The biochemical and molecular basis of resistance to acetolactate synthase (ALS)–inhibiting herbicides was investigated in eight resistant (R) and three susceptible (S) wild radish populations. In vitro enzyme assays revealed an ALS herbicide–resistant ALS enzyme in all R populations. ALS enzyme extracted from the shoots of all eight R populations was highly resistant to the ALS-inhibiting sulfonylurea herbicide chlorsulfuron (20- to 160-fold) and the triazolopyrimidine herbicide metosulam (10- to 46-fold) and moderately resistant to metsulfuron (three to eightfold). There was little or no cross-resistance to the imidazolinone herbicides imazapyr and imazethapyr. The ALS gene fragment covering potential mutation sites in these populations was amplified, sequenced, and compared. All eight R populations had point mutations in the codon for the proline residue in Domain A. However, the point mutations varied and encoded four different amino acid substitutions: histidine, threonine, alanine, and serine. No nucleotide difference in the DNA sequence of Domains C and D resulting in amino acid substitutions was observed between the R and S populations examined. In addition, a three- to fivefold higher ALS-specific activity was consistently observed in all R populations compared with S populations, whereas Northern blot analysis detected a similar level of ALS mRNA, suggesting a possible translational–posttranslational regulation of the enzyme. It is concluded that selection pressure from chlorsulfuron on eight separate wild radish populations has resulted in target site mutation at the same proline residue in the ALS gene. Higher ALS activity also may play a role in the resistance level.

Auxin-mediated responses of kochia biotypes resistant to dicamba (HRd) or resistant to dicamba and fluroxypyr (HRdf) were compared with those of two susceptible biotypes. Rates of shoot and root gravitropic response and patterns of apical dominance, as determined by lateral bud sprouting after decapitation, were determined in the absence of herbicide treatment. Shoots of susceptible plants reoriented toward vertical at a rate of 23.4° h−1, whereas the rates of HRd and HRdf shoot reorientation were significantly slower at 7.2° and 14.4° h−1, respectively. Root gravitropic responses were not different between resistant and susceptible biotypes. In contrast to susceptible biotypes, both apical and basal lateral buds on HRd plants elongated after decapitation, although differences between HRd and susceptible biotypes became smaller during succeeding weeks. The elongation pattern of HRdf lateral buds was intermediate to that of susceptible and HRd plants. Inhibition assays of root growth by natural and synthetic auxins showed that HRd root growth was less sensitive to dicamba, 2,4-D, naphthalene-1-acetic acid, and indole-3-acetic acid than was root growth of HRdf or the susceptible biotypes. Collectively, results support the hypothesis that auxin binding or signal transduction pathways are impaired in resistant biotypes and that HRd may contain different lesions than does HRdf.

A common sunflower population from Howard, SD (HSD) was previously determined to be cross-resistant to imazethapyr and chlorimuron-ethyl, both acetolactate synthase–inhibiting (ALS) herbicides. Experiments were conducted to determine if target-site polymorphisms could act as a mechanism of ALS-inhibitor herbicide resistance in the HSD common sunflower. Approximately 1,600 nucleotides were amplified by polymerase chain reaction and sequenced from putative ALS gene(s) in common sunflower and Jerusalem artichoke. In sunflower, two different amplification products were detected that differed by a nine-basepair deletion. This suggested the presence of at least two ALS genes in common sunflower that could contribute to the herbicide resistance phenotype. In addition, an Ala205 to Val205 substitution was observed in several clones from resistant common sunflower (amino acid position is relative to the full-length mouse-ear cress ALS protein). Previously documented mutations at this position in other species indicated that it might play a vital role in conferring resistance to one or more ALS-inhibitor herbicides.

Gene flow from imidazolinone (IMI)-resistant domestic sunflower to IMI-susceptible common sunflower and prairie sunflower was studied. Under greenhouse conditions, pollen from IMI-resistant domesticated sunflower was applied to flower heads of IMI-susceptible common and prairie sunflower. In addition, field studies were conducted in 2000 and 2001 near Manhattan, KS, to evaluate IMI-resistant gene flow from IMI-resistant domesticated sunflower to common and prairie sunflower under natural conditions. Common and prairie sunflower were planted in concentric circles at distances of 2.5, 5, 15, and 30 m around a densely planted IMI-resistant domesticated sunflower species. For both greenhouse and field studies, IMI-resistant gene flow was determined by treating the progeny of both wild species with 40 g ai ha−1 of imazamox. Greenhouse crosses made by hand showed that 94% of common sunflower and 79% of prairie sunflower were resistant or moderately resistant. The resistant plants were allowed to grow in the greenhouse and were backcrossed with the corresponding susceptible wild parents. Progeny of the backcross showed a 1:1 ratio of resistant to susceptible plants. In the field, gene flow was detected up to 30 m from the pollen source for both species, and it decreased as distance from the pollen source increased. In 2000, 11 to 22% of the progeny were resistant at 2.5 m from the pollen source and 0.3 to 5% were resistant at 30 m. In 2001, the number of resistant progeny did not exceed 7 and 2% at 2.5 and 30 m from the pollen source, respectively. The results of this study showed that IMI-resistant domesticated sunflower outcrosses with common and prairie sunflower over distances typically encountered near production fields. Also, backcrosses of resistant hybrids with wild parents are successful, further increasing the potential for the spread of IMI-resistant feral sunflowers.

Ecological niche modeling, a new methodology for predicting the geographic course of species' invasions, was tested based on four invasive plant species (garlic mustard, sericea lespedeza, Russian olive, and hydrilla) in North America. Models of ecological niches and geographic distributions on native distributional areas (Europe and Asia) were highly statistically significant. Projections for each species to North America—effectively predictions of invasive potential—were highly coincident with areas of known invasions. Hence, in each case, the geographic invasive potential was well summarized in a predictive sense; this methodology holds promise for development of control and eradication strategies and for risk assessment for species' invasions.

Experiments were conducted to determine the effects of temperature on seed germination and growth of redroot pigweed, Palmer amaranth, and common waterhemp. At 15/10 C day and night temperature, respectively, no seed germination was observed in any species. Seed germination increased gradually as temperature increased. Germination peaked at 25/20 C in common waterhemp and at 35/30 C in redroot pigweed and Palmer amaranth. Seed germination of all three species declined when temperatures increased above 35/30 C. All three species produced less biomass at 15/10 C than at 25/20 C and 35/25 C. Redroot pigweed and common waterhemp biomass were similar at 15/10 C and higher than that of Palmer amaranth. However, Palmer amaranth produced more biomass than redroot pigweed and common waterhemp at 25/20 and 35/30 C. At 45/40 C, redroot pigweed, common waterhemp, and Palmer amaranth plants died 8, 9, and 25 d after initiation of heat treatment, respectively. The largest root volume among the three species was in Palmer amaranth grown at 35/30 C, whereas the smallest root volume was produced by Palmer amaranth grown at 15/10 C. Potential quantum efficiency (F v/F max) of Palmer amaranth was higher than that of redroot pigweed and common waterhemp at higher temperature. The greater growth of Palmer amaranth at higher temperatures may be attributed in part to its extensive root growth and greater thermostability of its photosynthetic apparatus.

Field experiments were carried out at the Facultad de Agronomía, Universidad de Buenos Aires, Argentina (34°25′S, 58°25′W), to evaluate the possibility of reducing weed seedling emergence through the use of alfalfa cultivars with low levels of winter dormancy and by increasing plant density from 200 to 400 plants m−2. It was hypothesized that these treatments would alter the temperature regime and the red (R)–far-red (FR) ratio of radiation to which seeds were exposed. Responses to management treatments were recorded for bull thistle, cotton thistle, plumeless thistle, tall rocket, mustard, curly dock, and pigweed. During the alfalfa establishment year, pigweed and curly dock emergence was reduced by the nondormant cultivar established at high density. This reduction disappeared when soil beneath the canopy was fitted with heaters that mimicked bare-soil temperatures. Crop canopy presence during the establishment year was not effective in reducing mustard, cotton thistle, bull thistle, plumeless thistle, and tall rocket emergence. During the second and third years after crop establishment, the canopy of the nondormant alfalfa cultivar was effective in reducing germination of weed seeds placed on the soil surface during fall and winter. In contrast, the winter-dormant cultivar allowed the establishment of weeds during the winter period. These reductions in weed emergence were associated with a modification in the R–FR ratio perceived by the seeds located at the soil surface and could largely be removed by using FR filters to increase the R–FR ratio. These results suggest that the selection of a nondormant cultivar combined with an increase in plant density could effectively reduce weed populations in alfalfa.

Variation in spring emergence periodicity (both before and after crop seeding) of summer annual weeds is a potentially exploitable attribute that may be applied to weed management in canola. Tillage intensity, which is decreasing in the Great Plains of North America, may influence emergence periodicity of summer annual weeds. Emergence periodicity of common lambsquarters, field pennycress, green foxtail, redroot pigweed, wild buckwheat, wild mustard, and wild oat were monitored during the spring of 2000 in 17 producers' canola fields across southern Manitoba, Canada. The fields represented a region of approximately 2 million ha and included a broad range of soil types, agronomic practices, environmental conditions, and seedbank distributions. Fields were grouped into one of two broad tillage classifications (conventional or conservation). For most species, except redroot pigweed and wild mustard, conservation tillage promoted earlier emergence than conventional tillage in terms of both thermal and chronological time. The differences were significant even though there was only a limited range of tillage intensity for the two tillage classes within this region. Onset of canola crop emergence preceded that of all but one weed species in the conservation-tillage fields and five weed species in the conventional-tillage fields. This suggests that canola seeded in conservation- vs. conventional-tillage systems may have a competitive advantage by way of an earlier relative time of crop emergence. The influence of tillage system on weed emergence periodicity is likely due to the influence of tillage on the vertical origin of weed seedling recruitment because measurements of soil temperature and soil moisture did not help to fully explain the differences in emergence periodicity between tillage systems. The results from this study will facilitate weed control timing decisions in canola and provide validation data for weed emergence models.

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.

In western Canada, little is known about the seedbank ecology of volunteer canola. Therefore, integrated recommendations for the management of this weed are limited. In this study, we investigated the seedbank persistence and seedling recruitment of two spring canola genotype groups with different secondary seed dormancy potentials under contrasting tillage systems. The study was conducted at two locations with different soils in the Mixed Moist Grassland ecoregion of Saskatchewan. A single cohort seedbank was established in 1999 and was followed for 3 yr in successive wheat crops. In a separate laboratory study, the six canola genotypes examined were classified as those with high and those with medium potentials for the development of secondary seed dormancy (HD and MD, respectively). After one, two, and three winters, maximum persistence of 44, 1.4, and 0.2% of the original seedbank was observed among the treatments, respectively. In 2001, HD canola genotypes tended to exhibit 6- to 12-fold greater persistence than MD canola genotypes, indicating lower seedbank mortality in HD canola. Seedling recruitment of HD canola also was higher than MD canola when differences were observed between these genotype groups. Therefore, long-term seedbank persistence of canola can be reduced by growing genotypes with low inherent potential for the development of secondary seed dormancy. The proportion of persisting seeds tended to be higher under conventional tillage than under zero tillage because of lower seedbank mortality, but no clear distinction in seedbank persistence in terms of absolute time could be made between these two tillage systems. Volunteer canola seedling recruitment followed the pattern of a typical summer-annual weed, where seedling emergence was observed only during May and June.

Dwarf sorghum (C4) was grown at ambient and at projected levels of atmospheric carbon dioxide (250 mol mol−1 above ambient) with and without the presence of a C3 weed (velvetleaf) and a C4 weed (redroot pigweed), to quantify the potential effect of rising atmospheric carbon dioxide concentration [CO2] on weed–crop interactions and potential crop loss. In a weed-free environment, increased [CO2] resulted in a significant increase in leaf weight and leaf area of sorghum but no significant effect on seed yield or total aboveground biomass relative to the ambient CO2 condition. At ambient [CO2] the presence of velvetleaf had no significant effect on either sorghum seed yield or total aboveground biomass; however, at elevated [CO2], yield and biomass losses were significant. The additional loss in sorghum yield and biomass was associated with a significant (threefold) increase in velvetleaf biomass in response to increasing [CO2]. Redroot pigweed at ambient [CO2] resulted in significant losses in total aboveground biomass of sorghum but not in seed yield. However, as [CO2] increased, significant losses in both sorghum seed yield and total biomass were observed for sorghum–redroot pigweed competition. Increased [CO2] was not associated with a significant increase in redroot pigweed biomass (P = 0.17). These results indicate potentially greater yield loss in a widely grown C4 crop from weedy competition as atmospheric [CO2] increases.

Manipulation of cropping systems to improve weed management requires a better understanding of how crop- and soil-related factors affect weed life cycles. Our objective was to assess the impacts of timing of primary tillage and use of legume green manure on giant foxtail demography and soil properties. We measured giant foxtail seed survival and dormancy, seedling emergence and survival, and fecundity, in addition to soil phytotoxicity, chemical properties affecting soil fertility and soil water, in the transition between the wheat and corn phases of a wheat–corn–soybean crop sequence. Postdispersal predation of giant foxtail seeds was measured in all three phases of the crop sequence. Wheat was grown either as a sole crop (W) or underseeded with red clover (R), and residues from this phase were rototilled either in the fall (FT) or in spring (ST). There were strong interactions between Red clover and Tillage timing in their effects on giant foxtail recruitment and fecundity in corn. Giant foxtail seedling emergence was 30% lower, and time to 50% emergence was more than 1 wk later, in the ST/R treatment than in the ST/W, FT/W, and FT/R treatments, which did not differ. However, fecundity of giant foxtail was 200% greater in the ST/R treatment than in the other three treatments because of suppressed early corn growth. The net effect of the ST/R treatment on giant foxtail demography in corn was to greatly increase inputs to the seedbank compared with the ST/W, FT/W, and FT/R treatments. Giant foxtail demography in the wheat phase was also affected by Red clover. There was a 200% increase in daily rates of postdispersal seed predation in the wheat phase of the R treatment compared with the W treatment. High-seed predation in the wheat phase and low fecundity in the corn phase of the FT/R treatment suggest that population growth rate of giant foxtail will be lower in this treatment than in the other treatments. The degree of soil phytotoxicity from red clover residues, the changes in the amount of interference from the corn crop early in the growing season, and the differential suitability of crop residues in the different rotations as habitat for seed predators all contributed to changes in giant foxtail demography. Understanding the effects of cropping system characteristics on entire weed life cycles will facilitate the design of integrated suites of complementary weed management tactics.

Cropping system characteristics affect weed management by altering key demographic rates of weeds, including recruitment, seedling survival, fecundity, and seed survival. To facilitate the design and improvement of cropping systems that limit weed population growth, analytical methods are needed to identify weed management “choke points” (weed life stages that vary in response to management and whose variation strongly affects weed population growth rate). The objectives of this study were to (1) determine whether wheat–red clover green manure can limit giant foxtail population growth rate (λ) in a wheat–corn–soybean crop sequence and (2) identify choke points in the giant foxtail life cycle with respect to the green manure treatment. Demographic data were used to construct a periodic matrix model of giant foxtail population growth in a wheat–corn–soybean crop sequence, with either a wheat sole crop (W) or a wheat–red clover intercrop (R) in the wheat phase. Identification of choke points was accomplished by adapting the life-table response experiment (LTRE) design for retrospective perturbation analysis of the periodic matrix model. The difference in λ (Δλ) between the two treatments was decomposed into contributions from each parameter in each rotation phase of the periodic model. Each LTRE contribution was equal to the product of the sensitivity of λ to changes in a given parameter by the treatment difference in that parameter. Those parameters making large contributions to Δλ represented weed management choke points. Giant foxtail population growth rate in the simulation was more than twice as great in the W treatment (λ = 2.54) than in the R treatment (λ = 1.16). Retrospective perturbation analysis indicated that the proportion of seeds surviving predation in the wheat phase made the largest LTRE contribution (0.55) to Δλ, followed by seedling recruitment in the soybean (0.41) and corn (0.20) phases. By identifying weed management choke points within a given system, retrospective perturbation analysis can target research and management efforts for greater reductions in weed population growth.

Germination and longevity of purple witchweed seeds stored in nylon gauze bags in the soil in situ were tested in northern Bénin over a 2-yr period, covering the rainy seasons in 1994 and 1995. The seeds were collected at Ina Station in November 1993 from corn and sorghum fields. It appeared that germination percentages of the seeds, which were stimulated by GR24 to germinate, as well as their viability according to a tetrazolium test, decreased steadily in wet soil. During the 1994 rainy season, germination percentage of seeds, which reached maximum values of 30 to 74%, decreased to values of 11 to 17%. During the 1995 rainy season, the number of germinating seeds decreased further, and at the end of this season the germination percentage approached zero. Seed viability also decreased in line with the decrease in germination. In addition to the study on longevity under field conditions, seeds also were exposed to various water regimens in pots. In the pot experiment, purple witchweed seed viability and germination declined in moist soil treatments. The dying-off process observed contradicts the common opinion on longevity of Striga seeds in their natural environment. “Wet dormancy” was not observed in the course of the rainy season.

One approach to site-specific weed control is to map weeds within a field and then divide the field area into smaller grid units. The decision to apply a herbicide to individual grid units, or decision units, is made by using yield loss models to establish an economic threshold level. However, decision units often contain weed populations with aggregated distributions. Many yield loss models have not considered this because experiments dealing with weed–crop competition typically assume uniform weed distributions. Therefore, these models may overestimate yield losses. Field experiments conducted in 1999 and 2000 compared the effects of common ragweed having a uniform distribution vs. an aggregated distribution on soybean seed yield, moisture content, and dockage. Field experiment data were used to calculate and compare economic thresholds for both distributions. Economic thresholds that considered drying costs and dockage also were compared. There was no significant difference in I parameters (yield loss as density approaches zero) between the two ragweed distributions in either year. Seed moisture content and dockage increased with increasing common ragweed densities, but increases were not significant at the break-even yield loss level. Economic threshold values were similar for both distributions with differences between aggregated and uniform of 0.14 and 0.01 plants m−2 in 1999 and 2000, respectively. The economic threshold values were reduced by 0.01 to 0.06 plants m−2 when drying costs and dockage were considered.

Giant ragweed seeds have high nutritional value, consisting of 47% crude protein and 38% crude fat, and may be an important food source for rodent and invertebrate populations in agricultural and early successional ecosystems. We investigated temporal patterns of postdispersal giant ragweed seed predation on the soil surface of a no-tillage cornfield as affected by involucre (seed dispersal unit) size and presence or absence of crop residue. Cage exclusion experiments indicated that rodents and invertebrates were the principal predators of giant ragweed seed, and total predation of involucres over a 12-mo period beginning in November was 88%. Rodents were the greatest predators of giant ragweed involucres during fall and winter, and cumulative predation by February 1 in treatments with rodent access ranged from 39 to 43%. In contrast, giant ragweed involucre predation by invertebrates occurred mainly from May 1 to November 1. When rodent access to involucres was prevented, total involucre predation by invertebrates over a 12-mo period ranged from 57 to 78%. Rodents showed an initial preference for large involucres (> 4.8-mm diameter), and invertebrates preferred small involucres (< 4.8-mm diameter). Involucres covered with corn plant residue underwent less predation by rodents from November to February than uncovered involucres, but residue cover had no effect on seed predation by invertebrates. In a laboratory feeding trial, the carabid Harpalus pensylvanicus preferred seed of smooth pigweed and yellow foxtail to giant ragweed seed, suggesting that giant ragweed seed is an incidental rather than a preferred food source for some carabids. Because giant ragweed exhibits relatively low fecundity and short seed bank persistence, results of this study suggest that postdispersal predation may directly reduce giant ragweed recruitment the next year by reducing new seed bank inputs. However, seed losses from predation alone may be insufficient to maintain giant ragweed populations below economic threshold levels in no-tillage cornfields.

Kudzu is an aggressive, nonnative vine that currently dominates an estimated 810,000 ha of mesic forest communities in the eastern United States. To test an integrated method of weed control, abundances of kudzu and other plant species were compared during 4 yr after six herbicide treatments (clopyralid, triclopyr, metsulfuron, picloram + 2,4-D, tebuthiuron, and a nonsprayed check), in which loblolly pines were planted at three densities (0, 1, and 4 seedlings m−2) to induce competition and potentially delay kudzu recovery. This split-plot design was replicated on each of the four kudzu-dominated sites near Aiken, SC. Relative light intensity (RLI) and soil water content (SWC) were measured periodically to identify mechanisms of interference among plant species. Two years after treatment (1999), crown coverage of kudzu averaged < 2% in herbicide plots compared with 93% in the nonsprayed check, and these differences were maintained through 2001, except in clopyralid plots where kudzu cover increased to 15%. In 2001, pine interference was associated with 33, 56, and 67% reductions in biomass of kudzu, blackberry, and herbaceous vegetation, respectively. RLI in kudzu-dominated plots (4 to 15% of full sun) generally was less than half that of herbicide-treated plots. SWC was greatest in tebuthiuron plots, where total vegetation cover averaged 26% compared with 77 to 111% in other plots. None of the treatments eradicated kudzu, but combinations of herbicides and induced pine competition delayed its recovery.

Environmental legislation may impose limitations on the quantity of nitrogen (N) used in corn production on the basis of soil type and ground water flow. If N rates are reduced, this might influence the relative competitiveness of weed species. Therefore, the objectives of this research were to develop a surface response model to provide estimations of the effect of differing N rates on threshold values of green foxtail in corn and to use this model as a theoretical framework for hypothesis testing. Field experiments were conducted from 1999 to 2001 to examine the interaction of N rate and green foxtail density on corn grain yield. The experiment was designed as a two-factor factorial with N levels ranging from 0 to 200 kg N ha−1 and targeted green foxtail densities ranging from 0 to 300 green foxtail plants m−2. The addition of up to 200 kg N ha−1 increased corn grain yield in both weed-free and weedy treatments. Corn yield loss attributed to green foxtail ranged from 35 to 40% at 0 kg N ha−1 to 12 to 17% at 200 kg N ha−1. Ridge analysis of the response surfaces indicated that optimal corn grain yield could be achieved at derived values of 131 to 138 kg N ha−1 while maintaining a green foxtail density of 8 to 9 green foxtail plants m−2 on a sandy soil with less than 2% organic matter. The analyses of simulation results led to the generation of hypotheses of practical relevance to N management. On the basis of the generated hypotheses, a legislated reduction in N or an increase in the cost of N fertilizer would result in a lower threshold value for green foxtail in corn. If legislation were to ban the use of all herbicides in corn production, higher N rates or an increase in mechanical weed control measures would be required to offset yield losses caused by green foxtail. The human health and environmental consequences of such legislation would be significant.

A field study was conducted during 1997 to 2001 on a Dundee silt loam soil at Stoneville, MS, to examine the effects of rye and crimson clover residues on weeds, soil properties, soil microbial populations, and soybean yield in conventional tillage (CT) and no-tillage (NT) systems with preemergence (PRE)-only, postemergence (POST)-only, and PRE plus POST herbicide programs. Rye and crimson clover were planted in October, desiccated in April, and tilled (CT plots only) before planting soybean. Both cover-crop residues reduced density of barnyardgrass, broadleaf signalgrass, browntop millet, entireleaf morningglory, and hyssop spurge but did not affect yellow nutsedge at 7 wk after soybean planting (WAP) in the absence of herbicides. Densities of these weed species were generally lower with PRE-only, POST-only, and PRE plus POST applications than with no-herbicide treatment. Total weed dry biomass was lower when comparing CT (1,570 kg ha−1) with NT (1,970 kg ha−1), rye (1,520 kg ha−1) with crimson clover (2,050 kg ha−1), and PRE plus POST (640 kg ha−1) with PRE-only (1,870 kg ha−1) or POST-only (1,130 kg ha−1) treatments at 7 WAP. Soils with crimson clover had higher organic matter, NO3–N, SO4–S, and Mn, and lower pH compared with rye and no–cover crop soils. Total fungi and bacterial populations and fluorescein diacetate hydrolytic activity were higher in soil with crimson clover, followed by rye and no cover crop. Soybean yields were similar between CT (1,830 kg ha−1) and NT (1,960 kg ha−1), no cover crop (2,010 kg ha−1) and rye (1,900 kg ha−1), and rye and crimson clover (1,790 kg ha−1), but they were higher in PRE plus POST (2,260 kg ha−1) than in PRE-only (1,890 kg ha−1) or POST-only (1,970 kg ha−1) treatments.

The integration of cover crops with selected postemergence herbicides was evaluated on the basis of weed control and grain yields in no-till soybean and corn. Soybean was planted into wheat residue, whereas corn was planted into hairy vetch residue. Full, half, and quarter rates and sequential herbicide applications were made. The wheat cover crop did not increase weed suppression but increased soybean grain yields. Half rates of thifensulfuron plus quizalofop-P as single or split applications were as effective as full rates in reducing weed weight in soybean. Soybean grain yields were similar in the half- and full-rate treatments in 1994, but yield was highest in the full-rate treatment in 1995. The hairy vetch cover crop did not increase weed suppression but lowered corn stands and grain yields in 1995 and enhanced corn grain yields in 1996. Full, half, and quarter rates (1996 only) of nicosulfuron plus primisulfuron were equally effective in reducing weed weight. Corn grain yields were similar at all herbicide rates in 1995 but were inversely related to herbicide rate in 1996. Split herbicide applications did not improve weed suppression over single applications of the same herbicide rate in either crop. Results indicate that cover crops can improve crop productivity and reduced rates of environmentally benign herbicides can minimize the herbicide requirements in no-till corn and soybean.