Mounting cases of herbicide-resistant waterhemp [Amaranthus tuberculatus (Moq.) Sauer] in the U.S. Midwest have renewed the interest in nonchemical weed management strategies. Field experiments were conducted in 2021 and 2022 to quantify the effectiveness of a commercial combine equipped with a seed impact mill in preventing A. tuberculatus seed return to the soil seedbank in soybean [Glycine max (L.) Merr.]. Amaranthus tuberculatus seed shattering before crop harvest was quantified. Amaranthus tuberculatus started shattering seeds during the last week of August in both years. Overall, 51% of A. tuberculatus seeds were retained on the plant at harvest on October 23, 2021, compared with 61% at harvest on October 7, 2022. Viability of shattered A. tuberculatus seeds ranged from 84% to 94%. Additional seed shattering occurred when plants were disturbed by the combine header during soybean harvest, which caused 15% and 9% shattering in 2021 and 2022, respectively. Amaranthus tuberculatus seeds passed through the impact mill were grouped in three categories: no damage, moderate damage, and severe damage. In 2021, A. tuberculatus seeds with moderate damage had 26% lower germination and viability than seeds with no visible damage. In 2022, seed germination and viability of no-damage seeds did not differ from seeds with a moderate level of damage. No severely damaged seed germinated or tested viable in either year. Altogether, impact mill treatment reduced the number of germinable seeds by 87% compared with the no–impact mill treatment. These results indicate that seed impact mills can be a useful tool in Iowa soybean production to help manage multiple herbicide–resistant A. tuberculatus populations. However, A. tuberculatus seed shattering before crop harvest reduces the overall effectiveness of seed impact mills in preventing seedbank replenishments.

Two separate field experiments were conducted during the 2021-22 and 2022-23 growing seasons at Kansas State University Agricultural Research Center near Hays, KS to understand the emergence dynamics of glyphosate-resistant (GR) kochia [Bassia scoparia (L.) A. J. Scott] as influenced by fall- and spring-planted cover crops (CC) and residual herbicide. Study sites were under winter wheat (Triticum aestivum L.)-sorghum [Sorghum bicolor (L.) Moench]-fallow rotation with a natural seedbank of GR B. scoparia. In experiment 1, fall-planted CC mixture (triticale/winter peas/radish/rapeseed) was planted after wheat harvest and terminated at triticale [xTriticosecale Wittm. Ex A. Camus [Secale x Triticum] heading stage (next spring before sorghum planting). In experiment 2, spring-planted CC mixture (oats/barley/spring peas) was planted in sorghum stubbles and terminated at oats (Avena sativa L.) heading stage. Four treatments were established in each experiment: (1) nontreated control (no CC and no herbicide), (2) chemical fallow (no CC but glyphosate + acetochlor/atrazine or flumioxazin/pyroxasulfone + dicamba were used to control weeds), (3) CC terminated with glyphosate, and (4) CC terminated with glyphosate plus residual herbicide (acetochlor/atrazine for fall-planted CC and flumioxazin/pyroxasulfone for spring-planted CC). Results indicated that fall-planted CC delayed GR B. scoparia emergence by 3 to 5 weeks whereas spring-planted CC delayed emergence by 0 to 2 weeks compared to nontreated. Fall-planted CC terminated with glyphosate plus acetochlor/atrazine reduced the cumulative emergence of GR B. scoparia by 90 to 95% compared to nontreated across both yrs. Similarly, spring-planted CC terminated with glyphosate plus flumioxazin/pyroxasulfone reduced the cumulative emergence of GR B. scoparia by 83 to 90% compared to nontreated. These results suggest that fall- or spring-planted CC in combination with residual herbicide at termination can be utilized for GR B. scoparia suppression. Results from this study will help in developing prediction models for GR B. scoparia emergence under different CC strategies.

Palmer amaranth (Amaranthus palmeri S. Watson) is the most problematic weed of cotton (Gossypium hirsutum L.)-cropping systems in the U.S. Southeast. Heavy reliance on herbicides has selected for resistance to multiple herbicide mechanisms of action. Effective management of this weed may require the integration of cultural practices that limit germination, establishment, and growth. Cover crops have been promoted as a cultural practice that targets these processes. We conducted a 2-yr study in Georgia, USA, to measure the effects of two annual cover crops (cereal rye [Secale cereale L.] and crimson clover [Trifolium incarnatum L.]), a perennial living mulch (‘Durana®’ white clover [Trifolium repens L.]), and a bare ground control on A. palmeri population dynamics. The study was conducted in the absence of herbicides. Growth stages were integrated into a basic demographic model to evaluate differences in population trajectories. Cereal rye and living mulch treatments suppressed weed seedling recruitment (seedlings seed−1) 19.2 and 13 times and 12 and 25 times more than the bare ground control, respectively. Low recruitment was correlated positively with low light transmission (photosynthetic active radiation: above canopy photosynthetically active radiation [PAR]/below cover crop PAR) at the soil surface. Low recruitment rates were also negatively correlated with high survival rates. Greater survival rates and reduced adult plant densities resulted in greater biomass (g plant−1) and fecundity (seeds plant−1) in cereal rye and living mulch treatments in both years. The annual rate of population change (seeds seed−1) was equivalent across all treatments in the first year but was greater in the living mulch treatment in the second year. Our results highlight the potential of annual cover crops and living mulches for suppressing A. palmeri seedling recruitment and would be valuable tools as part of an integrated weed management strategy.

Field experiments were conducted over 2 yr (2019 to 2020) at two locations in Iowa to evaluate multi-tactic strategies for managing multiple herbicide–resistant (MHR) waterhemp [Amaranthus tuberculatus (Moq.) Sauer] in a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation. The effect of three herbicide programs on A. tuberculatus control was tested in corn (2019). The effects of the prior year’s corn weed control, a cereal rye (Secale cereale L.) cover crop, and soybean row spacing (38-cm vs. 76-cm wide) on A. tuberculatus density, biomass, and seed production were tested in soybean (2020). A herbicide program used in corn with two sites of action provided only 35% control of MHR A. tuberculatus compared with ≥97% control by a herbicide program with three sites of action. In soybean, adequate control of A. tuberculatus (≥90%) in the prior year’s corn crop and use of a cover crop or narrow rows reduced A. tuberculatus density by more than 60% at 3 and 9 wk after planting (WAP) compared with inadequate control (30%) in the prior year’s corn and no cover crop. Cover crop and narrow-row soybean reduced A. tuberculatus density by 44% at 3 WAP compared with no cover crop and wide-row soybean. Inclusion of a single control tactic, adequate control (≥90%) with multiple herbicides in the prior year’s corn, use of a cover crop, or narrow-row soybean reduced A. tuberculatus biomass and seed production at soybean harvest by at least 24% compared with inadequate control (30%) in the prior year’s corn, no cover crop, and wide-row soybean. The combination of all three control tactics reduced A. tuberculatus biomass and seed production at soybean harvest by at least 80%. In conclusion, diverse control tactics targeting A. tuberculatus at multiple life-cycle stages can make substantial contributions to the management of MHR populations.

Weeds compete with crops for soil moisture, along with other resources, which can impact the germination, growth, and seed production of weeds; however, this impact has not been systematically recorded and synthesized across diverse studies. To address this knowledge gap, a global meta-analysis was conducted using 1,196 paired observations from 86 published articles assessing the effect of water stress on weed germination, growth characteristics, and seed production. These studies were conducted and published during 1970 through 2020 across four continents (Asia, Australia, Europe, and North America). Imposed water stress was expressed as solution osmotic potential (ψsolution), soil water potential (ψsoil), or soil moisture as percent field capacity. Meta-analysis revealed that water stress inhibits weed germination, growth, and seed production, and the quantitative response intensified with increasing water stress. A ψsolution greater than −0.8 MPa completely inhibits germination of both grass and broadleaf weeds. A ψsolution from −0.09 to −0.32 MPa reduces weed germination by 50% compared with the unstressed condition. Moderate soil water stress, equivalent to 30% to 60% field capacity, inhibits growth characteristics (branches or tillers per plant, leaf area, leaves per plant, plant height, root, and shoot biomass) by 33% and weed seed production by 50%. Severe soil water stress, below 30% field capacity, inhibits weed growth by 51% and seed production by 88%. Although water stress inhibits weed growth, it does not entirely suppress the ability to germinate, grow, and produce seeds, resulting in weed seedbank accumulation. This creates management challenges for producers, because weed seeds can survive in the soil for many years, depending on weed species and environmental conditions. Quantitative information compiled in this meta-analysis can be instrumental to model the weeds’ multidimensional responses to water stress and designing integrated weed management strategies for reducing the weed seedbank.

This study was undertaken to investigate the integration effects of pretilachlor, oxadiazon, and dimethenamid with or without glyphosate in a stale seedbed method to control weedy rice in wet-seeded rice. The study, conducted in 2018 and 2019, comprised two seedbed treatments in main plots, with and without glyphosate (850 g ae ha−1), and four subplot treatments: pretilachlor, oxadiazon, dimethenamid, and unsprayed check. Fifteen days after glyphosate spray, each subplot was treated with preemergence herbicides at 500 g ai ha−1 under standing water conditions (2 to 3 in.), and the water level was maintained for 7 d. Pregerminated rice seeds (var. MR297) were hand-broadcasted in the moist soil at 120 kg ha−1 seeding rate. In 2019, the density and dry weight of weedy rice were 30% and 118% higher Gthan those observed in 2018, respectively. A stale seedbed with glyphosate reduced weedy rice dry weight by 12% as compared to what was observed in a stale seedbed without glyphosate. Addition of oxadiazon and pretilachlor to the stale seedbed drastically reduced weedy rice dry weight by 70% to 88% and 53% to 60% in both years. Dimethenamid contributed to a significant reduction of weedy rice dry weight of 19% in 2019 only but failed to provide a positive economic return. Integration of pretilachlor and oxadiazon in a stale seedbed with glyphosate gave profitable returns of $84.00 to 311.4 ha−1 and $175.70 to 483.8 ha−1, respectively. Without the presence of glyphosate, pretilachlor and oxadiazon contributed a positive return of $318.90 and $469.40, respectively, in 2018, but the economic returns were negative in 2019. These results suggest that integration of pretilachlor or oxadiazon in a stale seedbed with glyphosate is more crucial when weedy rice infestation is high, but glyphosate can be excluded from the management regime when the weedy rice populations are low.

Evolution of multiple herbicide–resistant Palmer amaranth warrants the development of integrated strategies for its control in the southcentral Great Plains (SGP). To develop effective control strategies, a better understanding of the emergence biology of Palmer amaranth populations from the SGP region is needed. A common garden study was conducted in a no-till (NT) fallow field at the Kansas State University Agricultural Research Center near Hays, KS, during the 2018 and 2019 growing seasons, to determine the emergence pattern and periodicity of Palmer amaranth populations collected from the SGP region. Nine Palmer amaranth populations collected from five states were included: Colorado (CO1, CO2), Oklahoma (OK), Kansas (KS1, KS2), Texas (TX), and Nebraska (NE1, NE2, NE3). During the 2018 growing season, the CO1 and KS1 populations displayed more rapid emergence rates, with greater parameter b values (−5.4, and −5.3, respectively), whereas the TX and NE3 populations had the highest emergence rates (b = −12.2) in the 2019 growing season. The cumulative growing degree days (cGDD) required to achieve 10%, 50%, and 90% cumulative emergence ranged from 125 to 144, 190 to 254, and 285 to 445 in 2018; and 54 to 74, 88 to 160, and 105 to 420 in the 2019 growing season across all tested populations, respectively. The OK population exhibited the longest emergence duration (301 and 359 cGDD) in both growing seasons. All tested Palmer amaranth populations had a peak emergence period between May 11 and June 8 in 2018, and April 30 and June 1 in the 2019 growing season. Altogether, these results indicate the existence of differential emergence pattern and peak emergence periods of geographically distant Palmer amaranth populations from the SGP region. This information will help in developing prediction models for decision-making tools to manage Palmer amaranth in the region.

Junglerice and feather fingergrass are major problematic weeds in the summer sorghum cropping areas of Australia. This study aimed to investigate the growth and seed production of junglerice and feather fingergrass in crop-free (fallow) conditions and under competition with sorghum planted in 50-cm and 100-cm row spacings at three sorghum planting and weed emergence timings. Results revealed that junglerice and feather fingergrass had greater biomass in early planting (November 11) compared to late planting times (January 11). Under fallow conditions, seed production of junglerice ranged from 12,380 to 20,280 seeds plant–1, with the highest seed production for the December 11 and lowest for the January 11 planting. Seed production of feather fingergrass under fallow conditions ranged from 90,030 to 143,180 seeds plant–1. Seed production of feather fingergrass under crop-free (fallow) conditions was similar for November 11 and December 11 planting times, but higher for the January 11 planting. Sorghum crop competition at both row spacings reduced the seed production of junglerice and feather fingergrass >75% compared to non-crop fallow. Narrow row spacing (50 cm) in early and mid-planted sorghum (November 11 and December 11) reduced the biomass of junglerice to a greater extent (88% to 92% over fallow-grown plants) compared to wider row spacing (100 cm). Narrow row spacing was found superior in reducing biomass of feather fingergrass compared to wider row spacing. Our results demonstrate that sorghum crops can substantially reduce biomass and seed production of junglerice and feather fingergrass through crop competition compared with growth in fallow conditions. Narrow row spacing (50 cm) was found superior to wider row spacing (100 cm) in terms of weed suppression. These results suggest that narrow row spacing and late planting time of sorghum crops can strengthen an integrated weed management program against these weeds by reducing weed growth and seed production.

The presence of a soil seedbank facilitates the persistence of annual weed species in arable fields. Soil weed seedbank is replenished by many sources, but the largest one is the seeds produced by uncontrolled late-season weed escapes. The estimation of weed seed production potential from late-season escapes may allow farmers to make appropriate management decisions to minimize seedbank replenishment. The objective of this research was to evaluate the feasibility of using unmanned aerial vehicle–based RGB and multispectral imagery for estimating seed rain potential in late-season weed escapes in crop fields. Three case studies were used to capture images of weed escapes before crop harvest: common waterhemp [Amaranthus tuberculatus (Moq.) Sauer] in soybean [Glycine max (L.) Merr.], Palmer amaranth [Amaranthus palmeri (S.) Watson] in cotton (Gossypium hirsutum L.), and johnsongrass [Sorghum halepense (L.) Pers.] in soybean. Randomly selected quadrats with different density gradients of weed escapes were sampled at the time of crop maturity. High-resolution RGB and multispectral images of the experimental area were collected using drones immediately before ground sample collection. Normalized difference vegetation index (NDVI), excess green index (ExG), and canopy volume estimates derived from canopy height models were used to obtain weed biological measurements (biomass and seed production). Among the indices investigated, NDVI and ExG had very strong correlations (0.71 to 0.97) with weed biomass. No specific remote sensing variable was ideal across the three cases examined here, suggesting that a generalized remote sensing approach may not offer robust estimations and case-specific applications are imperative. Nonetheless, drone imagery is a powerful tool for estimating seed production from uncontrolled weed escapes and assisting with management decision making.

Ethiopian mustard (Brassica carinata A. Braun) is a biofuel crop recently introduced in the southeastern United States. For this crop to be successful, integrated weed management strategies that complement its rotation with summer cash crops must be developed. The objectives of this research were to evaluate the effect of previous season summer crops on winter weed emergence patterns during Ethiopian mustard growing season and to assess the impact of planting Ethiopian mustard on the emergence patterns of summer weed species. Gompertz models were fit to winter and summer weed emergence patterns. All models represented more than 80% of the variation, with root mean-square error values less than 0.20. The emergence pattern for winter weed species was best described using growing degree-day accumulation, and this model can be utilized for implementing weed control strategies at the critical Ethiopian mustard growth stages. The results also showed that summer weeds can emerge during the winter in northern Florida but do not survive frost damage, which might create off-season seedbank reductions before the summer crop growing season.

Tillage regimes can influence weed population dynamics and, consequently, the choice of appropriate weed management practices. Studies were conducted in 2016 and 2017 in a long-term (36-yr) grain sorghum [Sorghum bicolor (L.) Moench ssp. bicolor] experiment at Texas A&M University, College Station, to determine the impact of long-term no-till (NT) and conventional till (CT) systems on weed species dynamics. Higher densities of johnsongrass [Sorghum halepense (L.) Pers.], prostrate spurge [Chamaesyce humistrata (Engelm. ex A. Gray) Small], waterhemp [Amaranthus tuberculatus (Moq.) Sauer], and henbit (Lamium amplexicaule L.) were recorded in the NT system compared with the CT system. Further, the NT system showed greater weed diversity (Shannon-Wiener index, H = 0.8) and species richness (S = 6.2), compared with the CT system (H = 0.6, S = 4.2). Seedling emergence of some dominant weed species was also delayed in the NT system. In the CT system, 50% emergence of S. halepense (8.5 C base temperature) and waterhemp (10 C base temperature) occurred at 59 and 63 growing degree days (GDD), respectively, whereas 68 and 75 GDD, respectively, were required in the NT system. Further, a greater proportion (61%) of the viable seedbank was present at the top 5 cm of the soil in the NT system compared with the CT system (46%). Overall, findings from this 36-yr-long tillage experiment have revealed that the NT system had greater weed densities (especially of the perennial weed S. halepense) and a high proportion of weed seeds (particularly small-seeded annuals) on the topsoil layer, corroborating some earlier reports that were based on short-term investigations. Findings indicate that growers transitioning to NT systems should be mindful of potential shifts in weed species dominance and develop appropriate management tactics.

In Australia, junglerice and feather fingergrass are problematic weeds in sorghum. The high seed production potential of these weeds increases their seedbank in the soil and makes weed control practices more difficult and expensive, particularly when weeds have evolved resistance to herbicides. A study was conducted to evaluate the seed production and seed retention behavior of junglerice and feather fingergrass at sorghum crop maturity following four transplanting times: 0, 2, 4, and 6 wk after sorghum emergence. Averaged across years, junglerice and feather fingergrass produced 4,060 and 5,740 seeds plant-1, respectively,when they were transplanted with the emergence of a sorghum crop. Seed retention ranged from 42% to 56% for junglerice and 67% to 75% for feather fingergrass when these weeds were transplanted from 0 to 4 wk after crop emergence. A positive correlation (r = 0.75 for junglerice; r = 0.44 for feather fingergrass) was found between seed production and weed biomass in both weeds, indicating that larger plants produced more seeds than smaller plants. However, no correlation was found between weed biomass and seed retention for junglerice. A weak positive correlation (r = 0.44) was found between feather fingergrass biomass and percent seed retention, indicating that seed retention was greater in larger plants compared with smaller plants. Our results suggest that feather fingergrass is a good candidate for harvest weed seed control (HWSC) tactics if crop harvest is timely. There is limited opportunity to use HWSC tactics for targeting junglerice seeds in sorghum crops, because most seeds dispersed before crop maturity. Additional research is required to evaluate seed retention levels of these weeds in other summer crops such as corn and soybean to determine the potential for HWSC for management of these species.

Dicamba-resistant (DR) kochia [Bassia scoparia (L.) A. J. Scott] has been reported in six U.S. states and one Canadian province. To develop effective B. scoparia control tactics, it is necessary to understand the seed germination pattern of DR B. scoparia. The objective of this study was to compare the germination characteristics of DR versus dicamba-susceptible (DS) B. scoparia populations from Montana and Kansas under constant (5 to 35 C) and/or alternating temperatures (5/10 to 30/35 C). DR B. scoparia lines from Montana were generated after three generations of recurrent selection of field-collected populations with dicamba. Seeds of DR or DS lines from Kansas were obtained after one generation of restricted self-pollination. DR B. scoparia lines from both Montana and Kansas had a lower maximum cumulative germination than the DS lines across all temperature treatments. A majority of DR B. scoparia lines from Montana showed a temperature-mediated seed germination response, with a higher thermal requirement (30 to 35 C or 25/30 to 30/35 C) to attain the maximum cumulative germination compared with DS lines. Germination rates at 5 to 30 C were lower for DR versus DS B. scoparia lines from Kansas. All DR lines from Montana took more time than DS lines to initiate germination at 5 and 10 C or 5/10 and 20/25 C. Similarly, there was a delayed onset of germination of the DR versus DS line from Kansas at 5, 10, 15, and 20 C. Furthermore, the DR B. scoparia from both Kansas and Montana had a slower germination pattern relative to the DS B. scoparia. Diversified crop rotations using winter wheat (Triticum aestivum L.), fall-sown cover crops, or early-spring planted crops (e.g., wheat or barley [Hordeum vulgare L.]) that are competitive against late-emerging B. scoparia in conjunction with strategic tillage and late-season weed control tactics should be used to facilitate depletion of DR B. scoparia seedbanks.

Evolution of kochia biotypes resistant to multiple herbicide sites of action is an increasing concern for growers across the U.S. Great Plains. This necessitates the development of integrated strategies for kochia control in this region based on improved forecasting of periodicity and patterns of kochia emergence in the field. Field experiments were conducted near Huntley, MT, in 2013 and 2014, and in Manhattan and Hays, KS, in 2013 to characterize the timing and pattern of emergence of several kochia populations collected from the U.S. Great Plains’ states. The more rapid accumulation of growing degree days (GDD) resulted in a shorter emergence duration (E90–E10) in 2014 compared with 2013 in Montana. Kochia populations exhibited an extended emergence period (early April through mid-July). Among all kochia populations, in 2013, Kansas-Garden City (KS-GC), Kansas-Manhattan (KS-MN), Oklahoma (OK), and Montana (MT) populations began to emerge earlier, with a minimum of 151 cumulative GDD to achieve 10% cumulative emergence (E10 values) in Montana. The New Mexico-Los Lunas (NM-LL) population exhibited a delayed onset but a rapid emergence rate, while the North Dakota (ND) and Kansas-Colby (KS-CB) populations emerged over a longer duration (E90–E10 of 556 and 547 GDD, respectively) in 2013 in Montana. In 2013 at the two locations in Kansas, kochia populations exhibited a similar emergence pattern, with no differences in the time to initiate germination (E10), rate of emergence (parameter b), or duration of emergence (E90–E10). At Hays, KS, the GDD for E50 and E90 were less for ND compared with KS-MN and KS-GC local populations. In 2014 the KS-MN kochia population exhibited an early (ED10 value of 215 GDD) but a more gradual emergence pattern (E90–E10=526 GDD) in Montana. In contrast, OK and New Mexico-Las Cruces (NM-LC) populations had an early and a more rapid emergence pattern (E90–E10=153 and 154 GDD, respectively). Kochia in Montana exhibited two to four emergence peaks. This differential emergence pattern of kochia populations reflects the occurrence of different emergence “biotypes” and emphasizes the need to adopt more location-specific and diversified weed control tactics to manage kochia seedbanks.

At a northern Iowa location, populations of 145, 203, 62, and 37 velvetleaf plants/m2 emerged in the four years following seed production by 0.4 velvetleaf plants/m2 growing in soybean. Annual velvetleaf densities in areas with no seed input averaged less than 6 plants/m2. At a central Iowa location, verticillium wilt reduced velvetleaf seed production, resulting in an average increase of 9 plants/m2 in the two years following seed production compared to the control treatment. This type of information is necessary to develop economic optimum thresholds, but the variability between locations demonstrates the difficulty in predicting crop and weed responses to management strategies implemented early in the growing season.

A study under glasshouse conditions determined the effect of germination stimulants, potassium nitrate and ethephon, on emergence of several pure lines of wild oat from soil. Effect of the two chemicals on stimulating emergence was variable and no synergism noted when used together. Examination of exhumed seed showed that these treatments reduced seed viability. By counting the number of dead and adding to those that emerged, total seed removed from the soil was obtained. All treatments removed seed from the soil seedbank, especially when seed was old, near the soil surface, was primary seed, or from low or intermediate dormancy lines. However, because of the insensitivity of a large proportion of the seed and the high rates required it is unlikely that either potassium nitrate or ethephon has practical use for field application.

WEEDSIM is a bioeconomic decision aid for management of annual weeds in corn and soybean. It was field-tested for 4 yr in Minnesota. The decision aid has two categories of management recommendations: soil-applied plus postemergence (PRE+), based on estimated weed seedbank composition and density; and postemergence (POST), based upon observed weed seedling composition and density. Weed densities, weed control, herbicide use, environmental impact of herbicide use, weed management costs, crop yields, and economic returns that resulted from PRE+ and POST recommendations were compared to those associated with herbicide management systems (HERB) that were standard for the region. After 4 yr of applying WEEDSIM recommendations to the same plots, there were no increases in annual weed densities (seedbanks, seedlings, established plants, or seed production) or decreases in weed control or crop (soybean, rotation corn, and continuous corn) yields, compared to HERB. WEEDSIM recommendations resulted in average annual herbicide applications of 1.1 kg ai ha−1 for PRE+ and 1.0 kg ai ha−1 for POST, compared to 3.5 kg ai ha−1 for HERB. Environmental impact indices associated with PRE+, POST, and HERB were 0.75, 0.71, and 0.54, with the lowest value indicating greater environmental risk than the two higher values. Similarly, average weed management costs were $24, $33, and $77 ha−1 for PRE+, POST, and HERB, respectively. Based on crop prices of $94 Mg−1 for corn and $220 Mg-1 for soybean, the average gross margins over weed control costs were higher for PRE+ ($509 ha−1) and POST ($522 ha−1) than for HERB ($455 ha−1). In general, WEEDSIM appeared to make management recommendations that adequately controlled weeds, maintained crop yields, reduced herbicide use, decreased environmental risk, lowered weed management costs, and increased gross margins over weed control costs compared to the use of herbicides standard for the region.

This research presents a modified floatation method to extract weed seed from soil samples and an automated imaging technique to enumerate seed following extraction. Rapid floatation of weed seed from soil in a K2CO3 solution was facilitated by centrifugation. High recovery of seed from soil samples was accomplished in a relatively short time. Recovery of seed of giant foxtail and velvetleaf from a silt loam soil was near 100% and recovery of redroot pigweed was 86%. Germination of common lambsquarters, giant foxtail, and velvetleaf seed was reduced by exposure to K2CO3 under certain conditions. However, these reductions do not appear to reduce the utility of this extraction technique. The automated image capture technique for counting seed showed similar limitations as with manual counting. Seed count accuracy generally decreased with increased number of seed and smaller seed sizes. In general, the image capture technique compared favorably with manual seed counting and has the potential to further improve seedbank assessment methods.

A long-term integrated pest management study initiated in 1980 and continued through 1991 was conducted to determine interactions of tillage, crop rotation, and herbicide use levels on weed seed populations, weed populations, and crop yield. This paper presents giant foxtail seed population and stand along with corn yield in continuous corn, corn rotated with soybean, or corn following wheat in a soybean-wheat-corn rotation. Increasing herbicide use levels above the minimum reduced giant foxtail seed in the 0- to 2.5-cm depth of soil. Reducing tillage from conventional moldboard plowing to chiseling to no-tilling increased giant foxtail seed in only the top 0 to 2.5 cm of soil. No-tilling increased giant foxtail seed over conventional tillage in each year data were collected. Growing corn in a soybean-corn or soybean-wheat-corn rotation reduced giant foxtail seed from corn grown continuously in all three soil depths sampled: 0 to 2.5 cm, 2.5 to 10 cm, and 10 to 20 cm. Although stands of giant foxtail tended to follow soil weed seed counts, crop rotation significantly reduced giant foxtail stand with maximum reduction in the soybean-wheat-corn rotation in all tillage systems. Giant foxtail stands were reduced following wheat in no-tilling, probably because of the allelopathic influence of wheat straw. Corn yields showed weed management levels above minimum control are not justified regardless of tillage and crop rotation.

Field research was conducted from 1987 to 1991 to evaluate the influence of four weed management systems on weed population and species dynamics in conventional-tillage and no-tillage grain sorghum production. These weed management systems included zero, low, medium, and high input systems. The weed seedbank increased faster in zero and low input weed management systems than in the high input weed management systems because of differences in weed control. Tillage influenced weed seed densities as well. Common ragweed, common lambsquarters, horseweed, and sicklepod seed densities often were greater in no-tillage than conventional-tillage plots. Common cocklebur and large crabgrass seed densities were usually greater in conventional-tillage than no-tillage plots. Smooth pigweed seed densities were not affected by tillage. Increasing weed management inputs diminished differences between tillage systems. Smooth pigweed dominated the weed populations after 4 yr in both tillage systems even in high input systems.