A common waterhemp biotype that was not controlled by triazine or acetolactate synthase (ALS)-inhibiting herbicides was isolated from a field in Bond County, IL, in the fall of 1996. Greenhouse and laboratory experiments determined resistance to atrazine and three ALS-inhibiting herbicides in this biotype. Based on whole-plant response, the Bond County common waterhemp biotype required over 1,000 times more imazethapyr relative to a susceptible biotype to reduce growth 50%. Cross-resistance to thifensulfuron, a sulfonylurea, and flumetsulam, a triazolopyrimidine sulfonanilide, was also detected. Based on in vivo enzyme assays, ALS in the Bond County common waterhemp biotype was 20-, > 8-, and 68-fold less sensitive than ALS in the susceptible biotype to imazethapyr, thifensulfuron, and flumetsulam, respectively. Whole-plant efficacy trials also indicated that the Bond County common waterhemp biotype required more than 20 kg ha−1 of atrazine to inhibit growth 50%. Chlorophyll fluorescence assays revealed that 100 nM atrazine inhibited photosynthesis in the susceptible biotype, whereas 10 M did not affect photosynthesis in the resistant biotype. Regions of the genes encoding ALS and D1 proteins were sequenced to determine the molecular basis for the resistances. Triazine resistance was conferred by a glycine for serine substitution at residue 264 of the D1 protein, while ALS resistance was conferred by a leucine for tryptophan substitution at residue 569 of ALS.

Imazethapyr-resistant biotypes of Palmer amaranth and common waterhemp were studied to determine the magnitude of resistance and cross-resistance to three acetolactate synthase (ALS)-inhibiting herbicides. Resistant biotypes of Palmer amaranth and common waterhemp demonstrated > 2,800- and > 130-fold resistance to phytotoxicity of imazethapyr compared to susceptible biotypes, respectively. Concentrations of imazethapyr required for 50% in vivo inhibition of ALS activity were at least > 13,100 and > 1,900 times greater for resistant biotypes of Palmer amaranth and common waterhemp, respectively, compared to susceptible plants. Resistant biotypes of both species demonstrated cross-resistance to the sulfonylurea herbicides thifensulfuron and chlorimuron at the whole plant and enzyme levels, indicating that a less sensitive ALS enzyme confers this resistance to these plants.

Field studies were conducted in 2007 and 2008 at seven sites in Ohio, Indiana, and Illinois to determine the effect of PRE herbicide and POST application timing on weed control and yield of glyphosate-resistant corn. Levels of PRE herbicide included none; low—atrazine; medium—atrazine and metolachlor; and high—atrazine, mesotrione, and metolachlor. Glyphosate was applied POST when corn was 30 cm tall, or 1 or 2 wk later. Common lambsquarters, giant foxtail, and giant ragweed infested at least six of the seven sites, and other weed species occurred at two to three sites. Control of weeds at the time of POST application ranged from 48 to 91%, 58 to 99%, and 87 to 100% for the low, medium, and high levels of PRE herbicide, respectively, averaged over POST application timing. Control of giant foxtail and redroot pigweed decreased by about 20% between the second and third POST timing, averaged over PRE herbicide, but control of other weeds was similar among timings. Late-season control of common ragweed, velvetleaf, common lambsquarters, and Pennsylvania smartweed exceeded 90%, regardless of PRE herbicide or POST timing. Control of redroot pigweed, ivyleaf morningglory, and giant ragweed was as low as 74, 67, and 83%, respectively, but the high level of PRE herbicide resulted in 90 to 97% control of these weeds. An interaction between PRE herbicide and POST timing for late-season control of giant foxtail, tall waterhemp, and yellow nutsedge reflected the more effective control among POST timings from the higher levels of PRE herbicide. The overall trend in this study was for more effective weed control in PRE/POST herbicide programs with more comprehensive PRE herbicides that have substantial activity on both grass and broadleaf weeds. Highest yield occurred where the PRE treatment consisted of a two- or three-way combination of herbicides applied at 50% of the recommended rate or higher. Yield was reduced at all POST timings with atrazine alone or in the absence of PRE herbicide.

Common waterhemp is a significant weed problem in Midwestern cropping systems partly because of its potential for multiple emergence events during the growing season. The effects of shade and time of emergence on this weed have not been characterized. In the field, common waterhemp vegetative and reproductive growth were evaluated under different irradiance levels at two emergence times. In full sunlight a common waterhemp plant emerging in late May produced 720 g of biomass and over one million seeds, and a plant emerging in late June produced 350 g of biomass and over 730,000 seeds. Plant biomass and seed production were lower as irradiance levels were decreased to 40, 68, and 99% shade. Mortality was high for common waterhemp grown in 99% shade; however, surviving plants produced some viable seed. Common waterhemp plants grown under reduced irradiance had higher leaf area ratios and lower relative growth rates.

Because soil-residual PRE herbicides reduce and delay annual weed emergence and decrease later weed growth, susceptible weeds surviving or recovering from herbicide treatment reduce crop yields less than do untreated weeds. Recently, corn yields were shown to be reduced differently by untreated weeds emerging in and between crop rows. However, equations have not been reported before that relate corn yield to in-row and between-row weed cover of mixed weed populations recovering from PRE soil-residual herbicides. Published data from PRE herbicide screening research for 3 site-yr in Missouri were reanalyzed to characterize this relation. In-row and between-row weed cover of mixed weed populations, chiefly giant foxtail and common waterhemp, were measured from photographs at midsummer. In 2 of 3 site-yr and with the 3 site-yr average, corn yields were a nonlinear function of both in-row and between-row weed cover recovering from various PRE soil-residual herbicide treatments. In 1 of 3 site-yr, corn yields were a nonlinear function of only between-row total weed cover. Subdividing weed cover into in-row and between-row subpopulations in equations accounted for more data variability in yield estimates than including either subpopulation alone. For all 3 site-yr after PRE herbicide treatment, corn yields were a nonlinear function of only between-row visually rated total weed control. Visual evaluation was less sensitive than photographic weed cover for measuring the contribution of in-row weeds to corn yield loss and characterizing the functional form of the equations.

Field studies were conducted from 1998 to 2000 at Belleville, IL, to evaluate tolerance and weed control in glyphosate-resistant soybean with sulfentrazone application. Sulfentrazone alone caused 14 to 16% height reduction 14 d after treatment (DAT), and sulfentrazone plus chlorimuron caused 26% height reduction 14 DAT. Minimal height reduction (0 to 11%) was observed 56 DAT. Sulfentrazone alone controlled giant foxtail 97 to 100%, yellow nutsedge 96 to 98%, common waterhemp 97 to 98%, common cocklebur 91 to 94%, and ivyleaf morningglory 100%. Sulfentrazone alone controlled common ragweed 63 to 89% and giant ragweed 50 to 72%. Sulfentrazone plus chlorimuron or cloransulam increased control of common and giant ragweed to 95% or greater. Sulfentrazone followed by glyphosate increased control of yellow nutsedge, common waterhemp, and ivyleaf morningglory compared with a single application of glyphosate. Sequential applications of glyphosate controlled weeds 93 to 100%. Sulfentrazone plus chlorimuron or cloransulam postponed the application of glyphosate at the 10-cm weed height by 12 d. Despite the injury, sulfentrazone did not reduce grain yield. Inadequate giant ragweed control reduced grain yield by approximately 48%.

A study was conducted to quantify the magnitude and sources of variation in common waterhemp temporal patterns of emergence over 1 yr. In 2008 and 2010, emergence patterns in the absence of soil disturbance were determined for replicated samples of maternal families (progeny from one individual) separately harvested during the previous year from four plants within each of four agricultural fields (16 maternal families yr−1) at a university research farm near Urbana, IL. Combining data across years, variance partitioning indicated that seed sample within maternal family explained 48% of total variation in the percentage of viable, buried seeds that produced seedlings. Differences within, rather than among, maternal families also accounted for large fractions (60 to 99%) of total variation in cumulative percentage emergence at specific points during the growing season. Within years, seed samples characterized by delayed or accelerated emergence patterns did not originate from specific maternal plants. These results indicate that common waterhemp seed populations are without strong maternal plant effects that limit emergence to narrow intervals within the overall emergence period. Thus, results of this study support the use of contemporary approaches for modeling herbicide resistance evolution in common waterhemp, which assume seedling cohorts contain offspring from all individuals occurring within the maternal population.

Greenhouse studies were conducted to examine the influence of temperature and relative humidity (RH) on the foliar activity of mesotrione on five weed species. Regression analysis was performed allowing for comparison of estimated GR50 (herbicide dose to inhibit growth by 50%) values for each weed response at either temperature (18 or 32 C) or RH level (30 or 85%). Temperature and relative humidity did not influence the response of ivyleaf morningglory, common cocklebur, and velvetleaf to mesotrione markedly. An increase in temperature or relative humidity increased the efficacy of mesotrione on common cocklebur and velvetleaf up to threefold. Conversely, common waterhemp and large crabgrass were six- and sevenfold more susceptible at 18 C than at 32 C, respectively. Common waterhemp and large crabgrass were four- and twofold more susceptible to mesotrione at 85% compared with 30% RH, respectively. The influence of temperature and RH on the efficacy of mesotrione in foliar applications is species dependent and may be an important consideration for field applications.

Greenhouse and field studies were conducted with a population of common waterhemp resistant to POST protoporphyrinogen oxidase (protox)-inhibiting herbicides to compare its response to PRE and POST applications of selected herbicides. In the greenhouse, a dose–response study of PRE applications of acifluorfen, fomesafen, or lactofen was conducted on protox-susceptible and -resistant common waterhemp. The protox-resistant biotype was approximately 6.3, 2.5, and 2.6 times more resistant than the susceptible biotype to acifluorfen, fomesafen, and lactofen, respectively. In a separate study under field conditions, protox-resistant common waterhemp were treated with PRE and POST applications of acifluorfen, azafenidin, flumioxazin, fomesafen, lactofen, oxyfluorfen, or sulfentrazone. At 14 and 28 d after POST treatment (DAPT) in 2002 and 2004, all PRE applications of herbicides gave greater control than did POST applications. At 14 DAPT, oxyfluorfen had the greatest difference in PRE and POST control, with 85 and 10% control in 2002, respectively. An additional field study was conducted to determine the stage of growth at which resistance to protox-inhibiting herbicides becomes most prevalent. Protox-resistant common waterhemp were treated with herbicides at the 2-leaf, 4- to 6-leaf, and 8- to 10-leaf growth stage. Acifluorfen and fomesafen at 420 g ha−1 gave greater than 90% control at the 2-leaf stage and 4- to 6-leaf stage, except in 2003 when control was 85% with acifluorfen. In 2003 and 2004, common waterhemp control at the 8- to 10-leaf stage ranged between 54 and 75% with acifluorfen or fomesafen. Results indicate that common waterhemp resistance to customary rates of POST protox-inhibiting herbicides becomes prevalent after the 4- to 6-leaf growth stage.

To minimize the chance of surface water contamination by herbicides, farmers need alternative ways to manage weeds in field crops, such as field corn, that reduce herbicide use. Zone herbicide application (ZHA) reduces herbicide use compared with conventional broadcast herbicide application by (1) banding low herbicide rates between corn rows (≤ 1× normal broadcast registered rate), (2) managing crops to favor crop competition, and (3) banding very low herbicide rates over crop rows (≪ 1× normal rate). The research goal was to compare the relative effectiveness of reduced-rate ZHA with broadcast herbicide application on in-row (IR) and between-row (BR) summer annual weed cover (chiefly giant foxtail and waterhemp species), grain yields, and net returns resulting from herbicide application in field corn. Preemergence ZHA of atrazine + metolachlor + clopyralid + flumesulam was made in zones (i.e., even width bands) at different rates between and over crop rows for three site-years in Missouri, and the 1× rate was 2.24 + 1.75 + 0.211 + 0.067 kg ai ha−1, respectively. Best ZHA treatments (0.29× to 0.30× IR herbicide rates + 0.74× to 0.80× BR herbicide rates) outperformed all reduced-rate broadcast herbicide treatments (0.25×, 0.5×, and 0.75×) based on net returns in partial budget analysis. Yields for highest yielding ZHA could not be distinguished from the 1× broadcast treatments in two of three site-years. Net returns due to herbicide application for the highest yielding ZHA were comparable with the 1× broadcast treatment in all three site-years. For the best ZHA, the 3-yr average for total herbicide applied per unit was 53% of the 1× broadcast rate. ZHA may provide row crop farmers with a new generic option for reducing herbicide rates and input costs while maintaining net returns and reducing the chance of surface water contamination by herbicides.

This study examined pollen morphological variation among Amaranthus species and interspecific hybrids. Ten weedy Amaranthus species, a cultivated grain species, and several putative hybrids resulting from interspecific mating between common waterhemp and Palmer amaranth were grown in a greenhouse. Mature pollen was collected, viewed, and photographed with a scanning electron microscope (SEM). The pollen grains were spherical shaped with polypantoporate, or golf ball-like, aperture arrangement. Differences were observed between the monoecious and dioecious Amaranthus species. Pollen grains of the dioecious species had a greater number of apertures on the visible surface. One exception to these trends was the dioecious species, Palmer amaranth, whose pollen was similar to that of the monoecious species spiny amaranth. However, pollen grain diameters did not differ between the monoecious and dioecious plants. Significant differences also were noted between the pollen from the putative common waterhemp × Palmer amaranth hybrids and the parental-type pollen grains. Pollen of the hybrids was similar in size to the maternal parent but had an aperture number that was intermediate between parents. This indicates that pollen characteristics may be controlled by the female and that hybrids may be more prevalent than originally thought.

Field studies were conducted in central Missouri and central Kansas to evaluate the crop tolerance and efficacy of various combinations of atrazine, flufenacet + isoxaflutole, flumetsulam + clopyralid, isoxaflutole, and S-metolachlor applied PPI or PRE in conventional-till corn. Application technique did not influence crop injury in Kansas. In Missouri, greater crop injury was observed with treatments containing isoxaflutole when applied PPI vs. PRE. Application technique influenced giant foxtail, ivyleaf morningglory, large crabgrass, Palmer amaranth, and common waterhemp control. In dry years, control of these weeds was usually either same or greater with PPI than it was with PRE treatments. In years with average to above average precipitation, isoxaflutole provided greater control as a PRE application than as a PPI application. Palmer amaranth and common waterhemp control was usually greater with atrazine, isoxaflutole, and S-metolachlor applied PRE than it was applied PPI. Differences in control of all weeds between PPI and PRE applications were less obvious with two or three herbicides compared with treatments with a single herbicide. In general, the corn yield was greater with most of the treatments having two, three, or four herbicides than it was with treatments having a single herbicide, which was due to better weed control with the tank-mixtreatments.

Field trials were initiated at three Missouri locations in the fall of 2004 and 2005 to compare the efficacy of fall and early spring herbicide applications on winter and summer annual weed populations in no-till soybean. All three experiments received applications of chlorimuron plus sulfentrazone plus 2,4-D; chlorimuron plus tribenuron plus 2,4-D; and glyphosate plus 2,4-D in the fall, 60 d prior to planting (60 PP), 30 days prior to planting (30 PP), and 7 d prior to planting (7 PP). Good control of all winter annual weeds, except for common chickweed, was observed at planting from fall applications of chlorimuron plus sulfentrazone plus 2,4-D and chlorimuron plus tribenuron plus 2,4-D. Fall, 60 PP, and 30 PP applications of chlorimuron plus sulfentrazone plus 2,4-D provided relatively poor control of common chickweed. Glyphosate plus 2,4-D applied in the fall and 60 PP provided adequate control of early fall germinating winter annuals, such as common chickweed and henbit. However, these treatments provided poor control of winter annual species that exhibited some degree of spring germination. Control of summer annual weed species 7 wk after planting (WAP) was highly variable. Relatively poor control of common waterhemp was obtained from all treatments except chlorimuron plus sulfentrazone plus 2,4-D applied at all three spring timings. Giant foxtail control was highly variable ranging from 0 to 95%. Measurements of weed biomass collected 7 WAP revealed larger amounts of biomass present in the fall application timing of each herbicide treatment. Emergence of winter and summer annual weed species from early March to 7 WAP varied by herbicide treatment. Emergence of total winter and summer annual weed species was reduced with both residual herbicide treatments when compared with glyphosate plus 2,4-D and the untreated control.

Use of composted swine manure produced in deep-bedded hoop structures is a promising approach for recycling farm waste products and improving soil fertility, but little is known about its effects on crop–weed interactions. A 2-yr study was conducted to evaluate the effect of compost amendments and tillage on soybean–common waterhemp competition. Experiments were conducted in no-tillage and chisel plow main plots with compost applied to one of two types of subplots. Common waterhemp and soybean growth was measured in sub-subplots accommodating weed-free soybean and soybean with common waterhemp sown at soybean planting, soybean emergence (VE), soybean second-node stage (V2), and soybean sixth-node stage (V6). Soybean heights were not influenced by compost or common waterhemp sowing time. Soybean stem diameters were influenced by year, tillage regime, and an interaction between compost and common waterhemp sowing time. In contrast, common waterhemp heights and basal diameters were greater when sown at planting and VE in compost-amended subplots than in compost-free subplots. Overall, there was a negative quadratic relationship between common waterhemp biomass and soybean yield (r 2 = 0.746). The extremely low common waterhemp emergence in V2 and V6 treatments suggested that early-season weed suppression was sufficient to protect soybean from common waterhemp competition. The sex determination of 2,557 common waterhemp plants showed a marginally higher male to female ratio in compost-amended treatments than in compost-free treatments (P = 0.0611). A linear-slope regression indicated that common waterhemp fecundity was positively related to individual plant biomass, with a change in slope occurring at 118.7 g. Under the conditions present in this experiment, compost did not enhance soybean yield but increased the competitive ability of waterhemp. Because composted swine manure can have a major influence on competition of common waterhemp with soybean, effective weed management practices should be in place when this soil amendment is used.

In a survey of herbicide responses among Illinois waterhemp half-sib populations, several were observed with differential responses to imazethapyr and thifensulfuron, two acetolactate synthase (ALS)–inhibiting herbicides. Plants from two waterhemp populations were verified resistant to imazethapyr, but susceptible to chlorimuron, using a nondestructive leaf-disc assay. Sequencing of the ALS gene revealed that imazethapyr-resistant waterhemp plants from both populations had inferred amino acid substitutions at position 653 of ALS. Depending on the population, the serine at position 653 of ALS was substituted with either asparagine (S653N) or threonine (S653T). Waterhemp lines were derived from each population to create uniformly imidazolinone-resistant (IR) waterhemp biotypes, designated IR-62 and IR-101. ALS-inhibitor responses of each IR biotype were compared with a previously identified ALS inhibitor–resistant biotype with a tryptophan to leucine substitution at position 574 (W574L) and an herbicide-susceptible control. Whole-plant dose–response experiments with waterhemp biotypes containing W574L, S653N, or S653T mutations indicated that each biotype was resistant to imazethapyr, but only the biotype with a W574L mutation was resistant to thifensulfuron. In vitro ALS-activity assays revealed unique patterns of cross-resistance among protein extracts derived from each biotype in response to imazethapyr, thifensulfuron, cloransulam, and pyrithiobac. In conclusion, three different forms of target-site–based resistance to ALS inhibitors have been identified in waterhemp.

POST combinations of photosystem II (PSII) and the 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors are effective for control of broadleaf weeds in corn. Field studies were conducted during 2007 and 2008 in Urbana and Dekalb, IL, to determine the nature of interactions between two PSII inhibitors, atrazine and bromoxynil, and the HPPD inhibitor mesotrione, based on control of common waterhemp, common lambsquarters, and giant ragweed. Two rates of each herbicide were evaluated, with the highest representing a typical recommended rate, and the lowest a fraction of that rate. Synergistic interactions occurred for common waterhemp control from 10 to 30 d after treatment (DAT) regardless of herbicide rates, rainfall accumulation, or plant height. Synergism between mesotrione and bromoxynil was not observed for common lambsquarters control at Urbana in 2008 at the lower herbicide rates, possibly due to taller weed heights at the time of herbicide application relative to 2007. Giant ragweed control indicated that a synergistic interaction occurred for all herbicides and rates in 2008. However, synergism between bromoxynil and mesotrione did not occur in 2007, likely due to limited rainfall before herbicide application. Reduced herbicide rates and adverse environmental conditions have the potential to regulate the expression of synergism between mesotrione and PSII inhibitors, and therefore may limit their effectiveness for weed management in corn.

The nitrogen (N) response of competing plants may be affected by photosynthetically active radiation (PAR) availability and maximum potential growth rate, which determine N requirements. The responses of two crop (corn and soybean) and six weed species (common lambsquarters, common waterhemp, giant foxtail, velvetleaf, wild mustard, and woolly cupgrass) in low and high (150 and 450 μmol m−2 s−1) PAR levels to daily fertilization with either low or high (0.2 or 7.5 mM) NH4NO3 levels were studied. Leaf area of all species responded positively to N by 8 d after emergence (DAE) when grown in high PAR; in low PAR, most species did not respond until 11 DAE. Dry weight and leaf area of all species at 18 DAE were greater with high than with low N. These responses to high N were also greater in high than in low PAR for all species. Dry weights with high N were up to 100% greater in low PAR and up to 700% greater in high PAR than dry weights with low N. These responses suggest that low PAR reduced the benefit of N to the plants. The regression of relative growth rate (RGR) with high N to RGR with low N had a slope that was less than unity (β = 0.79), indicating that species with a higher RGR with high N experienced greater decreases in RGR with low N. Similarly, the sensitivity (change in RGR) of plants grown with high and low N was positively related to RGR with high N. RGR differences among crop and weed species may be related to differences in N requirement that could be exploited for weed management. RGR and seed size were negatively correlated, which may explain previous observations that small-seeded weeds were more sensitive to environmental stress.

Plant model systems have contributed greatly to the dramatic progress in understanding the fundamental aspects of plant biology. Using model weeds will also help facilitate focused funding and research in the weed science community. Criteria for developing model weeds require attention to weedy characteristics that impart economic losses and a wide geographic distribution, attributes that present the potential for political and scientific support. Expressed sequence tag (EST) databases for model weeds are the most practical approach to identifying new genes and obtaining data on the gene expression underlying weedy characteristics. Weeds such as Canada thistle, eastern black nightshade, johnsongrass, jointed goatgrass, leafy spurge, waterhemp, and weedy rice are proposed as model systems.

Experiments were conducted to determine the efficacy, absorption, and translocation of glyphosate, glufosinate, and imazethapyr with selected weed species. In the greenhouse glyphosate, glufosinate, and imazethapyr were applied at 0.25, 0.5, and 1 times their label rates of 1,121, 410, and 70 g ha−1, respectively, on 10- to 15-cm black nightshade, common waterhemp, eastern black nightshade, field bindweed, giant ragweed, ivyleaf morningglory, prairie cupgrass, velvetleaf, and yellow nutsedge. Glyphosate applied at the 1-time rate caused injury greater than or similar to injury from the 1-time rate of glufosinate or imazethapyr on black nightshade, common waterhemp, eastern black nightshade, field bindweed, giant ragweed, prairie cupgrass, and velvetleaf. The 1-time rate of glufosinate injured ivyleaf morningglory and yellow nutsedge more than did the 1-time rate of glyphosate or imazethapyr. Under field conditions glyphosate caused the greatest injury to common waterhemp, prairie cupgrass, and velvetleaf across plant growth stages. Giant ragweed and ivyleaf morningglory injury was more dependent on growth stage, with the 15- and 30-cm growth stages more susceptible to glyphosate than to glufosinate or imazethapyr. Differential response of these weed species may be caused by differences in herbicide translocation. Glyphosate was translocated more in both giant ragweed and ivyleaf morningglory, and these species were injured more by glyphosate than by glufosinate or imazethapyr at the larger growth stages.

Field experiments were conducted in central Iowa to determine the growth of common waterhemp emerging after postemergence herbicide applications in soybean. Common waterhemp survival declined as emergence was delayed in relation to soybean. Ninety percent of plants emerging at approximately the same time as soybean survived, whereas only 13% of plants emerging approximately 50 d after planting (DAP) survived to maturity. Biomass accumulation declined rapidly as emergence was delayed in relation to soybean. Delaying emergence from 14 to 28 DAP resulted in a 50 to 80% reduction in shoot biomass. Common waterhemp emerging 50 DAP produced only 1 to 10% of the biomass of plants emerging at the same time as soybean. Plants emerging with soybean produced approximately 300,000 to 2.3 million seeds plant−1 depending on the location. Fecundity of common waterhemp plants was closely related to biomass accumulation and declined rapidly with delayed emergence. Although common waterhemp emerging after the V4 stage of soybean (40 DAP) are unlikely to affect crop yield because of high mortality levels and reduced growth, these plants may contribute significant seeds to the soil seed bank.