Early soybean planting and cover crop adoption in the U.S. Midwest prompt investigation into the impact of these practices on weed community dynamics and best management practices. While previous research has explored different aspects of giant ragweed control, the specific integration among soil management practices, including cover crop adoption, soybean planting timing, and herbicide use, has not been thoroughly investigated. This study assessed the effects of soil management, soybean planting time, and preemergence (PRE) herbicide application on giant ragweed control and soybean yield in Wisconsin and Nebraska in 2022 and 2023. The study included a factorial arrangement of four soil management treatments (conventional tillage, no-till, and fall-planted cereal rye early terminated and terminated at planting [planting green]), two soybean planting times, and two PRE herbicide treatments (PRE and no PRE). POST herbicides were applied when ∼50% of giant ragweed plants within each treatment reached ∼10 cm in height. In Nebraska, cereal rye and tillage treatments without a PRE had at least 67% lower giant ragweed density than no-till at POST. In no-till, densities were at least 60% lower with PRE compared to no PRE. In Wisconsin, cereal rye did not reduce giant ragweed density at POST compared to no-till, likely due to relatively low biomass accumulation. In contrast, delayed soybean planting reduced giant ragweed density for most treatments but lowered soybean yield in no-till and planting-green treatments. The PRE herbicides had either no effect or positive effects on reducing giant ragweed density and increasing soybean yield. Overall, this study suggests that soil management and soybean planting timing are crucial for effective giant ragweed management in Wisconsin, where biotypes with a long emergence window during the spring and summer are present, while in Nebraska, soil management and soybean planting timing are less critical due to giant ragweed biotypes with an early and short emergence window in the spring.

Intermediate wheatgrass (IWG) is a cool-season perennial grass developed as a dual-purpose grain and forage crop. One barrier to adopting this crop is a lack of information on the effects of herbicides on IWG for grain production. An experiment was conducted to evaluate herbicide effects on IWG grain yield, crop injury, and weed control over 2 yr (2019 to 2021) at sites in Wisconsin, Minnesota, New York, and North Dakota. This evaluation included broadleaf herbicides registered for use on wheat: 2,4-D amine, clopyralid, MCPA, and a mixture of clopyralid + MCPA (all are categorized as Group 4 herbicides by the Weed Science Society of America). Each herbicide or mixture was applied at 1× and 2× the labeled wheat application rate to newly planted and established (1- to 5-yr-old) IWG stands in the fall or spring. Herbicides were applied during IWG tillering or jointing stages in the fall or during the jointing stage in the spring. Across site years, application timing, herbicide, and application rate showed no effect on IWG grain yield or plant injury. Broadleaf weed control ranged from 71% to 92% across herbicide treatments relative to the nontreated check at the Wisconsin site, whereas weed control at the Minnesota site was variable among treatments. At the New York site, herbicides were equally effective for broadleaf weed suppression, whereas weed pressure was very low at the North Dakota site and treatments did not affect weed cover. The results show that newly planted and established stands of IWG are tolerant to the synthetic auxin herbicides 2,4-D amine, clopyralid, and MCPA when applied during tillering or jointing in the fall or during jointing in the spring. Synthetic auxins represent a potentially useful tool for weed control in IWG cropping systems, especially for problematic broadleaf weed species.

A comprehensive, Wisconsin state-wide assessment of waterhemp response to a diverse group of herbicide sites of action has not been conducted. Our objective was to characterize the response of a state-wide collection of waterhemp accessions to postemergence (POST) and preemergence (PRE) herbicides commonly used in corn and soybean in Wisconsin. Greenhouse experiments were conducted with more than 80 accessions from 27 counties. POST treatments included 2,4-D, atrazine, dicamba, fomesafen, glufosinate, glyphosate, imazethapyr, and mesotrione at 1× and 3× label rates. PRE treatments included atrazine, fomesafen, mesotrione, metribuzin, and S-metolachlor at 0.5×, 1×, and 3× label rates. Ninety-eight percent and 88% of the accessions exhibited ≥50% plant survival after exposure to imazethapyr and glyphosate POST 3× rate, respectively. Seventeen percent, 16%, and 3% of the accessions exhibited ≥50% plant survival after exposure to 2,4-D, atrazine, and dicamba, respectively, applied POST at the 1× rate. Survival of all accessions was ≤25% after exposure to 2,4-D or dicamba applied POST at the 3× rate, or glufosinate, fomesafen, and mesotrione applied POST at either rate evaluated. No plant of any accession survived exposure to glufosinate at either rate. Forty-five percent and 3% of the accessions exhibited <90% plant density reduction after exposure to atrazine applied PRE at the 3× rate and fomesafen PRE at the 1× rate, respectively. Plant density reduction of all accessions was ≥96% after exposure to fomesafen applied PRE at the 3× rate, or metribuzin, S-metolachlor, and mesotrione applied PRE at the 1× rate. Our results suggest that waterhemp resistance to imazethapyr and glyphosate applied POST is widespread in Wisconsin, whereas resistance to 2,4-D, atrazine, and dicamba applied POST is present to a lower extent. One accession (A75, Fond du Lac County) exhibited multiple resistance to imazethapyr, atrazine, glyphosate, and 2,4-D when applied POST. Overall, atrazine applied PRE was ineffective for waterhemp control in Wisconsin. Proactive resistance management and the use of effective PRE and POST herbicides are fundamental for waterhemp management in Wisconsin.

The continued dispersal of Palmer amaranth can impose detrimental impacts on cropping systems in Wisconsin. Our objective was to characterize the response of a recently introduced Palmer amaranth accession in southern Wisconsin to postemergence (POST) and preemergence (PRE) herbicides commonly used in corn and soybean. Greenhouse experiments were conducted with the Wisconsin putative herbicide-resistant accession (BRO) and two additional control accessions from Nebraska, a glyphosate-resistant (KEI2) and a glyphosate-susceptible (KEI3) accession. POST treatments were 2,4-D, atrazine, dicamba, glufosinate, glyphosate, imazethapyr, lactofen, and mesotrione at 1X and 3X label rates. PRE treatments were atrazine, mesotrione, metribuzin, S-metolachlor, and sulfentrazone at 0.5X, 1X, and 3X label rates. Plant survival of each accession was ≥63% after exposure to imazethapyr POST 3X rate. Survival of BRO and KEI2 was 44% (±13) and 50% (±13), respectively, after exposure to atrazine POST 3X rate. Survival of BRO was 69% (±12) after exposure to glyphosate POST 1X rate, whereas survival of KEI2 was 44% (±13) after exposure to glyphosate POST 3X rate. After exposure to 2,4-D POST 1X rate, KEI2 and KEI3 survival was 38% (±13) and 50% (±13), respectively. Survival of all accessions was ≤31% after exposure to 2,4-D POST 3X rate or dicamba, glufosinate, lactofen, and mesotrione POST at either rate. Plant density reduction of KEI2 was 77% (±13) after exposure to atrazine PRE 1X rate, whereas density reduction of BRO was 56% (±13) after exposure to atrazine PRE 3X rate. Plant density reduction of all accessions was ≥94% after exposure to mesotrione PRE 1X and 3X rates or metribuzin, S-metolachlor, and sulfentrazone PRE at either rate. Our results suggest that each accession is resistant (≥50% survival) to imazethapyr POST, that BRO and KEI2 are resistant to atrazine and glyphosate POST, and that KEI2 and KEI3 are resistant to 2,4-D POST. The recently introduced BRO accession exhibited multiple resistance to imazethapyr, atrazine, and glyphosate POST. In addition, atrazine PRE was ineffective for BRO control, suggesting that diversified resistance management strategies will be critical for its effective management.

Pollen-mediated gene flow (PMGF) refers to the transfer of genetic information (alleles) from one plant to another compatible plant. With the evolution of herbicide-resistant (HR) weeds, PMGF plays an important role in the transfer of resistance alleles from HR to susceptible weeds; however, little attention is given to this topic. The objective of this work was to review reproductive biology, PMGF studies, and interspecific hybridization, as well as potential for herbicide resistance alleles to transfer in the economically important broadleaf weeds including common lambsquarters, giant ragweed, horseweed, kochia, Palmer amaranth, and waterhemp. The PMGF studies involving these species reveal that transfer of herbicide resistance alleles routinely occurs under field conditions and is influenced by several factors, such as reproductive biology, environment, and production practices. Interspecific hybridization studies within Amaranthus and Ambrosia spp. show that herbicide resistance allele transfer is possible between species of the same genus but at relatively low levels. The widespread occurrence of HR weed populations and high genetic diversity is at least partly due to PMGF, particularly in dioecious species such as Palmer amaranth and waterhemp compared with monoecious species such as common lambsquarters and horseweed. Prolific pollen production in giant ragweed contributes to PMGF. Kochia, a wind-pollinated species can efficiently disseminate herbicide resistance alleles via both PMGF and tumbleweed seed dispersal, resulting in widespread occurrence of multiple HR kochia populations. The findings from this review verify that intra- and interspecific gene flow can occur and, even at a low rate, could contribute to the rapid spread of herbicide resistance alleles. More research is needed to determine the role of PMGF in transferring multiple herbicide resistance alleles at the landscape level.

Use of synthetic auxin herbicides has increased across the midwestern United States after adoption of synthetic auxin-resistant soybean traits, in addition to extensive use of these herbicides in corn. Off-target movement of synthetic auxin herbicides such as dicamba can lead to severe injury to sensitive plants nearby. Previous research has documented effects of glyphosate on spray-solution pH and volatility of several dicamba formulations, but our understanding of the relationships between glyphosate and dicamba formulations commonly used in corn and for 2,4-D remains limited. The objectives of this research were to (1) investigate the roles of synthetic auxin herbicide formulation, glyphosate, and spray additives on spray solution pH; (2) assess the impact of synthetic auxin herbicide rate on solution pH; and (3) assess the influence of glyphosate and application time of year on dicamba and 2,4-D volatility using soybean as bioindicators in low-tunnel field volatility experiments. Addition of glyphosate to a synthetic auxin herbicide decreased solution pH below 5.0 for four of the seven herbicides tested (range of initial pH of water source, 7.45–7.70). Solution pH of most treatments was lower at a higher application rate (4× the labeled POST rate) than the 1× rate. Among all treatment factors, inclusion of glyphosate was the most important affecting spray solution pH; however, the addition of glyphosate did not influence area under the injury over distance stairs (P = 0.366) in low-tunnel field volatility experiments. Greater soybean injury in field experiments was associated with high air temperatures (maximum, >29 C) and low wind speeds (mean, 0.3–1.5 m s−1) during the 48 h after treatment application. The two dicamba formulations (diglycolamine with VaporGrip® and sodium salts) resulted in similar levels of soybean injury for applications that occurred later in the growing season. Greater soybean injury was observed after dicamba than after 2,4-D treatments.

Previous research has shown that strip-tillage (ST) systems conserve soil, reduce production costs, and save time for growers compared with intensive-tillage systems. In contrast to these well-documented benefits, we have limited information on weed community dynamics and management risks in ST corn (Zea mays L.) production systems in the northern Corn Belt. Therefore, we conducted research in 2015 and 2016 to characterize weed community composition, emergence patterns, and aboveground productivity in an ST corn/no-tillage (NT) soybean [Glycine max (L.) Merr.] rotation that was established in 2007 compared with a long-term intensive-tillage chisel-plow (CP) continuous-corn system. Fifteen or more weed species were identified in nontreated quadrats in each cropping system in each year. Common lambsquarters (Chenopodium album L.) was the most abundant weed species across systems and years. Chenopodium album densities were similar between CP and ST corn phases and were approximately 2-fold greater compared with the NT soybean phase. Other abundant weed species occurred at much lower densities than C. album. In each year, cumulative emergence of nontreated weed communities was described best by a logistic function in each cropping system. Maximum weed community emergence was greater in CP corn than ST corn phases in 2015, but did not differ in 2016. In the ST corn phase, most (about 75%) weed community emergence occurred in the in-row (tilled) zone compared with the between-row (nondisturbed) zone. Total late-season weed shoot biomass did not differ between nontreated CP and ST corn phases in either year, with C. album accounting for >85% of total weed biomass in these phases. These results suggest that weed community composition, total emergence, and productivity were similar between CP and ST corn phases after 10 yr. Our findings, coupled with previous research that showed favorable agronomic performance and greater soil conservation associated with the long-term ST corn/NT soybean system, suggest that production risks are no greater than a CP corn system, while processes that underpin ecosystem services are enhanced. These results provide strong evidence to support grower adoption of ST practices as an alternative to intensive tillage.

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.

Field experiments were conducted in 1992 and 1993 to characterize the weed seed bank, to determine the influence of moldboard plowing and secondary soil disturbance on the emergence pattern of weeds, and to measure weed seed bank depletion by emergence in a long-term moldboard plow corn cropping system. Viable seeds of common lambsquarters, redroot pigweed, and each of 10 other species accounted for about 85, about 9, and less than 1%, respectively, of the total weed species in the seed bank. A negative binomial distribution described the spatial distribution of viable seeds of 10 species, but not of common lambsquarters or of redroot pigweed. Decreased density of seeds among species was associated with increased aggregation. Secondary soil disturbance increased the rate and magnitude of common lambs quarters emergence in 1992 but did not influence emergence in 1993. Secondary soil disturbance did not influence the magnitude and rate of emergence of redroot pigweed or velvetleaf. Whereas cumulative growing degree days from April through July were similar between years, the amount of rainfall was about 50% less in 1992 than in 1993. Secondary soil disturbance may have increased common lambsquarters emergence by increasing the availability of soil moisture and improving conditions for seed germination during the dry year. Even though seed bank depletion by seedling emergence was relatively low for all species, secondary soil disturbance in creased seed bank depletion of common lambsquarters and redroot pigweed about 7- and 3-fold, respectively, in 1992. Seasonal variation in the amount of rainfall may have influenced the effect of soil disturbance on emergence and seed bank depletion of common lambsquarters, which is the most abundant species in the weed community.

A large crabgrass population (PW2) that demonstrated resistance to fluazifop-P and sethoxydim was identified in Wisconsin during 1992. Dose-response experiments were conducted in the greenhouse to determine the level of resistance of a PW2 large crabgrass accession to aryloxyphenoxypropionate (APP), cyclohexanedione (CHD), and other herbicide chemistries relative to a large crabgrass accession (A310) which was susceptible to APP and CHD herbicides. Based on shoot dry biomass reduction, the PW2 accession was 337- and 59-fold resistant to sethoxydim and fluazifop-P, respectively, relative to the A310 accession. Resistance of the PW2 accession to fenoxaprop, haloxyfop, quizalofop, and diclofop ranged from 18- to 29-fold. The PW2 accession was only 7-fold resistant to clethodim. Both large crabgrass accessions were susceptible to imazethapyr and linuron. These results suggest that APP and CHD herbicides will be ineffective for the management of the PW2 accession. An integrated approach including cultural, mechanical, and alternative chemical methods, should be implemented for the management of the PW2 accession and to prevent additional resistance problems.

The influence of secondary soil disturbance on the emergence pattern and seed bank depletion of an annual weed community in a long-term, no-tillage corn cropping system was determined in 1992 and 1993. As a component of this research, the seed bank was characterized prior to implementation of soil disturbance treatments. The seed bank was initially composed of common lambsquarters, redroot pigweed, and giant foxtail, with approximately 55, 36, and 8% of the total viable seeds, respectively. The remaining 1% was comprised of five other species in 1992 and eight in 1993. The spatial distribution of viable seeds of each species, except common lambsquarters and redroot pigweed, was described by a negative binomial distribution. Three dispersion indices indicated that seeds of individual and total weed species were aggregated and that the level of aggregation of viable seeds of a species was associated with seed density; at lower seed densities, the level of aggregation was greater. Soil disturbance increased common lambsquarters emergence 6-fold in 1992 relative to nondisturbed soil, but did not influence emergence in 1993. Rainfall was about 50% less in 1993. In contrast, soil disturbance increased giant foxtail and redroot pigweed emergence approximately 6- and 3-fold in 1992 and 1993, respectively. Seedling emergence associated with soil disturbance, relative to nondisturbed soil, increased seed bank depletion of common lambsquarters 16-fold in 1992, and giant foxtail and redroot pigweed and average of 6- and 3-fold in 1992 and 1993, respectively. These results indicated that soil disturbance increased seedling emergence and seed bank depletion of the predominant species in the weed community of a long-term, no-tillage system, but that this response was dependent on rainfall for common lambsquarters.

In greenhouse studies, the calculated I50 (herbicide application resulting in 50% inhibition of shoot regrowth) in tall fescue was approximately 0.004 kg/ha for both sethoxydim and haloxyfop. In red fescue, the I50 for sethoxydim and haloxyfop was 9.4 kg/ha and 0.04 kg/ha, respectively. As measured in crude cell extracts of tall fescue, incorporation of 14C-acetyl-coenzyme A into fatty acids was inhibited 62 and 71% by 10 μM sethoxydim and 10 μM haloxyfop, respectively. In red fescue, 10 μM haloxyfop inhibited 14C-acetyl-CoA incorporation into fatty acids by 29%, whereas 10 μM sethoxydim had no effect. The I50 for inhibition of acetyl-coenzyme A carboxylase activity in tall fescue with sethoxydim and haloxyfop was 6.9 and 5.8 μM, respectively. In red fescue the I50 for haloxyfop was 118 μM. Sethoxydim concentrations as high as 1 mM had little effect on acetyl-coenzyme A carboxylase activity in red fescue. These results suggest that acetyl-coenzyme A carboxylase is a sensitive site of action for sethoxydim and haloxyfop in tall fescue, and that tolerance to these herbicides in red fescue is due to the presence of a more tolerant form of the enzyme.

Peak germination and emergence of common lambsquarters usually occur in early to mid-spring, but both processes can occur during summer and fall. Seeds produced by different common lambsquarters cohorts (seedlings that emerge at nearly the same time) may vary in dormancy status, response to environmental conditions, and response to management factors. Therefore, experiments were conducted to determine the influence of different cohorts on common lambsquarters demography. Field experiments determined plant density, biomass, and seed production of different common lambsquarters cohorts within a crop-free community of annual weed species that included redroot pigweed, giant foxtail, and velvetleaf. Common lambsquarters plant density and aboveground biomass were greater for a mid-May cohort than for early June, late June, mid-July, or early August cohorts, but seed production of the mid-May and early June cohorts did not differ (about 192,000 seeds m−2) and was greater than that of other cohorts (111,500 seeds m−2 or less). In the laboratory, percent germination prior to stratification (exposure of seeds to low temperatures) was less for seeds harvested from early May and late May cohorts (≥ 9%) than those of mid-June or early July cohorts (≤ 75%). After stratification in the field, percent emergence (seedlings per number of planted seeds) and mean emergence time were similar among early May, late May, mid-June, and early July cohort seed sources, and were not influenced by shallow burial in soil. These results suggest that recruitment from seeds produced by different common lambsquarters cohorts is similar, but proportional to the number of seeds produced by each cohort.

Experiments were conducted to determine the productivity, inter-, and intraspecific competitive ability of giant foxtail accessions resistant (PCW1) and susceptible (AC1) to acetyl-coenzyme A carboxylase (ACCase) inhibitors. Under noncompetitive conditions in the field, shoot dry biomass, plant height, and leaf area over time were similar between the PCW1 and AC1 accessions. The instantaneous relative growth rate and instantaneous net assimilation rate did not differ between the accessions; however, the instantaneous leaf area ratio was slightly greater for the AC1 accession than the PCW1 accession. The seed yield was similar between the PCW1 and AC1 accessions. Addition series experiments were conducted in the field to determine the intraspecific competitive ability of the PCW1 and AC1 accessions. Regression surface analysis of reciprocal mean shoot dry biomass and seed yield indicated that the relative competitive ability of the PCW1 and AC1 accessions was similar. Replacement series experiments were conducted in the greenhouse with or without corn to determine the interspecific competitive ability of the accessions. The relative growth rate, shoot dry biomass, and seed yield of the PCW1 and AC1 accessions were greater without than with corn competition. The relative competitive ability of the PCW1 and AC1 accessions was similar with or without corn competition based on shoot dry biomass or seed yield. These results indicated that the intra- and interspecific competitive ability of the PCW1 and AC1 giant foxtail accessions are similar and suggest that resistance to ACCase inhibitors has not reduced the relative fitness of the PCW1 accession.

Studies were conducted to determine the relative competitive ability and productivity of an atrazine-resistant biotype (WRB1) and an atrazine-susceptible accession (WSA1) of velvetleaf from Wisconsin. Noncompetitive productivity studies conducted in the field and greenhouse during 1992 and 1993 showed that instantaneous relative growth rate, net assimilation rate, and leaf area ratio of the WRB1 biotype and WSA1 accession were similar. There were no consistent differences between the WRB1 biotype and WSA1 accession in plant height, shoot dry biomass, and leaf area over time. The WRB1 biotype and WSA1 accession did not differ in noncompetitive seed yield. A replacement series experiment conducted in the field in 1992 and 1993 showed that the WRB1 biotype and WSA1 accession did not differ in shoot dry biomass or seed yield at densities of 36, 64, and 100 plants m−2. Results suggest that resistance to atrazine has not reduced the noncompetitive productivity or the intraspecific competitive ability of the WRB1 biotype relative to the WSA1 accession. Thus, even in the absence of atrazine selection intensity, the frequency of atrazine resistance gene(s) is unlikely to decrease in the velvetleaf population from which the WRB1 biotype was selected.

Field and greenhouse research was conducted to quantify the level of resistance to atrazine in Wisconsin (WRB1) and Maryland (MRB) velvetleaf biotypes and to determine cross-resistance of the WRB1 and MRB biotypes to other selected herbicides as compared to a Wisconsin atrazine-susceptible velvetleaf accession (WSA1). In field studies, the WRB1 and MRB biotypes survived atrazine applied POST at dosages as high as 4.5 kg ha−1. In contrast, the WSA1 accession had 50% survival following a 1.1 kg ha−1 POST atrazine application. The WRB1 biotype demonstrated neither cross-resistance nor negative cross-resistance to alachlor, bentazon, bromoxynil, cyanazine, dicamba, linuron, metribuzin, or thifensulfuron. The MRB biotype demonstrated neither cross-resistance nor negative cross-resistance to alachlor, bentazon, dicamba, metribuzin, or thifensulfuron; slight negative cross-resistance was demonstrated to bromoxynil, cyanazine, and linuron. In greenhouse studies, the WRB1 and MRB biotypes were approximately 100-fold more resistant to atrazine than the WSA1 accession; the WRB1 and MRB biotypes demonstrated neither cross-resistance nor negative cross-resistance to alachlor, ametryn, bentazon, bromoxynil, cyanazine, dicamba, linuron, metribuzin, pendimethalin, terbacil, or thifensulfuron. Absence of cross-resistance to cyanazine, ametryn, metribuzin, and terbacil in the WRB1 and MRB biotypes of velvetleaf is in contrast to most other atrazine-resistant weed biotypes.

Management of perennial weeds is a major concern in reduced-tillage cropping systems. Field research was conducted at Nashua, IA, from 1977 through 1990 to evaluate the long-term impacts of tillage and cropping patterns on perennial weed populations in corn and soybean production. Continuous corn and a corn/soybean rotation were conducted utilizing moldboard plow, chisel plow, ridge tillage, and no-tillage systems. The research area was free of established perennial weed species at the initiation of the experiment in 1977. Hemp dogbane was observed by 1980, with the greatest densities in no-tillage. By 1990, continuous corn had greater hemp dogbane densities with no-tillage than other tillage system by crop rotation treatments. American germander densities were not affected by tillage systems in 1980 and 1981, but by 1990, corn/soybean rotations had greater densities in moldboard plow than other tillage systems. Field bindweed developed primarily in the corn/soybean rotations with the greatest densities occurring in no-tillage. Greater and more diverse populations of perennial weeds developed in reduced-tillage systems than in the moldboard plow system. However, practices used to control annual weeds and environmental factors interacted with tillage to regulate perennial weed populations.

Previous research has shown that haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} and sethoxydim {2-[1-(ethoxyimino)butyl-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} give less control of quackgrass [Agropyron repens (L.) Beauv. # AGRRE] when applied at the five- to seven-leaf stage compared to the three- to four-leaf stage. Current research indicates that reduced control of quackgrass in the eight-leaf stage, compared to the four-leaf stage, was not due to reduced herbicide retention on leaves or to changes in residual herbicide activity in the soil following postemergence applications. The efficacy of both herbicides on rhizome bud kill did not change between the four- and eight-leaf stages following application to the foliage. However, the efficacy of both herbicides on crown tissue bud viability was less at the eight-leaf stage. Rhizome sink intensity (the capacity to accumulate radiolabel) was similar at both stages of development following 14C-haloxyfop and 14C-sethoxydim application to either upper or lower leaves. Crown tissue sink intensity was significantly less at the eight-leaf stage compared to the four-leaf stage following 14C-sethoxydim application to either upper or lower leaves and following 14C-haloxyfop application to lower leaves. These data suggest that crown bud regrowth contributes substantially to reduced control of quackgrass in advanced stages of growth following applications of haloxyfop and sethoxydim.

Many growers perceive that reduced herbicide inputs and greater reliance on mechanical methods will result in increased weed management problems over time. Previous research has shown short-term benefits of integrated weed management strategies, but information concerning the long-term implications of these strategies is lacking. Therefore, research was conducted from 1992 through 1995 to determine the influence of full-rate preemergence broadcast, half-rate preemergence broadcast plus cultivation, and full-rate preemergence band plus cultivation treatments on weed population and seedbank dynamics in no-tillage (NT), chisel plow (CP), and moldboard plow (MP) systems in both continuous corn (CC) and corn—soybean (CS) rotation. Seventeen weed species were identified in the plant population and seedbank across all treatments, but common lambsquarters, giant foxtail, and redroot pigweed dominanted. Vertical distribution of weed seeds in soil was influenced by tillage, with 74, 59, and 43% of the total viable seed of these species less than 10 cm deep in NT, CP, and MP, respectively. In contrast, seed dormancy in the spring was not influenced greatly by tillage and averaged 79, 10, and 42% for common lambsquarters, giant foxtail, and redroot pigweed, respectively, across tillage methods. Crop rotation was not a major factor influencing weed population and seedbank dynamics. From 1992 to 1995, reduction of shoot biomass and seed production of the predominant weed species were similar among weed management treatments in most tillage and crop rotation systems. The influence of weed management treatments on the seedbank was also similar, with up to 50, 95, and 92% less seed of common lambsquarters, giant foxtail, and redroot pigweed, respectively, in the seedbank in 1995 than in 1992. Corn and soybean grain yields were similar among weed management treatments and were greater than the nontreated check in each tillage and crop rotation treatment. These results indicate that reduced herbicide inputs plus interrow cultivation were as effective as full-rate herbicide inputs for the management of several annual weed species in both conventional- and conservation-tillage systems over 4 yr.

Experiments were conducted to determine the productivity and the competitive ability of a large crabgrass accession (PW2) resistant to acetyl-coenzyme A carboxylase (ACCase) inhibitors relative to a susceptible accession (A310). Under noncompetitive conditions in the field, shoot dry biomass and plant height over time were similar for the two accessions. Leaf area was slightly greater for A310 than for PW2. The instantaneous relative growth rate was similar for the accessions. The instantaneous net assimilation rate was slightly greater for A310 than for PW2; however, the instantaneous leaf area ratio was slightly greater for PW2 than for A310. At maturity, the seed yield was similar for the accessions. The inter-accession competitive ability of PW2 and A310 plants in the field was similar, based on shoot dry biomass or seed yield. In the greenhouse, interspecific competitive ability of the accessions with corn was similar, based on shoot dry biomass or seed yield. These results suggest that resistance to ACCase inhibitors is not associated with reduced fitness of the PW2 accession.