Rice was planted at 78 kg/ha with different densities of creeping rivergrass. Creeping rivergrass at stand densities (SD) of 10,000 to 70,000 stolon segments/ha produced 5.4 to 6 stolons per introduced segment; however, 130,000 through 520,000 SD produced 1.4 to 2.1 stolons per segment. Stolon production was 60,000 plants/ha and increased to 760,000 stolons/ha at planting densities of 10,000 to 520,000/ha, respectively. The 260,000 and 520,000 SD produced similar stolon lengths of 217 and 318 km/ha; however, stolon length was less at lower SD. Total node production was 290,000 nodes per ha with an average of 29 nodes per segment in the 10,000 SD and 5.4 to 9.8 nodes per segment with 70,000 or greater SD. Total biomass increased as SD increased. Creeping rivergrass shoot emergence from soil was 31, 63, 44, and 25% for segments planted at 0, 1.3, 2.5, and 5 cm deep, respectively. In a greenhouse study, glyphosate at 1,260 g ai/ha controlled creeping rivergrass 91% and biomass production was 19% of the nontreated creeping rivergrass. Bispyribac, cyhalofop, fenoxaprop/s, fenoxaprop/s plus fenoxaprop, glufosinate, imazethapyr, penoxsulam, propanil, and quinclorac were less effective than glyphosate.

Aquatic plants provide many benefits to the environment, but must be managed when growth reaches nuisance levels or when invasive plant species are released. Management tactics include biological, chemical, cultural, mechanical, and physical tools. Each specific management technique has advantages and disadvantages. In addition, the implementation of these techniques can become complicated because of the multiple users, managers, and stakeholders that may be present on large bodies of water. As an example, hydrilla is the most economically damaging aquatic weed in the United States. It reproduces through fragmentation, turions, and occasionally seed and can colonize a wide variety of aquatic environments. The most common management tactics for hydrilla include biological, chemical, and mechanical tools. Triploid grass carp have been the primary biological control agent, whereas fluridone has been the only systemic herbicide used. Because of heavy utilization of fluridone, biotypes have developed resistance to this herbicide in Florida. Although several acetolactate synthase-inhibiting herbicides are in development, herbicides with additional modes of action are needed for resistance management. Other aquatic plant management needs include additional control tactics for algae and additional extension resources for public education.

Field studies were conducted in 2005 and 2006 to determine the most effective chemical options within three individual herbicide-based burndown programs, glyphosate, paraquat and glufosinate, for controlling glyphosate-resistant horseweed in Mississippi. Burndown treatments were applied April 5, 2005 and March 15, 2006 to horseweed plants 15 to 30 cm in height. Glyphosate at 0.86 kg ae/ha alone provided 60 to 65% horseweed control 4 wk after treatment (WAT). Control 4 WAT ranged from 73 to 74% when the glyphosate rate was increased to 1.25 kg/ha. Glyphosate at 0.86 kg/ha applied in combination with 2,4-D at 0.84 kg ae/ha or dicamba at 0.28 ae/ha maximized control of horseweed (≥ 90%) 4 WAT and soybean yield. Horseweed control 4 WAT with paraquat alone at 0.84 kg ai/ha ranged from 55 to 63% and control did not improve by increasing the rate to 0.98 kg/ha. Addition of 2,4-D or dicamba to paraquat maximized horseweed control both years (78 to 89%), whereas soybean yield was maximized with addition of dicamba or metribuzin at 0.42 kg ai/ha. Glufosinate applied alone at 0.47 kg ai/ha resulted in at least 88% control of horseweed and maximized soybean yield. Results indicate that effective management of glyphosate-resistant horseweed can be obtained in glyphosate-resistant soybean in glyphosate-, paraquat-, and glufosinate-based preplant weed control programs.

Glyphosate-resistant alfalfa offers new weed control options for alfalfa establishment. Field studies were conducted in 2004 and 2005 to determine the effect of establishment method and weed control method on forage production and alfalfa stand establishment. Seeding methods included clear seeding and companion seeding with oats. Herbicide treatments included glyphosate, imazamox, imazamox + clethodim, and no herbicide. Temporary stunting from the glyphosate treatments was observed (< 7%); however, injury did not reduce forage yield or stand density in 2004. No glyphosate injury was observed in 2005. Weed control with glyphosate was more consistent than with imazamox or imazamox + clethodim. In 2004, total seasonal forage yield, which consisted of alfalfa, weeds, and oats (in some treatments), was the highest where no herbicide was applied in the oat companion crop and was reduced where herbicides were applied in both establishment systems. In 2005, seeding method or weed control method did not affect total seasonal forage production. Alfalfa established with the clear-seeded method and treated with glyphosate yielded the highest alfalfa dry matter in both years. Imazamox injury reduced first-harvest alfalfa yield in the clear-seeded system in both years. When no herbicide was applied, alfalfa yield was higher in the clear-seeded system. The oat companion crop suppressed alfalfa yield significantly in both years. Alfalfa established with an oat companion crop had a lower weed biomass than the clear-seeded system where no herbicide was applied in both years.

Alfalfa–orchardgrass hay is popular in the Western United States because of an expanding horse-hay market. However, weed control in mixed alfalfa–orchardgrass stands is problematic, as herbicides must be safe for both species. Most growers rely solely on the competitiveness of the crop for weed control, which is often insufficient, especially in older stands. Field experiments were established in northern California to determine the efficacy and crop safety of several herbicides for winter annual weed control in established alfalfa–orchardgrass. Metribuzin at 560 or 840 g/ha and hexazinone at 420 g/ha applied in late fall provided at least 95% control of shepherd's purse and at least 80% control of downy brome without crop injury. Imazethapyr at 70 g/ha applied shortly after crop green-up provided more than 85% control of emerged mustards without crop injury. Paraquat at 560 g/ha applied shortly after crop green-up gave 50 to 82% weed control and caused significant injury to orchardgrass, which was still noticeable at first cutting.

Field experiments were conducted at Warooka and Rudall on the Yorke and Eyre Peninsula of South Australia during 2004 and 2005 to investigate the effectiveness of metribuzin for the selective control of rigid brome in no-till sown barley. Metribuzin (135 to 203 g ai/ha) incorporated by sowing (IBS) was more effective in controlling rigid brome (> 67%) than the same herbicide dose applied POST. Although IBS metribuzin at the highest rate (270 g/ha) provided effective weed control (82 to 90%), it was more phytotoxic to barley, reducing crop density by 23% relative to the nontreated control at Rudall. Soil at Rudall had low clay and organic matter content. In contrast, tank mixtures of metribuzin (203 g/ha) with pendimethalin, applied IBS, provided reliable rigid brome control (89 to 93%) and resulted in little crop damage (< 5%). Over the site-by-year combinations studied, all herbicide-treated barley at Warooka and Rudall yielded 6 to 50% more grain than the nontreated crop. Although metribuzin provides growers with an opportunity to selectively control rigid brome in no-till barley, high rates (≥ 203 g/ha) of this herbicide on sandy textured soils can result in significant crop damage.

Selected fatty acids, such as pelargonic acid and caprylic acid, are effective nonselective herbicides for a wide spectrum of annual weed species. Greenhouse and field studies evaluated the potential of certain food-grade organic acids to enhance herbicide activity of pelargonic acid-based formulations. The addition of diammonium succinate and succinic acid improved the efficacy of a pelargonic acid formulation from 117 to 200% in the greenhouse. L-Lactic acid and glycolic acid enhanced the efficacy of pelargonic acid formulations on velvetleaf and common lambsquarters in the greenhouse by 126 to 168% and in the field by 117 to 138%. Thus, combinations of fatty and organic acids with selected emulsifiers can be highly effective as safe, nonselective herbicides.

Field experiments were conducted in Missouri in 2004 through 2006 to compare the efficacy of fall and spring herbicide treatments for control of winter and summer annual weeds in no-tillage corn. Applications in the fall and 45 d before planting (45 PP) of 2,4-D and simazine; rimsulfuron, thifensulfuron, and 2,4-D; or glyphosate and 2,4-D controlled at least 90% of all winter annual weeds through 1 wk after corn planting (WAP), with a few exceptions. Applications made 30 d before planting (30 PP) of 2,4-D and glyphosate or rimsulfuron, thifensulfuron, and 2,4-D provided greater than 90% control of most winter annual weeds 1 WAP. Winter annual weed control was reduced with herbicide treatments applied 7 d before planting (7 PP), compared with earlier application timings. Summer annual weed control 5 WAP increased as the time between application and planting decreased. Fall applications resulted in greater weed biomass 5 WAP, compared with spring applications.

Populations of plumeless thistle and musk thistle were mowed at various growth stages at two separate sites in Maryland during a 6-yr period to elucidate relationships among seed rain, soil seed banks, and population recruitment. The majority of seeds (96%) in the soil profile were distributed within 7.6 cm of the surface at both sites. Mowing plumeless thistle when most of the flower heads were at the full bud or postbloom stage did not reduce seed bank or plant densities, unlike mowing at full bloom, which significantly and quickly reduced both. Musk thistle responded differently, with plant density declining only with the postbloom mowing treatment, which occurred after the parent plant had dispersed seeds and died. Seed bank densities were unchanged by this treatment. This indicates that other factors may limit the recruitment and maintenance of musk thistle, such as allelochemical production by parents and interspecific plant competition. Density of musk thistle declined over an 11-yr period at one monitoring site, whereas plumeless thistle remained unchanged. The disproportionate seed-destroying activities of the weed biological control agent Rhinocyllus conicus on musk thistle may explain this difference.

A field experiment was conducted in Okinawa, Japan from 2005 to 2007 to evaluate the effect of purple nutsedge and other weeds on turmeric growth and yield and determine the effective weed-control period to minimize crop interference. Treatments consisted of all weeds removed at 70, 115, and 160 d sequentially after planting (DAP), all weeds except purple nutsedge removed at these three dates, plus weedy and weed-free controls. Purple nutsedge density increased until 115 DAP when grown alone with turmeric and thereafter decreased markedly, whereas it increased only until 70 DAP when grown with other weeds. Weed reinfestation in turmeric that was sequentially weeded was high until 115 DAP, but thereafter was negligible. In the weedy control, the weed infestation was severe until 160 DAP. Plant height and leaf and tiller number per plant of turmeric increased rapidly from 70 to 160 DAP. Season-long weed infestation significantly reduced shoot biomass and rhizome yield of turmeric, but both were similarly high among treatments of weed-free turmeric, all weed removal, and removal of all weeds except purple nutsedge. Purple nutsedge (> 3,000 plants/m2) did not significantly reduce turmeric yield, whereas the combined weed species reduced yield by greater than 40%. Thus, crop interference by purple nutsedge was not high, and other weeds could be removed during 70 to 160 DAP for reduced labor requirements and higher yield of turmeric.

Field studies were conducted near Lewiston–Woodville and Rocky Mount, NC to evaluate the effects of mixed weed species on peanut yield. A combination of broadleaf and grass weeds were allowed to interfere with peanut for various intervals to determine both the critical timing of weed removal and the critical weed-free period. These periods were then combined to determine the critical period of weed control in peanut. The effects of various weedy intervals on peanut yield were also investigated. The predicted critical period of weed control, in the presence of a mixed population of weeds, was found to be from 3 to 8 wk after planting (WAP). Peanut yield decreased as weed interference intervals increased, demonstrating the need for weed control throughout much of the growing season in the presence of mixed weed populations.

Studies were conducted to evaluate the effects of grass and broadleaf weeds on peanut growth and peanut yield. In separate studies, grass or broadleaf weeds were allowed to compete with peanut for various intervals to determine both the critical timing of weed removal and the critical weed-free period. Hand-weeding and selective herbicides were used at appropriate times to remove and terminate weed growth. These periods were then used to determine the critical period of weed control. The effects of various weedy intervals on peanut yield were also investigated. The critical period of grass weed control was found to be from 4.3 to 9 wk after planting (WAP), whereas the critical period of broadleaf weed control was from 2.6 to 8 WAP. Peanut yields decreased as weed interference intervals for both grass and broadleaf weeds increased, demonstrating the need for control of both grass and broadleaf weeds throughout much of the growing season.

Field studies were conducted in 2002 and 2003 to determine the effects of herbicides applied below suggested use rates on weed management, yield, and estimated net return of peanut in both narrow and wide row-spacing regimes. In the single row-spacing regime study, diclosulam at 6 g/ha (1/4×) (1/4 labeled use rate) plus flumioxazin at 26 g/ha (1/4×) applied PRE followed by (fb) imazapic POST at 17, (1/4×), 35, (1/2×), or 70, (1×) g/ha was the lowest herbicide input that provided at least 80% control of sicklepod, yellow nutsedge, Florida beggarweed, and hairy indigo at 21 d after treatment (DAT). By 42 DAT, weed control was ≥ 80%, similar to herbicide treatments applied at labeled rates. In the twin row-spacing regime study at Jay, FL, and Citra, FL, in 2003, yellow nutsedge control was lowest (83 to 85%) from diclosulam applied at 1/4× in combination with flumioxazin at 1/4× PRE and flumioxazin at 1/4× or 52 g/ha (1/2×) PRE alone. All other herbicide treatments provided > 90% control. Diclosulam at 13 g/ha (1/2×) fb imazapic at 1/2× and flumioxazin at 1/4× fb imazapic at 1/2× was the lowest rate combination that provided > 80% control of sicklepod. Florida beggarweed control was > 90% with all rate combinations except diclosulam PRE alone, diclosulam at 1/4× PRE fb imazapic at 1/4× POST, and imazapic at 1/4× alone POST. Net return in the single-row spacing regime was highest for flumioxazin at 105 g/ha (1×) PRE alone, diclosulam at 1/4× applied with flumioxazin at 1/4× PRE alone or fb imazapic at 1× ($1,114, $1,094, and $990/ha, respectively). The twin row-spacing regime net return was highest with diclosulam PRE alone at 1/4, 1/2, or 26 g/ha (1×) ($2,063, $1,846, $1,734/ha, respectively). Diclosulam applied with flumioxazin at 1/2× fb imazapic at 1/2× was the lowest herbicide tank-mix input providing high net return at $1,162/ha.

Field trials were conducted in 2005 and 2006 to evaluate application of glyphosate alone or plus the plant growth regulator mepiquat chloride with 20 different insecticides to second-generation glyphosate-resistant cotton at the pinhead square or first bloom growth stages. At 7 DAT, averaged across cotton growth stages and herbicide treatments, combination with insecticides profenofos and methomyl resulted in 5 and 9% plant injury, respectively, and were the only insecticide combinations that resulted in injury greater than glyphosate or glyphosate plus mepiquat chloride applied alone. By 14 DAT, cotton injury was less than 2% for all treatments. Averaged across cotton growth stages and insecticides, addition of mepiquat chloride to glyphosate resulted in a 4 and 6 cm height reduction at 7 and 28 DAT, respectively. Seed cotton yield and percent first harvest were similar for all treatments, indicating that cotton injury and height reductions observed after application did not result in yield reductions or maturity delays. Glyphosate combined with insecticides and mepiquat chloride, in accordance with herbicide labeling for second-generation glyphosate-resistant cotton, offers producers the ability to integrate pest and crop management strategies and reduce application costs with minimal effect on the crop.

Managing weeds in watermelon is challenging because of the limited availability of herbicides approved for use in this crop. Field experiments on efficacy and crop tolerance were conducted to determine the potential for halosulfuron use in watermelon in Georgia and North Carolina. Halosulfuron was applied PRE, early POST (EPOST; one-leaf watermelon), and late POST (LPOST; watermelon with 30-cm runners) at 26, 39, and 52 g ai/ha. Under weed-free conditions, PRE treatments did not injure watermelon. EPOST and LPOST treatments caused 45 and 34% injury 2 wk after treatment, respectively, averaged over halosulfuron rate. EPOST treatments reduced watermelon fruit number and total weight by 15 and 22%, respectively, and LPOST treatments reduced total fruit weight 12%. Halosulfuron PRE at 39 or 52 g/ha provided 94% or greater control of carpetweed, Palmer amaranth, and smooth pigweed. EPOST treatments controlled 84 and 88% of yellow nutsedge and smooth pigweed, respectively, but LPOST treatments controlled less than 83% of all weed species. Sequential applications of halosulfuron at 26 g/ha PRE and 26 g/ha LPOST controlled 89 to 99% of carpetweed, coffee senna, Palmer amaranth, smooth pigweed, and yellow nutsedge. Our data suggest growers can effectively use halosulfuron PRE in seeded watermelon. However, POST applications should be made only after watermelon has 30-cm runners and as a salvage spot treatment where previous weed control strategies have failed to provide adequate control.

Thirty-one Ohio vineyards were surveyed in 2004 to document weeds that persisted following weed control practices. Weeds were identified and density was determined during visits to each vineyard. Herbicide-use history, grape varieties, and grape age were recorded during interviews with the growers. Data were analyzed by SAS 9.1 with the use of the generalized linear model (GLM), and means were compared according to Student–Newman–Keuls (SNK) at the 0.05 level. Crabgrass, dandelion, pigweed, foxtail, fall panicum, clover, chickweed, common ragweed, smartweed, and oxalis were the most prevalent 10 weeds in Ohio vineyards based on relative abundance values. The frequency and density of crabgrass, dandelion, fall panicum, oxalis, and common purslane were significantly higher in vineyards in which glyphosate was the only herbicide used than in vineyards where other herbicides were applied. The number of species and density were higher in vinifera vineyards that had been hilled for winter protection than in vineyards that had not been hilled.

Field studies were conducted during the 2000 and 2001 growing seasons to evaluate imazapic plus 2,4-D for weed control and bermudagrass tolerance. Imazapic at 140 g ai/ha plus 2,4-D at 280 g ai/ha or greater controlled dallisgrass at least 82% and provided at least 90% control of field sandbur, johnsongrass, and field mint. A mixed stand of Texas panicum and large crabgrass was controlled less than 75% and bahiagrass less than 25% with all rates of imazapic plus 2,4-D. Coastal bermudagrass injury was greater than 24% at three of four locations and Tifton 85 injury was greater than 50% at 3 wk after treatment with rates of imazapic plus 2,4-D as low as 70 plus 140 g/ha. Coastal bermudagrass yield was reduced at the first cutting with imazapic plus 2,4-D at 140 plus 210 g/ha and 240 plus 420 g/ha. Imazapic plus 2,4-D at 200 plus 350 g/ha reduced Tifton 85 forage production at the first two cuttings.

Influence of aqueous leaf extracts, leaf residue, and leached-leaf residue of houndstongue, a noxious rangeland weed, on seedling emergence of forage grasses was studied. Ultraviolet-B (UV-B) effects during houndstongue growth on subsequent germination and growth-inhibitory activity of leaf extracts were investigated. Addition of glasshouse-grown houndstongue leaf extract to mineral soil decreased emergence of crested wheatgrass by 13% and prairie junegrass by 20% at 14 d after sowing. Idaho fescue emergence was unaffected. Incorporation of houndstongue leaf- and leached-leaf residue into soil (0.4 g residue : 20 g soil) delayed emergence of forage grasses. At 14 d after sowing, houndstongue leaf residue spread on the soil surface (0.2 g residue : 20 g soil) tended to inhibit seedling emergence more than leaf residues incorporated into soil. In separate experiments, houndstongue plants were grown at 0, 4, 7, and 11 kJ/m2/d biologically effective UV-B radiation for 6 wk, and leaf extracts (0.5, 1, 2, and 4% wt/v) were prepared. Exposure of houndstongue to increasing UV-B dose during plant growth generally increased the inhibitory activity of their leaf extract on prairie junegrass germination. Crested wheatgrass and Idaho fescue seedlings incubated in extracts of houndstongue leaves exposed to UV-B, compared with leaves grown in a UV-B–free environment, had decreased root lengths. Leaf extracts of plants exposed to elevated UV-B levels had higher absorbance at 300 nm, indicating greater concentration of UV-B–absorbing compounds. This study suggests houndstongue leaf extracts and residues inhibit seed germination and seedling emergence and that UV-B may enhance their allelopathic influence on some forage grasses. Field studies are needed to confirm the allelopathic influence of houndstongue under rangeland conditions.

Failure of glyphosate to control Palmer amaranth was first reported in Arkansas in Mississippi County in June, 2005. The objectives of this research were to (a) confirm glyphosate-resistant Palmer amaranth in Arkansas, and (b) determine the effectiveness of 15 postemergence- (POST) applied herbicides comprising eight modes of action in controlling the glyphosate-resistant biotype compared to glyphosate-susceptible accessions. The LD50 values were similar among three susceptible Palmer amaranth accessions, ranging from 24.4 to 35.5 g ae/ha glyphosate. The resistant biotype had an LD50 of 2,820 g/ha glyphosate, which was 79- to 115-fold greater than that of the susceptible biotypes and 3.4 times a normal glyphosate-use rate of 840 g/ha. The glyphosate-resistant biotype was effectively controlled with most of the evaluated herbicides, but the use of acetolactate synthase-inhibiting herbicides such as pyrithiobac, trifloxysulfuron, and imazethapyr is not a viable option for control of this Palmer amaranth population.

Studies were conducted in 2003 and 2004 over seven environments evaluating rice root growth inhibition (RGI) and foliar injury from penoxsulam at 30 and 60 g ai/ha and bispyribac-sodium at 30 g ai/ha applied to four- to five-leaf rice at three flood timings, 1, 7, and 14 d after herbicide treatment (DAT), for five rice cultivars, ‘Bengal’, ‘Cypress’, ‘Wells’, ‘Cocodrie’, and ‘XP712’. Flooding at 1 and 7 DAT resulted in greater RGI compared with flood at 14 DAT when evaluated 1 wk after flood (WAF). By 2 WAF, RGI was greater with flooding at 1 DAT compared with flooding at 7 DAT for cultivars Bengal, Cypress, and Wells. Analyzing flood timing 1 DAT, bispyribac-sodium reduced root growth of Bengal and Cypress compared with penoxsulam at 30 g/ha at 1 week after treatment (WAT). At 2 WAT, RGI for Cocodrie was higher following penoxsulam at 60 g/ha when compared with bispyribac-sodium. By 3 WAT, RGI was higher following penoxsulam at 60 g/ha when compared with penoxsulam at 30 g/ha for Cocodrie and greater than bispyribac-sodium and penoxsulam at 30 g/ha for Cypress. Foliar injury following penoxsulam at both rates was less than injury following bispyribac-sodium for all cultivars except XP712 at 1 WAT. XP712 resulted in < 5% RGI and < 6% foliar injury at each evaluation. Rice grain yield was not affected by herbicide treatment for any cultivar compared with the standard treatment of propanil plus quinclorac.