Glyphosate’s efficacy is influenced by the amount absorbed and translocated throughout the plant to inhibit 5-enolpyruvyl shikimate-3-phosphate synthase (EPSPS). Glyphosate resistance can be due to target-site (TS) or non–target site (NTS) resistance mechanisms. TS resistance includes an altered target site and gene overexpression, while NTS resistance includes reduced absorption, reduced translocation, enhanced metabolism, and exclusion/sequestration. The goal of this research was to elucidate the mechanism(s) of glyphosate resistance in common ragweed (Ambrosia artemisiifolia L.) from Ontario, Canada. The resistance factor for this glyphosate-resistant (GR) A. artemisiifolia biotype is 5.1. No amino acid substitutions were found at positions 102 or 106 of the EPSPS enzyme in this A. artemisiifolia biotype. Based on [14C]glyphosate studies, there was no difference in glyphosate absorption or translocation between glyphosate-susceptible (GS) and GR A. artemisiifolia biotypes. Radio-labeled glyphosate metabolites were similar for GS and GR A. artemisiifolia 96 h after application. Glyphosate resistance in this A. artemisiifolia biotype is not due to an altered target site due to amino acid substitutions at positions 102 and 106 in the EPSPS and is not due to the NTS mechanisms of reduced absorption, reduced translocation, or enhanced metabolism.

Glufosinate inhibits glutamine synthetase (GS), a key enzyme for amino acid metabolism and photorespiration. Protoporphyrinogen oxidase (PPO) inhibitors block chlorophyll biosynthesis and cause protoporphyrin accumulation, a highly photodynamic intermediate. Both herbicides ultimately lead to plant death by a massive accumulation of reactive oxygen species (ROS) through different mechanisms. We investigated a potential synergistic effect by the mixture of the two herbicide mechanisms of action (MoAs). The tank mix between a low rate of glufosinate (280 g ai ha−1) with an ultra-low dose of saflufenacil (1 g ha−1) provided enhanced herbicidal activity compared with the products applied individually on Palmer amaranth (Amaranthus palmeri S. Watson). The synergism between the two herbicides was also confirmed by isobole analysis and field trials. The herbicide combination provided high levels of efficacy when applied at low temperature and low humidity. Mechanistically, glufosinate caused a transient accumulation of glutamate, the building block for chlorophyll biosynthesis. Consequently, inhibition of both GS and PPO resulted in greater accumulation of protoporphyrin and ROS, forming the physiological basis for the synergism between glufosinate and PPO inhibitors. While the synergy between the two herbicide MoAs provided excellent efficacy on weeds, it caused low injury to PPO-resistant waterhemp [Amaranthus tuberculatus (Moq.) Sauer] and high injury to both glufosinate-resistant and glufosinate-susceptible soybean [Glycine max (L.) Merr.]. Glufosinate enhances the activity of PPO inhibitors through glutamate and protoporphyrin accumulation, leading to increased levels of ROS and lipid peroxidation. The synergism between the two herbicide MoAs can help to overcome environmental effects limiting the efficacy of glufosinate. Future research is needed to optimize the uses for this herbicidal composition across different cropping systems.

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.

Glyphosate-resistant (GR) kochia has been reported across the western and midwestern United States. From 2011 to 2014, kochia seed was collected from agronomic regions across Colorado to evaluate the frequency and distribution of glyphosate-, dicamba-, and fluroxypyr-resistant kochia, and to assess the frequency of multiple resistance. Here we report resistance frequency as percent resistance within a population, and resistance distribution as the percentage and locations of accessions classified as resistant to a discriminating herbicide dose. In 2011, kochia accessions were screened with glyphosate only, whereas from 2012 to 2014 kochia accessions were screened with glyphosate, dicamba, and fluroxypyr. From 2011 to 2014, the percentages of GR kochia accessions were 60%, 45%, 39%, and 52%, respectively. The percentages of dicamba-resistant kochia accessions from 2012 to 2014 were 33%, 45%, and 28%, respectively. No fluroxypyr-resistant accessions were identified. Multiple-resistant accessions (low resistance or resistant to both glyphosate and dicamba) from 2012 to 2014 were identified in 14%, 15%, and 20% of total sampled accessions, respectively. This confirmation of multiple glyphosate and dicamba resistance in kochia accessions emphasizes the importance of diversity in herbicide site of action as critical to extend the usefulness of remaining effective herbicides such as fluroxypyr for management of this weed.

Glyphosate-resistant (GR) Palmer amaranth (Amaranthus palmeri S. Watson) is considered one of the most troublesome weeds in the southern and central United States, but results of previous research to determine the mode of inheritance of this trait have been conflicting and inconclusive. In this study, we examined segregation patterns of EPSPS gene-copy numbers in F1 and F2 generations of A. palmeri and found no evidence of a Mendelian single-gene pattern of inheritance. Transgressive segregation for copy number was exhibited by several F1 and all of the F2 families, most likely the product of EPSPS copy-number variation within each plant. This variation was confirmed by assaying gene-copy number across clonal generations and among individual shoots on the same plant, demonstrating that EPSPS amplification levels vary significantly within a single plant. Increases and decreases in copy number occurred in a controlled, stress-free environment in the absence of glyphosate, indicating that EPSPS gene amplification is a random and variable process within the plant. The ability of A. palmeri to gain or lose EPSPS gene copies is a valuable adaptive trait, allowing this species to respond rapidly to selection pressures and changing environments.

Glyphosate-resistant (GR) goosegrass [Eleusine indica (L.) Gaertn.] was recently identified in Brazil, but its resistance mechanism was unknown. This study elucidated the resistance mechanism in this species and developed a molecular marker for rapid detection of this target-site resistance trait. The resistance factor for the resistant biotype was 4.4-fold compared with the glyphosate-susceptible (GS) in greenhouse dose–response experiments. This was accompanied by a similar (4-fold) difference in the levels of in vitro and in planta shikimate accumulation in these biotypes. However, there was no difference in uptake, translocation, or metabolism of glyphosate between the GS and GR biotypes. Moreover, both biotypes showed similar values for 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) copy number and transcription. Sequencing of a 330-bp fragment of the EPSPS gene identified a single-nucleotide polymorphism that led to a Pro-106-Ser amino acid substitution in the enzyme from the GR biotype. This mutation imparted a 3.8-fold increase in the amount of glyphosate required to inhibit 50% of EPSPS activity, confirming the role of this amino acid substitution in resistance to glyphosate. A quantitative PCR–based genotyping assay was developed for the rapid detection of resistant plants containing this Pro-106-Ser mutation.

Timing of weed emergence and seed persistence in the soil influence the ability to implement timely and effective control practices. Emergence patterns and seed persistence of kochia populations were monitored in 2010 and 2011 at sites in Kansas, Colorado, Wyoming, Nebraska, and South Dakota. Weekly observations of emergence were initiated in March and continued until no new emergence occurred. Seed was harvested from each site, placed into 100-seed mesh packets, and buried at depths of 0, 2.5, and 10 cm in fall of 2010 and 2011. Packets were exhumed at 6-mo intervals over 2 yr. Viability of exhumed seeds was evaluated. Nonlinear mixed-effects Weibull models were fit to cumulative emergence (%) across growing degree days (GDD) and to viable seed (%) across burial time to describe their fixed and random effects across site-years. Final emergence densities varied among site-years and ranged from as few as 4 to almost 380,000 seedlings m−2. Across 11 site-years in Kansas, cumulative GDD needed for 10% emergence were 168, while across 6 site-years in Wyoming and Nebraska, only 90 GDD were needed; on the calendar, this date shifted from early to late March. The majority (>95%) of kochia seed did not persist for more than 2 yr. Remaining seed viability was generally >80% when seeds were exhumed within 6 mo after burial in March, and declined to <5% by October of the first year after burial. Burial did not appear to increase or decrease seed viability over time but placed seed in a position from which seedling emergence would not be possible. High seedling emergence that occurs very early in the spring emphasizes the need for fall or early spring PRE weed control such as tillage, herbicides, and cover crops, while continued emergence into midsummer emphasizes the need for extended periods of kochia management.

Wild-proso millet control in furrow-irrigated corn was evaluated in Colorado and Nebraska in 1986 and 1987. No single herbicide alone controlled wild-proso millet all season. In Colorado, EPTC applied preplant incorporated followed by cyanazine plus pendimethalin applied early postemergence controlled 94% of wild-proso millet. Acetochlor applied preemergence followed by cyanazine plus pendimethalin applied early postemergence controlled wild-proso millet better all season (93%) than alachlor (81%) or metolachlor (71%) followed by the same early postemergence strategy. Successful Colorado wild-proso millet management treatments (>85% season-long control) increased corn yields an average of 3260 kg ha-1 compared to the untreated control. To obtain wild-proso millet control of 90% or more in Nebraska in 1986, alachlor, cycloate, EPTC, and metolachlor applied had to be combined with cyanazine plus pendimethalin applied early postemergence. Average corn yields in herbicide-treated areas in Nebraska were 2980 kg ha-1 higher than those recorded in the untreated control.

Dicamba, 2,4-D, picloram, and commercially available premixes of glyphosate plus 2,4-D or glyphosate plus dicamba were evaluated alone and in combination for field bindweed control in a winter wheat-fallow system in Colorado, Wyoming, Kansas, and Montana. Approximately one year after application, herbicide mixtures containing picloram at 0.14 or 0.28 kg ai ha-1 provided the best control. In five of seven locations, the control provided by picloram in herbicide mixtures was greater than the control provided by glyphosate plus 2,4-D, 2,4-D, or dicamba when these products were mixed with picloram. Glyphosate plus 2,4-D or glyphosate plus dicamba premixes, or 2,4-D added to dicamba were less effective for long-term control of field bindweed than mixtures containing 0.14 kg ai ha-1 or more of picloram. Under drought conditions in Kansas in 1988, picloram did not control field bindweed as well as in Colorado, Wyoming, or Montana where rainfall was normal.

A 3-yr field study was conducted to compare an in-row cultivator versus a standard row-crop cultivator to decisions made with WEEDCAM, a weed/corn management computer decision aid, for controlling annual weeds within the row in irrigated corn. In the absence of herbicides, weeds were always controlled better with the in-row cultivator than with the standard row-crop cultivator. However, grain yield and gross margin were affected only in 1991 when weeds emerged simultaneously with corn, and rain delayed the first cultivation 10 d. The in-row cultivator plots not only averaged 34% more grain ha-1 than the standard row-crop cultivator plots, but gross margin was $143 ha-1 more. Weed densities each year were about 95% less in plots managed in accordance with the computer model WEEDCAM simulations than in the non-herbicide treated post-planting tillage plots. Grain yields and gross margins were not affected by weed seedbank density, pre-cultivation tillage, or type of cultivator when weed management decisions were based on WEEDCAM simulation ranking. In the absence of herbicides, weeds can be controlled successfully in corn with an in-row cultivator, but success will depend on such factors as weed seedbank density, cultivation timeliness, and relative time of weed and corn emergence.

The impact of weed density and weed distribution on irrigated corn yield was investigated in Colorado. Weed densities examined were 0,33,50, or 100% of the indigenous weed population. A series of weed distribution treatments were achieved by varying the length of the weed-free and weedy zones within the corn row while maintaining a constant weed population of 33 or 50% of the indigenous weed level. Grain yield was affected by weed density, but not by weed distribution. Each additional weed reduced corn yield 8.5 and 2.3 kg ha−1 in 1991 and 1992, respectively. When corn yields were estimated with a computer weed/corn management model, weed densities 5 to 8 wk after planting provided a better yield reduction estimate than weed densities immediately before harvest.

Laboratory experiments were conducted to assess the influence of surfactants applied with or without nitrogen on MON 37500 foliar absorption by Bromus tectorum, Bromus japonicus, Aegilops cylindrica, Triticum aestivum, Chorispora tenella, and Lactuca serriola. MON 37500 absorption in B. tectorum and B. japonicus increased from 40% 24 h after treatment (HAT) to 48% 48 HAT, averaged across surfactants with no added nitrogen. Averaged across nitrogen source and species, nonionic surfactant, ethylated seed oil, and organosilicate provided comparable enhancement of MON 37500 absorption (56 to 68%), whereas crop oil concentrate provided only 27 to 29% absorption under the same conditions. Averaged across species and surfactant class, urea ammonium nitrate had the greatest effect on MON 37500 absorption (68%), compared to ammonium sulfate (59%) or no nitrogen (40%). Nitrogen, regardless of the type, significantly improved foliar absorption of MON 37500. MON 37500 absorption by species was 71, 63, 57, 57, 49, and 38% in C. tenella, B. japonicus, T. aestivum, A. cylindrica, B. tectorum, and L. serriola, respectively, when averaged across surfactants and nitrogen. Densely pubescent B. japonicus leaves did not retain significant amounts of MON 37500 following a primary leaf wash.

A methodological approach to determine the optimum time to control weeds that integrates aspects of weed biology, weed-crop competition, and economics is presented. The approach is based on the concept of Time Density Equivalent: this is defined as the density of weed plants that germinate with the crop and compete until harvest that causes the same yield loss caused by a group of weeds with a given density, time of emergence, and time of removal. A model was developed that accounts for pattern of weed emergence and permits determination of timing of weed control that minimizes economic loss due to weeds emerging both before and after treatments. The outcomes of the model are presented with two examples: corn in competition with velvetleaf and soybean in competition with Amaranthus cruentus. For both crops, six different weed control strategies involving preemergence, chemical, and mechanical postemergence treatments are considered. The results obtained with the model are compared with the calculation of net margin based on assumptions of simultaneous emergence of crop and weeds and no effect of different times of control. Different control strategies are compared considering not only maximum net margin but also its dependence on time of control, because a strategy with a lower value of maximum net margin, but a flatter net margin curve, allows more flexibility of time of control.

Two greenhouse experiments were conducted to compare the growth of individual sulfonylurea-resistant and -susceptible kochia under noncompetitive conditions. Aboveground leaf and stem dry weight, and leaf area per plant were measured weekly 14 times, starting 14 d after planting. Data were fitted using the Richards function for shoot dry weight per plant, a polynomial exponential for leaf area per plant and splined function to calculate leaf area ratio, leaf weight ratio, and leaf/stem ratio. Absolute and relative growth rates, and net assimilation rate were derived from these functions. Growth and development of individual sulfonylurea-resistant and -susceptible kochia plants under noncompetitive conditions was the same. Final shoot dry weight and leaf area were unaffected by aceto lactate synthase enzyme differences in the kochia biotypes. However, more resources were partitioned to leaves than stems in resistant than in susceptible kochia. If competitive abilities of sulfonylurea-resistant and -susceptible kochia are different, it is not the consequence of differential growth and ontogeny of the two types of plants.

The life cycle of Notaris bimaculatus Fab. and the influence of this weevil on the control of quackgrass [Agropyron repens (L.) Beauv.] with glyphosate [N-(phosphonomethyl) glycine] were studied. Observations on this insect in 1978 and 1979 showed that its life cycle occurs in close association with quackgrass. The adult weevils feed on quackgrass culms and caryopses and use the inside of the culms for ovipositing. Adult populations, measured in quackgrass infestations during the summer months, ranged from 3 to 44/25 sweeps of an insect net. Larvae emerge from the eggs after 2 weeks, feed down the inside of the culms, chew an exit hole, and move into the soil where they attack the rhizomes. Larval numbers ranged from two to six/28 dm3 of soil during the summer months. Larvae feed on the rhizome surface or enter the rhizomes where they devour the vascular and cortical tissue. In dense quackgrass sods treated with glyphosate at 1.7 kg/ha, feeding damage on quackgrass rhizomes caused by the larvae reduced the control of quackgrass by disrupting the translocation of glyphosate in the rhizomes. Although soil-borne larvae of several insects were found in quackgrassinfested soil, larvae of N. bimaculatus were always present. Controlling these soil-borne insect larvae with a soil-applied insecticide for 2 months before applying glyphosate resulted in significantly increased quackgrass control. Shoot regrowth several months after the application of glyphosate at 1.4 kg/ha to quackgrass grown in cages infested with 400 adult weevils was 298 and 611 shoots/m2 in the 1979 and 1980 experiments, respectively. Similar glyphosate applications to weevil-free quackgrass resulted in only 26 and 15 shoots/m2 in the 1979 and 1980 experiments.

Crop yield loss–weed density relationships critically influence calculation of economic thresholds and the resulting management recommendations made by a bioeconomic model. To examine site-to-site and year-to-year variation in winter Triticum aestivum L. (winter wheat)–Aegilops cylindrica Host. (jointed goatgrass) interference relationships, the rectangular hyperbolic yield loss function was fit to data sets from multiyear field experiments conducted at Colorado, Idaho, Kansas, Montana, Nebraska, Utah, Washington, and Wyoming. The model was fit to three measures of A. cylindrica density: fall seedling, spring seedling, and reproductive tiller densities. Two parameters: i, the slope of the yield loss curve as A. cylindrica density approaches zero, and a, the maximum percentage yield loss as A. cylindrica density becomes very large, were estimated for each data set using nonlinear regression. Fit of the model to the data was better using spring seedling densities than fall seedling densities, but it was similar for spring seedling and reproductive tiller densities based on the residual mean square (RMS) values. Yield loss functions were less variable among years within a site than among sites for all measures of weed density. For the one site where year-to-year variation was observed (Archer, WY), parameter a varied significantly among years, but parameter i did not. Yield loss functions differed significantly among sites for 7 of 10 comparisons. Site-to-site statistical differences were generally due to variation in estimates of parameter i. Site-to-site and year-to-year variation in winter T. aestivum–A. cylindrica yield loss parameter estimates indicated that management recommendations made by a bioeconomic model cannot be based on a single yield loss function with the same parameter values for the winter T. aestivum-producing region. The predictive ability of a bioeconomic model is likely to be improved when yield loss functions incorporating time of emergence and crop density are built into the model's structure.

Metsulfuron sorption, dissipation, and leaching were studied in six Colorado soils. Sorption was studied in the laboratory by batch equilibration of soil horizons from the surface to 1 m deep found in each study area. Kd was correlated to several soil parameters with pH (-0.773) and percent organic matter (OM) (0.666) the strongest, although low. Kd generally decreased with depth at each site and ranged from 0.10 to 0.83 among surface soils. The lowest Kd was in the soil with the highest pH. Leaching and dissipation were studied by high-performance liquid chromatography/ultraviolet analysis of field samples collected 1 m deep. Bromide ion as conservative tracer was applied concurrently with metsulfuron at the four dryland and two irrigated sites. In most cases, metsulfuron was found only in upper horizons, and the half-life ranged from 11.8 to 27.7 d, the shortest being in the soil with highest percent OM and lowest pH. Very little leaching below the surface horizon was detected in any soil.

Laboratory and greenhouse experiments were conducted to examine the absorption and fate of quinclorac in field bindweed and to assess the importance of quinclorac soil activity for field bindweed control. No foliar absorption of 14C-quinclorac occurred when applied alone, but absorption increased to 24% when quinclorac was applied with 2,4-D, 28% urea ammonium nitrate (UAN), and methylated seed oil (MSO). Quinclorac translocation in field bindweed was limited, as < 18% of the total amount of absorbed radiolabeled material translocated out of the treated leaves 168 hours after treatment (HAT). Quinclorac metabolism in the treated leaves was minimal; 95% of the recovered 14C was intact herbicide 168 HAT. Quinclorac soil activity on field bindweed was demonstrated in preemergence and soil subsurface applications. Preemergence application of 35, 70, 140, or 280 g ha−-1 quinclorac reduced field bindweed shoot growth. Field bindweed shoots exhibited auxinic herbicide symptoms at all quinclorac rates. Subsurface layering of quinclorac below the root system at rates of 35 and 280 g ha−-1 also reduced shoot and root growth. Both herbicide rates induced malformation in root structure with a proliferation of lateral branching, swollen and fused root tips, and malformed root buds. Shoot growth from surviving roots replanted in untreated media was also reduced in both herbicide treatments. These findings suggest quinclorac soil activity may be important for field bindweed control.

The crop-weed interference relationship is a critical component of bioeconomic weed management models. Multi-year field experiments were conducted at five locations to determine the stability of corn-velvetleaf interference relationships across years and locations. Two coefficients (I and A) of a hyperbolic equation were estimated for each data set using nonlinear regression procedures. The I and A coefficients represent percent corn yield loss as velvetleaf density approaches zero, and maximum percent corn yield loss, respectively. The coefficient I was stable across years at two locations, but varied across years at one location. The coefficient A did not vary across years within locations. Both coefficients, however, varied among locations. Results do not support the use of common coefficient estimates for all locations within a region.