbicide treatments were evaluated for control of volunteer potato in field corn grown under no-till and conventional tillage in 1994 and 1995. In mid-June, potato control was greater in conventionally tilled corn than no-till corn primarily due to the reservoir tillage that followed postemergence (POST) herbicide applications in the conventional system. Final potato tuber weight was not different between tillage levels in 1994 or 1995. Herbicide treatments reduced potato tuber weight 64 to 96% in no-till corn and 85 to 99% in conventionally tilled corn compared to nontreated checks. Atrazine applied preemergence at 1.1 kg ai/ha followed by dicamba plus 2,4-D applied POST at 0.28 plus 1.1 kg ae/ha, respectively, reduced potato tuber weight greater than 95% in both years and both tillage systems. Corn yield was not affected by tillage level or herbicide treatments in 1994 or 1995 and averaged 10.5 MT/ha in 1994 and 15.1 MT/ha in 1995. In a separate experiment, glyphosate, dicamba plus 2,4-D, or fluroxypyr applied at the eight-leaf stage to potato grown without a corn crop, and followed by cultivation 10 d later, reduced the number of potato tubers produced 79 to 95% compared to nontreated potato.

Sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio) propyl]-3-hydroxy-2-cyclohexen-1-one} at 0.34 kg ai/ha, fluazifop {(+)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} at 0.13 kg ai/ha, and SC-1084 {3-hydroxy-4-[4-[[5-trifluoromethyl)-2-pyridinyl] oxy] phenoxy] pentanoic acid} at 0.28 kg ai/ha controlled ‘Tifway’ bermudagrass [Cynodon transvaalensis Burtt-Davy #3 CYNTR] x [Cynodon dactylon (L.) Pers. # CYNDA] nearly 100% when applied for 2 consecutive years. Tall fescue (Festuca arundinacea Schreb. ‘Ky 31’ # FESAR) tolerated fluazifop at 0.13 kg/ha and SC-1084 at 0.07 to 0.28 kg/ha; however, centipedegrass [Eremochloa ophiuroides (Munro) Hack, ‘common’ #ERLOP] and zoysiagrass (Zoysia japonica Steud. # ZOYJA x Zoysia tenuifolia Willd. ex trin. ‘Emerald’ #ZOYTE did not tolerate these treatments. Centipedegrass tolerated sethoxydim, zoysiagrass tolerance was intermediate, and tall fescue was injured severely. Sethoxydim at 0.22 kg/ha discolored leaves of zoysiagrass, but the turf recovered fully. Thus, bermudagrass, when mixed with either tall fescue, centipedegrass, or zoysiagrass, can be controlled selectively with herbicides applied postemergence.