Field experiments were conducted to determine suitable application timings and rates of MON 37500 for downy brome control in winter wheat. MON 37500 applied preemergence (PRE) or in fall postemergence (POST) provided better control of downy brome than when applied spring POST. MON 37500 at rates ranging from 19 to 33 g ai/ha applied PRE or fall POST controlled downy brome >85%. MON 37500 applied spring POST at 60 g/ha only suppressed growth of downy brome. Winter wheat was not visibly injured and maturity was not delayed by MON 37500 applied up to 60 g/ha at any of the application timings. Winter wheat yield responded positively to all MON 37500 treatments but PRE or fall POST applications usually resulted in greater yields than spring POST applications. Wheat yields progressively increased with increasing rates of MON 37500 up to 30–40 g/ha, above which yields tended to plateau or, in two instances, decline slightly. MON 37500 is an important new herbicide that should enable growers to better manage downy brome in winter wheat production systems.

Common St. Johnswort is widespread in eastern Canada but it seldom constitutes a serious weed problem. A demographic study conducted in 1993 and 1994 at four typical undisturbed sites indicated that 36 to 96% of established St. Johnswort shoots died during the growing season. Mortality was always associated with infection by a host-specific Colletotrichum gloeosporioides. The leaf-feeding beetle Chrysolina hyperici occurred at all sites and caused maximum midsummer defoliation of 27% in 1993 and 51% in 1994. Healthy plants readily recovered from defoliation during pupation of the fourth instar of the insect in June and following adult estivation in August. Although widespread, C. hyperici populations appear transient and alone do not cause sustained feeding pressure resulting in weed control. When Chrysolina larvae and adults were collected at six field sites and placed on healthy seedlings under controlled conditions, up to 36% of the plants became infected with C. gloeosporioides. Scanning electron micrographs commonly showed Colletotrichum conidia among the setae on legs, tarsal pads, and antennae of adults and larvae. In a series of three experiments conducted under controlled conditions in which Chrysolina larvae and adults were placed on healthy plants after feeding on diseased ones, the incidence of infection ranged from 63 to 100%. Hence, under favorable conditions Chrysolina adults may selectively transmit the pathogen in the field. This study demonstrated the potential of enhancing biological control of weeds by insects with the integration of an effective, host-specific pathogen.

An experimental turfgrass growth regulator, V-10029, was compared with trinexapac-ethyl, a growth regulator used commonly on highly maintained turfgrasses, to evaluate growth suppression patterns and suppression of seedhead formation; the latter was evaluated on turfgrass species and annual bluegrass, a weedy species that appeared and developed seedheads in tall fescue. Plugs of creeping red fescue, Kentucky bluegrass, perennial ryegrass, and creeping bentgrass were taken from the field into a greenhouse. V-10029 at three rates (0.015, 0.029, and 0.059 kg/ha) was compared to an untreated control and trinexapac-ethyl at a label rate (0.382 kg/ha for perennial ryegrass and 0.287 kg/ha for the other four species). Of the eight replications for each treatment, four were not mowed for the purpose of evaluating suppression of seedhead formation, and four were used for weekly clipping collection to evaluate growth suppression. Compared to the untreated control, V-10029 at all three applied rates caused significant seedhead suppression in both tall fescue infested with annual bluegrass (> 90%) and perennial ryegrass (50 to 80%). Trinexapac-ethyl was not as effective, causing seedhead suppression levels of 57% in annual bluegrass found in tall fescue and 43% in perennial ryegrass. V-10029 caused unacceptable levels of discoloration (> 20%) in all turfgrass species, and discoloration increased with increasing rates of application. Patterns of growth suppression for tall fescue, Kentucky bluegrass, and perennial ryegrass, in response to V-10029 at all rates and to trinexapac-ethyl, were similar on a percent of suppression basis. Growth of creeping bentgrass was suppressed by V-10029 only at high rates. In contrast, creeping red fescue was significantly injured by V-10029 at all rates. The greatest growth suppression, in response for all treatments, occurred 2 to 3 wk after application. The effect of trinexapac-ethyl dissipated after 4 wk, while V-10029 was effective until 5 to 7 wk after treatment. Observed effects of V-10029 were consistent with its activity as an ALS-inhibiting herbicide and, therefore, as a Class D turfgrass growth regulator.

Field experiments evaluated pitted morningglory, velvetleaf, and barnyardgrass control in soybean with imazethapyr applied alone and with either crop oil concentrate at 1.0% v/v, a blend of methylated seed oil and organosilicone surfactant, or a blend of methylated seed oil, phosphate esters, and organosilicone surfactant at 0.5% v/v, each with and without a 28% N mixture of urea and ammonium nitrate (URAN) applied at 2.3 L/ha. Treatments were applied at spray volumes of 94 and 9.4 L/ha. Control of each species was often increased with the addition of both oil adjuvants and URAN. The blend of methylated seed oil, phosphate esters, and organosilicone surfactant and the blend of methylated seed oil and organosilicone surfactant were each more effective than crop oil concentrate on pitted morningglory and barnyardgrass, while each oil adjuvant was similar on velvetleaf. The URAN effectively increased imazethapyr control of all species. Control of all species was similar at the spray volume of 9.4 L/ha and at 94 L/ha.

The practical application of yield loss prediction models using relative leaf area of weeds is limited due to the lack of a quick and accurate method of leaf area estimation. Leaf cover (the vertical projection of plant canopy on the ground) can be used to approximate leaf area at early stages of plant development. An automated digital image analysis system for measuring leaf cover has been developed. The system has an operator-assisted module aimed at validating the automated functions. The objective of this research was to demonstrate the accuracy of the operator-assisted module under different weed–crop conditions. A laboratory experiment was conducted using simulated weed–crop populations. Two additional field experiments were conducted using corn in competition with: (1) common lambsquarters, barnyardgrass, or a mixture of both species, and (2) a natural weed community. In the laboratory experiment, a narrow linear relation was observed between leaf cover estimated with the operator-assisted module and leaf area measured with an optical area meter (r 2 > 0.98). In field experiments, the regression between corn leaf cover estimated by the operator-assisted module and corn leaf area measured with the optical area meter was not as good (r 2 < 0.55). The poor performance of the module was probably due to the overlapping and the architecture of corn leaves (especially unexpanded leaves). Nevertheless, the system showed high precision in estimating leaf area of both grassy weeds and broadleaf weeds (r 2 > 0.89). Generally, the accuracy of the estimates decreased as the growth stage became more advanced. Apart from its initial purpose as a calibration tool for the automated system, the operator-assisted module can have several potential research applications. It can be used: (1) as an alternative to destructive leaf area measurement at early stages of plant development, (2) as a tool in the study of plant competitive ability, and (3) as an objective and quantitative support to visual observations.

Sulfometuron at 17 g ai/ha in the planting furrow (13% of the anticipated fallow-field application rate) inhibited sugarcane emergence and development and ultimately reduced sugar yields in the initial production year by 13% when compared to a weed-free control that contained no herbicide in the planting furrow. Residual levels of metribuzin in the planting furrow representing 100% of the standard fallow-field application rate of 1,680 g ai/ha had no adverse effect on sugarcane development or sugar yield. When applied only to the soil surface immediately after planting, sulfometuron did not injure sugarcane, and sugar yields were equivalent to standard, at-planting, preemergence (PRE) applications of either metribuzin at 2,020 g/ha or a mixture of pendimethalin plus atrazine each at 2,240 g ai/ha. To minimize sugarcane injury, sulfometuron should be kept out of the germinating zone of lateral buds along planted sugarcane stalks.

In field studies, in 1993, 1994, and 1995, clomazone at 390 g ai/ha and rimsulfuron at 35 g ai/ha were evaluated preplant incorporated (PPI) and postemergence (POST), respectively, for weed control and crop response in transplanted ‘Keystone RG3’ bell pepper. Clomazone did not injure bell pepper. Common lambsquarters and jimsonweed control was 77 to 95% by clomazone treatments but was variable by rimsulfuron treatments at 41 to 93% common lambsquarters control and 42 to 68% jimsonweed control; common ragweed control by both herbicides was unacceptable. Injury by rimsulfuron to bell pepper in the field was 19 to 47% at 21 DAT. In the greenhouse, injury to Keystone RG3 and three additional bell pepper varieties, ‘Camelot,’ ‘Jupiter,’ and ‘Memphis’ was similar at 44 to 62% by rimsulfuron at 17 to 35 g/ha POST, and bell pepper had lower height and dry weight than untreated controls. In the greenhouse, jimsonweed control was below 49% and black nightshade control was below 23% by 17 to 35 g/ha rimsulfuron. In these studies, clomazone controlled common lambsquarters and jimsonweed without injury to bell pepper. The solanaceous weeds treated in this study were not well controlled by rimsulfuron, and bell pepper was not sufficiently tolerant to rimsulfuron to permit its use in this crop.

The effect of adding a spray adjuvant to SAN 582H tank mixtures with fluazifop-P, imazethapyr, and sethoxydim was evaluated. SAN 582H synergistically increased broadleaf signalgrass control with reduced rates of all three postemergence (POST) herbicides when no spray adjuvant was used and when crop oil concentrate was added. For example, broadleaf signalgrass control increased from 50% to 83% when SAN 582H was tank-mixed with 52 g ai/ha sethoxydim and crop oil concentrate. In another experiment, several formulations of SAN 582H, including blank solvent-only formulations (no SAN 582H), were evaluated in combination with a reduced rate of sethoxydim to determine the source of synergism from tank mixtures. The SAN 582H molecule, not the carrier solvents in formulated product, was determined to be the source of synergism. The synergistic properties of SAN 582H were compared to other chloroacetamides. Synergism from acetochlor was similar to SAN 582H when applied POST with a reduced rate of either fluazifop-P, imazethapyr, or sethoxydim for grass control. Metolachlor also synergistically increased the control of grasses with the POST herbicides; however, metolachlor caused considerable phytotoxicity when applied alone and synergistic interactions were detected less frequently. The efficacy of sethoxydim mixed with SAN 582H was evaluated under different soil moisture conditions. Broadleaf signalgrass control increased from 81% to 93% under dry, moisture-stressed conditions when 210 g/ha sethoxydim was tank- mixed with SAN 582H.

Field studies to evaluate the response of cotton to flame cultivation were conducted in 1994, 1995, and 1996. Flame cultivation at different liquid propane-gas (LP-gas) pressure settings (100 and 175 kPa) with or without watershield protection was applied to cotton at two stages of growth (20 to 25 cm and 40 to 45 cm tall). In 1994, younger cotton was more susceptible to flame cultivation than older cotton, and flaming at an LP-gas pressure of 175 kPa without watershield protection increased plant injury regardless of growth stage, whereas injury was not evident in 1995 and 1996. Injury caused by flame cultivation was a transient browning of lower leaf margins. The use of a watershield sprayed above the burner reduced injury. Flame cultivation applied to cotton 20 to 25 cm tall slightly elevated the first fruiting node, but no effects were noted on other vegetative and reproductive growth parameters or yield regardless of pressure setting or watershield protection. Flame cultivation applied to cotton 40 to 45 cm tall did not affect the position of the first fruiting node. Although flame cultivation caused some injury one year, total seed cotton and cotton lint yields were not reduced.

Nine corn inbreds were selected from a test on 250 inbreds based on distinctly different response to herbicides. These inbreds showed up to 50,000-fold differences, which varied by inbred and herbicide. Only the inbred GA209 was sensitive to bentazon and the four sulfonylurea herbicides. GA209 and B90 were the most sensitive inbreds to the sulfonylurea herbicides. F2R was generally the most tolerant inbred. The pattern of herbicide tolerance varied greatly. For example, the relative tolerance of F2 to thifensulfuron was 12-fold more than to rimsulfuron while Mo17 was 56-fold more tolerant to rimsulfuron than thifensulfuron. F2 was fivefold more tolerant of chlorsulfuron than chlorimuron, whereas B80 was 100-fold the reverse. These inbreds provide a small subset of varieties to evaluate the large and inherent differences in herbicide tolerance of corn.

Producers rely on cultural practices to manage downy brome, jointed goatgrass, and feral rye in winter wheat because there are no effective herbicides for in-crop control. This study characterized seedling emergence, growth, and development of these winter annual grasses, with the goal of suggesting or improving cultural control strategies. Feral rye seedlings emerged within 4 wk, whereas downy brome and jointed goatgrass seedlings emerged over a 10-wk period. Emergence patterns of these grasses suggest that delay of winter wheat planting may be effective in reducing feral rye densities, but this strategy most likely will be ineffective with downy brome or jointed goatgrass. Downy brome began anthesis 1 to 2 wk earlier than the other two grasses and winter wheat. Both downy brome and jointed goatgrass were shorter than winter wheat during the growing season, whereas feral rye was at least as tall as wheat. Producers mow infested wheat to prevent weed seed production, but this practice may not be effective with jointed goatgrass and downy brome because of their short stature and downy brome's earlier development. Conversely, mowing has potential in preventing feral rye seed production. The grasses produced between 340 and 770 seeds/ plant.

Field experiments were carried out from 1987 to 1990 to develop an integrated weed control management system for the control of blackgrass in winter wheat in Greece. The weed control treatments included both herbicides and cultural measures. The performance of preemergence (PRE) herbicides against blackgrass was better than postemergence (POST) herbicides; selectivity was dependent on application rate. Late timing of wheat sowing resulted in significant reduction of the blackgrass population competing with wheat. The critical period for blackgrass infestation occurred 4.0 to 5.5 wk after wheat emergence. Blackgrass density of 170 to 1,170 plants/m2 caused wheat yield reduction equal to 10 and 30%, respectively. A quadratic relationship describing the percent reduction of wheat yield due to competition with different blackgrass densities was developed. The economic threshold level for blackgrass varied between 100 and 125 plants/m2. The results emphasize the importance of using a combination of chemical and cultural controls, including critical periods for herbicide application in maintaining an integrated approach for blackgrass control in wheat.

Purple nutsedge collections from 13 states within the continental United States and from 21 other locations around the world were studied to compare their reproduction and morphological characteristics. Differences occurred in the number of shoots produced from single tubers, the number of leaves per shoot, and the length and width of leaves. Differences were also found with respect to flowering, length of culms supporting the inflorescences, and number, length, and width of involucral bracts at the apex of the culms. Variations occurred in the flower parts, including the number and length of rachises and the length of rachillae and spikelets. There were differences in the growth patterns of the leaves, with some collections having leaves mostly erect and others having leaves lie more closely to the ground. The color of the leaves varied between light and dark green, and the color of the inflorescence varied between light and dark purplish-brown. Greater variation was found among plants from different locations around the world than from within the continental United States.

Glyphosate was evaluated at 0.8, 1.3, and 1.7 kg ae/ha applied at the two-leaf, four-leaf, or two- to three-tiller growth stage for red rice control. In addition, red rice seedheads were counted concurrently with soybean harvest at each of three locations to assess treatment effect on seedhead reduction. Field studies were conducted at Starkville, MS, in 1994 and 1995 and Shaw, MS, in 1995. A significant rate response was not observed for red rice control 2 and 4 wk after treatment (WAT) or for seedhead reductions. Glyphosate controlled red rice 88, 91, and 88% 2 WAT when applied to two-leaf, four-leaf, or two- to three-tiller red rice, respectively. Due to subsequent seedling emergence, control from glyphosate applications to two- or four-leaf red rice 4 WAT was 51 and 84%, respectively. Red rice treated at the two- to three-tiller stage was controlled 91% 4 WAT. When compared to the nontreated control, seedheads were reduced 97% by two- to three-tiller applications, compared to 87 and 56% reductions from four- and two-leaf applications, respectively.

The contribution of nitrogen (N) fertilizer placement to management of annual weeds in spring wheat grown under two tillage systems was examined at sites located in three ecoregions of the northern Great Plains from 1994 to 1996. The effect of three different fertilizer placements on narrowleaf and broadleaf weed emergence, growth, and N uptake were determined in a no-till and a conventional tillage system. Fertilizer was broadcast, subsurface-banded in the spring prior to seeding, or side-banded at the time of seeding. Tillage system generally did not influence weed and crop response to fertilizer placement. Broadcast-applied fertilizer was more effective than banded fertilizer in promoting wild oat and broadleaf weed emergence and growth over the growing season. Weed densities, biomass, and N uptake measured early in the growing season averaged 20 to 40% less, and grain yield of wheat at maturity averaged 12% higher where fertilizer was side-banded compared to broadcast. In contrast, the addition of fertilizer, regardless of placement, was detrimental to green foxtail because of enhanced crop competitiveness. Banding fertilizer at recommended rates can be an effective cultural practice for managing weeds in no-till and conventional tillage wheat-cropping systems in semiarid to subhumid regions of the northern Great Plains, but it is not reliable when used as the sole method of weed management.

Little is known about the biology and control of hophornbeam copperleaf, a weed of increasing importance in the Midwest. More than 2 wk of cold stratification and a 0.2% KNO3 solution increased germination of hophornbeam copperleaf. Germination at constant 30 C was 47% and alternating 30/20 C was 65%. Scarification did not increase hophornbeam copperleaf germination. Within the first 600 growing degree days after soybean planting, plant height, leaf area, and dry matter accumulation of hophornbeam copperleaf grown in soybean and alone were similar. Subsequently, leaf area and dry matter accumulation of hophornbeam copperleaf grown alone were greater than of those grown in soybean. In contrast, plant height of hophornbeam copperleaf grown in soybean was greater than when grown alone. Hophornbeam copperleaf grown alone produced up to 12 510 seeds/plant, whereas hophornbeam copperleaf grown with soybean produced 980 seeds/plant. Of 13 postemergent herbicides evaluated on hophornbeam copperleaf in soybean, only lactofen, acifluorfen, and fomesafen controlled 80% or more. Lactofen at 210 g ai/ha consistently controlled more than 95% of the hophornbeam copperleaf.

The objectives of this study were to determine how the timing of weed management treatments in winter wheat stubble affects weed control the following season and to determine if spring herbicide rates in corn can be reduced with appropriately timed stubble management practices. Field studies were conducted at two sites in Ohio between 1993 and 1995. Wheat stubble treatments consisted of glyphosate (0.84 kg ae/ha) plus 2,4-D (0.48 kg ae/ha) applied in July, August, or September, or at all three timings, and a nontreated control. In the following season, spring herbicide treatments consisted of a full rate of atrazine (1.7 kg ai/ha) plus alachlor (2.8 kg ai/ha) preemergence, a half rate of these herbicides, or no spring herbicide treatment. Across all locations, a postharvest treatment of glyphosate plus 2,4-D followed by alachlor plus atrazine at half or full rates in the spring controlled all broadleaf weeds, except giant ragweed, at least 88%. Giant foxtail control at three locations was at least 83% when a postharvest glyphosate plus 2,4-D treatment was followed by spring applications of alachlor plus atrazine at half or full rates. Weed control in treatments without alachlor plus atrazine was variable, although broadleaf control from July and August glyphosate plus 2,4-D applications was greater than from September applications. Where alachlor and atrazine were not applied, August was generally the best timing of herbicide applications to wheat stubble for reducing weed populations the following season.

Drift of glyphosate may cause injury to nontarget plant species. Herbicidal damage caused by glyphosate may be attributed to the herbicides that inhibit acetohydroxyacid synthase (AHAS) or acetyl CoA carboxylase (ACCase), because the time taken to show herbicidal damage and the visual injury symptoms caused by these three types of herbicides are similar. Glyphosate inhibits the biosynthesis of aromatic amino acid biosynthesis which causes accumulation of shikimate in plants. A modified, simpler method of extraction and assay for detection of shikimate in plants has been adapted from previously published procedures. With this assay, rapid accumulation of shikimate was observed in glyphosate-sensitive plants that have been treated with glyphosate. Furthermore, the accumulation of shikimate in herbicide-sensitive plants was only due to glyphosate and not due to an AHAS or an ACCase inhibitor. Therefore, high levels of shikimate in a damaged plant indicate glyphosate injury. With this assay, no accumulation of shikimate was seen in a glyphosate-resistant variety of soybean. Therefore, lack of accumulation of shikimate in a glyphosate-treated weed or crop species indicates that the plant is resistant to glyphosate.

Producers in the northern Great Plains are exploring alternative crop rotations, with the goal of replacing spring wheat-fallow. We characterized the weed associations occurring with tillage system and nitrogen level in two rotations, spring wheat (SW)-fallow (F) and SW-winter wheat (WW)-sunflower (SUN). Weed density was measured 10 yr after initiation of the study. With both rotations, weed community density was highest with no-till. For SW-F, green foxtail, yellow foxtail, and fairy candelabra comprised 99% of the weed community, whereas 13 species were observed in SW-WW-SUN. Fairy candelabra, a rangeland species, was observed only in the no-till system of SW-F. In SW-WW-SUN, no-till favored kochia, Russian thistle, and foxtails, whereas common lambsquarters and annual sowthistle were more common in tilled systems. Nitrogen fertilizer increased crop competitiveness in SW-WW-SUN with no-till, subsequently reducing weed density. Cultural strategies that disrupt weed associations will aid producers in managing weeds.

Field experiments were conducted to compare barnyardgrass control and rice grain yield following a single postemergence (POST) application of propanil plus clomazone with single or repeat POST applications of propanil alone or single POST applications of propanil plus pendimethalin, molinate, quinclorac, or thiobencarb. In four of 10 experiments, propanil plus clomazone controlled barnyardgrass better than single or repeat applications of propanil alone or single applications of propanil plus pendimethalin, molinate, quinclorac, or thiobencarb. The most consistent increase in rice yield over a single application of propanil occurred where clomazone was applied in mixture with propanil.