Seven half-day regional listening sessions were held between December 2016 and April 2017 with groups of diverse stakeholders on the issues and potential solutions for herbicide-resistance management. The objective of the listening sessions was to connect with stakeholders and hear their challenges and recommendations for addressing herbicide resistance. The coordinating team hired Strategic Conservation Solutions, LLC, to facilitate all the sessions. They and the coordinating team used in-person meetings, teleconferences, and email to communicate and coordinate the activities leading up to each regional listening session. The agenda was the same across all sessions and included small-group discussions followed by reporting to the full group for discussion. The planning process was the same across all the sessions, although the selection of venue, time of day, and stakeholder participants differed to accommodate the differences among regions. The listening-session format required a great deal of work and flexibility on the part of the coordinating team and regional coordinators. Overall, the participant evaluations from the sessions were positive, with participants expressing appreciation that they were asked for their thoughts on the subject of herbicide resistance. This paper details the methods and processes used to conduct these regional listening sessions and provides an assessment of the strengths and limitations of those processes.

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.

The influence of relative planting date on the growth of common cocklebur (Xanthium pensylvanicum Wallr. ♯ XANST), common ragweed (Ambrosia artemesiifolia L. ♯ AMBEL), sicklepod (Cassia obtusifolia L. ♯ CASOB), and redroot pigweed (Amaranthus retroflexus L. ♯ AMARE) grown in competition with soybean [Glycine max (L.) Merr. ‘Bragg’] was studied in the greenhouse. Increases in dry matter and height were slower for the five weed species than for soybean throughout the period of the study. The root: shoot ratio of soybean was the highest of any plant in the study, while common ragweed, common cocklebur, common lambsquarters, and sicklepod were intermediate, and redroot pigweed was the lowest. Soybean dry weight was always reduced when grown in competition with a weed. Soybean dry-matter production was reduced most when weeds were planted 2 weeks before soybean, especially with common cocklebur and common lambsquarters. Weed dry-matter content was severely reduced when the weed seed were planted simultaneously with or following soybean. Soybean height was usually reduced by competition with the weeds. The height of common ragweed was increased, however, when planted simultaneously with soybean. Common lambsquarters, redroot pigweed, and common ragweed heights were increased when planted 2 weeks prior to soybean.

Field experiments were conducted to determine the effect of DPX-PE350 on the efficacy of fluazifop-P, sethoxydim, and quizalofop. DPX-PE350 severely antagonized the activity of fluazifop-P on large crabgrass. Antagonism of sethoxydim and quizalofop was dependent on rate of graminicide and large crabgrass stage of growth at application. DPX-PE350 did not reduce control of johnsongrass with quizalofop. Sethoxydim alone controlled 36 to 71% more johnsongrass than sethoxydim mixed with DPX-PE350 at 35 to 105 g ai ha-1. Antagonism of fluazifop-P activity was dependent upon the rate of DPX-PE350 and fluazifop-P applied. Applying DPX-PE350 3 d after fluazifop-P or sethoxydim reduced antagonism. Grass species, growth stage, rate, and sequential application timing all appear to play a role in the severity of antagonism.

There is no question that weeds negatively impact human endeavors, both in agricultural and non-agricultural environments. The need for weed population management is inarguable. The important issue is “how will we manage weeds in the future”? Will it be business as usual, or will we be able to develop and implement a fully integrated approach to weed management? If Weed Science is to become a full partner in utilizing the concepts of Integrated Pest Management, there are at least four research areas that must be significantly strengthened. These research areas include: biology and ecology of weeds and crop/weed interactions, economic thresholds for weeds, alternative weed control methodology, and information delivery systems. New or non-traditional sources of funding will be required to adequately address these important issues.

The effects of sethoxydim {2-[1-(ethoxyimino) butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexene-1-one} rate, order, and timing of sequential herbicide applications, and the addition of oil concentrate on sethoxydimbentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H-one 2,2-dioxide] antagonism were studied on broadleaf signalgrass [Brachiaria platyphylla (Griseb.) Nash. ♯3 BRAPP], fall panicum (Panicum dichotomiflorum Michx. ♯ PANDI), and large crabgrass [Digitaria sanguinalis (L.) Scop. ♯ DIGSA] in field experiments during 1980 and 1981. Tank-mixing bentazon with sethoxydim reduced control of these species compared to applications of sethoxydim alone. Sequential applications of the herbicides prevented the antagonism. Variations in time between and order of sequential applications had no consistent effect on efficacy. Increases in sethoxydim rate reduced the severity of the antagonism in some cases, while the addition of oil concentrate had no effect.

Greenhouse and laboratory experiments were conducted to determine the effect of DPX-PE350 on the herbicidal activity of fluazifop-P on large crabgrass. Greenhouse experiments indicated that fluazifop-P activity was antagonized by DPX-PE350. This antagonism was reduced by increasing fluazifop-P rates or by applying DPX-PE350 after fluazifop-P. Applying DPX-PE350 24 h prior to fluazifop-P reduced fluazifop-P activity more than applying it 10 min prior. The effect of DPX-PE350 application to large crabgrass on extractable acetyl-CoA carboxylase activity, and the effect of DPX-PE350 on acetyl-CoA carboxylase activity in vitro also was studied. Applying DPX-PE350 to large crabgrass prior to protein extraction did not reduce fluazifop-P inhibition of acetyl CoA carboxylase activity in vitro. The presence of DPX-PE350 in the enzyme assay mix did not affect fluazifop-P inhibition of acetyl-CoA carboxylase activity. The effect of DPX-PE350 on uptake, translocation, and metabolism of fluazifop-P was considered. DPX-PE350 did not affect uptake of fluazifop-P by large crabgrass. Applying DPX-PE350 24 h prior to fluazifop-P decreased translocation of fluazifop-P out of the treated leaf 44 to 53% whereas mixing DPX-PE350 with fluazifop-P decreased fluazifop-P translocation only 15 to 17%. Metabolism of fluazifop-P was not affected by applications of DPX-PE350. These studies suggest that the antagonism of fluazifop-P activity by DPX-PE350 is due to decreased translocation of fluazifop-P out of the treated leaf.

Field studies were conducted during 1983 and 1984 to determine the single and interactive effects of trifluralin, soybean thrips, and the sodium salt of acifluorfen on soybean. Increased soybean injury was observed in 1983 when acifluorfen at 0.6 kg ai/ha was applied to soybeans infested with soybean thrips versus plants where soybean thrips were controlled. Soybean injury measured by percent defoliation and visual injury ratings was reduced when thrips were controlled versus soybeans where thrips were not controlled with carbaryl at 0.9 kg ai/ha in 1983. Soybean thrips alone did not reduce soybean seed yield in this study. Acifluorfen reduced soybean photosynthetic rate, shoot weight, root weight, and seed yield. Trifluralin had no effect on soybean growth parameters measured in this study.

In 1982 and 1983, effects of soybean row spacing and irrigation on weed resurgence following postemergence herbicide applications were evaluated. Also, ‘Forrest’ and ‘Ransom’ cultivar effects and early and late planting date influences on weed resurgence were evaluated in 1982 and 1983, respectively. In both years, as row spacing increased, weed resurgence increased. Photosynthetically active radiation measurements indicated the amount of weed resurgence coincided closely with the differing amounts of light penetrating to the soil surface. Irrigation, planting date, and cultivars showed no significant effect on weed resurgence. In 1982, a significant cultivar by row spacing interaction was obtained. In 1983, the late planting date showed a trend for less weed resurgence under irrigated conditions, although the difference was not significant. Soybean canopy measurements indicated development was greater under irrigation.

Root development and the associated acquisition of water and nutrients are an important part of weed competitiveness. Characterization of root morphological development, however, is inherently problematic because of the complexities of soil–plant interactions. In this study, we used hydroponically grown plants and digital imaging to examine root characteristics of Glycine max and the competing weeds Senna obtusifolia and Amaranthus palmeri. The purpose was to define inherent differences in root length and surface area that would contribute to growth responses during the establishment phase in the field. The methodology involved growing plants for 16 to 22 d, dissecting and staining root segments, mounting subsamples on slides, and imaging using a stereomicroscope and digital camera. Microscopy was required because of the small diameters of a significant proportion of the weed roots. With plants of similar root fresh weights (4.5 to 5.0 g), counting of individual roots revealed that 5. obtusifolia and A. palmeri had 2 and 3.7 times more roots than G. max (4,616 and 7,781 vs. 2,120, respectively). The imaging analyses indicated that roots of S. obtusifolia and A. palmeri had 2.9 and 5 times more length than G. max (10,042 and 17,192 cm vs. 3,418 cm, respectively). Furthermore, the analysis of length in different root diameter classes indicated that weed roots were noticeably finer then those of G. max. Approximately 84% of S. obtusifolia root length was contributed by roots in the 0.1- to 0.25-mm range, whereas 45% of the G. max roots were in the 0.1- to 0.25-mm range and 48% were in the 0.25- to 0.75-mm range. In contrast, 68% of A. palmeri length was contributed by roots smaller than 0.1 mm in diameter with 26% in the 0.1- to 0.25-mm range. Based on the expression of root characteristics observed here, root systems of these weed species would have finer roots with much greater length that would occupy a much larger volume of soil than those of G. max. Presumably, this would result in a competitive advantage in the acquisition of water and nutrients, especially when availability is limited.

Differential response of three weed species to prosulfuron was examined through studies of acetolactate synthase (ALS) sensitivity, uptake, translocation, and metabolism of the herbicide. Sensitivity of common cocklebur, common lambsquarters, and sicklepod ALS to prosulfuron was similar with I50 values of 3.3, 4.5, and 8.8 nM, respectively. Absorption of the herbicide increased with time for all three weed species. Percent absorption 2 d after application for common cocklebur, common lambsquarters, and sicklepod was 8, 33, and 26%, respectively. An average of 53% of the absorbed herbicide remained in the treated leaves of common cocklebur, 80% in common lambsquarters, and 93% in sicklepod. Approximately 33, 15, and 5% of the absorbed herbicide was translocated into the upper shoots of common cocklebur, common lambsquarters, and sicklepod, respectively. Metabolism of 14C-prosulfuron 1, 2, and 4 d after application was greater in sicklepod than in common lambsquarters or common cocklebur. The results suggest a combination of differential rates of translocation and metabolism may account for the differing sensitivity of these weeds at the whole plant level.

A 3-yr study was initiated in 1982 to determine the effects of herbicides and crop rotations on large crabgrass [Digitaria sanguinalis (L.) Scop. # DIGSA] and broadleaf signalgrass [Brachiaria platyphylla (Griseb.) Nash # BRAPP] population dynamics. Regardless of the crop rotation sequence, broadleaf signalgrass immediately became the predominant weed where standard herbicide programs were used. Large crabgrass became the predominant species after two growing seasons if no herbicides were applied. Domination by large crabgrass appeared to be due to greater seed production. The domination by broadleaf signalgrass in plots treated with herbicides was attributed to its tolerance to the primary grass herbicide alachlor [2-chloro-N-(2,6-diethylphenyl)-N-methoxymethyl)acetamide]. Broadleaf signalgrass emergence from soil treated with 2.2 kg ai/ha was not statistically different from that in untreated soil, while large crabgrass and fall panicum [Panicum dichotomiflorum (L.) Michx. # PANDI] emergence was significantly reduced at the same rate.

Broadleaf weeds apparently have patchy distributions within a field while POST control decisions are made assuming a regular spatial distribution. As a result, yield loss from weed competition may be overestimated, possibly leading to mistakes in choosing the optimal control treatment. Data on distribution of broadleaf weeds in 14 soybean fields were used in simulation experiments to investigate the potential for improving decision making with information about weed patchiness. The feasibility of modeling weed distribution in individual fields was also examined. Overall, the cost of assuming a regular distribution when making POST decisions was found to be low. Errors that occurred most often involved recommending more intensive control than was actually required, although in a few cases less intensive control was recommended. Error in the yield loss estimated for the uncontrolled population did not indicate the potential for a mistake in decision making for a field. Accurately modeling distribution of weeds within fields may be difficult as a result of correlations between distributions of individual species within a field and variation in distributions between fields.

Field experiments were conducted in 1979, 1980, and 1981 to determine the level of interference five broadleaf weed species exert on soybean [Glycine max (L.) Merr.]. Weed species studied were common cocklebur (Xanthium pensylvanicum Wallr. ♯ XANST), common ragweed (Ambrosia artemesiifolia L. ♯ AMBEL), common lambsquarters (Chenopodium album L. ♯ CHEAL), sicklepod (Cassia obtusifolia L. ♯ CASOB), and redroot pigweed (Amaranthus retroflexus L. ♯ AMARE). The following soybean seed yield reductions were observed with a density of 16 weeds/10-m row: redroot pigweed 22%, common lambsquarters 15%, common ragweed 12%, and sicklepod 5%, respectively. At a density of 8 weeds/10-m row, common cocklebur reduced soybean yield 11%. No single weed growth parameter predicted soybean seed yield reduction for all weed species. Soybean height was reduced by sicklepod competition; was not affected by competition from common lambsquarters, common ragweed, or common cocklebur; and was increased in the presence of redroot pigweed at 12 weeks after planting, when measured 30 cm from the weeds. Leaf area of soybean was higher at greater distances from the weed for all weed species. The range of soybean leaf area reductions occasioned by proximity to individual weed species corresponded fairly well with differences in soybean yield reduction.

Foliar absorption of 14-chlorimuron at 6, 12, 24, and 48 h after application to common cocklebur was reduced when applied in combination with acifluorfen. A maximum reduction of 10% was observed at 48 h. 14C translocation to meristematic tissue above the treated leaves also was reduced by acifluorfen in common cocklebur at 24 and 48 h after treatment. Absorption and translocation of 14C-chlorimuron in sicklepod was not affected by acifluorfen. Thin layer chromatographic analysis of treated leaf extracts showed two metabolites in common cocklebur (Rf values of 0.0 and 0.42) and four metabolites in sicklepod (Rf values of 0.0, 0.16, 0.33, and 0.50). All metabolites were more polar than the parent compound (Rf value of 0.64). Addition of acifluorfen did not affect chlorimuron metabolism in common cocklebur but reduced metabolism in sicklepod. Absorption, translocation, and metabolism of acifluorfen in common cocklebur or sicklepod were not affected by the addition of chlorimuron.

The Southern Weed Science Society has conducted an annual survey of the most troublesome weeds in several major crops since 1971. The objective of this summary was to characterize shifts in weed populations over a 22-yr period in four major agronomic crops. For corn, soybean, cotton, and peanut, the largest increases in rank as the most troublesome weeds were found with sicklepod and bermudagrass. The largest decreases were found with johnsongrass, crab-grasses, and common cocklebur. Morningglories and nutsedges remained relatively constant weed problems over the 22-yr period. Sicklepod, nutsedges, and morningglories were the three most troublesome weeds averaged over all crops because they are so well established and relatively difficult to control. Pigweeds (Palmer amaranth, sandhills amaranth, tumble pigweed, and water-hemps) have become increasingly important in soybean, peanut, and cotton in a limited number of states.

Experiments were conducted to investigate the interactions of tank-mix combinations of sethoxydim plus the sodium salt of bentazon, the sodium salt of acifluorfen, fomesafen, imazaquin, or the ethyl ester of chlorimuron. Antagonistic interactions were observed with tank-mixes of sethoxydim plus bentazon, imazaquin, or chlorimuron applied for fall panicum, large crabgrass, and goosegrass control in field experiments. Antagonism was observed in greenhouse experiments with tank-mixes of sethoxydim plus bentazon or imazaquin applied to goosegrass. Bentazon, acifluorfen, and fomesafen reduced 14C-sethoxydim uptake by large crabgrass. However, imazaquin and chlorimuron did not affect 14C-sethoxydim uptake. In field experiments, no interactions occurred with tank-mixes of sethoxydim plus any of the broadleaf weed control herbicides applied to entireleaf or tall morningglory.

The number of computer applications for purposes of weed control decisions has increased dramatically in the past eight yr. During this time, many efficacy-, and population-based weed control decision aids have been developed. The complexity of decisions range from those based on optimal weed control (independent of net profitability) to those predicting weed population effects before most profitable treatments are selected. While the number of software applications has increased sharply, national software databases have not been funded and tracking has been made difficult. The purpose of this paper is to review the criteria upon which efficacy- and population-based weed control decision aids are founded, and to identify software currently available in each of the two categories.

Studies were conducted in North Carolina to determine if johnsongrass [Sorghum halepense (L.) Pers.] rhizomes in the soil exude or produce substances that can affect soybean [Glycine max (L.) Merr.] seedling growth. Fresh and dry weights of soybean seedlings were dependent on the concentration of rhizomes present in the soil before soil samples were used for soybean growth, and on the time of year when soil samples were collected. In studies on rhizome residues, soybean seedling dry weight decreased as percent of dry, decayed rhizomes in the soil increased. For each rhizome concentration, soybean growth inhibition decreased as decay time increased. Fresh and dry weights of soybean plants were reduced by diluted fresh rhizome extract used for irrigation once a week for 3 weeks. Since no rhizomes were present in most of the soils during soybean growth and since nutrient concentrations, organic matter, and pH were not different among the soils used, it is suggested that johnsongrass rhizomes living or decaying in the soil exude, contain, or produce substances that exhibit allelopathic characteristics to soybean growth.