In recent years, glyphosate-resistant (GR) giant ragweed has become an increasingly problematic weed in soybean in Missouri and throughout many areas of the Midwest. Field trials were conducted in 2010 and 2011 to determine the influence of various application timings and glyphosate tank-mix combinations on the survival of GR giant ragweed in soybean. Glyphosate was applied alone and in combination with selected tank-mix combinations early POST (EPOST) to 10-, 20-, or 30-cm GR giant ragweed. Treatments received late POST (LPOST) applications approximately 3 wk after EPOST applications, once GR giant ragweed resumed growth. GR giant ragweed survival 2 wk after EPOST applications (2 WAEPOST) ranged from 60 to 100%. Glyphosate plus fomesafen applications to 10-cm plants resulted in 60% survival of GR giant ragweed, which was the lowest survival observed across all treatments and application timings. However, GR giant ragweed survival ranged from 37 to 98% 4 wk after LPOST applications (4 WALPOST). Glyphosate plus fomesafen applications to 10-cm plants resulted in 37% survival of GR giant ragweed 4 WALPOST, which was the lowest survival observed across all treatments and application timings. Few differences in soybean yields were observed between herbicide treatments within application timings in either year, and all herbicide treatments resulted in yields higher than the nontreated control. In 2011, soybean yields were reduced following glyphosate tank-mix combinations to larger plants. Overall, higher soybean injury and GR giant ragweed survival was observed in response to glyphosate tank-mix combinations made to taller plants. Results from this research indicate that additional methods of GR giant ragweed control, other than POST-only glyphosate tank-mix combinations, will be needed for effective management of GR giant ragweed in GR soybean.

Three field experiments were conducted in 2010 and 2012 in a soybean production field near West Memphis, AR, containing glyphosate-resistant johnsongrass. The goal of this research was to develop effective herbicide programs for glyphosate-resistant johnsongrass in glufosinate-resistant soybean. Control of the resistant johnsongrass was greater with glufosinate at 590 and 740 g ai ha−1 than at 450 g ha−1. Sequential glufosinate applications were more effective than a single application, irrespective of rate. A PRE application of flumioxazin at 71 g ai ha−1 immediately after planting provided no more than 26% johnsongrass control 6 wk after soybean emergence (WAE). The addition of clethodim at 136 g ai ha−1 to sequential applications of glufosinate at 450 g ha−1 improved control over sequentially applied glufosinate alone. Herbicide programs containing imazethapyr or imazamox in combination with glufosinate followed by clethodim plus glufosinate controlled johnsongrass at least 94% at 10 WAE and provided three distinct mechanisms of action, a highly effective resistance management strategy. Results from this research indicate that a high level of glyphosate-resistant johnsongrass control can be achieved through the use of several herbicide options in glufosinate-resistant soybean.

Crop Protection Online (CPO) is a decision support system, which integrates decision algorithms quantifying the requirement for weed control and a herbicide dose model. CPO was designed to be used by advisors and farmers to optimize the choice of herbicide and dose. The recommendations from CPO for herbicide application in spring barley in Denmark were validated through field experiments targeting three levels of weed control requirement. Satisfactory weed control levels at harvest were achieved by a medium control level requirement generating substantial herbicide reductions (∼ 60% measured as the Treatment Frequency Index (TFI)) compared to a high level of required weed control. The observations indicated that the current level of weed control required is robust for a range of weed scenarios. Weed plant numbers 3 wk after spraying indicated that the growth of the weed species were inhibited by the applied doses, but not necessarily killed, and that an adequate level of control was reached later in the season through crop competition.

Palmer amaranth is a difficult-to-control broadleaf weed that infests corn and soybean fields in south-central and southwestern Nebraska and several other states in the United States. The objectives of this research were to confirm triazine and 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide-resistant Palmer amaranth in Nebraska and to determine sensitivity and efficacy of POST-applied corn herbicides for control of resistant and susceptible Palmer amaranth biotypes. Seeds from a putative HPPD-resistant Palmer amaranth biotype from Fillmore County, NE were collected from a seed corn production field in fall 2010. The response of Palmer amaranth biotypes to 12 rates (0 to 12×) of mesotrione, tembotrione, topramezone, and atrazine was evaluated in a dose–response bioassay in a greenhouse. On the basis of the values at the 90% effective dose (ED90) level, the analysis showed a 4- to 23-fold resistance depending upon the type of HPPD-inhibiting herbicide being investigated and susceptible biotype used for comparison. This biotype also had a 9- to 14-fold level of resistance to atrazine applied POST. Results of a POST-applied herbicide efficacy study suggested a synergistic interaction between atrazine and HPPD-inhibiting herbicides that resulted in > 90% control of all Palmer amaranth biotypes. The resistant biotype had a reduced sensitivity to acetolactate synthase inhibiting herbicides (halosulfuron and primisulfuron), a photosystem-II inhibitor (bromoxynil), and a protoporphyrinogen oxidase inhibitor (fluthiacet-methyl). Palmer amaranth biotypes were effectively controlled (≥ 90%) with glyphosate, glufosinate, and dicamba, whereas 2,4-D ester provided 81 to 83% control of the resistant biotype and > 90% control of both susceptible biotypes.

The goal of this research was to develop herbicide programs for controlling acetolactate synthase (ALS)–, propanil-, quinclorac-, and clomazone-resistant barnyardgrass. Two applications of imazethapyr alone at 70 g ai ha−1 failed to control the ALS-resistant biotype more than 36%; however, when imazethapyr at 70 g ha−1 was applied early POST (EPOST) followed by imazethapyr at 70 g ha−1 plus fenoxaprop at 120 g ai ha−1 immediately prior to flooding (PREFLD), barnyardgrass control improved to 78% at 10 wk after planting. When imazethapyr was applied twice following PRE or delayed PRE applications of clomazone at 336 g ai ha−1, quinclorac at 560 g ai ha−1, pendimethalin at 1,120 g ai ha−1, or thiobencarb at 4,480 g ai ha−1 control was 92 to 100%. A single-pass program consisting of a delayed PRE application of clomazone at 336 g ha−1 plus quinclorac at 560 g ha−1 plus pendimethalin at 1,120 g ha−1 plus thiobencarb at 4,480 g ha−1 controlled all herbicide-resistant barnyardgrass biotypes at the same level as a standard multiple application program.

Sedges are problematic weeds that reduce quality of turfgrass sod, and herbicides may be needed for control prior to harvesting. The objective of this research was to evaluate application timing of halosulfuron and sulfentrazone on sod quality, tensile strength, and postharvest rooting of four warm-season turfgrasses. Bermudagrass injury from herbicides was minimal (< 10%), but injury to centipedegrass, St. Augustinegrass, and zoysiagrass was detected before harvesting and generally increased with sulfentrazone rate from 0.21 to 0.84 kg ai ha−1. Sod tensile strength was not reduced from the nontreated for bermudagrass and centipedegrass treated with herbicides, but tensile strength was reduced 15 and 22% following herbicide applications 1 wk before harvesting (WBH) St. Augustinegrass and zoysiagrass, respectively. Sulfentrazone at 0.84 kg ha−1 reduced zoysiagrass tensile strength 20% from the nontreated, but other treatments did not reduce measurements. Turfgrass injury from halosulfuron was negligible on all species, and treatments did not affect sod tensile strength of the four species. Treatments did not affect root mass of any species at 4 wk after sod transplanting. Sod quality after transplanting was reduced from the nontreated on several dates for centipedegrass and St. Augustinegrass when treated with sulfentrazone 1 and 2 WBH, and zoysiagrass quality was reduced from treatments 4 WBH. Bermudagrass quality after transplanting was not reduced from the nontreated by any herbicides. Results suggest sod harvesting should be delayed 1, > 2, > 2, and ≥ 4 weeks after sulfentrazone applications at labeled rates for bermudagrass, centipedegrass, St. Augustinegrass, and zoysiagrass, respectively. Sod harvesting should be delayed 1, 2, ≥ 4, and 2 wk after halosulfuron treatments at 0.07 kg ai ha−1 for bermudagrass, centipedegrass, St. Augustinegrass, and zoysiagrass, respectively.

Glyphosate-resistant (GR) Palmer amaranth is a widespread problem in southeastern cotton production areas. Herbicide programs to control this weed in no-till cotton commonly include flumioxazin applied with preplant burndown herbicides approximately 3 wk before planting followed by fomesafen applied PRE and then glufosinate or glyphosate applied POST. Flumioxazin and fomesafen are both protoporphyrinogen oxidase (PPO) inhibitors. Multiple yearly applications of PPO inhibitors in cotton, along with widespread use of PPO inhibitors in rotational crops, raise concerns over possible selection for PPO resistance in Palmer amaranth. An experiment was conducted to determine the potential to substitute diuron for one of the PPO inhibitors in no-till cotton. Palmer amaranth control by diuron and fomesafen applied PRE varied by location, but fomesafen was generally more effective. Control by both herbicides was inadequate when timely rainfall was not received for activation. Palmer amaranth control was more consistent when programs included a preplant residual herbicide. Applied preplant, flumioxazin was more effective than diuron. Programs with diuron preplant followed by fomesafen PRE were as effective as flumioxazin preplant followed by fomesafen only if fomesafen was activated in a timely manner. Programs with flumioxazin preplant followed by diuron PRE were as effective as flumioxazin preplant followed by fomesafen PRE at all locations, regardless of timely activation of the PRE herbicide. As opposed to flumioxazin preplant followed by fomesafen PRE, which exposes Palmer amaranth to two PPO-inhibiting herbicides, one could reduce selection pressure by using flumioxazin preplant followed by diuron PRE without sacrificing Palmer amaranth control or cotton yield.

Pyroxasulfone dissipation and mobility in the soil was evaluated and compared to S-metolachlor in 2009 and 2010 at two field sites in northern Colorado, on a Nunn fine clay loam, and Olney fine sandy loam soil. Pyroxasulfone dissipation half-life (DT50) values varied from 47 to 134 d, and those of S-metolachlor ranged from 39 to 63 d. Between years, herbicide DT50 values were similar under the Nunn fine clay loam soil. Under the Olney fine sandy loam soil, dissipation in 2009 was minimal under dry soil conditions. In 2010, under the Olney fine sandy loam soil, S-metolachlor and pyroxasulfone had half-lives of 39 and 47 d, respectively, but dissipation rates appeared to be influenced by movement of herbicides below 30 cm. Herbicide mobility was dependent on site-year conditions, in all site-years pyroxasulfone moved further downward in the soil profile compared to S-metolachlor.

Understanding the persistence of PRE-applied pendimethalin is important in determining timing of subsequent weed management programs in sugarcane on organic soils in the Everglades Agricultural Area (EAA). Dissipation of oil- and water-based pendimethalin formulations applied PRE at 2, 4, and 8 kg ai ha−1 were compared in 2011 and 2012 on organic soils in the EAA. The rate of dissipation of both formulations was very similar. Both formulations had an initial rapid rate of dissipation followed by a slower rate of dissipation. However, the initial amount of pendimethalin in the soil was higher with the water-based compared to the oil-based formulation, most likely because of the lower volatility of the water-based formulation. The half-lives (DT50s) of the oil-based formulation were 32, 18, and 10 d and 8, 8, and 12 d at 2, 4, and 8 kg ha−1, respectively, in 2011 and 2012, respectively. The DT50s of the water-based formulation were 20, 13, and 10 d and 12, 12, and 14 d at 2, 4, and 8 kg ha−1, respectively in 2011 and 2012, respectively. These DT50 values were attributed to low soil water content as well as the absence of incorporation following application. Our results suggest that dissipation of pendimethalin is rapid on organic soils irrespective of the formulation when applied under dry soil conditions with no incorporation into the soil.

Herbicide-resistant barnyardgrass has become widespread in the rice production systems of the midsouthern United States, leaving few effective herbicide options for controlling this weed. The acetolactate synthase (ALS)- and acetyl-CoA carboxylase (ACCase)-inhibiting herbicides remain largely effective in Clearfield® rice production, but strategies need to be developed to protect the long-term utility of these options. A two-trait model was developed to understand simultaneous evolution of resistance in barnyardgrass to the ALS- and ACCase-inhibiting herbicides in Clearfield rice. The model was used to predict resistance under a number of common weed management scenarios across 1,000 hypothetical rice fields in the Mississippi Delta region and answer some key management questions. Under an ALS inhibitor–only program consisting of three annual applications of imidazolinone herbicides (imazethapyr or imazamox) in continuous Clearfield rice, resistance was predicted within 4 yr with 80% risk by year 30. Weed management programs that consisted of ALS- and ACCase-inhibiting herbicides such as fenoxaprop and cyhalofop greatly reduced the risk of ALS-inhibiting herbicide resistance (12% risk by year 30), but there was a considerable risk for ACCase resistance (evolving by year 14 with 13% risk by year 30) and multiple resistance (evolving by year 16 with 11% risk by year 30) to both of these mechanisms of action. A unique insight was that failure to stop using a herbicide soon after resistance evolution can accelerate resistance to the subsequent herbicide option. Further, a strong emphasis on minimizing seedbank size is vital for any successful weed management strategy. Results also demonstrated that diversifying management options is not just adequate, but diversity combined with timely herbicide applications aimed at achieving high efficacy levels possible is imperative.

WideStrike® Acala cotton is a two-gene, in-plant trait that provides broad-spectrum and season-long control of lepidopteran insect pests, and the varieties available in California also have resistance to glyphosate. There have been indications that WideStrike cotton has some glufosinate tolerance as well, so the level of tolerance to glufosinate needed to be ascertained. A 2-yr (2008 and 2009) study was conducted in California to evaluate the potential crop injury caused by three different rates (0.59, 0.88, and 1.76 kg ai ha−1) of glufosinate–ammonium at four different growth stages (cotyledon, 2-node, 5- to 6-node, and 18- to 19-node stages) of WideStrike Acala cotton. The effects of these treatments on the cotton plants and yield were closely monitored. Glyphosate at 1.54 kg ae ha−1 was applied at all cotton growth stages as a standard application, and a nontreated control was included. The greatest level of injury (58%) was observed with the highest rate of glufosinate applied at both the cotyledon and the two-node stage of cotton. However, injury was less than 10% following glufosinate at 0.59 kg ha−1 applied at the 18- to 19-node stage. The level of injury increased with the higher application rate of glufosinate at all crop growth stages. In 2008 and 2009, the glufosinate treatments had no effect on cotton lint yield. Therefore, the study showed that glufosinate can be applied safely topically at 0.59 kg ha−1 at the cotyledon- to 2-node stage or as POST-directed spray between the 5- to 19-node stages. Although injury occurred at this rate, the plants recovered within 2 to 3 wk of the treatment. Increasing glufosinate rates beyond 0.59 kg ha−1 can increase the possibility of greater crop injury.

In 2008, a population of johnsongrass collected from a soybean field near West Memphis, AR, in Crittenden County was confirmed resistant to glyphosate. This was the first documented case of glyphosate-resistant johnsongrass in Arkansas. The purpose of this study was to determine the geographical distribution of glyphosate-resistant johnsongrass in Arkansas crops and screen for resistance to additional herbicides. A total of 141 johnsongrass accessions were collected from 14 counties in Arkansas in the fall of 2008, 2009, and 2010 and screened for resistance to four of the most commonly used POST herbicides for johnsongrass control—imazethapyr, glyphosate, clethodim, and fluazifop. One accession potentially resistant to glyphosate (J12) and another with apparent resistance to imazethapyr (J14) were further evaluated in a dose–response experiment. The lethal dose required to kill 50% of the plants from the putative glyphosate-resistant and imazethapyr-resistant accessions was higher than that of a biotype known to be susceptible to these herbicides. The J12 accession had an LD50 of 1,741 g ae ha−1 glyphosate, which was 8.5-fold greater than the susceptible biotype. The J14 accession had an LD50 of 73 g ai ha−1 imazethapyr, which was 3.7-fold greater than the LD50 of the susceptible biotype. All other accessions were effectively controlled by the four evaluated herbicides. Widespread herbicide-resistant johnsongrass was not found in Arkansas, although accession J12 was resistant to glyphosate and J14 resistant to imazethapyr.

Herbicide-resistant kochia is an increasing concern for growers in the northwestern United States. Four suspected glyphosate-resistant (Gly-R) kochia accessions (referred to as GIL01, JOP01, CHES01, and CHES02) collected in fall 2012 from four different chemical-fallow fields in northern Montana were evaluated. The objectives were to confirm and characterize the level of glyphosate resistance in kochia accessions relative to a glyphosate-susceptible (Gly-S) accession and evaluate the effectiveness of various POST herbicides for Gly-R kochia control. Whole-plant dose–response experiments indicated that the four Gly-R kochia accessions had 7.1- to 11-fold levels of resistance relative to the Gly-S accession on the basis of percent control ratings (I50 values). On the basis of shoot dry weight response (GR50 values), the four Gly-R kochia accessions exhibited resistance index (R/S) ratios ranging from 4.6 to 8.1. In a separate study, the two tested Gly-R accessions (GIL01 and JOP01) showed differential response (control and shoot dry weight reduction) to various POST herbicides 21 d after application (DAA). Paraquat, paraquat + linuron, carfentrazone + 2,4-D, saflufenacil alone or with 2,4-D, and bromoxynil + fluroxypyr effectively controlled (99 to 100%) and reduced shoot dry weight (88 to 92%) of the GIL01 accession, consistent with the Gly-S kochia accession; however, bromoxynil + MCPA and bromoxynil + pyrasulfotole provided 76% control and 83% shoot dry weight reduction of the GIL01 accession and were lower compared with the Gly-S accession. The JOP01 accession exhibited lower control or shoot dry weight reduction to all herbicides tested, except dicamba, diflufenzopyr + dicamba + 2,4-D, paraquat + linuron, and bromoxynil + pyrasulfotole, compared with the Gly-S or GIL01 population. Furthermore, paraquat + linuron was the only treatment with ≥ 90% control and shoot dry weight reduction of the JOP01 kochia plants. Among all POST herbicides tested, glufosinate was the least effective on kochia. This research confirms the first evolution of Gly-R kochia in Montana. Future research will investigate the mechanism of glyphosate resistance, inheritance, ecological fitness, and alternative strategies for management of Gly-R kochia.

Field experiments were conducted across two locations during 2011 and 2012 to evaluate herbicide options for the control of glyphosate-resistant (GR) giant ragweed and GR waterhemp in dicamba-resistant (DR) soybean. All herbicide treatments provided 91 to 100% control of GR giant ragweed 3 wk after treatment (WAT). Flumioxazin plus dicamba plus glyphosate applied preplant provided greater control and density reduction of GR giant ragweed than flumioxazin plus 2,4-D plus glyphosate. When flumioxazin plus dicamba plus glyphosate were applied preplant, the addition of dicamba to glyphosate at either the early-postemergence (EPOST) or mid-postemergence (MPOST) timing provided greater control and density reduction of GR giant ragweed than glyphosate alone. Regardless of the preplant treatment, delay of EPOST dicamba to the MPOST timing did not influence GR giant ragweed control or density reduction. In the GR waterhemp experiment, dicamba plus glyphosate applied sequentially provided 88 to 89% control and 90% density reduction at the EPOST and MPOST timings compared to only 24% control and 42% density reduction in response to glyphosate applied sequentially. Control and GR waterhemp density reduction did not improve with the addition of acetochlor to either the EPOST or late-postemergence (LPOST) timings. Flumioxazin plus chlorimuron applied PRE followed by dicamba plus glyphosate or dicamba plus glyphosate plus acetochlor provided greater control of GR waterhemp than glyphosate plus fomesafen or glyphosate alone applied EPOST. Results from this research indicate that dicamba applied once or sequentially and when timed appropriately to match the biology of the weed species can be utilized as a component of an integrated program for the management of GR weeds like giant ragweed and waterhemp in DR soybean.

Field experiments were conducted in 2009 and 2012 at Fayetteville, AR, to compare four herbicides across three application timings for johnsongrass control and to evaluate the effect of late-season herbicide applications on johnsongrass control, fecundity, seed viability, and progeny emergence. In the first experiment, glyphosate at 840 g ae ha−1 and clethodim at 68 and 136 g ai ha−1 provided 75 to 94% control of 15- to 60-cm tall johnsongrass and up to 98% stand reduction 4 wk after treatment (WAT). Glufosinate and nicosulfuron were generally effective on ≤30-cm tall johnsongrass; however, efficacy declined as johnsongrass size increased. In the second experiment, glyphosate at 840 g ha−1 provided at least 89% johnsongrass control when applied at boot or after panicle emergence and up to 95% stand reduction 3 WAT. Applications of glyphosate at 420 and 840 g ha−1, clethodim at 68 and 136 g ha−1, and glufosinate at 740 g ai ha−1 at the boot stage reduced viable seed production of treated plants 94 to 99%. Irrespective of rate, glyphosate and clethodim applied at boot stage reduced progeny emergence by 77 to 95% and 100-seed weight of treated plants 62 to 96% compared to the nontreated check. This research demonstrates the importance of herbicide selection, particularly for controlling johnsongrass plants larger than 30 cm. Additionally, the results demonstrate the benefits of a single application of glyphosate or clethodim at the boot stage of johnsongrass for decreasing the soil seedbank and reducing the success of johnsongrass progeny in future years.

Kochia is a troublesome weed in the western Great Plains and many accessions have evolved resistance to one or more herbicides. Dicamba-resistant soybean is being developed to provide an additional herbicide mechanism of action for POST weed control in soybean. The objective of this study was to evaluate variation in response to dicamba among kochia accessions collected from across Nebraska. Kochia plants were grown in a greenhouse and treated when they were 8 to 12 cm tall. A discriminating experiment with a single dose of 420 g ae ha−1 of dicamba was conducted on 67 accessions collected in Nebraska in 2010. Visual injury estimates were recorded at 21 d after treatment (DAT) and accessions were ranked from least to most susceptible. Four accessions representing two of the most and least susceptible accessions from this screening were subjected to dose-response experiments using dicamba. At 28 DAT, visible injury estimates were made and plants were harvested to determine dry weight. An 18-fold difference in dicamba dose was necessary to achieve 90% injury (I90) between the least (accession 11) and most susceptible accessions. Approximately 3,500 g ha−1 of dicamba was required in accession 11 to reach a 50% dry weight reduction (GR50). There was less than twofold variation among the three more susceptible accessions for both the I90 and GR90 parameters, suggesting that most kochia accessions will be similarly susceptible to dicamba. At 110 DAT, accession 11 had plants that survived doses of 35,840 g ha−1, and produced seed at doses of 17,420 g ha−1. The identification of one resistant accession among the 67 accessions screened, and the fact that dicamba doses greater than 560 g ha−1 were required to achieve GR80 for all accessions suggest that repeated use of dicamba for weed control in fields where kochia is present may quickly result in the evolution of dicamba-resistant kochia populations.

Field studies were conducted at Clinton, NC (2009, 2010), and Kinston, NC (2010), to determine ‘Covington' sweetpotato tolerance to flumioxazin applied after transplanting. Flumioxazin was directed to 25% of the sweetpotato vine beginning at the distal end (shoot tip), 25% of the vine beginning at the proximal end (crown), or to the entire vine (over-the-top) and was applied at 2 or 5 wk after transplanting (WAP). Applications made at 2 WAP resulted in 10 to 16% foliar necrosis at 3 WAP. Necrosis was transient and ≤ 2% by 6 WAP. Stunting injury at 6 WAP for flumioxazin applied at 2 WAP was greatest (12%) with the over-the-top application, followed by crown (5%), and shoot tip (1%) applications. Applications made at 5 WAP resulted in 35, 23, and 15% foliar necrosis at 6 WAP for over-the-top, crown, and shoot tip applications, respectively. By 12 WAP, stunting injury for all treatments was ≤ 3%. No. 1, jumbo, canner, and total marketable sweetpotato yield of the nontreated check was 36,670; 7,610; 7,170; and 51,450 kg ha−1, respectively. No. 1 and total marketable sweetpotato yields were reduced when flumioxazin was applied at 2 or 5 WAP. No. 1 sweetpotato yield was reduced when flumioxazin was applied to the crown or over-the-top (27,240 and 28,330 kg ha−1, respectively). Sweetpotato receiving flumioxazin applied to the shoot tip had similar no. 1 (31,770 kg ha−1) yields as the nontreated check, crown, and over-the-top applications. Total marketable sweetpotato yield was reduced by flumioxazin application to shoot tip, crown, and over-the-top (45,350; 40,100; 40,370 kg ha−1, respectively). Neither flumioxazin application timing nor placement influenced either jumbo- or canner-grade sweetpotato yields. Currently, after-transplant applications of flumioxazin do not appear to be a suitable fit for POST weed control in North Carolina sweetpotato production systems.

Small burnet is a hardy, relatively long-lived evergreen forb with the potential to improve grazing lands, particularly to extend grazing into late fall and winter. Small burnet was evaluated for tolerance to spring and fall POST applications of aminopyralid, bromoxynil, clethodim, clopyralid, dicamba, dimethenamid-P, imazamox, metribuzin, pendimethalin, quinclorac, and 2,4-DB. Injury, seed yield, seed viability, and dry matter yield (DMY) were measured in the spring following application. Injury was observed in response to all spring and fall herbicide applications. Injury from aminopyralid was the highest for both spring and fall applications at 24 and 79%, respectively. Fall application of imazamox and dicamba resulted in 57 and 31% injury, respectively. Spring-applied aminopyralid and 2,4-DB both reduced DMY by 16%, whereas fall applications of imazamox, dicamba, and aminopyralid reduced DMY by 36, 12, and 67%, respectively. Fall applications of imazamox and aminopyralid reduced seed yield by 33 and 65%, respectively. Fall-applied aminopyralid reduced seed germination by 43%. None of the spring-applied herbicides affected seed yield or seed germination. Small burnet is severely injured by aminopyralid and to a lesser degree by imazamox and dicamba. Bromoxynil, clethodim, clopyralid, dimethenamid-P, metribuzin, pendimethalin, and quinclorac did not affect small burnet DMY, seed yield, or germination the year after application.

A commercial intrarow rotating cultivator was tested for weed removal and impact on hand-weeding times in bok choy, celery, lettuce, and radicchio. The rotating cultivator was tested as an automated crop thinner and weeder in direct-seeded bok choy and lettuce as an alternative to hand-thinning and -weeding. The rotating cultivator utilized machine-vision guidance to align a rotating disk with the crop plant to be saved and to remove weeds and undesired crop plants. The rotating cultivator was compared to a standard interrow cultivator, which could not remove weeds from the plant line. Main plots were cultivator type, rotating, or standard, and subplots were herbicides: pronamide for lettuce or prometryn for celery. Weed densities, hand-weeding times, crop stand, and yields were monitored. Economic analysis was performed on a subset of the data. The intrarow rotating cultivator was generally more effective than the standard interrow cultivator for reducing weed densities and hand-weeding times. However, the rotating cultivator reduced seeded lettuce stands by 22 to 28% when compared to hand-thinning and standard cultivation, resulting in lower yields and net returns. In transplanted celery, lettuce, and radicchio, the rotating cultivator removed more weeds than the standard cultivator, and reduced stands by just 6 to 9% when compared to the standard cultivator. In transplanted lettuce, the rotating cultivator was more precise and did less damage to the crop. Because transplanted crops were larger than the weeds, they were more easily differentiated using this technology. Net returns were therefore similar between the two cultivators. What is needed for celery and leafy vegetables is an effective intrarow weed removal system that reduces or eliminates the need for hand-weeding yet does not reduce yields. The rotating cultivator was developed for transplanted crops, where it performs adequately, but it cannot be recommended in the seeded crops evaluated.

Field experiments were conducted in 2010 and 2011 at two locations in Michigan to determine the effects of nitrogen and weed removal on glyphosate-resistant sugarbeet yield and quality. Nitrogen rates were 0, 67, 100, 134, and 67 : 67 kg N ha−1, and weeds were removed when they were < 2, 8, 15, and 30 cm tall. At the beginning of the growing season, weeds responded to N sooner than sugarbeet. Nitrogen assimilation by weeds was three times greater than sugarbeet at 0, 67, 100, and 134 kg N ha−1 and four times greater than sugarbeet with the split application of N (67 : 67 kg N ha−1) averaged over the weed removal timings. Higher N rates increased N sufficiency index values and sugarbeet canopy closure; weeds 30 cm tall had lower N sufficiency index values and a smaller sugarbeet canopy. The effect of N on root yields varied, but the highest N rates (134 kg N ha−1 or 67 : 67 kg N ha−1) were among the highest sugarbeet yields at all locations. Highest yields were achieved when weeds were controlled before reaching 2 cm tall at three of the four site-years. Delaying weed control until weeds were 8 or 15 cm tall reduced yield by 15%, whereas 30-cm-tall weeds reduced yield up to 21%. Recoverable white sucrose ha−1 (RWSH) also was reduced by 8 to 16% if weeds were 8 cm tall. These results indicate that weeds are highly competitive with sugarbeet and can assimilate large quantities of N early in the growing season, especially at larger growth stages. However, it appears that sugarbeets were able to scavenge sufficient N at the N rates used in this study to overcome N removal effects from larger weeds, resulting in no interaction between N rate and weed removal timing for sugarbeet root yield, quality, or RWSH.