Effect of herbicide and cover crop on weed control in no-tillage jack-o-lantern pumpkin (Cucurbita pepo L.) production

Field experiments were conducted to evaluate cover crop (none, winter wheat, or winter rye) and pre-emergence (PRE) applications of clomazone plus ethalfluralin alone or with halosulfuron [PRE or post-emergence (POST)] for smooth crabgrass [Digitaria ischaemum (Schreb. ex Schweig) Schreb. ex Muhl.] and redroot pigweed (Amaranthus retroflexus L.) control in no-tillage ‘Aspen’ jack-o-lantern pumpkin (Cucurbita pepo L.). At pumpkin harvest, cover crops had reduced smooth crabgrass density, but not redroot pigweed. Although PRE or POST applications of halosulfuron alone were more effective at reducing redroot pigweed density than clomazone plus ethalfluralin PRE, the combination of halosulfuron plus clomazone and ethalfluralin PRE reduced redroot pigweed density to the greatest extent. Pumpkin yields were not affected by cover crop, although average pumpkin sizes were greater with the inclusion of cover crop residues. Pumpkin fruit sizes and yields were the greatest with clomazone and ethalfluralin PRE in combination with halosulfuron applied PRE or POST. Overall, cover crop had relatively little influence on pumpkin yields compared with herbicide treatments. The addition of halosulfuron to clomazone and ethalfluralin provided greater broadleaf weed control resulting in greater jack-o-lantern pumpkin yields.     

Herbicide options for effective weed management in dry direct-seeded rice under scented rice-wheat rotation of western Indo-Gangetic Plains

Farmers' participatory field trials were conducted at Madhuban, and Taraori, the two participatory experimental sites/locations of the Cereal Systems Initiative for South Asia (CSISA), a collaborative project of IRRI and CIMMYT in Karnal district of Haryana, India, during Kharif (wet season) 2010 and 2011. This research aimed to evaluate preemergence (PRE) and postemergence (POST) herbicides for providing feasible and economically viable weed management options to farmers for predominant scented rice varieties. Treatments with pendimethalin PRE fb bispyribac-sodium + azimsulfuron POST had lower weed biomass at 45 days after sowing (DAS). At Madhuban, highest grain yield of scented basmati rice (3.43 t ha⁻¹) was recorded with the sequential application of pendimethalin PRE fb bispyribac-sodium + azimsulfuron POST. However, at Taraori, yields were similar with pendimethalin or oxadiargyl PRE fb bispyribac-sodium and/or azimsulfuron POST. Applying oxadiargyl by mixing with sand onto flooded field was less effective than spray applications in non-flooded field. The benefit-cost ratio of rice crop was higher with herbicide treatments at both sites as compared with the non-treated weed-free check except single PRE and POST applications and sequential application of oxadiargyl PRE fb oxadiargyl PRE. In a separate experiment conducted at Nagla and Taraori sites, scented rice cultivars' ('CSR 30′ and 'Pusa 1121′) tolerance to three rates of azimsulfuron (15, 25, and 35 g ai ha⁻¹) was evaluated over two years (2010 and 2011). CSR 30 (superfine, scented) was more sensitive to higher rates (35 g ai ha⁻¹) of azimsulfuron as compared to Pusa 1121 (fine, scented). Crop injuries were 8 and 28% in case of CSR 30; 5 and 15% in Pusa 1121 when applied with azimsulfuron 25 and 35 g ai ha⁻¹, respectively. Azimsulfuron applied at 35 g ai ha⁻¹ reduced yield in both cultivars but in CSR 30 yield reduction was twofold (11.5%) as that of Pusa 1121 (5.2%).     

Weed control and potato (Solanum tuberosum) tolerance with dimethenamid isomers and other herbicides

Two research studies were conducted to evaluate weed control in potato with dimethenamid and dimethenamid-p. No significant injury was observed from most applications of dimethenamid prior to potato emergence, but injury was 20% to 38% with dimethenamid when emerging potatoes were covered slightly by soil during “drag-off” and rain occurred within 24 h. Common lambsquarters (Chenopodium album L.) and common ragweed (Ambrosia artemisiifolia L.) control with dimethenamid preemergence (PRE) did not exceed 68%. Dimethenamid-p plus metribuzin or dimethenamid-p followed by (fb) rimsulfuron postemergence (POST) controlled common lambsquarters 95% to 96% and common ragweed 71% to 92%. Annual grass control was greater with S-metolachlor alone than with dimethenamid isomers alone. Broadleaf and grass control was similar with dimethenamid and dimethenamid-p.     

How long can a herbicide remain in the spray tank without losing efficacy?

Five field trials were conducted over a three-year period (2006–2008) at two locations in southwestern Ontario to determine the length of time herbicides can remain in the spray tank prior to application in the field without impacting efficacy. Four pre-emergence and five post-emergence herbicides were mixed at their labelled rates and then applied in maize. Herbicides were either applied immediately after mixing in water, or after being held in solution for 1, 3 or 7 days. The most common weed species in the trials were Abutilon theophrasti, Amaranthus retroflexus, Ambrosia artemisiifolia, and Chenopodium album. Holding the herbicides in spray solution for up to 7 days did not affect the efficacy of the post-emergence herbicides in this study. Similarly, control of A. retroflexus and C. album with the pre-emergence herbicides was not affected holding the herbicides in spray solution for up to 7 days. However, control of A. theophrasti was decreased when isoxaflutole plus atrazine, dimethenamid plus dicamba/atrazine, or rimsulfuron plus S-metolachlor plus dicamba was in spray solution for more than 1 day. Nonetheless, there were no decreases in yield for any of the treatments evaluated. These data provide valuable information which growers can use to make informed decisions on whether to apply herbicides in non-ideal weather or to postpone application. The results of this study suggest that for the herbicides and weed species tested it is better to postpone application rather than make applications under non-ideal conditions.     

Weed control research in Colorado potatoes—A review

Studies have been conducted for three years to determine the herbicides best adapted for the control of annual broadleaf and grass weeds in Colorado potato fields. The most effective registered herbicides are EPTC, EPTC + trifluralin, and metobromuron. Promising herbicides that are not registered are alachlor and 4-amino-6-t-butyl-3-(methylthio)-as-triazin-5-(4H)-one (BAY-94337). These experiments show the efficacy of several herbicides. The data indicate little need for the presently accepted pattern of extensive post planting cultivation when weeds are chemically conrolled.     

Weed control with selected herbicides in acetolactate synthase-resistant sorghum

The recent development of grain sorghum hybrids with resistance to acetolactate synthase (ALS)-inhibiting herbicides has allowed for the use of several post-emergence applied (POST) ALS-inhibitors to control weeds in the crop. Field experiments were conducted at four sites in Kansas in 2008 to evaluate the efficacy of nicosulfuron and nicosulfuron + rimsulfuron applied alone or in combination with dicamba, metsulfuron methyl, and atrazine. All POST treatments slightly injured sorghum 2 weeks after treatment (WAT) at Garden City and Hesston, whereas at Hays and Manhattan, only treatments that included dicamba caused injury. Nicosulfuron + rimsulfuron applied alone provided 41, 83, 74, and 93% control of grasses 4 WAT at Garden City, Hays, Hesston, and Manhattan, respectively. However, to obtain the highest level broadleaf weed control, nicosulfuron or nicosulfuron + rimsulfuron need to be applied with other broadleaf herbicides. POST treatment of nicosulfuron + metsulfuron methyl + dicamba + atrazine provided 90% or greater control of all broadleaf weeds at sorghum flowering. Sorghum grain yield was greater following all herbicide treatments compared with the weedy check. The POST treatment that provided the highest yield at Garden City was nicosulfuron + rimsulfuron + atrazine, whereas in Hesston and Manhattan, nicosulfuron + metsulfuron methyl + dicamba + atrazine provided the highest yields. This research showed that many grasses can be effectively controlled with POST applications of nicosulfuron or nicosulfuron + rimsulfuron in ALS-resistant sorghum. The research also indicated that broadleaf weed control is greater when nicosulfuron or nicosulfuron + rimsulfuron are applied with other broadleaf-control herbicides such as dicamba, metsulfuron methyl, and atrazine.     

Tolerance of winter wheat (Triticum aestivum L.) to pre-emergence and post-emergence application of saflufenacil

Saflufenacil is a new herbicide being developed for pre-plant burndown for non-selective removal of broadleaf weeds and pre-emergence (PRE) broadleaf weed control in field crops, including maize, soybean, sorghum and wheat. As part of studying the potential use pattern of this herbicide, four field studies were conducted in 2006 and 2007 at Concord, northeast Nebraska, to determine winter wheat tolerance to PRE and post-emergence (POST) applications of saflufenacil. The fall POST applications were conducted at the 2–3 leaf stage (5 cm height) whereas the spring POST and tank-mixes studies were sprayed at the 4th node stage (40 cm height) of crop. Dose-response curves based on log-logistic model were used to determine the ED (effective dose) values of saflufenacil for visual ratings of crop injury and relative yield. There was no crop injury or yield reduction with PRE applied saflufenacil dose of up to 400 g a.i. ha⁻¹. However, there was significant crop injury in the POST applications in the fall (up to 95%) and in the spring (up to 67%). There was also yield reduction of as much as 66% in the fall and 58% in the spring POST applications. Addition of adjuvants also increased crop injury levels. For example, at 14 days after treatment in the fall applications, about 5% visual crop injury (ED₅) was evident with 82, 67 and 10 g a.i. ha⁻¹ of saflufenacil compared with 51, 30 and 11 g a.i. ha⁻¹ in the spring, with no adjuvant, or non-ionic surfactant (NIS), or crop oil concentrate (COC), respectively. Saflufenacil at half the proposed used dose of 25 g a.i. ha⁻¹ was safe to mix with the currently used POST herbicides of wheat with no visible crop injury and yield reduction. PRE applications of saflufenacil would be safe for use in winter wheat; however, the POST application of saflufenacil alone or with the adjuvant NIS or COC produces unacceptable injury and yield loss. These results are similar to the proposed PRE use pattern of saflufenacil. In addition, the proposed label does not suggest the POST use of saflufenacil in winter wheat, or any other cereal crops, which is similar to what we have concluded from this study.     

Comparison of various weed control programs for potatoes

All weed control programs resulted in satisfactory weed control. Costs of controlling weeds ranged from $10 to $93/ha. Cultivation alone was the cheapest method of controlling weeds. Controlling weeds by using herbicides alone with no cultivation resulted in the highest weed control costs but tended to give the best weed control at harvest. Herbicides saved one to two cultivations. EPTC (S-ethyl dipropylthiocarbamate) gave slightly better control of green foxtail [Setaria viridis (L.) Beauv.] and slightly poorer control of broadleaf weeds than trifluralin (α,α,α,-trifluoro-2,6-dinitro-N, N-dipropyl-p-toluidine). Cultivation helped control weeds but had no other apparent beneficial or adverse effect on potatoes. The correlation coefficient between total tuber yield and dry weight of weeds was ?0.97 at Grand Forks, North Dakota and ?0.85 at Casselton, North Dakota.     

Pre and postemergence herbicides for weed control in castor crop

Weed management is among the main factors limiting cultivation of castor (Ricinus communis) in extensive fields, particularly when labor is scarce or expensive. This experiment evaluated the efficiency of weed management programs using preemergence (clomazone, pendimethalin, and trifluralin) and a postemergence herbicide (chlorimuron-ethyl) applied at 20 days after emergence in castor plants cv. BRS Energia under rainfed conditions in Apodi, Brazil. No phytotoxicity was observed on the castor plants, and the postemergence herbicide significantly increased castor seed yield to 1466 kg ha⁻¹ complementing the weed control of preemergence herbicides treatments in which seed yield was 1207 kg ha⁻¹. Seed yield on weedy and weed-free treatments was 760 and 1971 kg ha⁻¹, respectively. Weeds were kept under a satisfactory control up to 40 days after emergence. This program resulted in reasonable weed control because the preemergence herbicides controled monocotyledon weeds, while the postemergence herbicide controlled broad leafed species being selective to castor plants.     

Response of spring planted barley (Hordeum vulgare L.), oats (Avena sativa L.) and wheat (Triticum aestivum L.) to mesotrione

There is little information on the response of spring planted barley (Hordeum vulgare L.), oats (Avena sativa L.) and wheat (Triticum aestivum L.) to mesotrione under Ontario environmental conditions. Four field studies were conducted in Ontario, Canada over a two-year period (2008 and 2009) to evaluate the sensitivity of spring planted cereals (barley, oats, and wheat) to pre-emergence (PRE) and post-emergence (POST) applications of mesotrione at 50, 100, and 150 g ai ha⁻¹. Mesotrione applied PRE caused minimal visible injury at 3, 7, 14 and 28 days after emergence (DAE) and had no adverse effect on plant height or yield of barley, oats and wheat. Mesotrione applied POST caused as much 11% injury and reduced plant height as much as 6% in spring planted cereals. Injury was higher in wheat compared to barley or oats. Mesotrione applied POST had no adverse effect on the yield of barley or oats but decreased the yield of wheat as much as 14%. Based on this study, mesotrione applied PRE at 50, 100 or 150 g ai ha⁻¹ can be safely used in spring planted barley, oats, and wheat. Mesotrione applied POST at the proposed dose of 50, 100 or 150 g ai ha⁻¹ can also be safely used in spring planted barley and oats. However, mesotrione applied POST results in unacceptable injury in spring planted wheat.     

Applications of pre-emergent pyroxasulfone,flufenacet and their mixtures with triallate for the control of Bromus diandrus (ripgut brome) in no-till wheat (Triticum aestivum) crops of southern Australia

Four field experiments were conducted over a three-year period in Victoria and South Australia to investigate the effectiveness of pre-emergence (PRE) applications of pyroxasulfone, flufenacet and their mixtures with triallate for the control of Bromus diandrus in spring wheat. Herbicide mixtures of pyroxasulfone plus triallate and flufenacet plus triallate applied PRE to wheat provided consistently high levels of B. diandrus control (≥85%). In contrast, applications of pyroxasulfone and flufenacet applied alone along with trifluralin plus metribuzin (a common farmer practice in southern Australia) provided more variable control of B. diandrus (33–90%). Pyroxasulfone plus triallate treatments had a much lower (≤47 panicles m⁻²) panicle density of B. diandrus than trifluralin plus metribuzin (42–318 panicles m⁻²) and the non-treated control (118–655 panicles m⁻²). PRE herbicides which were safe to spring wheat and provided the greatest level of control of B. diandrus resulted in significantly (P < 0.05) higher grain yields at Culgoa (120%) and Gama (13%) than non-treated wheat (720 and 1740 kg ha⁻¹). Although flufenacet was effective against B. diandrus, crop phytotoxicity at the higher dose (900 g ai ha⁻¹) reduced spring wheat grain yield. Based on these results, PRE pyroxasulfone plus triallate could play an important role in the management of B. diandrus in spring wheat. However, high cost of these herbicides (AUS$35-$70 ha⁻¹) may limit their adoption in low rainfall and low yielding wheat environments in southern Australia where B. diandrus is most prevalent.     

Weed control and sensitivity of oats (Avena sativa) with various doses of saflufenacil

Saflufenacil is a new herbicide being developed by BASF for broadleaved weed control in maize, soybean and other crops prior to crop emergence. Six field studies were conducted in Ontario, Canada over a three year period (2008-2010) to evaluate the potential of saflufenacil applied pre-emergence (PRE) at various doses for broadleaved weed control in oats. Saflufenacil applied PRE caused minimal visible injury at 1, 2 and 4 weeks after emergence (WAE) in oats. At 4 WAE, the dose of saflufenacil required to provide 95% control of Ambrosia artemisiifolia (common ragweed), Chenopodium album (common lambsquarters), Polygonum convolvulus (wild buckwheat), Polygonum scabrum (green smartweed) and Sinapsis arvensis (wild mustard) was 72 to >100, >100, 74, 58 and >100 g ai ha⁻¹, respectively. Generally, similar saflufenacil dose-response trends were seen at 8 WAE. The doses of saflufenacil required to provide 95% reduction in density and dry weight ranged from 95 to >100 and 42 to >100 g ai ha⁻¹ respectively for A. artemisiifolia, C. album, P. convolvulus, P. scabrum and S. arvensis. Oat yield showed no sensitivity to saflufenacil at the doses evaluated. Based on this study, saflufenacil applied PRE can be safely used in spring planted oats for the control of some troublesome annual broadleaved weeds.     

Evaluation of herbicides to manage herbicide-resistant corn poppy (Papaver rhoeas) in winter cereals

Corn poppy is the most important broad-leaved weed in winter cereals in southern Europe. It is an increasing problem due to the appearance of herbicide-resistant biotypes. Experiments were conducted in north-eastern Spain with three corn poppy populations to determine the level of resistance to 2,4-D and tribenuron using seed-germination tests. Field experiments were conducted at sites from where the tested populations had been collected to test the effectiveness of various herbicides applied pre-emergence (PRE) or post-emergence (POST) in winter cereals. In the seed-germination tests, two populations were found to be 2,4-D-resistant and one resistant to both 2,4-D and tribenuron. In field trials, the efficacy of 2,4-D at 900 g a.i. ha⁻¹ ranged from 25 to 76% of reduction of plant densities, depending on season and population, indicating that all populations were 2,4-D-resistant in field conditions. Moreover, at 15 g a.i. ha⁻¹ tribenuron, one of the three populations (CU2) was only reduced by 65% in two seasons, indicating also the presence of resistance to tribenuron in this population in addition to 2,4-D. Florasulam at 6.25 g a.i. ha⁻¹ + 2,4-D at 300 g a.i. ha⁻¹ resulted in poor control (75–85%) in CU2 suggesting possible cross-resistance to this triazolopyrimidine. On average, all the other herbicides provided 90–100% control of corn poppy. Only PRE herbicides and the mixture 150 g a.i. ha⁻¹ bromoxynil + 150 g a.i. ha⁻¹ ioxynil + 450 g a.i. ha⁻¹ mecoprop in POST performed consistently well (>90% control). The results of this study indicate that 2,4-D and tribenuron-resistant corn poppy populations in winter cereals can be controlled by application of PRE or POST herbicides with alternative modes of action.     

Selectivity of fomesafen based systems for preemergence weed control in cucurbit crops

Experiments at two sites during two years evaluated the selectivity of preemergence fomesafen in cucurbit crops of winter and summer squash, zucchini, cantaloupe, cucumber, and pumpkin. Cucumbers were the most sensitive of the cucurbit crops to fomesafen and produced little or no fruit in two out of three experiments when applied at 0.28 kg ai ha⁻¹. Fomesafen also reduced cantaloupe yield. Visual damage was noted on the other crops tested, but crop yield was not impacted by fomesafen at 0.28–0.35 kg ai ha⁻¹. With the exception of cucumbers, injury caused by fomesafen to cucurbit crops was transitory even when fomesafen-treated soil splashed onto the leaves of emerging cucurbits during a powerful thunderstorm at one of the test sites. Control of redroot pigweed (Amaranthus retroflexus), Powell amaranth (Amaranthus powellii) and other Amaranthus spp., lambsquarters (Chenopodium album), hairy nightshade (Solanum physafolium), common purslane (Portulaca oleraceae), and velvetleaf (Abutilon theophrastii) ranged from 92 to 100% with fomesafen applied at 0.28 kg ai ha⁻¹. The excellent efficacy on these difficult to control weed species suggests that lower rates of fomesafen may be appropriate and improve crop tolerance, particularly if fomesafen is tankmix-applied with other preemergence herbicides such as s-metolachlor or dimethenamid-p. Weed control with these combinations was excellent for all weed species in these experiments.     

Weed control,environmental impact and profitability with trifluralin plus reduced doses of imazethapyr in dry bean

Field experiments were conducted in 2003, 2006, and 2007 in Ontario to determine if reduced doses of imazethapyr combined with trifluralin applied pre-plant incorporated (PPI) can be used as an economically and environmentally feasible weed management strategy for broad spectrum weed control in white and kidney bean. There was minimal injury (<5%) in white or kidney bean from imazethapyr applied alone or in combination with trifluralin, regardless of dose. The dose of imazethapyr required for 80 and 95% control of Setaria viridis (L.) Beauv. (green foxtail), Chenopodium album L. (common lambsquarters) and Ambrosia artemisiifolia L. (common ragweed) was reduced when combined with trifluralin (600 g ai ha⁻¹). There was a trend for increased yield of white and kidney bean with increasing doses of imazethapyr applied alone and in combination with trifluralin. Combining trifluralin with imazethapyr increased the environmental impact (EI) by more than ten-fold compared to imazethapyr alone. The lowest dose of imazethapyr used in this study resulted in the lowest environmental risk. The doses of imazethapyr that maximized profit were 38 g ai ha⁻¹ for white bean and 47 g ai ha⁻¹ for kidney bean. Combining imazethapyr with trifluralin will provide growers with a weed management strategy that provides acceptable weed control, with only a small increase in environmental impact, and has the potential to increase yields and net returns.     

Management of herbicide-resistant Phalaris minor in wheat by sequential or tank-mix applications of pre- and post-emergence herbicides in north-western Indo-Gangetic Plains

Development of cross resistance or multiple cross resistance in Phalaris minor in wheat will continue to increase, as the weed develops mechanisms of resistance against new herbicides. This weed is a major threat to wheat productivity in north-western India, and as such needs to be addressed with integrated weed management approaches, including crop and herbicide rotations, herbicide combinations along with cultural and mechanical methods. Three field experiments were conducted during 2008–09 to 2012–13 along with large plot adaptive trials during 2012–13 with the objective to evaluate the efficacy of sequential applications of pendimethalin applied pre-emergent followed by clodinafop, sulfosulfuron, or pinoxaden applied post-emergent and tank-mix applications of metribuzin with these post-emergence herbicides for the management of herbicide-resistant P. minor in wheat. Clodinafop 60 g ha⁻¹ or sulfosulfuron 25 g ha⁻¹ at 35 days after sowing (DAS) and pendimethalin 1000 g ha⁻¹ as pre-emergence did not provide consistently effective control of P. minor in wheat. An increase in the dose of clodinafop from 60 to 75 g ha⁻¹ and of sulfosulfuron from 25 to 30 g ha⁻¹ also did not improve their efficacy to a satisfactory level. However, pinoxaden 50 g ha⁻¹ provided effective control (97–100%) of P. minor but not of broadleaf weeds. The tank-mix application of metribuzin with clodinafop 60 g ha⁻¹ or sulfosulfuron 25 g ha⁻¹ at 35 DAS and the sequential application of pendimethalin 1000 g ha⁻¹ or trifluralin 1000 g ha⁻¹ just after sowing followed by clodinafop 60 g ha⁻¹ or sulfosulfuron 25 g ha⁻¹ at 35 DAS provided 90–100% control of P. minor along with broadleaf weeds in wheat, thus resulting in improved grain yields (4.72–5.75 t ha⁻¹) when compared to clodinafop 60 g ha⁻¹ (3.85–5.60 t ha⁻¹) or sulfosulfuron 25 g ha⁻¹ alone (3.95–5.10 t ha⁻¹). The efficacy of mesosulfuron + iodosulfuron (a commercial mixture) 14.4 g ha⁻¹ against P. minor was not consistent across the experiments and over the years. The ready-mix combination of fenoxaprop + metribuzin (100 + 175 g ha⁻¹) at 35 DAS provided effective control of weeds but its varietal sensitivity needs to be determined before its use in field conditions. The tank-mix or sequential application of herbicides would be a better option than their applications alone to manage the serious problem of herbicide-resistant P. minor in wheat.     

Weed control efficacy and winter wheat safety of a novel herbicide HW02

HW02, a pyruvate dehydrogenase inhibitor, is a newly developed herbicide for broadleaf weed control in wheat, maize and turf in China. Greenhouse and field experiments were conducted to evaluate its efficacy against weeds and safety to winter wheat. In the greenhouse experiment, this herbicide had higher activities than 2,4-D against Descurainia sophia (L.) Schur., Amaranthus retroflexus L., Capsella bursa-pastoris (L.) Medic., and Malachium aquaticum (L.) Fries.. When it was applied at late tillering stage of winter wheat in spring, the herbicide provided weed biomass reduction of 98%–100% at the rates 225–525 g a.i. ha⁻¹ and was safe to the crop at the rate of up to 900 g a.i. ha⁻¹. These results showed HW02 could be an alternative herbicide for resistant weed management because its mode of action is different from herbicides presently used.     

Spring-applied saflufenacil and imazapic provided longer lasting Euphorbia esula L. control than fall applications

Euphorbia esula is a serious weed problem in North American rangeland and pasture. Saflufenacil is a new herbicide being primarily developed for pre-plant and pre-emergence broadleaf weed control in several field crops. Imazapic is commonly used for E. esula control as a fall treatment, because spring applications do not provide satisfactory control and may cause grass injury. Field experiments were conducted during the spring and fall of 2007 and 2008 at O'Neill, Nebraska, USA with the objective to describe dose–response curves of saflufenacil and imazapic applied alone and tank-mixed in order to determine the best ratios of the two herbicides for E. esula control. The response of E. esula to saflufenacil for each imazapic treatment was evaluated in terms of plant density reduction over time, based upon DR₉₀ (effective dose that provides 90% density reduction). In general, saflufenacil or imazapic applied alone did not provide satisfactory long-term E. esula control, regardless of application timings. Overall, E. esula control increased when saflufenacil was applied with imazapic, especially as the doses of both herbicides increased. In the spring study, a 90% density reduction of E. esula for 24 months after treatment (MAT) was achieved with saflufenacil at 127 g ha⁻¹ tank-mixed with 105 g ha⁻¹ of imazapic. When applied in the fall, a significantly higher saflufenacil dose (418 g ha⁻¹) was needed in a tank-mix with 105 g ha⁻¹ of imazapic to obtain 90% density reduction for up to 23 MAT. This result indicates that spring applications of imazapic plus saflufenacil provided better E. esula control than fall treatments.     

Response of weeds and soil microorganisms to imazaquin and pendimethalin in cowpea and soybean

Herbicides applied to combat weeds and increase crop yields may also have undesired effects on beneficial soil microorganisms. Field studies were conducted in 2005 and 2006 in Zaria, Nigeria, to evaluate the response of weeds and soil microorganisms to imazaquin applied at 0.05, 0.10, 0.20 and 0.40 kg a.i./ha and pendimethalin applied at 1.0, 2.0, 4.0, and 8.0 kg a.i./ha in cowpea and soybean. Hoe-weeded and unweeded (no herbicide) plots were controls. Both herbicides significantly reduced weed biomass in both crops, when compared to the unweeded control, which had the highest weed biomass at all sampling dates. Treatments with 0.40 kg a.i./ha of imazaquin, 2.0 and 4.0 kg a.i./ha of pendimethalin, and the hoe-weeded control, had the highest cowpea grain yield. The unweeded control had the lowest grain yield which was comparable to that in all other herbicide treatments. All treatments except 4.0 and 8.0 kg a.i./ha of pendimethalin had higher soybean grain yield than the unweeded control. Soybean yield was lowest in the unweeded control, and treatments that received 4.0 and 8.0 kg a.i./ha of pendimethalin. All rates of imazaquin gave similar soybean grain yields that were 29–41% higher than that from pendimethalin. The hoe-weeded control had the highest yield, which was 79% more than that in the unweeded control. Higher rates of imazaquin and pendimethalin reduced nodulation, nitrogen fixation, and vesicular arbuscular mycorrhizal (VAM) fungi colonisation in both crops. VAM fungi species diversity and species richness in cowpea rhizosphere soil and species diversity in soybean rhizosphere soil were reduced relative to the controls due to application of both herbicides with the rates of 0.10, 0.20, and 0.40 kg a.i./ha of imazaquin and 8.0 kg a.i./ha of pendimethalin being significantly effective.     

Weed control with pre-emergence herbicides in vegetable soybean (Glycine max L. Merrill)

Field and laboratory experiments were conducted in the early and late rainy seasons in Thailand to evaluate the effect of pre-emergence application of herbicides and determine the herbicide residues on vegetable soybean (Glycine max L. Merrill cv. No. 75) production. No visible crop injury was observed after application of alachlor 469 g a.i./ha, clomazone 1080 g a.i./ha, metribuzin 525 g a.i./ha, pendimethalin 1031.25 g a.i./ha, tank-mixed clomazone 960 g a.i./ha + pendimethalin 928 g a.i./ha, or tank-mixed metribuzin 350 g a.i./ha + pendimethalin 928 g a.i./ha. However, acetochlor 1875 g a.i./ha, isoxaflutole 75 g a.i./ha, and oxadiazon 1000 g a.i./ha caused visible crop injury. Plant bioassay of herbicide residues in the soil after harvest showed no phytotoxic effect on baby corn (Zea mays Linn. cv. Suwan 3), cucumber (Cucumis sativus L. cv. Pijit 1), pak choi (Brassica chinensis Jusl. cv. Chinensis), and soybean (G. max L. Merrill cv. CM 60). Gas Chromatography-Mass Spectrometry (GC–MS) analysis showed no significant herbicide residues on crop yield (or MRLs < 0.01 ppm) for all herbicides used in this study. The application of metribuzin at 525 g a.i./ha was sufficient to provide satisfactory full-season control of several weed species and gave the highest crop yield. In addition, pendimethalin at 1031.25 g a.i./ha, and tank-mixed metribuzin at 350 g a.i./ha + pendimethalin at 928 g a.i./ha can provide a similar level of weed control as an alternative to reduce herbicide dosage thereby increasing food and environmental safety in vegetable soybean production.