In strip-tillage peanut production, situations occur when dinitroaniline herbicides are not applied in a timely manner. In these cases, dinitroaniline herbicides would be applied days or weeks after seeding. However, there is no information that documents the effects of delayed applications on weed control. Trials were conducted in 2004, 2005, and 2007 in Georgia to determine the weed control efficacy of delayed applications of pendimethalin in strip-tillage peanut production. Treatments included seven timings of pendimethalin application and three pendimethalin-containing herbicide combinations. Timings of application were immediately after seeding (PRE), vegetative emergence of peanut (VE), 1 wk after VE (VE 1wk), VE 2wk, VE 3wk, VE 4wk, and a nontreated control. Pendimethalin containing herbicide programs included pendimethalin plus paraquat, pendimethalin plus imazapic, and pendimethalin alone. Among the possible treatment combinations was a current producer standard timing for nonpendimethalin weed control programs in peanut, which was either imazapic or paraquat alone applied VE 3wk. Pendimethalin alone did not effectively control Texas millet regardless of time of application (69 to 77%), whereas southern crabgrass was controlled by pendimethalin alone PRE (87%). Delayed applications of pendimethalin controlled Texas millet and southern crabgrass when combined with either paraquat or imazapic, with imazapic being the preferred combination due to better efficacy on southern crabgrass than paraquat at most delayed applications. Peanut yield was improved when any of the herbicide combinations were applied PRE compared to later applications. Across all times of application, pendimethalin plus imazapic effectively maximized peanut yield with interference from annual grasses.

Nomenclature:Imazapic; paraquat; pendimethalin; southern crabgrass, Digitaria ciliaris (Retz.) Koel; Texas millet, Urochloa texana (Buckl.) R. Webster; peanut, Arachis hypogaea L., ‘C99R’

The present work aimed to evaluate plant injury caused by residues in the soil of the formulated mixture of imazethapyr and imazapic to a nontolerant genotype of rice (IRGA 417) drilled at 371 and 705 d after herbicide application (DAA). Herbicide carryover reduced up to 55% of the grain yield of the IRGA 417 drilled at 371 DAA, and plant injury was still evident at 705 DAA but without grain yield reduction.

Nomenclature: Imazethapyr; imazapic; rice, Oryza sativa L. ‘IRGA 417’

Horseweed (Conyza canadensis) is a common weed in no-till crop production systems. It is problematic because of the frequent occurrence of biotypes resistant to glyphosate and acetolactate synthase (ALS)-inhibiting herbicides and its ability to complete its life cycle as a winter or summer annual weed. Tactics to control horseweed while controlling other winter annual weeds routinely fail; herbicide application timing and spring emergence patterns of horseweed may be responsible. The objectives of this experiment were to (1) determine the influence of fall and spring herbicides with and without soil residual horseweed activity on spring-emerging glyphosate-resistant (GR) horseweed density and (2) evaluate the efficacy and persistence of saflufenacil on GR horseweed. Field studies were conducted in southern Indiana and Illinois from fall 2006 to summer 2007 and repeated in 2007 to 2008. Six preplant herbicide treatments were applied at four application timings: early fall, late fall, early spring, and late spring. Horseweed plants were counted every 2 wk following the first spring application until the first week of July. Horseweed almost exclusively emerged in the spring at both locations. Spring horseweed emergence was higher when 2,4-D glyphosate was fall-applied and controlled other winter annual weeds. With fall-applied 2,4-D glyphosate, over 90% of the peak horseweed density was observed before April 25. In contrast, only 25% of the peak horseweed density was observed in the untreated check by April 25. Starting from the initiation of horseweed emergence in late March, chlorimuron tribenuron applied early fall or early spring, and spring-applied saflufenacil at 100 g ai/ha provided greater than 90% horseweed control for 12 wk. Early spring–applied saflufenacil at 50 g ai/ha provided 8 wk of greater than 90% residual control, and early spring–applied simazine provided 6 wk of greater than 90% control. When applied in late spring, saflufenacil was the only herbicide treatment that reduced horseweed densities by greater than 90% compared to 2,4-D glyphosate. We concluded from this research that fall applications of nonresidual herbicides can increase the rate and density of spring emerging horseweed. In addition, spring-applied saflufenacil provides no-till producers with a new preplant herbicide for foliar and residual control of glyphosate- and ALS-resistant horseweed.

Nomenclature: Chlorimuron; glyphosate; saflufenacil; simazine; tribenuron; 2,4-D ester; horseweed, Conyza canadensis L. ERICA

Sulfentrazone is commonly used for weed control in soybeans and tobacco, and vegetable crops and cotton are often rotated with soybeans and tobacco. Studies were conducted to evaluate the potential for sulfentrazone to carryover and injure several vegetable crops and cotton. Sulfentrazone was applied PRE to soybean at 0, 210, 420, and 840 g ai/ha before planting bell pepper, cabbage, cotton, cucumber, onion, snap bean, squash, sweet potato, tomato, and watermelon. Cotton, known to be susceptible to sulfentrazone carryover, was included as an indicator species. Cotton injury ranged from 14 to 18% with a 32% loss of yield in 1 of 2 yr when the labeled use rate of sulfentrazone (210 g/ha) was applied to the preceding crop. High use rates of sulfentrazone caused at least 50% injury with yield loss ranging from 36 to 100%. Bell pepper, snap bean, onion, tomato, and watermelon were injured < 18% by sulfentrazone at 840 g/ha. Squash was injured < 3% and < 36% by sulfentrazone at 210 and 840 g/ha, respectively. Yield of these crops was not affected regardless of sulfentrazone rate. Cabbage and cucumber were injured < 13% by sulfentrazone at 210 and 420 g/ha, and yields were not affected. Sulfentrazone at 840 g/ha injured cabbage up to 46% and reduced yield in 1 of 2 yr. Sulfentrazone injured cucumber up to 63% and reduced yield of No. 2 grade fruits. Sulfentrazone at 210 and 420 g/ha injured sweet potato < 6% and did not affect yield. Sulfentrazone at 840 g/ha injured sweet potato 14% and reduced total yield 26%. Our results suggest little to no adverse effect on bell pepper, cabbage, cucumber, onion, snap bean, squash, sweet potato, tomato, or watermelon from sulfentrazone applied at registered use rates during the preceding year.

Nomenclature: Sulfentrazone; bell pepper, Capsicum annuum L. ‘Jupiter’ cabbage, Brassica oleracea L. var. capitata ‘Conquest’; cotton, Gossypium hirsutum L. ‘DP-51’; cucumber, Cucumis sativus L. ‘Calypso’; onion, Allium cepa L. var. cepa ‘Tuffball’; snap bean, Phaseolus vulgaris L. ‘Strike’; soybean, Glycine max (L.) Merrill ‘9711’; squash, Cucurbita pepo L. ‘Early Prolific’; sweet potato, Ipomoea batatas (L.) Lam. ‘Beauregard’; tobacco, Nicotiana tabacum L.; tomato, Lycopersicon esculentum Mill. ‘Mountain Spring’; watermelon, Citrullus lanatus (Thumb.) Matsum and Nakai ‘Sangria’

In the southeastern United States, Florida betony continues to be a problem weed in both turfgrass and ornamentals. Several herbicides including atrazine, dichlobenil, and glyphosate can provide good control (greater than 70%) of Florida betony, but their uses are limited. Over the past several years, many additional herbicides have been added to the turf market. New herbicides evaluated in this study included the sulfonylurea herbicides foramsulfuron, metsulfuron, and trifloxysulfuron; the picolinic acids clopyralid and fluroxypyr; and the aryl triazinone herbicide carfentrazone in combination with 2,4-D, dicamba, and mecoprop. In both the 2004 and 2005 trials, all sulfonylurea herbicides provided greater than 83% control of Florida betony at 10 wk after treatment. Other herbicides that provided less than 80% control of Florida betony in 2004 and 2005 included clopyralid, fluroxypyr, and the carfentrazone combination treatment. Selective control of Florida betony in ornamentals, however, still remains a challenge, as none of these herbicides are labeled for ornamentals.

Nomenclature: 2,4-D; atrazine; carfentrazone; clopyralid; dicamba; fluroxypyr; foramsulfuron; glyphosate; mecoprop; metsulfuron; and trifloxysulfuron; Florida betony, Stachys floridana Shuttlew. ex Benth. STAFL

Tolerance of sprigged ‘Tifsport’ and ‘Tifdwarf’ bermudagrass, ‘Meyer’ zoysiagrass, and ‘Salam’ seashore paspalum to oxadiazon (2,240 g/ha) or quinclorac (840 g/ha) applied 1 wk before sprigging (WBS), at sprigging (AS), 2 wk after sprigging (WAS), and 4 WAS was investigated in the field. Weed control was also evaluated. For both herbicides only the AS application timing injured the turfgrass greater than 22%, and injury for the other application timings ranged from 9 to 19% 5 WAS. When evaluated 8 WAS turfgrass injury following the AS application timing remained at 19%, and injury for all other timings was 8% or less. Eight WAS the 1 WBS, AS, 2 WAS, and 4 WAS application timings achieved 89, 79, 94, and 99% plot coverage, respectively, when averaged over all turfgrass species/cultivars and herbicides. By 13 WAS, all species/cultivars achieved at least 90% plot coverage. Presprigging applications of oxadiazon provided 98 to 100% goosegrass and old world diamond-flower control. Quinclorac applied AS provided greater than 70% control of these weeds. Results indicate that oxadiazon and quinclorac applied AS will cause unacceptable turfgrass injury. If goosegrass and/or old world diamond-flower are problematic, oxadiazon is a feasible choice for control of these weeds, but quinclorac is not.

Nomenclature: Oxadiazon; quinclorac; goosegrass, Eleusine indica (L.) Gaertn.; old world diamond-flower, Oldenlandia corymbosa L.; bermudagrass, Cynodon dactylon (L.) Pers. × Cynodon transvaalensis Burtt-Davey ‘Tifsport’, ‘Tifdwarf’ CYNDA; zoysiagrass, Zoysia japonica L. ‘Meyer’ ZOYJA; seashore paspalum, Paspalum viginatum Swartz, ‘Salam’ PASVA

Machine-vision cultivator guidance systems are commercially available to growers, but little work has been done to determine if these guidance systems can improve integrated weed management systems in vegetable crops. Studies were conducted in 2005 and 2006 in broccoli and lettuce to evaluate band-applied DCPA or pronamide, respectively, and four noncultivated bands ranging from 5.1 to 12.7 cm. DCPA or pronamide were applied in bands centered on the seed line at 0, 7.6 or 12.7 cm wide. A commercial machine-vision system was used to guide a commercial cultivator. Generally, weed densities and hand-weeding times were less where the DCPA band in broccoli or the pronamide band in lettuce were 7.6 or 12.7 cm wide compared to no herbicide. Weed densities were lowest in both crops where the noncultivated band width was 5.1 cm compared to 12.7-cm noncultivated bands. For broccoli in both 2005 and 2006, net returns above production costs were generally higher in the 7.6- and 12.7-cm-wide DCPA bands compared with the no-herbicide band. In lettuce in both years, the no-pronamide treatment had higher net returns, when compared with the 7.6- and 12.7-cm pronamide bands. Lettuce yields and higher net returns in the no-pronamide treatment compared to the 7.6- and 12.7-cm pronamide bands may be due to slight yield reduction from pronamide. Results suggest that pronamide was not needed during the dry months of the year when weed management tools such as hand-weeding and cultivation work very well. However, in periods of rainy weather when cultivation and hand-weeding are not possible, then pronamide would likely provide the greatest economic benefit. Given the large impact of cultivation on vegetable weed management programs, the greatest potential benefit of machine-vision guided cultivators is if they facilitate more timely and effective cultivation.

Nomenclature: DCPA; pronamide; broccoli, Brassica oleracea L. var. botrytus L. ‘Marathon’; lettuce, Lactuca sativa L. ‘Sniper’, ‘PIC 714’, and ‘Darkland’

Field research was conducted in 2000 and 2001 to determine the effect of yellow nutsedge emergence timing and plant density on soybean yield and on yellow nutsedge propagation the following year. Yellow nutsedge tubers were planted at 0-, 7.5- (13/m²), 15- (8.6/m²), 30- (4.3/m²), 60- (2.2/m²), and 90-cm (1.5/m²) in-row spacings with soybean. Yellow nutsedge densities from 2.2 to 13 plants/m² in a high-yield year (2000) and 4.3 to 13 plants/m² in a low-yield year (2001) reduced grain yields 9 to 34%. In a separate experiment, tubers were planted 0, 2, 4, 6, and 8 wk after planting at a 15-cm (8.6/m²) in-row spacing. Seedlings that emerged with the crop and until 2 wk after planting reduced yield 9 to 11%. Yellow nutsedge densities from 1.5 to 13 plants/m² contributed to significant aboveground biomass production, even with a competitive crop, such as soybean. For every gram of aboveground yellow nutsedge biomass produced in the fall, there were more than four shoots present the following spring.

Nomenclature: Yellow nutsedge, Cyperus esculentus L. CYPES; soybean, Glycine max (L.) Merr. ‘Asgrow 3701’

Glyphosate-resistance evolution in weeds is evident globally, especially in areas where transgenic glyphosate-resistant crops dominate. Resistance to glyphosate is currently known in 16 weed species, including rigid ryegrass in Australia. Following the first report of glyphosate resistance in 1998, there are now 78 documented glyphosate-resistant populations of rigid ryegrass in grain-growing regions of southern Australia. In some regions where glyphosate-resistance evolution has already occurred in rigid ryegrass, transgenic glyphosate-resistant canola was introduced in 2008, further highlighting the need to monitor glyphosate-resistance evolution in weeds. A rigid ryegrass population (WALR70) was collected in 2005 from a crop field in Esperance, Western Australia, after it had survived applications of glyphosate. Dose–response experiments confirmed resistance in the population, with the glyphosate rate resulting in 50% mortality (LD₅₀) for WALR70 being 11 times greater than that for a susceptible biotype. The WALR70 population also had low levels of resistance to some acetyl coenzyme A carboxylase (ACCase)- and acetolactate synthase (ALS)-inhibiting herbicides (diclofop, fluazifop, clodinafop, tralkoxydim, chlorsulfuron, and imazethapyr), but was susceptible to other herbicide modes of action, such as atrazine, trifluralin, and paraquat. Two other rigid ryegrass populations assessed in this study were also confirmed to be resistant to glyphosate. The increasing number of glyphosate-resistant rigid ryegrass populations in Australia is of concern to growers because of the importance of glyphosate in intensive cropping systems and the introduction of glyphosate-resistant canola to this region.

Nomenclature: Glyphosate; rigid ryegrass, Lolium rigidum Gaudin

Weeds cause crop loss indirectly by reducing the quantity of resources available for growth. Quantifying the effects of weed interference on nitrogen (N) supply, crop growth, and N nutrition may assist in making both N and weed management decisions. Experiments were conducted to quantify the effect of N addition and weed interference on soil nitrate-N (NO₃-N) over time and the dependence of corn growth on NO₃-N availability, determine the corn N nutrition index (NNI) at anthesis, and evaluate if relative chlorophyll content can be utilized as a reliable predictor of NNI. Urea was applied at 0, 60, and 120 kg N/ha to establish N treatments. Season-long weedy, weed-free, and five weed interference treatments were established by delaying weed control from time of crop planting to the V3, V6, V9, V15, or R1 stages of corn development. Soil NO₃-N ranged from 20 kg N/ha without N addition to 98 kg N/ha with 120 kg N/ha added early in the season, but crop and weed growth reduced soil NO₃-N to 10 kg N/ha by corn anthesis. Weed presence reduced soil NO₃-N by up to 50%. Average available NO₃-N explained 29 to 40% of the variation in corn shoot mass at maturity. Weed interference reduced corn biomass and NNI by 24 to 69%. Lack of N also reduced corn NNI by 13 to 46%, but reduced corn biomass by only 11 to 23%. Nondestructive measures of relative chlorophyll content predicted corn NNI with 65 to 85% accuracy. Although weed competition for factors other than N may be the major contributor to corn biomass reduction, the chlorophyll meter was a useful diagnostic tool for assessing the overall negative effects of weeds on corn productivity. Further research could develop management practices to guide supplemental N applications in response to weed competition.

Nomenclature: Corn, Zea mays L. ‘DK589RR’

Field studies were conducted in Powell, WY in 2006 and 2007 to determine the influence of season-long interference of various wild buckwheat densities and duration of interference on sugarbeet. Percent sucrose content was not affected by wild buckwheat interference. Root and sucrose yield loss per hectare increased as wild buckwheat density increased. The estimated percent yield loss as wild buckwheat density approaches infinity was 64 and 61% for root and sucrose yield loss, respectively. The estimated percent yield loss per unit weed density at low weed densities was 6% for both root and sucrose yield loss. Greater durations of wild buckwheat interference had a negative effect on sugarbeet root yield. The critical timing of weed removal (CTWR) to avoid 5 and 10% root yield loss was 32 and 48 d after sugarbeet emergence (DAE), respectively. These results show that wild buckwheat is competitive with sugarbeet and should be managed appropriately to forestall any negative effects on sugarbeet root and sucrose yield.

Nomenclature: Wild buckwheat, Polygonum convolvulus L. POLCO; sugarbeet, Beta vulgaris L

Seeds of a suspected glyphosate-resistant giant ragweed biotype from Lauderdale County, TN, were collected from a continuous cotton field in fall 2007 after plants were nonresponsive to multiple glyphosate applications. The objectives of this research were to (1) confirm resistance by quantifying the response of the putative resistant biotype to glyphosate compared to a susceptible biotype from a nonagricultural area, (2) quantify shikimate accumulation over time in both biotypes, and (3) determine the effectiveness of POST-applied herbicides labeled for use in cotton in controlling both biotypes at three growth stages. The susceptible biotype had a 50% lethal dose of 407 g ae/ha of glyphosate compared with 2,176 g/ha for the resistant biotype when treated at the four-node stage, a 5.3-fold level of resistance. The resistant biotype accumulated 3.3- to 9.8-fold less shikimate than the susceptible biotype at 1 to 7 d after treatment. The resistant biotype was less responsive to glyphosate as treatment was delayed past the two-node stage, much more than the susceptible biotype. Glufosinate, MSMA, and diuron controlled both biotypes by at least 90%, regardless of size at application. Prometryn, flumioxazin, carfentrazone-ethyl, fomesafen, and trifloxysulfuron controlled both biotypes by at least 89% when applied at the two-node stage, but control generally diminished with later application timings. Pyrithiobac was not effective in controlling either biotype, regardless of size at application. Hence, there are effective herbicide options for controlling glyphosate-resistant giant ragweed in cotton, and the resistant biotype does not appear to exhibit multiple resistances to other herbicides.

Nomenclature: Carfentrazone-ethyl; diuron; flumioxazin; fomesafen; glufosinate; glyphosate; MSMA; prometryn; pyrithiobac; trifloxysulfuron; giant ragweed, Ambrosia trifida L. AMBTR; cotton, Gossypium hirsutum L

Organic soil amendments are known to affect the composition and density of annual weed communities. The objective of this research was to measure the effect on emergence and growth of redroot pigweed seedlings when soil was amended with composted dairy manure at 18, 36, and 54 T/ha, or with raw dairy manure at 41, 82, and 123 T/ha. Data recorded (1) seedling emergence over 12 days, (2) number of leaves and total leaf area, (3) shoot and root dry weight, and (4) seed number. Maximum seedling emergence (32%) occurred in nonamended soil (the control). Emergence declined in a linear fashion when soil was amended with manure or with compost. Compost additions affected seedling emergence more severely than did manure additions. For every measure of redroot pigweed growth except seed production, amendment with manure at 123 T/ha retarded growth compared to soil alone or compost-amended mixes. Manure applied at 82 T/ha reduced leaf area and plant height relative to other treatments. Growth of redroot pigweed in soil amended with compost at 36 and 54 T/ha was always equal to or greater than growth in soil that was not amended. Seed production in one of two runs of the experiment was more than double in soils amended with compost at 36 and 54 T/ha compared to the nonamended soil. These results suggest that amending soils with raw dairy manure may decrease the competitiveness of redroot pigweed, whereas amending with composted manure is likely to increase competitiveness.

Nomenclature: Redroot pigweed, Amaranthus retroflexus L. AMARE