Studies were conducted with one glyphosate-resistant and three conventional soybean cultivars to determine the effects of reducing row spacing from 76 to 38 and 19 cm, in combination with increased soybean population and herbicide applications (single and sequential), on sicklepod control and soybean yield. ‘Hutcheson’ was the only cultivar, and only under adverse conditions (1998), for which the 19-cm rows provided greater sicklepod control than the 38-cm rows. It was only under optimum conditions (1997), and only with conventional cultivars and sequential herbicide applications, that the 19-cm–row yield of 3,350 kg/ha was 21% greater than the 38-cm–row yield in medium population (455,375 plants/ha), and 21 and 64% greater than the 19- and 76-cm–row yield in low populations (241,000 plants/ha), respectively. In 1998, the same treatment showed similar late-season sicklepod control (> 80%) and a yield of 1,890 kg/ha, with no difference between the 19- and the 38-cm rows; but the yields were 15 and 24% greater than the yield of the narrow (19 and 38 cm) and that of the 76-cm row in low populations, respectively. Low populations with sequential applications in narrow rows and in the 76-cm row showed similar late-season sicklepod control (59 to 70%) and yield in 1998; but in 1997, the narrow rows showed 18 and 32% greater late-season control and yield than the 76-cm row, respectively. In 1997 and 1998, for ‘Hartz 5088RR’ (glyphosate resistant) with medium population and narrow rows, the single and sequential applications provided similar late-season sicklepod control (> 80%) and yield, and at least 10 and 24% greater yield than the narrow and the 76-cm rows in low populations, respectively.

A field study was conducted over 2 yr to evaluate red morningglory control and sugarcane injury with herbicides applied below the sugarcane canopy in late May to early June following the layby cultivation. Rainfall to activate herbicides was received within the first 2 wk after application at the two sites in 1997, but not at the two sites in 1998. Red morningglory control for most herbicides 21 d after treatment (DAT) in both years was at least 90% and was attributed to postemergence activity of the herbicides and limited weed reinfestation following cultivation and herbicide application. At 45 DAT, red morningglory control with sulfentrazone was maximized (87 to 100%) at 0.14 kg ai/ha at three of four sites. Control with azafeniden was less consistent and, to achieve the same level of control as for sulfentrazone, 0.42 kg ai/ha azafeniden was needed in 1997 and 0.84 kg/ha in 1998. The currently registered herbicides atrazine, diuron, metribuzin, and terbacil controlled red morningglory no more than 83%, 45 DAT at three of the four sites. Of these herbicides, terbacil was the least effective. Even in 1997 at one site when the herbicide was rainfall activated early, red morningglory control with sulfentrazone at 0.14 kg/ha and azafeniden at 0.42 kg/ha 45 DAT was still greater than with the currently registered herbicides. Sugarcane injury expressed as foliage discoloration 21 DAT was 7 to 18% for sulfentrazone at 0.14 kg/ha and 15 to 31% for azafeniden at 0.42 kg/ha. Plant recovery was rapid and none of the herbicide treatments evaluated reduced sugarcane stalk height or population in September.

Field experiments were conducted in 1999 and 2000 to evaluate reduced-rate split-applied treatments of imazamethabenz and ICIA 0604 for wild oat control in hard red spring wheat. Single herbicide treatments were applied at standard timing to wild oat at the two- to four-leaf stage. The first split-applied treatments were made when most of the wild oat had emerged but prior to the two-leaf stage. The second split-applied treatments were made after the surviving wild oat had visibly recovered and prior to the two-leaf stage of late-emerging wild oat. Split applications of imazamethabenz or ICIA 0604 allowed the herbicide rate to be reduced by half while maintaining or improving wild oat control. Both imazamethabenz and ICIA 0604 were most effective with methylated vegetable oil adjuvants. Wheat yield and net returns were greatest for half-rate split applications of imazamethabenz and ICIA 0604 in both years, likely because of less early-season wild oat competition and reduced wheat injury caused by ICIA 0604.

A population model was constructed to simulate the development of imazamox-resistant jointed goatgrass (AEGCY) in imazamox-resistant (Clearfield™) wheat. The model computed changes in the surface and in the buried AEGCY seed banks for both resistant and susceptible biotypes. Simulations started with an initial density of 1,000 susceptible and zero resistant seeds/m2 in each seed bank. Simulation of continuous, no-till Clearfield wheat production resulted in rapid development of resistant AEGCY without hybridization with wheat and in extremely rapid resistance development with hybridization. In less than 10 yr, the resistant population was growing exponentially in both simulations. Adding a fallow year with tillage into the simulated rotation did not substantially slow down the appearance of resistance but did delay the rate of resistant population increase by several orders of magnitude over 10 yr. Alternating Clearfield and a nonresistant winter wheat in combination with fallowing prevented the establishment of a significant resistant AEGCY population and prevented the susceptible seed population from increasing exponentially. These projections suggest that imazamox-resistant wheat can be a tool for managing AEGCY populations especially if combined with rotations that include fallow and crops other than Clearfield winter wheat.

Two field trials were established in Virginia during 1998, 1999, and 2000 to evaluate long-term mugwort control with varying rates of dicamba, triclopyr, clopyralid, picloram, metsulfuron, glufosinate, glyphosate, and the dimethylamine salt and the isooctyl ester of 2,4-D. A logarithmic sprayer was utilized in the first field trial to evaluate mugwort control with each of these herbicides, and, except for metsulfuron, rates ranged from 0.28 to 8.9 kg ai/ha. Mugwort control with metsulfuron was evaluated at a lower rate range. In addition, mugwort control was evaluated with a logarithmic range of pelargonic acid rates in combination with constant rates of glyphosate, glufosinate, and the dimethylamine salt of 2,4-D. Complete control of mugwort plants and rhizomes was achieved at 1 yr after treatment with picloram at rates ≥ 0.28 kg ai/ha, with clopyralid at rates ≥ 4.4 kg ai/ha, and with glyphosate at 8.9 kg ai/ha. Greater than 80% mugwort control was also achieved at 1 yr after treatment with clopyralid at rates ≥ 0.28 kg/ha, with glyphosate at rates ≥ 4.4 kg/ha, and with dicamba at 8.9 kg ai/ha. However, all rates (≤ 8.9 kg ai/ha) of glufosinate, triclopyr, and the dimethylamine salt and the isooctyl ester of 2,4-D provided less than 50% mugwort control at 1 yr after treatment. Similar results were obtained with metsulfuron at rates ≤ 0.063 kg ai/ha. The addition of pelargonic acid to glyphosate, glufosinate, or the dimethylamine salt of 2,4-D did not significantly enhance mugwort control when compared with applications of these herbicides alone. Additionally, progressively higher rates of pelargonic acid in combination with glyphosate, glufosinate, or the dimethylamine salt of 2,4-D resulted in antagonism of these herbicides. In separate field trials conducted in 2000, reduced rates of picloram were evaluated. In these dose–response trials, picloram at 0.28 kg/ha was the lowest rate that consistently provided greater than 90% control of mugwort at 3 mo after treatment. Collectively, the results from both trials indicate that excellent long-term mugwort control may be achieved with relatively low use rates of picloram (0.28 kg/ha) and clopyralid (0.28 kg/ha), but higher rates were needed for glyphosate (8.9 kg/ha) and dicamba (8.9 kg/ha). Results also indicate that little to no long-term control of mugwort will be achieved with applications of glufosinate, metsulfuron, triclopyr, or the dimethylamine salt and the isooctyl ester of 2,4-D, even at exceptionally high use rates.

Common cocklebur is an adaptable species found in diverse environments. We grew 28 common cocklebur accessions from 14 states at Urbana, IL, to compare their growth, physiology, and morphology in a common environment to determine if regional control recommendations of specific biotypes might be practical. Accessions were evaluated for midseason and maximum heights and widths, flowering date, insect damage, and petiole color in 1998 and 1999. Weight of aboveground tissue, photosynthetic rate, stomatal conductance, chlorophyll content, and leaf and bur morphology descriptions were also taken in 1999. Midseason and maximum widths, midseason height, flowering date, and insect damage differed significantly based on the year of evaluation. The year by accession interaction was a significant source of variation for midseason and maximum heights and widths, flowering date, insect damage, and petiole color. Within each year, accessions differed significantly for all measured variables except maximum width in 1998 and stomatal conductance in 1999. Accessions from northern states usually flowered earlier, and flowering date was negatively correlated with latitude of origin (1998 r = −0.76; 1999 r = −0.60). Nevertheless, some of the earliest flowering accessions were from the southern states. Results from cluster analysis indicated that several accessions had phenotypes more similar to accessions from distant sites than to nearby accessions. We conclude that the accessions differ for traits that may affect weediness, but proximal accessions differ too frequently to recommend regional, biotype-specific control strategies.

Field studies were conducted in 1998 and 1999 to evaluate the response of seven cotton cultivars to CGA-362622 applied postemergence at 7.5 and 15 g ai/ha to three- to five-leaf cotton. The cultivars evaluated included Deltapine 51, Deltapine NuCotn 33B, Paymaster 1220 RR, Paymaster 1220 BG/RR, Stoneville bromoxynil-resistant 47, Stoneville 474, and Sure-Grow 125. At 1 to 2 wk after treatment (WAT), CGA-362622 at 7.5 and 15 g/ha injured all cotton cultivars 7 to 9% and 13 to 15%, respectively. Cotton injury symptoms included chlorosis and minor stunting. At 3 to 4 WAT, injury from CGA-362622 at 7.5 and 15 g/ha was 2 to 6% and 7 to 9%, respectively. Except for Paymaster 1220 RR, Deltapine NuCotn 33B, and Stoneville 474, all cotton cultivars were injured more by the higher rate than by the lower rate of CGA-362622. Injury was not visibly apparent 6 to 8 WAT. CGA-362622 at either rate had no effect on cotton lint yield.

Separate field experiments were conducted to evaluate weed control and cotton response to flumioxazin in North Carolina. Flumioxazin postemergence directed (PD) at 70 g ai/ha applied alone or mixed with glyphosate or MSMA completely controlled common lambsquarters, common ragweed, entireleaf morningglory, ivyleaf morningglory, Palmer amaranth, pitted morningglory, prickly sida, sicklepod, smooth pigweed, and tall morningglory 4 wk after treatment. Glyphosate at 1,120 g ai/ha controlled sicklepod and entireleaf, ivyleaf, pitted, and tall morningglory less than flumioxazin. Weed-free experiments were conducted to evaluate cotton injury, fresh biomass reduction, and yield response to flumioxazin at 70 g ai/ha preplant (PP) and two rates PD. Nine PP applications were made at various timings between 0 and 10 wk prior to planting. Cotton was stunted 12% initially, and midseason cotton biomass was reduced when flumioxazin was applied at planting in 1 yr. Flumioxazin did not injure 15- or 30-cm–tall cotton when applied PD at 36 or 70 g/ha. Cotton yield differed between years, but was not affected by various flumioxazin treatments compared with commercial standards and nontreated controls.

Field studies were conducted at three locations in North Carolina to evaluate clomazone preemergence (PRE) in glyphosate-tolerant cotton. Cotton was injured by fluometuron PRE at one location. Clomazone PRE controlled common ragweed, goosegrass, large crabgrass, and prickly sida better than trifluralin preplant incorporated (PPI). Trifluralin controlled Palmer amaranth better than clomazone. The addition of a late post-directed (LAYBY) treatment of cyanazine plus MSMA improved the control of goosegrass. Glyphosate at 0.8 kg ai/ha, used postemergence as needed, controlled tall morningglory and entireleaf morningglory at least 84%. Common ragweed was controlled with all herbicides, except trifluralin PPI. Prickly sida was controlled 94% or greater in glyphosate-containing systems. Cotton yields and net returns were similar for all glyphosate systems, regardless of soil-applied herbicides and LAYBY treatment options.

Genetically modified, herbicide-resistant crop (HRC) cultivars, which allow for simplified weed control decisions compared with conventional cultivars, have considerable potential in Latin America. The number of herbicide applications can be reduced in HRCs, and otherwise difficult-to-control species, including red rice in rice or herbicide-resistant weeds, can potentially be managed. The American tropics include the centers of origin of several crops, such as corn and potato, and natural or agrestal floras containing wild near relatives of introduced crops, including rice and cotton, for which HRCs could be used. Potential direct impacts of HRC adoption on biodiversity in Latin America include changes in the genetic diversity of crops, increased volunteer crop problems, and invasion by resistant cultivars of natural areas beyond the farm boundary. Additionally, there is a risk associated with the escape of transgenes from HRCs, involving introgression to weedy relatives on a field scale leading to amplification effects of existing weeds or modification of gene pools of crop progenitors in the centers of origin or diversity. Possible indirect effects of HRCs include the potential expansion of agriculture into uncleared wild areas made economically attractive by the more efficient cropping system, and adverse effects on nontarget organisms and ecosystem processes. The occurrence of spontaneous hybrids of crop–weed or near-relative complexes is known in the United States for rice and sunflower, and the development of HRC–weed hybrids would provide a particularly difficult set of weed management problems. For crops in which HRCs are currently in use or under development, the greatest risks in Latin America appear to be with corn, cotton, and potato. However, some of the genetic and geographical barriers reduce the risk of hybridization between these crops and their wild relatives. Furthermore, unlike the case of feral rapeseed on road verges in Europe, there is no known example of a conventionally bred crop or interfertile hybrid with a near relative becoming established outside of cultivation in Latin America. Soybean, the most widely adopted HRC in Latin America to date, is an exotic beyond the range of its wild relatives. With the exception of O. glumaepatula, there appears to be little threat to endemic wild species from HRC rice as strong infertility barriers should prevent transgene flow, but possible transgene movement to the conspecific red rice may become an issue. Beyond the field boundary, a key question concerns the possible persistence of HRC x wild relative hybrids and the manner in which resistance traits affect their overall fitness. Designing practical risk assessment protocols on which to base HRC release approval is a considerable challenge, and it is vital that Latin American states continue to build and maintain functional biosafety regulatory structures.

Herbicide-resistant crops (HRCs), particularly glyphosate-resistant soybean, are increasingly important in Latin America. Prior to commercial release of these crops, their short- and long-term risks should be thoroughly assessed. A risk assessment should include the identification and characterization of potential hazards and an estimation of the likelihood of these hazards occurring. For HRCs the agro-ecological hazards are mostly related to the occurrence of herbicide-resistant (HR) weeds and crop volunteers and the adverse effects from the use of pesticides within the agricultural area. Herbicide-resistant rice is used as a case study to visualize the key components of such a risk assessment. For this purpose, a model that simulates a typical rain-fed rice production system in Central America was developed. The model was used to investigate the selection of HR weedy rice populations under various scenarios. Scenarios included contrasting weed management practices, hybridization levels between the commercial HR cultivated and weedy rice, and seed predation rates. Because risks may become apparent only after long-term cultivation of HR rice, simulations were run for a 10-yr period. In a cropping system relying on glufosinate-resistant rice for weed control, the model predicted that resistance to glufosinate would occur after 3 to 8 yr of monoculture. Increasing the hybridization level from 1 to 5% decreased the time for resistance to occur by 1 to 3 yr. Increasing annual rate of weedy rice seed predation at the soil surface delayed the development of resistance. Tillage as a weed control tactic also delayed the occurrence of resistance when compared with a no-till situation.

Laboratory experiments were conducted in 55-L aquaria to evaluate the efficacy of the aquatic herbicide endothall and the fungal pathogen Mycoleptodiscus terrestris (Gerd.) Ostazeski, applied alone and in combination against hydrilla. Treatments included 0.25, 0.50, and 1.25 mg ae/L endothall, 100, 200, and 400 colony-forming units (CFU)/ml M. terrestris, simultaneous integrated treatments of 0.25, 0.50, and 1.25 endothall + 100 or 200 CFU/ml M. terrestris, sequential integrated treatments of 100 and 200 CFU/ml M. terrestris + 0.25 and 0.50 mg ae/L endothall, and untreated controls. By 42 d after treatment (DAT), all treatments had significantly reduced shoot biomass levels of hydrilla compared with the untreated controls. Combining the two lowest herbicide rates with M. terrestris provided better hydrilla control than either treatment alone. Based on these results, an outdoor mesocosm study was conducted to evaluate the efficacy and selectivity of endothall and the pathogen applied alone and in combination against hydrilla, Illinois pondweed, American pondweed, and vallisneria. Treatments included 0.25 and 0.50 mg ae/L endothall, 100 and 200 CFU/ml M. terrestris, integrated treatments of 0.25 and 0.50 mg ae/L endothall + 100 and 200 CFU/ml M. terrestris, and untreated controls. Unlike the laboratory results, none of the treatments controlled hydrilla 100%. The combined treatments worked better than either treatment applied alone. By 42 DAT, all the combined treatments except 0.25 mg ae/L endothall + 100 CFU/ml M. terrestris had reduced above-ground hydrilla biomass by ≥ 90% compared with the untreated controls. All nontarget species sustained varying amounts of injury from endothall and M. terrestris applied alone or in combination.

The allelopathic potential of hoary cress was evaluated by exposing the seeds and the germinated seeds of winter wheat, alfalfa, crested wheatgrass, bluebunch wheatgrass, and hoary cress to a water extract of dried, hoary cress roots under controlled conditions in an environmental chamber. Germination for all species was reduced in the hoary cress root extract when compared with distilled water, with winter wheat and hoary cress being more tolerant than the other species. Root length of all species was reduced by the extract when compared with distilled water. These data show the presence of phytotoxic chemical(s) that may inhibit germination and initial seedling growth in natural environments. Three glucosinolate compounds were identified in hoary cress root extracts.

Autumn desiccation of warm-season perennial grasses is one method to enhance the establishment and early production of overseeded cool-season annuals. Potential herbicides were compared with paraquat, glyphosate, and dalapon to desiccate a dallisgrass–bermudagrass sod for overseeding annual ryegrass over a 2-yr period. Fluazifop-P at ≥ 0.14 kg/ha, glufosinate at ≥ 1.12 kg/ha, and haloxyfop at ≥ 0.07 kg/ha provided good dallisgrass desiccation and ryegrass yields similar to those with dalapon and glyphosate. Dallisgrass recovery was inversely related to desiccation rating for all treatments except for paraquat. None of the herbicide treatments had the desired combination of acceptable dallisgrass desiccation, improved early ryegrass production, and acceptable dallisgrass recovery.

A survey was conducted to document the significance of crabgrass species among cropping systems and geographical regions in the continental United States. Surveys were mailed to 117 weed scientists, at least one in each state, including, where possible, extension weed scientists with responsibilities in each of the major agronomic and horticultural crops plus turfgrass systems. A 62% response rate was achieved. Large, smooth, and southern crabgrasses were considered at least occasionally important, with India and blanket crabgrasses considered rarely or not important. Smooth crabgrass and large crabgrass were considered to be important species in most of the latitudinal range, whereas southern crabgrass was considered to be important only in the southern states. Crabgrasses were considered to be more problematic in turf than in other cropping systems. From a regional perspective in turf, large crabgrass was the most important species in the Southeast, followed by southern crabgrass and smooth crabgrass. In the Northeast, smooth crabgrass was perceived as the more important species, but large crabgrass was more important in the north-central states.

A tractor-mounted, air-pressurized, herbicide application system was designed and constructed for use in weed management research on cropland, pastures, and rangeland. The spray system was designed to minimize wind-induced spray pattern distortion, to enable accurate application of multiple treatments, to withstand the stresses of use on uneven and rough terrain, to apply herbicide treatments reliably, to be easy to transport, to be constructed with readily available spray system components, and to enable quick diagnosis and resolution of operational problems. The spray system has a total shield frame length of 5.2 m. The shielded frame comprises three sections. The two outside sections are connected to the center section by hinges so that they can be folded up and over the center section for transport or storage. Four spray booms are mounted inside the shield with a 50-cm distance between nozzles. Herbicides are usually applied with the bottom of the shield placed 20 cm above the soil or plant surface. This height provides 100% overlap of the spray pattern between adjacent nozzle tips. The spray system has been a durable and reliable tool that accurately and quickly applies herbicide treatments.

Foxtail, foxtail, where have you been? Been down to China, and I come back again! Foxtail, foxtail, where are you goin'? Any old where that the wind is a' blowin'!

James A. Duke, 1992.