Field experiments were conducted in 2004 and 2005 at Clemson, SC, and in 2004 at Clinton, NC, to quantify Palmer amaranth and large crabgrass growth and interference with plasticulture-grown bell pepper over multiple environments and develop models which can be used on a regional basis to effectively time removal of these weeds. Experiments at both locations consisted of an early and a late spring planting, with the crop and weeds planted alone and in combination. Daily maximum and minimum air temperatures were used to calculate growing degree days (GDD, base 10 C) accumulated following bell pepper transplanting and weed emergence. Linear and nonlinear empirical models were used to describe ht, canopy width, and biomass production as a function of accumulated GDD. Palmer amaranth reduced bell pepper fruit set as early as 6 wk after transplanting (WATP) (648 GDD), whereas large crabgrass did not significantly reduce fruit set until 8 WATP (864 GDD). Using the developed models and assuming Palmer amaranth and large crabgrass emergence on the day of bell pepper transplanting, Palmer amaranth was predicted to be the same ht as bell pepper at 287 GDD (20 cm tall) and large crabgrass the same ht as bell pepper at 580 GDD (34 cm tall).

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 experiments were conducted in 1999, 2000, and 2001 to investigate PRE and POST applications of halosulfuron-methyl in combination with clomazone plus ethalfluralin for control of sedge and smooth pigweed in summer squash. Halosulfuron was applied PRE or POST to summer squash at 9, 18, or 27 g ai/ha in combination with a PRE application of clomazone at 175 g ai/ha plus ethalfluralin at 630 g ai/ha. Smooth pigweed control by addition of halosulfuron at 18 and 27 g/ha in combination with clomazone plus ethalfluralin PRE was greater than 89% independent of application method. Yellow nutsedge control was greater than 83% with POST applications of halosulfuron at 18 and 27 g/ha in combination with clomazone plus ethalfluralin PRE. Yellow nutsedge control was greater than 60% from all POST halosulfuron applications at 9, 18, or 27 g/ha in the greenhouse. In a separate field study without ethalfluralin PRE, rice flatsedge control was more than 85% from halosulfuron applied POST at 18 and 27 g/ha. Yellow summer squash and zucchini squash were injured as much as 52 and 47%, respectively, from inclusion of halosulfuron PRE or POST at 27 g/ha in treatments. Summer squash yields were generally not affected by halosulfuron rate, and were comparable to or higher than summer squash treated by only the mixture of clomazone plus ethalfluralin. In these studies, summer squash were injured by halosulfuron applied at 9 to 27 g/ha PRE or POST, yet rapidly recovered, making this herbicide acceptable for use in combination with clomazone and ethalfluralin for controlling several common weed species.

Field trials were conducted in 2003/2004 and 2005/2006 at Reidsville, Georgia, to evaluate the effects of previously applied residual herbicides on onion growth and bulb production. Before transplanting onion, preplant applications of imazapic at 18 and 36 g ai/ha, diclosulam at 7 and 14 g ai/ha, pyrithiobac at 27 and 54 g ai/ha, trifloxysulfuron at 6.6 and 13.2 g ai/ha, diuron at 224 and 448 g ai/ha, and cloransulam at 22 g ai/ha were made. An untreated control was included for comparison. Trifloxysulfuron at 13.2 g/ha, diclosulam at 14 g/ha, pyrithiobac at 54 g/ha, and cloransulam at 22 g/ha injured onion 26, 73, 86, and 86% in 2003/2004, respectively, and 13 to 44% injury in 2005/2006. These same herbicides also reduced yield. Imazapic and diuron injured transplanted onion 6% during both seasons but did not reduce yield. This research suggests imazapic and diuron restrictions could possibly be reduced. However, the onion rotational restrictions for diclosulam, pyrithiobac, trifloxysulfuron, and cloransulam are accurate.

The use of POST herbicides has been limited in sweet corn because of the narrow spectrum of weed control or potential crop injury. Field experiments were conducted to evaluate the 4-hydroxyphenyl pyruvate dioxygenase (HPPD)-inhibiting herbicides mesotrione, tembotrione, and topramezone applied POST in sweet corn at three locations. Efficacy of mesotrione, tembotrione, and topramezone applied alone or mixed with atrazine was compared to other labeled POST herbicides following PRE S-metolachlor. Giant foxtail control was greater with tembotrione or topramezone than mesotrione alone or mixed with atrazine. Common lambsquarters, velvetleaf, and common ragweed were controlled 98% or greater with the HPPD-inhibiting herbicides when mixed with atrazine. Tolerance of six sweet corn hybrids was determined in the field when treated with 1× and 2× rates of these herbicides mixed with atrazine. Tolerance of six sweet corn hybrids to these herbicides was determined in the greenhouse when treated with 0.5, 1, 2, 4, 8, and 16 times the labeled rate. Differential hybrid tolerance to each herbicide was observed in both the field and greenhouse evaluations. Tembotrione killed ‘Merit’ in both evaluations. Excluding Merit, hybrids generally had good tolerance to tembotrione and topramezone in the field, but had differential tolerance to mesotrione. With the exception of Merit, hybrids generally had greater tolerance to tembotrione than topramezone and less tolerance to mesotrione in the greenhouse. These HPPD-inhibiting herbicides provide POST weed control, but the potential for sweet corn injury varies among the herbicides and hybrids and warrants further characterization.

Sulfosulfuron was recently registered for grassy weed control in creeping bentgrass, but turf sensitivity is a concern for intensively managed golf courses. Field and growth chamber experiments in New Jersey investigated creeping bentgrass growth responses and tolerance to sulfosulfuron. Creeping bentgrass chlorosis increased with sulfosulfuron rate but turf had less chlorosis from sequential sulfosulfuron applications compared to bispyribac–sodium. Herbicide-treated turf had similar root weight compared to untreated turf on six sampling dates. In growth-chamber experiments, creeping bentgrass treated with sulfosulfuron had chlorosis and clipping weight reductions exacerbated by reductions in temperature from 25 to 15 C. Overall, creeping bentgrass appears to tolerate sequential sulfosulfuron applications better than or comparable to bispyribac-sodium in early summer, whereas creeping bentgrass sensitivity to sulfosulfuron increases at cooler temperatures.

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.

Swallowwort, a twining perennial of the Asclepiadaceae family, has become increasingly invasive in cultivated fields, orchards, fence rows, natural areas, and roadsides of some provinces in Iran. The ability of this weed to establish and develop from seed was studied in laboratory and greenhouse experiments. Optimum germination temperature was 25 C. Germination did not occur at ≤ 15 C either in dark or light. In temperature fluctuation experiments, maximum germination occurred at 30/22 C, but light fluctuations had no effect on swallowwort seed germination. At pH 4 to 9 seed germination was 44 to 66%. Osmotic potential up to −0.2 MPa had little effect on germination. Seed germination was less than 5% at −0.8 Mpa. Swallowwort seeds were not able to germinate at 300 mM NaCl; however, there was 12% germination at the 200 mM concentration. Maximum emergence occurred when the seeds were planted at 1.5 cm depth. Seedlings did not emerge when planted at 6.5 cm depth. Germination percentage of the seeds kept under immersed (water) conditions was similar to those of nonimmersing periods; however, seedling growth was greater in treatments without immersing periods.

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.

Minimizing inputs such as fertilizers, herbicides, or tillage may be sought by producers to satisfy economic as well as environmental goals. One of the challenges in reducing inputs, whether synthetic fertilizers or herbicides, or substituting a synthetic nutrient with an organic source, is to identify practices that will provide optimum growing conditions for the crop while maintaining an adequate level of weed control. Our objective was to measure the cumulative effects of 12 yr of nitrogen (N) and phosphorus (P) fertilization treatments applied to two tillage systems [conventional tillage (CT) vs. no tillage (NT)] in a corn–soybean rotation on weed communities and crop yields. Residual (postherbicide treatment) weed species assembly was determined by multivariate analysis and was influenced mainly by tillage, with weeds more strongly associated with NT than with CT. Diversity of weed communities as measured by richness, evenness (E), and a diversity index (H′), and total weed biomass were greater for NT than for CT. Nutrient treatments had little or no effect on these parameters. Corn yields were reduced by 70% in the absence of N and by 25% in NT compared to CT treatments. Soybean yields were reduced in NT with increasing P rates compared to other treatments, but reductions never exceeded 10%. Overall, corn and soybean had different responses to treatments, with corn yields being far more affected by fertilization and tillage than soybean yields. Conversely, the absence of tillage had a much greater effect than the absence of nutrient input on weed community assembly and biomass, suggesting the importance of a weed management program specifically tailored for NT systems.

In the semiarid northern Great Plains, the adoption of zero tillage improves soil water conservation, allowing for increased crop intensification and diversification. Zero-tillage crop production relies heavily on herbicides for weed management, particularly the herbicide glyphosate, increasing selection pressure for herbicide-resistant weeds. Barley is well adapted to the northern Great Plains, and may be a suitable herbicide-free forage crop in zero-tillage systems. A 2-yr field study was conducted to determine if planting date influenced crop and weed biomass, water use (WU), and water-use efficiency (WUE) of barley and weed seed production in three preplant weed management systems: (1) conventional preplant tillage with a field cultivator (TILL); (2) zero tillage with preemergence glyphosate application (ZTPRE); and (3) zero tillage without preemergence glyphosate (ZT). None of the systems included an in-crop herbicide. Planting dates were mid-April (early), late May (mid), and mid-June (delayed). Early planting of ZT barley resulted in excellent forage yields (7,228 kg/ha), similar to those from TILL and ZTPRE. Early planting resulted in a small accumulation of weed biomass, averaging 76 kg/ha, and no weed seed production regardless of preplant weed management system. Early planting resulted in higher WU than delayed planting, averaging 289 and 221 mm, respectively, across management systems and years. The WUE of crop and total biomass did not differ among preplant weed management systems at harvest from the early planting date. Delayed planting resulted in decreased forage yield with high amounts of weed biomass and seed production, especially in ZT. A pre-emergence glyphosate application was not necessary for early-planted ZT forage barley. Early planting of herbicide-free barley for forage can be an excellent addition to northern Great Plains cropping systems as part of a multitactic approach for improved weed and water management.

Applications of metribuzin plus MSMA have been used to control goosegrass in bermudagrass turf for over 20 yr. In 2003, two goosegrass biotypes on the island of Kauai, Hawaii were found to be resistant to applications of metribuzin plus MSMA. Metribuzin plus MSMA applied at rates of 0.28 kg ai/ha plus 2.2 kg/ha, respectively, followed by MSMA at 2.2 kg/ha 7 d later, provided 100% control of two susceptible goosegrass biotypes, but no control of two resistant biotypes. At the flowering growth stage, metribuzin applied at a rate of 0.28 kg/ha controlled both susceptible biotypes (> 92%), but did not control the two resistant biotypes. Two applications of MSMA applied at a rate of 2.2 kg/ha (7 d apart) did not control any of the four biotypes at the flowering growth stage. The two resistant biotypes were approximately 100 to 200 times less sensitive to increasing rates of metribuzin than the two susceptible biotypes. The two biotypes that were resistant to metribuzin plus MSMA were also resistant to applications of simazine plus MSMA. However, all four biotypes were susceptible to applications of glyphosate and foramsulfuron.

Diflufenzopyr is an auxin-transport inhibitor that can increase the phytotoxicity of certain auxin-mimicking herbicides such as dicamba on broadleaf species. Dicamba is commonly used alone and in combination with other auxin herbicides for broadleaf weed control in various species of turfgrass. Dicamba efficacy applied over a series of rates either alone or as an admixture with either 20 or 40% by weight of diflufenzopyr relative to the weight of dicamba was evaluated on purple cudweed and common lespedeza. The 20% admixture reduced the LD50 of dicamba on purple cudweed from 23 to 20 g/ha. Similarly, LD50 on common lespedeza was reduced from 36 and 27 g/ha. The 20% admixture was 13 and 25% more active than dicamba alone for these two weed species, respectively. However, the synergistic benefit was limited to a relatively narrow range of rates that are below the minimal registered rate of dicamba. Turfgrass injury, as expressed by the suppression of foliage growth, was similar whether dicamba was applied alone or with diflufenzopyr for all species evaluated except St. Augustinegrass. The admixture was less injurious than dicamba alone in St. Augustinegrass. The synergistic benefit with respect to weed control was obtained without a corresponding increase in injury on the turfgrasses.

Late-season weed infestations often do not affect yields and are allowed to mature and contribute seed to the soil seedbank, ensuring the future establishment of competitive weed complexes. Effective long-term weed management strategies must incorporate practices to reduce late-season weed seed production by weed complexes. Field studies were conducted to determine the effects of late-season glyphosate applications on seed production of barnyardgrass, Palmer amaranth, pitted morningglory, prickly sida, and sicklepod. Although sequential 0.42-kg ae/ha glyphosate applications initiated when the first weed species in the complex flowered and repeated every 10 d was the most effective treatment and reduced seed production of all species by ≥ 95%, the most practical treatment was a single 0.84-kg/ha glyphosate application at pitted morningglory flowering, suppressing seed production of barnyardgrass, Palmer amaranth, pitted morningglory, prickly sida, and sicklepod by 88, 83, 98, 95, and 99%, respectively. This research demonstrates that annual contributions by a weed complex to the soil seedbank can be significantly and practically reduced by a single late-season glyphosate application.

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.

Open field production of fruit and nut-tree nursery stock depends upon preplant soil fumigation, extensive tillage, and hand-labor throughout the growing season for adequate weed control. Because methyl bromide, the favored fumigant, is being phased out because of environmental concerns and the costs of both fuel and labor continue to rise, herbicides are likely to become a more important weed management tool in the tree nursery industry. Two trials were conducted to evaluate weed control and crop safety with several herbicides applied following fumigation with methyl bromide or 1,3-dichloropropene in central California stone-fruit nurseries. PRE and POST-directed applications of several labeled and unlabeled materials were applied in a band over seeded peach rootstock or applied after emergence with a drop-nozzle spray boom. Crop productivity and weed control were monitored throughout the 1-yr growing season. PRE oryzalin and dithiopyr treatments provided the best weed control with very little crop injury. PRE applications of flumioxazin, rimsulfuron, and sulfentrazone did not have adequate crop safety at the rates and timings tested. However, POST-directed applications of flumioxazin and rimsulfuron were much safer to the peach and almond crops and should be evaluated in future trials. Additional herbicides and application techniques are needed to find acceptable, safe control measures for weeds, such as California burclover, common mallow, and redstem filaree, which often are poorly controlled with preplant fumigation in tree nurseries.

Indiana growers who purchase restricted-use pesticides are required to show the agrochemical dealer their private pesticide applicator license before the transaction can be completed. To keep the license current, growers may attend three private applicator recertification meetings or retake the exam every 5 yr. The meetings are county-based and must be a minimum of 2 h in length; they must include a topic mandated by the Office of the State Chemist. During the 2005/2006 private applicator recertification program (PARP) cycle, off-site movement of atrazine into surface water was the regulatory topic presented to 2,887 participants at 69 meetings. A team of individuals from different disciplines prepared educational materials to support the regulatory topic of the year. Surveys to assess grower awareness were conducted at the meetings and 1 yr later to monitor the long-term impact of the educational effort, and to evaluate which tactics were being adopted to reduce off-site movement of atrazine. Growers farming more than 800 ha had a higher degree of concern than small growers regarding the loss of atrazine as a weed management tool. Eighty-nine percent of the growers thought there would be a 314 to 1,255 kg/ha yield loss if atrazine was removed from the marketplace. Eighty-four percent of the growers estimated that weed control costs would increase $15 to $25/ha if no other products were available to replace atrazine. The three most acceptable management strategies to reduce atrazine movement were: more attention to label setback distances, establishment of grass filter strips around surface water, and reducing atrazine rates by tank mixing with other herbicides. The results of this project indicated that statewide programs such as this are effective in increasing awareness of an issue and documenting the impact of extension education programs.

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.

Smutgrass is a clump-type grassy weed that is commonly found in grazed pastures throughout the Southeast. Currently, the only herbicide registered for control of smutgrass is hexazinone and it is unknown if hexazinone negatively impacts bermudagrass forage growth and quality. Experiments were conducted to determine the impact of hexazinone rate on bermudagrass yield and crude protein. All rates of hexazinone reduced bermudagrass yield at 4 wk after treatment (WAT). From 6 to 12 WAT, no bermudagrass yield reduction was observed except for the 2 kg/ha treatment. The yield reducing effects of hexazinone were transitory at the 0.5 and 1.0 kg/ha rates; cumulative yield for the season was reduced by 17 and 23%, respectively. Although yield was negatively influenced by hexazinone, crude protein generally increased in a linear fashion with increased hexazinone rate.

Field trials were conducted at three California locations near Oxnard, Salinas, and Watsonville from 2002 to 2006 to evaluate broadleaf weed control and tolerance of strawberry to oxyfluorfen. Oxyfluorfen applied at 0.3 and 0.6 kg/ha before strawberry transplanting reduced densities of broadleaf weeds such as California burclover, hairy nightshade, little mallow, shepherd's-purse, and yellow sweetclover 70 to 100% compared with nontreated plots but did not control horseweed. Oxyfluorfen application resulted in 9% and 19% greater visible injury to strawberry for the two rates, respectively, compared with nontreated plants in 1 yr but did not reduce strawberry yield. After oxyfluorfen application at 0.6 kg/ha, strawberry plants had 5 to 48% more injury than nontreated plants in subsequent years but early-season yields were similar. Hand-weeding time was reduced 30 to 50% compared with nontreated plots regardless of oxyfluorfen rate. Both water-based and solvent-carrier formulations of oxyfluorfen resulted in similar weed control, strawberry injury, and fruit yield. Plastic mulch installation after oxyfluorfen application but before planting reduced injury to strawberry more than 50% compared with nonmulched beds. Oxyfluorfen applied 30 d before strawberry transplanting had similar crop injury and yield to applications made 15 and 7 d before planting. These results suggest that oxyfluorfen can be used safely in California plasticulture strawberry production for control of common weed species and to reduce labor inputs associated with hand weeding.