Herbicides that inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD) can be mixed with herbicides that generate reactive oxygen species (ROS) to enhance the spectrum, level, speed, and consistency of weed control efficacy; however, mixtures of these herbicides can increase corn injury. A total of five field trials were conducted from 2021 to 2023 in Ridgetown, Ontario, to determine the sensitivity of two corn hybrids (‘DKC39-97’ and ‘B79N56PWE’) to tolpyralate plus ROS-generating herbicides (atrazine, bromoxynil, bentazon, or glufosinate) applied postemergence at the recommended rate (1×) and sequentially to represent a spray overlap (2×) in the field. Tolpyralate plus atrazine, bromoxynil, bentazon, or glufosinate (2× rates) caused greater corn injury to DKC39-97 than B79N56PWE corn at 1, 2, and 4 wk after treatment (WAT). Tolpyralate plus atrazine, bromoxynil, bentazon, or glufosinate (2× rates) caused 38%, 36%, 29%, and 18% injury to DKC39-97 corn, but only 5%, 20%, 9%, and 2% injury to B79N56PWE corn, respectively at 1 WAT. Corn injury to both hybrids decreased over time with ≤2% injury at 8 WAT. Tolpyralate + atrazine, bromoxynil, or bentazon (2× rates) caused a 17%, 16%, and 13% height reduction, respectively, of DKC39-97 corn at 2 WAT; however, tolpyralate + glufosinate did not reduce DKC39-97 corn height. Tolpyralate + bromoxynil or bentazon (2× rates) caused a 12% and 10% height reduction of B79N56PWE corn, respectively, at 2 WAT; however, tolpyralate + atrazine or glufosinate did not reduce B79N56PWE corn height. Tolpyralate + atrazine or glufosinate (2× rates) caused a greater corn height reduction of DKC39-97 corn than B79N56PWE corn at 2 WAT. Grain yield was on average 2% lower from DKC39-97 than B79N56PWE corn. Tolpyralate + bromoxynil or bentazon (2× rates) caused 7% and 6% corn grain yield reduction compared to tolpyralate plus glufosinate (2× rate). Results indicate that tolpyralate + ROS-generating herbicides can cause corn injury, which is influenced by corn hybrid and ROS-generating herbicide. Corn producers need to consider the differential sensitivity of corn hybrids and ROS-generating herbicides when using an HPPD-inhibiting herbicide for weed management.

Six field experiments were established in southwestern Ontario in 2021 and 2022 to evaluate whether the addition of a grass herbicide (acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor) to tolpyralate + atrazine improves late-season weed control in corn. Tolpyralate + atrazine caused 12% and 5% corn injury at 1 and 4 wk after herbicide application (WAA); corn injury was not increased with the addition of a grass herbicide. Weed interference reduced corn yield 60%. The addition of a grass herbicide to tolpyralate + atrazine did not enhance velvetleaf control. The addition of acetochlor or dimethenamid-p to tolpyralate + atrazine enhanced pigweed species control 4% 4 WAA; the addition of other grass herbicides tested did not increase pigweed species control. The addition of acetochlor enhanced common ragweed control 5% at 4 WAA, and the addition of acetochlor or dimethenamid-p enhanced common ragweed control 8% at 8 WAA; the addition of other grass herbicides did not improve common ragweed control. The addition of acetochlor to tolpyralate + atrazine enhanced common lambsquarters control up to 4%; there was no enhancement in common lambsquarters control with the addition of the other grass herbicides. Tolpyralate + atrazine controlled barnyardgrass 90% and 78% at 4 and 8 WAA, respectively; the addition of a grass herbicide enhanced barnyardgrass control 9% to 10% and 21% at 4 and 8 WAA, respectively. Tolpyralate + atrazine controlled green or giant foxtail 80% and 69% at 4 and 8 WAA, respectively; the addition of a grass herbicide enhanced foxtail species control 15% to 19% and 24% to 29% at 4 and 8 WAA, respectively. This research shows that adding a grass herbicide to tolpyralate + atrazine mixture can improve weed control efficacy, especially increased annual grass control in corn production.

Ethofumesate is a broad-spectrum, soil-applied herbicide used to control broadleaf and grass weeds in sugarbeet crops. Ethofumesate is commonly applied preemergence at rates ranging from 1.25 to 4.2 kg ai ha−1, or applied postemergence (POST), up to 0.38 kg ai ha−1. The Generic Crop Science company has developed a new Ethofumesate 4SC label that has increased ethofumesate POST rates up to 4.48 kg ha−1 in sugarbeet with more than two true leaves per plant. Field and greenhouse experiments were conducted in 2018 and 2019 to evaluate sugarbeet tolerance and herbicide efficacy. Field tolerance experiments indicated sugarbeet stature from ethofumesate applied POST at 0.28, 0.56, and 1.12 kg ha−1 was the same as that of the nontreated control, but ethofumesate at 2.24 kg ha−1 reduced sugarbeet stature, although that rate did not affect yield components. Ethofumesate applied POST at 4.48 kg ha−1 reduced sugarbeet stature and affected sugarbeet yield components. Ethofumesate applied alone POST provided weed control of up to 85%, 76%, and 84% on common lambsquarters, redroot pigweed, and waterhemp, respectively, in field efficacy experiments. Mixing ethofumesate at 1.12 kg ha−1 with glyphosate does not provide a second effective herbicide for POST control of common lambsquarters or redroot pigweed, but it does provide residual control of these weeds when at least one-half inch of penetrating rainfall occurs, following application. In greenhouse experiments, ethofumesate alone or ethofumesate plus glyphosate applied to common lambsquarters, redroot pigweed, or waterhemp at heights of less than 2.5 cm provided the best combination of burndown and soil residual control compared with weeds that were 2.5 to 5 cm tall. Ethofumesate applied POST at 1.12 kg ha−1 plus glyphosate provided the best combination of tolerance and efficacy, especially on waterhemp.

Small-seeded vegetable crop production is challenged by poor emergence, stand establishment and canopy development, as well as a lack of effective weed control options. The potential use of plant growth regulators such as gibberellic acid (GA) could enhance early emergence and growth rates while potentially synchronizing weed germination with control tactics. In response, a controlled environment study was conducted to investigate the effects of GA on garden beet, cabbage, carrot, and onion. At 7 d after seeding (DAS) carrot emergence was greater when carrot seeds were treated with 2, 4, 8, 16, or 64 ppm GA compared with nontreated seeds. Total carrot emergence over the study period was 14% greater when seeds were treated with 4 ppm GA compared with nontreated seeds. Treatment of cabbage with as low as 2 to 4 ppm GA increased cabbage emergence rate and total plant emergence over the study period relative to nontreated seeds. Onion response to GA treatment was variable and unremarkable and was hypothesized to be influenced by seed dormancy because emergence was also low with the nontreated seeds. The GA rates that stimulated vegetable crop seed germination and emergence were then explored with three common weed species to determine whether a similar response was observed. If so, GA could be used to stimulate weed emergence in synchrony with management tactics. Palmer amaranth emergence was strongly affected by GA treatment, whereby the total number of emerged plants was 48% greater when 4 ppm GA was applied than in the nontreated check. Velvetleaf emergence at 3 DAS with the 4 and 8 ppm GA was 2.9 and 3.0 plants pot−1, respectively, compared to no emergence in the nontreated pots. Redroot pigweed emergence was not affected by GA treatment at any rate.

Tolpyralate is an herbicide that is usually mixed with atrazine for broad-spectrum weed control in corn. Previous research has provided information on the effective dose (ED) of tolpyralate applied alone and in a 1:33.3 mixture with atrazine; however, tolpyralate is commercially applied at a dose of 30 to 40 g ai ha−1 with a minimum of 560 g ai ha−1 of atrazine. Therefore, five field trials were conducted over 3 yr (2019 to 2021) to determine the ED of atrazine to complement 30 g ai ha−1 of tolpyralate to achieve 80%, 90%, and 95% control of seven weed species 2, 4, and 8 wk after application (WAA). Tolpyralate was applied alone and in a mixture with atrazine doses ranging from 50 to 2,000 g ai ha−1. At 8 WAA, the ED of atrazine for 95% control of velvetleaf, common ragweed, common lambsquarters, and wild mustard was below the minimum label dose of atrazine on the commercial tolpyralate label, ranging from 430 to 520 g ai ha−1, which supports the use of the minimum label dose of atrazine. In contrast, redroot pigweed required 1,231 g ai ha−1 of atrazine to complement tolpyralate for 95% control 8 WAA. At 8 WAA, barnyardgrass and a mixture of green foxtail and giant foxtail (Setaria spp.) were not controlled by 80%, 90%, or 95% with tolpyralate applied alone or co-applied with any dose of atrazine evaluated in this study. The results of this study conclude that tolpyralate + atrazine is highly efficacious on several weed species at atrazine doses of 40 to 130 g ai ha−1 below the label dose of 560 g ai ha−1, but the use of the higher dose of tolpyralate or another herbicide may be required to improve control of redroot pigweed and grass weed species.

Residual herbicides are routinely applied to control troublesome weeds in pumpkin production. Fluridone and acetochlor, Groups 12 and 15 herbicides, respectively, provide broad-spectrum PRE weed control. Field research was conducted in Virginia and New Jersey to evaluate pumpkin tolerance and weed control to PRE herbicides. Treatments consisted of fomesafen at two rates, ethalfluralin, clomazone, halosulfuron, fluridone, S-metolachlor, acetochlor emulsifiable concentrate (EC), acetochlor microencapsulated (ME), and no herbicide. At one site, fluridone, acetochlor EC, acetochlor ME, and halosulfuron injured pumpkin 81%, 39%, 34%, and 35%, respectively, at 14 d after planting (DAP); crop injury at the second site was 40%, 8%, 19%, and 33%, respectively. Differences in injury between the two sites may have been due to the amount and timing of rainfall after herbicides were applied. Fluridone provided 91% control of ivyleaf morningglory and 100% control of common ragweed at 28 DAP. Acetochlor EC controlled redroot pigweed 100%. Pumpkin treated with S-metolachlor produced the most yield (10,764 fruits ha–1) despite broadcasting over the planted row; labeling requires a directed application to row-middles. A separate study specifically evaluated fluridone applied PRE at 42, 84, 126, 168, 252, 336, and 672 g ai ha–1. Fluridone resulted in pumpkin injury ≥95% when applied at rates of ≥168 g ai ha–1; significant yield loss was noted when the herbicide was applied at rates >42 g ai ha–1. We concluded that fluridone and acetochlor formulations are unacceptable candidates for pumpkin production.

Transgenic crops are being developed with herbicide resistance traits to expand innovative weed management solutions for crop producers. Soybean with traits that confer resistance to the hydroxyphenylpyruvate dioxygenase herbicide isoxaflutole is under development and will provide a novel herbicide mode of action for weed management in soybean. Ten field experiments were conducted over 2 years (2017 and 2018) on five soil textures with isoxaflutole-resistant soybean to evaluate annual weed control using one- and two-pass herbicide programs. The one-pass weed control programs included isoxaflutole plus metribuzin, applied PRE, at a low rate (52.5 + 210 g ai ha−1), medium rate (79 + 316 g ai ha−1), and high rate (105 + 420 g ai ha−1); and glyphosate applied early postemergence (EPOST) or late postemergence (LPOST). The two-pass weed control programs included isoxaflutole plus metribuzin, applied PRE, followed by glyphosate applied LPOST, and glyphosate applied EPOST followed by LPOST. At 4 weeks after the LPOST application, control of common lambsquarters, pigweed species, common ragweed, and velvetleaf was variable at 25% to 69%, 49% to 86%, and 71% to 95% at the low, medium, and high rates of isoxaflutole plus metribuzin, respectively. Isoxaflutole plus metribuzin at the low, medium, and high rates controlled grass species evaluated (i.e., barnyardgrass, foxtail, crabgrass, and witchgrass) 85% to 97%, 75% to 99%, and 86% to 100%, respectively. All two-pass weed management programs provided 98% to 100% control of all species. Weed control improved as the rate of isoxaflutole plus metribuzin increased. Two-pass programs provided excellent, full-season annual grass and broadleaf weed control in isoxaflutole-resistant soybean.

A field study was conducted for the 2014 and 2015 growing season in Arkansas, Indiana, Illinois, Missouri, Ohio, and Tennessee to determine the effect of cereal rye and either oats, radish, or annual ryegrass on the control of Amaranthus spp. when integrated with comprehensive herbicide programs in glyphosate-resistant and glufosinate-resistant soybean. Amaranthus species included redroot pigweed, waterhemp, and Palmer amaranth. The two herbicide programs included were: a PRE residual herbicide followed by POST application of foliar and residual herbicide (PRE/POST); or PRE residual herbicide followed by POST application of foliar and residual herbicide, followed by another POST application of residual herbicide (PRE/POST/POST). Control was not affected by type of soybean resistance trait. At the end of the season, herbicides controlled 100 and 96% of the redroot pigweed and Palmer amaranth, respectively, versus 49 and 29% in the absence of herbicides, averaged over sites and other factors. The PRE/POST and PRE/POST/POST herbicide treatments controlled 83 and 90% of waterhemp at the end of the season, respectively, versus 14% without herbicide. Cover crop treatments affected control of waterhemp and Palmer amaranth and soybean yield, only in the absence of herbicides. The rye cover crop consistently reduced Amaranthus spp. density in the absence of herbicides compared to no cover treatment.

In laboratory experiments, we studied barnyardgrass, common lambsquarters, and redroot pigweed seed dormancy release by alternating temperatures under different soil water regimes, to determine whether that response serves as a mechanism for assuring seedling establishment. Alternating temperatures were important in breaking seed dormancy in the three weeds, but the species' response to alternating temperatures differed with soil water content. For redroot pigweed and newly dispersed seeds of barnyardgrass, dormancy breaking by alternating temperatures occurred only when soil water content was high enough for germination. Moreover, adverse incubation conditions (constant temperature and low water content) induced secondary dormancy in these seeds. Seventy percent of seeds incubated in open boxes emerged before the soil reached wilting point, when seeds were incubated at temperatures high enough for a high germination rate. A dormancy breaking mechanism that triggers a high rate of germination may be important in assuring success of seedling establishment where rainfall patterns result in wetting and drying cycles of the soil, especially in soil types that lose water rapidly from the surface layers.

Leaf tissue and leaf-tissue extracts from annual wormwood and pure artemisinin were evaluated for their effects on plant growth when incorporated into sandy loam soil. Dried leaf tissue was successively extracted with methylene chloride (MeCl2), ethanol (EtOH), and water, and the extracts and residue were reduced to dryness and stored at −20 C. Leaf tissue was incorporated in soil at rates equivalent to 0, 0.37, 0.73, or 1.1% (w/w) based on soil dry weight. Peat moss treated with extracts or artemisinin was incorporated into soil at a rate equivalent to the 0.73% (w/w) treatment. Inhibition of growth was species-specific; estimated reduction of dry weight by 0.73% (w/w) leaf tissue was 82, 49, 25, and 9% for redroot pigweed, common lambsquarters, soybean, and corn, respectively. The effects of the MeCl2 extract, which contained all of the extractable artemisinin, on germination and growth of redroot pigweed were similar to that of leaf tissue. Annual wormwood leaf tissue and MeCl2-extract treatments were the only treatments that resulted in a reduction in seedling survival. Artemisinin at levels equivalent to that contained in the MeCl2 extract and leaf-tissue treatments had significantly less effect on seedling survival, germination, and growth of redroot pigweed than the MeCl2 extract. Furthermore, the aqueous extract, which did not contain artemisinin, and the extract residue had activities similar to that of the artemisinin treatment. Thus, the allelopathic effects of annual wormwood can not be attributed to artemisinin alone.

Field experiments were conducted in 2008 and 2010 to determine crop tolerance and weed control efficacy of the POST herbicides bentazon, flumioxazin, and oxyfluorfen applied to direct-seeded dry bulb onions on organic soil. Postemergence application of oxyfluorfen at 0.071 kg ai ha−1 resulted in less than 20% onion injury when applied at the 2 and 4 onion leaf stages and provided good control of ladysthumb and common lambsquarters. Oxyfluorfen EC caused slightly higher visual injury than oxyfluorfen SC, but there was no difference in onion yield among the treatments. Application of flumioxazin at 0.036 of 0.072 kg ai ha−1 alone or in combination with pendimethalin ACS resulted in minimal onion injury and no yield reduction. Combining flumioxazin in a tank mix with pendimethalin EC, dimethenamid-P EC, or S-metolachlor EC resulted in significant onion injury and yield reduction. Flumioxazin plus S-metolachlor, dimethenamid-P, or pendimethalin improved ladysthumb control in one of two years. Bentazon applied at 0.56 kg ai ha−1 produced moderate onion injury and did not control yellow nutsedge adequately. Bentazon applied at 1.12 kg ai ha−1 provided good control of yellow nutsedge but caused serious onion injury and yield loss.

A four site-year study was conducted in North Carolina to evaluate the effects of soybean planting timing and row spacing on soil moisture, weed density, soybean lodging, and yield in a cover crop-based no-till organic soybean production system. Soybean planting timing included roll-kill/planting and roll-kill/delayed planting where soybean planting occurred either on the same day or approximately 2 wk later, respectively. Soybean row spacing included 19, 38, and 76 cm, and all treatments included a weedy check and weed-free treatment. Rye biomass production averaged above 10,000 kg ha−1 dry matter, which resulted in good weed control across all sites. Despite having good weed control throughout all treatments, weed coverage was highest in the 76-cm row-space treatment when compared to both the 19-cm and 38-cm row spacing in two of the four site-years. Soybean lodging is a potential consequence of no-till planting of soybeans in high residue mulches, and of the three row spacings, the 19-cm spacing exhibited the greatest incidence of lodging. Row spacing also influenced soybean yield; the 19- and 38-cm row spacing out yielded the 76-cm spacing by 10%. Soil volumetric water content (VWC) was higher in the cereal rye mulch treatments compared to the no rye checks. Furthermore, delaying soybean planting lowered soil water evaporation. However, the increased soil VWC in the rolled-rye treatment did not translate into increased soybean yield. The rolled-rye treatment exhibited significant (P < 0.01) increases in soil VWC when compared to the no-rye treatment at three of the four site-years. These results highlight planting date flexibility and potential risk to lodging that producers face when no-till planting organic soybeans.

Corn gluten meal (CGM) and white mustard seed meal (MSM) can release biologically active allelochemicals and have been demonstrated to be useful as PRE alternative weed control products. The objective of this study was to compare the effects of CGM and MSM on the emergence and aboveground dry weight of five broadleaf and two grass weed species. Greenhouse experiments were conducted using 26 by 53 cm plastic trays filled with a mix of field soil and potting soil (4 : 1 by wt). CGM and MSM were mixed with 1.5 kg of soil mix and applied at rates equivalent to 2,240, 4,480, and 6,720 kg ha−1. Overall, MSM was more effective than CGM for controlling weeds. Averaged over application rates and compared to the nontreated control, emergence rates were 17, 27, and 34% for kochia, common lambsquarters, and barnyardgrass, respectively, in CGM-amended soil, and 14, 13, and 6% for kochia, common lambsquarters, and barnyardgrass, respectively, in MSM-amended soil. Averaged over application rates, green foxtail and common lambsquarters aboveground dry biomass were 40 and 25% of the nontreated control, respectively, in CGM-amended soil. Green foxtail and common lambsquarters shoot biomass in MSM-amended soil was 13 and 5% of the nontreated control, respectively. Significant interactions were observed for meal by rate on redroot pigweed seedling emergence and redroot pigweed, barnyardgrass (Moscow), and annual sowthistle (Moscow) aboveground dry biomass. These interactions can be attributed to the fact that herbicidal effects were less evident in response to higher application rates using MSM compared to higher CGM application rates. Overall, this greenhouse study indicates MSM is more effective than or at least equal to CGM for broadleaf and grass weed control at the same application rate.

Nonsynthetic herbicides offer a potentially useful addition to the suite of weed management tools available to organic growers, but limited information is available to guide the optimal use of these products. The objectives of this research were to (1) evaluate the efficacy of clove oil– and vinegar-based herbicides on weeds across multiple states, and (2) assess the potential role of temperature, relative humidity (RH), and cloud cover in explaining inter-state variations in results. From 2006 to 2008, a total of 20 field trials were conducted in seven states using an identical protocol. Seeds of brown mustard were sown and herbicides applied to both mustard and emerged weeds when mustard reached the three- to four-leaf stage. Treatments included clove oil at 2.5, 5, 7.5, and 10% v/v concentrations at 54 L ha−1, and vinegar at 5, 10, 15, and 20% v/v concentrations at 107 L ha−1. Results varied widely across trials. In general, concentrations of at least 7.5% for clove oil and 15% for vinegar were needed for adequate control of mustard. Both products were more effective at suppressing mustard than Amaranthus spp. or common lambsquarters. Poor control was observed for annual grasses. No significant effects of cloud cover on the efficacy of either product were detected. In contrast, RH was positively correlated with control of brown mustard by both clove oil and vinegar with improved control at higher RH. Temperature had no detectable effect on the efficacy of clove oil, but higher temperatures improved control of brown mustard by vinegar.

Teff is a warm-season C4 annual grass crop grown for forage and food grain that has recently increased in production in parts of the United States. Hay from teff is well suited for livestock, especially horses. The objective of this study was to evaluate teff and weed response to selected herbicides in field studies conducted at the Malheur Experiment Station, Ontario, OR in 2009 and 2010. Herbicides were applied POST when teff was at the four-leaf stage. Broadleaf weed control at 21 d after treatment was greater than 91% across herbicide treatments. Only the premix of 2.5 g ai ha−1 florasulam + 99 g ae ha−1 fluroxypyr + 15 g ai ha−1 pyroxsulam provided acceptable control of barnyardgrass. Due primarily to barnyardgrass competition, teff treated with a premix of 2.5 g ha−1 florasulam + 99 g ha−1 fluroxypyr + 15 g ha−1 pyroxsulam produced 7,200 kg ha−1 of teff hay compared with 4,800 kg ha−1 of teff hay for 2,4-D and dicamba and 4,200 kg ha−1 teff hay when no herbicides were used. Teff grain production was greater with 2.5 g ha−1 florasulam + 99 g ha−1 fluroxypyr + 15 g ha−1 pyroxsulam compared with any of the other treatments. The use of a premix of florasulam + fluroxypyr + pyroxsulam would improve broadleaf and grass weed control in ‘Tiffany' and ‘Dessie' teff varieties, improve hay and grain yield, and reduce production costs.

There is an urgent need to accelerate the development and implementation of effective organic-compliant herbicides that are environmentally safe and that help the producer meet increasing consumer demand for organic products. Therefore, greenhouse experiments were conducted to evaluate the effectiveness of acetic acid (5%), acetic acid (30%), citric acid (10%), citric acid (5%) + garlic (0.2%), citric acid (10%) + garlic (0.2%), clove oil (45.6%), and corn gluten meal (CGM) compounds as natural-product herbicides for weed control. The herbicides were applied to the broadleaf weeds stranglervine, wild mustard, black nightshade, sicklepod, velvetleaf, and redroot pigweed and to narrowleaf weeds crowfootgrass, Johnsongrass, annual ryegrass, goosegrass, green foxtail, and yellow nutsedge. The herbicides were applied POST at two weed growth stages, namely, two to four and four to six true-leaf stages. CGM was applied PPI in two soil types. Citric acid (5%) + garlic (0.2%) had the greatest control (98%) of younger broadleaf weeds, followed by acetic acid (30%) > CGM > citric acid (10%) > acetic acid (5%) > citric acid (10%) + garlic (0.2%), and clove oil. Wild mustard was most sensitive to these herbicides, whereas redroot pigweed was the least sensitive. Herbicides did not control narrowleaf weeds except for acetic acid (30%) when applied early POST (EPOST) and CGM. Acetic acid (30%) was phytotoxic to all broadleaf weeds and most narrowleaf weeds when applied EPOST. Delayed application until the four- to six-leaf stage significantly reduced efficacy; acetic acid was less sensitive to growth stage than other herbicides. These results will help to determine effective natural herbicides for controlling weeds in organic farming.

Dose–response curves for acetochlor with and without timely rotary hoeing (two passes) were derived for corn (Zea mays) fields over 2 yr. The fields were dominated by green foxtail (Setaria viridis), which constituted 73 to 86% of the weed vegetation, but also contained minor populations of common lambsquarters (Chenopodium album), Pennsylvania smartweed (Polygonum pensylvanicum), and redroot pigweed (Amaranthus retroflexus). In the absence of herbicide, rotary hoeing achieved about 50% weed control. In the absence of rotary hoeing, weed control averaged > 90% at the full label rate of acetochlor (3 kg ai/ha on clay loam soil). With two timely rotary hoeings, however, this same level of control was achieved with only 1 kg/ha acetochlor. Given the suite of weed species present in these experiments, timely rotary hoeing substituted for 67% of the label rate of acetochlor. Timeliness of rotary hoeing operations also provided consistency of results from one year to the next.

Waterhemp has emerged as one of the most problematic weeds in agronomic crops in the Midwest because of an extended germination period and widespread occurrence of biotypes resistant to atrazine and sulfonylurea herbicides. However, the competitive effects of late-emerging cohorts on corn yield are not known. Field studies were conducted in 2001 and 2002 at Columbia, Novelty, and Albany, MO, to determine the effects of late-emerging waterhemp interference on corn growth, nitrogen (N) accumulation, and yield. Waterhemp emerged approximately 20 d after planting (DAP) and was treated at heights of 8, 15, 23, 31, 38, or 46 cm with directed applications of dicamba + diflufenzopyr followed by hand hoeing. Soil water status, corn leaf chlorophyll content, and corn and common waterhemp height were recorded at the time of waterhemp removal. N stress was detected with a chlorophyll meter at four of six removal timings at high waterhemp densities (362 or more plants/m2) but only at one of six removal timings at lower densities (82 or less plants/m2). Water stress was observed at five of the six removal timings at high densities but at none of the removal timings at low densities. High waterhemp densities reduced corn yield when allowed to reach 15 cm before removal, and yields were reduced 36% when not controlled. At low densities, yield losses did not occur unless waterhemp was allowed to remain with corn season long. Our research suggests that waterhemp is less competitive with corn than redroot pigweed, smooth pigweed, and Palmer amaranth. In addition, low densities of late-emerging waterhemp would not warrant removal to protect corn yield.

Propane flaming could be an effective alternative tool for weed control in organic cropping systems. However, response of major weeds to broadcast flaming must be determined to optimize its proper use. Therefore, field experiments were conducted at the Haskell Agricultural Laboratory, Concord, NE in 2007 and 2008 using six propane doses and four weed species, including green foxtail, yellow foxtail, redroot pigweed, and common waterhemp. Our objective was to describe dose–response curves for weed control with propane. Propane flaming response was evaluated at three different growth stages for each weed species. The propane doses were 0, 12, 31, 50, 68, and 87 kg ha−1. Flaming treatments were applied utilizing a custom-built flamer mounted on a four-wheeler (all-terrain vehicle) moving at a constant speed of 6.4 km h−1. The response of the weed species to propane flaming was evaluated in terms of visual ratings of weed control and dry matter recorded at 14 d after treatment. Weed species response to propane doses were described by log-logistic models relating propane dose to visual ratings or plant dry matter. Overall, response of the weed species to propane flaming varied among species, growth stages, and propane dose. In general, foxtail species were more tolerant than pigweed species. For example, about 85 and 86 kg ha−1 were the calculated doses needed for 90% dry matter reduction in five-leaf green foxtail and four-leaf yellow foxtail compared with significantly lower doses of 68 and 46 kg ha−1 of propane for five-leaf redroot pigweed and common waterhemp, respectively. About 90% dry matter reduction in pigweed species was achieved with propane dose ranging from 40 to 80 kg ha−1, depending on the growth stage when flaming was conducted. A similar dose of 40 to 60 kg ha−1 provided 80% reduction in dry matter for both foxtail species when flaming was done at their vegetative growth stage. However, none of the doses we tested could provide 90% dry matter reduction in foxtail species at flowering stage. It is important to note that foxtail species started regrowing 2 to 3 wk after flaming. Broadcast flaming has potential for control or suppression of weeds in organic farming.

Weed management can be difficult and expensive in organic agricultural systems. Because of the potentially high cost of the natural product herbicides vinegar and clove oil, their efficacy with regard to weed species growth stages needs to be determined. A further objective was to identify anatomical and morphological features of redroot pigweed and velvetleaf that influence the effectiveness of vinegar and clove oil. Research was conducted on greenhouse-grown cotyledon, two-leaf, and four-leaf redroot pigweed and velvetleaf. Dose–response treatments for vinegar included 150-, 200-, 250-, and 300-grain vinegar at 318 L/ha and at 636 L/ha. Clove oil treatments included 1.7, 3.4, 5.1, and 6.8% (v/v) dilutions of a clove oil product in water (318 L/ha), and a 1.7% (v/v) dilution in 200-grain vinegar (318 L/ha). An untreated control was included. Separate plantings of velvetleaf and pigweed were treated with vinegar or clove oil and were used to study anatomical and morphological differences between the two species. Redroot pigweed was easier to control with both products than velvetleaf. Whereas 200-grain vinegar applied at 636 L/ha provided 100% control (6 d after treatment [DAT]) and mortality (9 DAT) of two-leaf redroot pigweed, this same treatment on two-leaf velvetleaf provided only 73% control and 18% mortality. The obtuse leaf blade angle in velvetleaf moved product away from the shoot tip, whereas in pigweed, the acute leaf blade angle, deep central leaf vein, and groove on the upper side of the leaf petiole facilitated product movement toward the stem axis and shoot tip. For both species, and at all application timings, 150-grain vinegar at 636 L/ha provided control equal to that of 300-grain vinegar at 318 L/ha. As growth stage advanced, control and biomass reduction decreased and survival increased. Application timing will be critical to maximizing weed control with vinegar and clove oil.