Weeds are one of the greatest challenges to snap bean (Phaseolus vulgaris L.) production. Anecdotal observation posits certain species frequently escape the weed management system by the time of crop harvest, hereafter called residual weeds. The objectives of this work were to (1) quantify the residual weed community in snap bean grown for processing across the major growing regions in the United States and (2) investigate linkages between the density of residual weeds and their contributions to weed canopy cover. In surveys of 358 fields across the Northwest (NW), Midwest (MW), and Northeast (NE), residual weeds were observed in 95% of the fields. While a total of 109 species or species-groups were identified, one to three species dominated the residual weed community of individual fields in most cases. It was not uncommon to have >10 weeds m−2 with a weed canopy covering >5% of the field’s surface area. Some of the most abundant and problematic species or species-groups escaping control included amaranth species such as smooth pigweed (Amaranthus hybridus L.), Palmer amaranth (Amaranthus palmeri S. Watson), redroot pigweed (Amaranthus retroflexus L.), and waterhemp [Amaranthus tuberculatus (Moq.) Sauer]; common lambsquarters (Chenopodium album L.); large crabgrass [Digitaria sanguinalis (L.) Scop.]; and ivyleaf morningglory (Ipomoea hederacea Jacq.). Emerging threats include hophornbeam copperleaf (Acalypha ostryifolia Riddell) in the MW and sharppoint fluvellin [Kickxia elatine (L.) Dumort.] in the NW. Beyond crop losses due to weed interference, the weed canopy at harvest poses a risk to contaminating snap bean products with foreign material. Random forest modeling predicts the residual weed canopy is dominated by C. album, D. sanguinalis, carpetweed (Mollugo verticillata L.), I. hederacea, amaranth species, and A. ostryifolia. This is the first quantitative report on the weed community escaping control in U.S. snap bean production.

The ability to use the protoporphyrinogen oxidase (PPO)-inhibiting herbicides fomesafen, flumioxazin, and sulfentrazone in potato is limited regionally or by soil texture, largely because of crop injury noted in research in the 1990s. With that in mind, we evaluated whether reducing the herbicide rates could maintain weed control while providing more consistent crop safety. Studies were conducted on a silt loam and a coarse-textured loamy sand soil. Soil texture played a greater than anticipated role in PPO-inhibitor herbicide injury risk as it relates to high-precipitation events. For example, in 2020 at the silt loam location, there were five precipitation events across the season that exceeded 2.5 cm, including one 6 d after treatment (DAT), and a seasonal total precipitation that was over 10 cm greater than the previous year. Despite excessive moisture and initial potato injury as high as 27% where flumioxazin was applied at the high rate with S-metolachlor, by 29 DAT injury was less than 10% in all treatments, and marketable tuber yield was similar among treatments. In contrast, in 2020 at the loamy sand location, there were four precipitation events across the season that exceeded 2.5 cm, and potato injury was as much as 60%. In 2020 the high amount of injury from flumioxazin was hypothesized to be caused by precipitation before herbicide application and not after, suggesting a need for more research in this area. This work documents the fine line between yield reduction presumably caused by reduced weed control and yield reduction assumed to be related to herbicide injury. This delineation between adequate weed control and consistent crop safety may differ by soil texture and environmental conditions, supporting the notion that custom-tailored weed management may become more necessary as high-precipitation events become more common in upper midwestern U.S. agricultural systems.

Weed management in carrot is challenging, given slow and inconsistent crop emergence and early-season growth and the lack of practical season-long management tools such as herbicides. We investigated holistic carrot production systems with a focus on minimizing inputs while optimizing resource use. In an overall sense, results of this work were consistent between years, and stark. The choice of carrot variety had a moderate influence on carrot foliar canopy development and, subsequently, weed density. For example, ‘Cupar’ carrot formed a complete crop canopy sooner than the other dicer-type ‘Canada’ variety. Likely as a result, density of weed species such as spotted ladysthumb and common lambsquarters was less where ‘Cupar’ was grown compared with where ‘Canada’ was grown. Gibberellic acid as a foliar application was not successful in these studies and, in a few cases, may have even increased weed-seed germination and establishment. Adding two carrot rows to the current regional industry-standard three-row bed system not only enhanced competitiveness with weeds but also improved carrot yield without additional fertilizer, water, or pest management inputs. By far, though, the most successful strategy to reduce weed density while maintaining or improving carrot yield was to delay seeding by 17 to 19 d. We anticipate more holistic production system research that integrates low-input alternatives in other crops as herbicide-resistant weeds proliferate while few new herbicides are developed. As was demonstrated in this research, such novel approaches can be successful without adding significant economic burden to the farmer or increasing risk of crop failure.

Linuron herbicide has been a mainstay of carrot weed management for years, but uncertainty around regulatory registration review and an increased prevalence of herbicide-resistant weeds have spurred interest in identifying alternatives that can be readily adopted in production. With this context in mind, herbicide programs were evaluated on a coarse-textured, low organic matter soil in 2015 and 2016. Season-long weed control without compromising yield was possible with weed management programs that included prometryn POST instead of linuron. With that said, a PRE herbicide such as pendimethalin was critical to establish an early-season competitive advantage for carrot plants over weeds, and careful attention should be paid to the prometryn rate, as selectivity is marginal. Carrot is often interseeded with a grain nurse crop to mitigate risk of wind erosion. Nurse crop injury was minimal where S-metolachlor, pendimethalin, or prometryn was applied at rates labeled for PRE use in carrot, with the exception of where prometryn was applied at rates above 1.1 kg ai ha−1.

Economically solvent fresh-market potato production is predominantly dependent on the ability to efficiently produce consistent tuber quality and high yield, and the ability to meet first-to-market demand with early-season potatoes. Unfortunately, these two qualifiers often work against each other in terms of production management. In response, we studied integrated potato vine management programs that support timely early-season potato harvest. Vine management with a flail chopper, roller, and flame burner used alone or followed by diquat was evaluated when initiated 21 or 14 d prior to harvest in 2015 and 2016. Potato varieties included ‘Yukon Gold’ and ‘Dark Red Norland’. Potato leaf and stem management, as well as tuber skinning, stolon separation, grade, and yield were quantified. Among mechanical methods, potato leaf and stem management were poor when vines were rolled or mowed but better when flail chopped. In general, vine management and tuber skin set was better when treatments were initiated 21 d prior to harvest as opposed to 14 d. Vine management, tuber skin set, and yield were comparable when potato vines were flail chopped followed by flame burning and where diquat was applied twice, offering a viable program for smaller scale or organic production.

Soybean yield gain over the last century has been attributed to both genetic and agronomic improvements. Recent research has characterized how breeding efforts to improve yield gain have also secondarily impacted agronomic practices such as seeding rate, planting date, and fungicide use. To our knowledge, no research has characterized the relationship between weed–soybean interference and genetic yield gain. Therefore, the objectives of this research were to determine whether newer cultivars would consistently yield higher than older cultivars under increasingly competitive environments, and whether soybean breeding efforts over time have indirectly increased soybean competitiveness. Field research was conducted in 2014, 2015, and 2016 in which 40 maturity group (MG) II soybean cultivars released between 1928 and 2013 were grown season-long with three different densities of volunteer corn (0, 2.8, and 11.2 plants m−2). Soybean seed yield of newer cultivars was higher than older cultivars at each volunteer corn density (P<0.0001). Soybean seed yield was also higher in the weed-free treatment than at low or high volunteer corn seeding rates. However, soybean cultivar release year did not affect late-season volunteer corn shoot dry biomass at either seeding rate of 2.8 or 11.2 seeds m−2. The results indicate that while soybean breeding efforts have increased yield potential over time, they have not increased soybean competitiveness with volunteer corn. These results highlight the importance of other cultural practices such as planting date and crop row spacing for weed suppression in modern soybean production systems.

Slow carrot emergence and canopy development render the crop a poor competitor with weeds. In this study, the ability to suppress weeds and maintain yield in the presence of weeds was compared among nine carrot varieties that included those selected by plant breeders for rapid vegetative canopy development compared to traditional varieties. Two weed management treatments were compared: handweeding for 21 d after carrot seeding versus handweeding for the entire carrot season. In years and locations with low to moderate weed pressure, such as in the 2014 study, differences among carrot varieties in weed competitiveness or tolerance were less apparent and therefore less relevant. Maximum carrot yield loss to weed competition among varieties was 28% in 2014. Yield loss in the presence of weeds was 15% or less with six of the nine carrot varieties. However, when weed pressure was intense in the 2015 study, both carrot plant density and carrot canopy development were inversely related to weed biomass. Carrot yield loss in the presence of weeds ranged from 38 to 87%. Despite correcting seeding populations for differences in germination among carrot varieties, carrot stand establishment varied greatly and would likely affect subsequent weed control measures such as timely cultivation or herbicide application. Future research efforts are warranted that consider carrot stand establishment factors and their relationship with integrated weed management programs.

Five cultivation implements (two flex-tine cultivators, brush hoe, and rolling and shovel cultivators) were evaluated for weed control and effect on broccoli, snap bean, and sweet corn yield. Each implement was used alone or in combination with flex-tine cultivators. Standard broadcast herbicide treatments were: broccoli, metolachlor at 2.3 kg ai/ha posttransplant; snap bean, metolachlor preemergence (PRE) at 2.3 kg ai/ha followed by fomesafen postemergence (POST) at 0.4 kg ai/ha; sweet corn, atrazine at 1.1 kg ai/ha plus metolachlor at 2.3 kg ai/ha PRE. In the snap bean trials, an additional treatment consisted of metolachlor at 2.3 kg ai/ha PRE applied in a 25-cm band over the crop row followed by fomesafen POST at 0.4 kg ai/ha directed at the crop row, with a subsequent shovel cultivation. In the sweet corn trials, all cultivation treatments received an application of atrazine at 1.1 kg ai/ha plus metolachlor at 2.3 kg ai/ha PRE in a 25-cm band over the crop row. In broccoli, weed control and yields with all cultivation treatments were comparable to those with broadcast herbicide. In snap bean, flex-tine cultivators failed to control weeds and prevent yield reductions compared to the broadcast herbicide treatment. When either the brush hoe or the shovel cultivator was preceded by flex-tine cultivation, weed control and snap bean yield were comparable to those with broadcast herbicides. Banded herbicides followed by shovel cultivation also controlled weeds and prevented yield reductions. In sweet corn, banded herbicides controlled in-row weeds, and despite variable between-row weed control, all cultivation treatments yielded as well as the broadcast herbicide treatment. Relative performance of the cultivators was influenced by soil type and rainfall as well as crop canopy characteristics.

Red beet growers have expressed interest in adopting the microrate herbicide approach originally implemented in sugarbeet to achieve season-long weed management. Several red beet herbicides were first labeled for use in sugarbeet and lack substantial residual weed control. In response, red beet herbicide programs were evaluated that included a PRE application followed by up to three POST applications of various herbicide combinations. This research, however, indicated that herbicide programs that included PRE herbicides followed by as few as one or two POST applications that involve multiple active ingredients can provide season-long weed control. This observation was consistent across a broad spectrum of weeds, between two study locations that varied in soil type, and during two growing seasons. Herbicide programs that included only a PRE and six-leaf red beet growth stage application were successful at two locations in maintaining weed control and crop yield relative to hand-weeded red beet. Furthermore, these herbicide programs reduced the number of applications by 50% compared with the full programs, reducing crop injury risk and grower cost.

Field research was conducted to evaluate the effectiveness of herbicides and carrot mowing for swamp dodder control. Herbicide evaluation indicated the highest carrot yield and lowest crop injury with pendimethalin compared to the industry standard linuron. Swamp dodder control with pendimethalin was greater than 80% at 56 and 70 d after planting (DAP). Other herbicides controlled swamp dodder, but crop injury was unacceptable. Carrot yield was greater where pendimethalin or s-metolachlor was applied compared to all other herbicides. Carrot mowing once 72, 86, or 100 DAP and mowing twice (72 plus 100 DAP) reduced the percentage of carrots infected with swamp dodder. Carrot infection level was least when mowed 100 DAP, and mowing did not increase yield compared to the non-treated check. These results suggest that the integration of pendimethalin for early-season swamp dodder control, followed by mowing 100 DAP, could reduce the impact of swamp dodder on carrots.

Small broomrape is an annual, parasitic weed that was discovered recently in Oregon's red clover seed production system. Field experiments were conducted in 2002 and 2003 at two locations to evaluate 10 herbicide treatments applied after small broomrape emergence in red clover. Bentazon, bromoxynil, glyphosate, imazamox, imazamox plus bentazon, imazethapyr, MCPA, and pendimethalin were evaluated. Small broomrape density, small broomrape seed viability after treatment, and clover injury and seed yield were quantified. Small broomrape control with imazamox, glyphosate, and imazamox plus bentazon treatments was greater than the nontreated check in both years. However, imazamox and imazamox plus bentazon treatments were the only herbicide treatments that consistently exhibited a high level of crop safety, reduced small broomrape density, and did not reduce red clover yield. Herbicide treatments did not prevent production of viable small broomrape seeds. Future research is needed to develop control options that will prevent red clover yield loss and viable small broomrape seed production when applied before small broomrape emergence.

Experiments compared the effect on weed control and potato yield of banded applications of metolachlor plus linuron with or without flex-tine, rolling, and shovel cultivation prior to hilling. Cultivation without banded herbicide resulted in greater prehilling in- and between-row weed densities and reduced late-season weed control as compared to broadcast herbicides or cultivation with banded herbicides. Although the flex-tine and rolling cultivators were expected to provide improved in-row weed control, there were few differences between these and the other cultivation implements. Despite reduced weed control with cultivation alone, potato yields were not reduced.

There is significant interest from processors in producing organic sweet corn and snap bean. However, large-scale production is necessary for this to be a practical and economical venture for processors. This study focused on the feasibility of managing weeds in organic sweet corn and snap bean, utilizing methods that are practical in large hectarage. Tactics such as rotary hoe, interrow cultivation, and a stale seedbed were evaluated alone or in combination. Hand-weeded and herbicide-based treatments were included for each crop for comparison. Percentage weed control, weed biomass, and crop yield were quantified, and net profit was calculated for each treatment. Organic weed management was feasible in snap bean, with yields similar among several of the organic treatments and the herbicide treatment in all 3 yr of the study. Interrow cultivation was the most effective means of organic weed control in snap bean. Organic weed management was possible in snap bean because it is a short-season crop and an effective competitor with weeds in the crop row. Organic weed management was more difficult in sweet corn because of the longer crop season and poor competition with weeds in the crop row. In sweet corn, the organic treatment involving three interrow cultivations was the only one consistently similar in yield to the herbicide treatments. Higher net profits were attained for most of the organic treatments in both crops because of the organic premium. Market saturation and organic premium adjustments are factors for grower consideration in this potential industry, particularly for sweet corn production.

Broomrapes (Orobanche spp.) are chlorophyll-lacking root parasites of many dicotyledonous species and cause severe damage to vegetable and field crops from several botanic families such as Fabaceae, Solanaceae, Compositae, and Umbelliferae. In Oregon, small broomrape has been identified as a parasite of red clover. In Oregon field studies, small broomrape control was excellent when imazamox was applied postemergence to red clover but preemergence to small broomrape. Temperature is one of the main factors that affect broomrape development. The objective of this study was to optimize small broomrape chemical control in red clover based on growing degree days (GDD). The study was conducted in controlled temperature conditions. Red clover plants were grown in soil artificially infested with small broomrape seeds. Imazamox was applied at 800, 1,000, 1,200, and 1,400 GDD. There was no injury to red clover from any imazamox treatment at any of the application timings. Small broomrape shoot emergence was reduced where imazamox was applied compared to the untreated control. Early imazamox applications reduced small broomrape biomass more than later applications. Control was greatest when imazamox was applied at 20 g ai ha−1 at 1,000 GDD. This application controlled small broomrape for 800 GDD after initial treatment. However, season-long control would require an additional treatment. This model predicts the optimal timing and rate of imazamox application for small broomrape control in red clover.

Small broomrape is a holoparasitic plant that attaches to the roots of red clover as well as several other host plants. Hosts and false hosts produce stimulants that induce small broomrape germination but small broomrape does not attach to a false host. Wheat has been identified as a false host for small broomrape; therefore, studies were conducted to investigate the effect of red clover and wheat root exudates on small broomrape germination. In one study, the effect of exudates from red clover and wheat at multiple growth stages on small broomrape germination was evaluated. Red clover induced small broomrape germination at all growth stages tested but was greatest (78%) in the presence of exudates from red clover at the three-trifoliolate stage. Maximum small broomrape germination was 25% when exposed to exudates produced by one-leaf-stage wheat. In a second study, the relationship between small broomrape germination and host growth condition was evaluated using root exudates from red clover or wheat grown under several temperature conditions for either 4 or 8 wk. For the different temperatures, there were no differences in small broomrape germination when exudates of red clover grown for 4 wk were used. Small broomrape germination was reduced when exposed to exudates from red clover plants grown for 8 wk at 10 C compared with plants grown at 15, 20, and 25 C. Differences in small broomrape seed germination were observed with temperature under which wheat was grown for 4 wk, but not for 8 wk. Although wheat exudates resulted in less small broomrape seed germination than red clover exudates, growing wheat as a false host in a small broomrape-infested field could be an important component of an integrated management plan.

Small broomrape is an annual holoparasitic weed that was recently discovered in red clover production fields in Oregon. Imidazolinone herbicides such as imazamox control small broomrape; however, the mechanism of uptake by the parasite is largely unknown. Studies were conducted to determine the imazamox route of uptake by small broomrape in red clover, and to determine the potential for imazamox to be exuded from red clover and the subsequent effect on small broomrape. Small broomrape control was best at 90% when imazamox was foliar-applied, and worst at 42% or less when imazamox was soil-applied. The presence of activated charcoal to adsorb imazamox at the soil surface did not affect efficacy of broadcast foliar treatment. Small broomrape control was also evaluated when a foliar-treated red clover plant was grown in the same pot as a nontreated, parasitized red clover plant that was bagged during herbicide application. Activated charcoal was spread on the soil surface to adsorb imazamox, thus limiting herbicide uptake routes to the foliage of one of two red clover plants in the pot. Small broomrape attachment decreased on nontreated red clover when the other red clover plant in the pot was treated, suggesting roots exuded the herbicide or an active metabolite.

Soybean planting has occurred earlier in the Midwestern United States in recent years; however, earlier planting subjects the crop to longer durations of weed interference. This may change the optimum timing of POST glyphosate applications, or increase the need for residual herbicides applied PRE to optimize yield. A field study was conducted in 2012 and 2013 near Arlington, WI to determine the effect of planting date, residual herbicide use, and POST glyphosate timing on weed control and soybean yield. Planting dates were late April, mid-May, and early June. A PRE application of sulfentrazone plus cloransulam was applied to half the plots following each planting date. Glyphosate was applied POST to all plots at the V1, V2, V4, or R1 soybean growth stage. Planting date and glyphosate timing did not affect soybean yield in this study. However, averaged across years, planting dates, and POST glyphosate timings, yield increased from 3,280 to 3,500 kg ha−1 when a PRE herbicide with residual soil activity was used. In POST-only treatments, delaying the planting date to June decreased weed density at POST application timing from 127 to 5 plants m−2 (96%) and from 205 to 42 plants m−2 (80%) in 2012 and 2013, respectively. Where a PRE was used, total weed density at POST application timing was always less within planting date, and also declined from early to late planting date 26 to 3 plants m−2 (89%) and 23 to 6 plants m−2 (74%) in 2012 and 2013, respectively. In conclusion, both PRE herbicide use and delayed soybean planting were effective strategies to reduce the number of in-crop weeds exposed to POST glyphosate and should be considered as strategies to reduce the number of weeds exposed to POST herbicides for resistance management.

Annual ryegrass has been proposed as a cover crop in the corn–soybean cropping systems of the U.S. Midwest because of its low seed cost, rapid establishment, contribution to soil quality, weed suppressive abilities, and susceptibility to common broad-spectrum herbicides. However, cover crops can reduce the subsequent main crop yield by creating unfavorable germination and emergence conditions, harboring pests, and if not controlled, competing with the main crop. This study, conducted in Illinois, Oregon, and Tennessee, investigated the efficacy of glyphosate for annual ryegrass winter cover crop removal. Glyphosate at 415, 830, and 1,660 g ae/ha was applied to annual ryegrass at late tiller, second node, boot, and early flowering stages. Annual ryegrass control was consistently maximized with the highest glyphosate rate applied at the boot or early flower stage. Annual ryegrass biomass was generally the lowest with the highest rate of glyphosate applied at the earlier stages. Overall, no single application timing at any glyphosate rate provided complete control or biomass reduction of the annual ryegrass cover crop. A sequential herbicide program or a glyphosate plus a graminicide tank-mix probably will be needed for adequate annual ryegrass stand removal.

Small broomrape is a parasite of several broadleaf plant species. Consequences of small broomrape infestation in host cropping systems include seed contamination, reduction in crop seed yield, and host plant death. The effect of small broomrape parasitism on the biomass partitioning of its primary host, red clover, has not been documented. Greenhouse experiments were conducted to determine the relationship between small broomrape and red clover biomass accumulation. Total biomass of parasitized red clover plants was 15 to 51% less than nonparasitized red clover plants. Small broomrape parasitism reduced the amount of dry matter allocated to red clover inflorescences by 50 to 80%. Small broomrape dry matter accumulation was strongly related to total red clover–small broomrape dry matter accumulation. Small broomrape attachment number per red clover plant was a poor indicator of relative small broomrape dry weight accumulation. The results of this study indicated that small broomrape accumulated resources from red clover at the greatest expense to the economically important reproductive tissues.

Small broomrape, a holoparasitic weed, was recently introduced to the Pacific Northwest and contaminates a limited number of red clover fields in Oregon. Greenhouse and field studies were conducted to evaluate small broomrape response to common crop and weed species in the Pacific Northwest, and to evaluate a quick-screening method for plant species response to the parasite. In greenhouse studies, plants were grown in a hydroponic polyethylene bag system to allow for continuous visibility of their roots and monitoring of small broomrape seed germination and tubercle development. Results of the greenhouse study were validated in a field contaminated with small broomrape. In the greenhouse polyethylene bag study, small broomrape germinated and tubercles developed on alfalfa, arrowleaf clover, carrot, celery, crimson clover, lettuce, prickly lettuce, red clover, spotted catsear, subterranean clover, white clover, and wild carrot. Small broomrape germinated but did not develop tubercles when grown with barley, birdsfoot trefoil, common vetch, creeping bentgrass, cucumber, field corn, red fescue, flax, Italian ryegrass, nasturtium, oats, orchardgrass, perennial ryegrass, snap bean, sugar pea, sunflower, sweet corn, tall fescue, tomato, and wheat. In the field study, tubercles developed on alfalfa, arrowleaf clover, common vetch, crimson clover, red clover, subterranean clover, and white clover, but small broomrape flower stalks did not emerge in arrowleaf clover or crimson clover. Common vetch was a host for small broomrape in the field study but not in the hydroponic polyethylene bag system, thus, caution is warranted in using the quick screening method to elucidate potential small broomrape host species.