Repeated application of acetyl-coA carboxylase–inhibiting herbicides, such as sethoxydim, may select for resistant (R) weed populations, making a rapid and reliable seedling bioassay a useful tool. Such a bioassay was developed to determine shoot and root responses of giant foxtail seedlings to sethoxydim. Root and shoot elongation of susceptible (S) and R giant foxtail seedlings was measured at 3 and 6 d after exposure to 0.1 to 100 mg/L sethoxydim. A bioassay concentration of 10 mg/L sethoxydim easily discriminated between S and R biotypes of giant foxtail at 6 d after exposure, with R:S shoot and root growth ratios of 3 and 10, respectively.

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.

Field studies were conducted over 2 yr to evaluate weed control, yield, and net returns of glyphosate-resistant soybean using total postemergence (5 wk) (POST) herbicide systems with glyphosate–isopropylamine (Ipa) or glyphosate–trimethylsulfonium (Tms) alone, tank mixed with fomesafen, or in sequential treatments with bentazon, fomesafen, Ipa, or Tms. Soybean early-season injury ranged from 0 to 28% across the test. Although Ipa did not injure soybean, glyphosate–Tms early postemergence (3 wk) (EPOST) injured soybean from 7 to 17% depending on the rate. Glyphosate–Tms mixed with fomesafen EPOST injured soybean from 20 to 28%. Red morningglory control by Ipa and Tms at 0.8 kg ae/ha was no more than 88%. Sequential applications of Tms or Ipa controlled red morningglory 78% or less. Fomesafen improved red morningglory control by Ipa and Tms. Bentazon did not affect the control of red morningglory by these herbicides. Sicklepod, smooth pigweed, and large crabgrass control was 81, 93, and 79%, respectively, or greater for all herbicide treatments. By midseason, narrow-row soybeans had canopied, and competition from weeds was minimal. Overall, the net returns were reflective of soybean yield, and maximum net returns were recorded for treatments with reduced herbicide inputs. Conversely, sequential application of herbicides as EPOST followed by POST treatments resulted in lower net returns because of increased herbicide and application costs.

No more the grassy brook reflects the day,

But, chocked with sedges, works its weedy way;

The Deserted Village, Oliver Goldsmith, 1770.

Yellow nutsedge (Cyperus esculentus L.) is a perennial herbaceous sedge native to the eastern Mediterranean (Mabberley 1997; Tutin et al. 1980; Zeven and Zhukovsky 1975). Cultivated in ancient Egypt for its tasty and oil-rich tubers, the wild type has spread as a weed to all the continents of the world and adapted to climates from tropical to subarctic. Adaptability to many agricultural habitats and the great reproductive capacity of yellow nutsedge have propelled it to rank as the 16th worst weed in the world (Holm et al. 1991a, 1991b; Negbi 1992).

“All the herbe, his branches, leaves, and sede, do cleave and sticke fast to every thing that it toucheth: it is so sharp, that being drawen along the tongue, it wil make it to bleede.”

A Neiwe Herball, or Historie of Plantes, D. Rembert Dodoens, 1578

Catchweed bedstraw, Galium aparine L., is a winter or summer annual herb native to Eurasia and North America. The weed is more commonly known as “cleavers” in most of the English-speaking world outside the United States. Both names refer to the small hook-like bristles that cover the plant causing it to cling to almost anything that it touches. The plant is present on all of the world's continents in temperate zones, in warmer arctic zones, and at higher elevations in the tropics (Barkley 1986; Holm et al. 1991b). Catchweed bedstraw has numerous medicinal properties, has been used for animal feed and food processing, is a persistent weed in small grains and other crops, and is above all an accomplished world traveler and “hitch-hiker,” as we shall see.

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.

We demonstrate how direct (constrained) ordination methods can be used to analyze differential control by different weed management tactics. Weed biomass data of individual weed species were compiled for 110 trials on potatoes, each of which included a mechanical control treatment, a herbicide treatment, and an untreated control. We used “trial id” as 110 categorical dummy covariables, thereby eliminating between-trial variation, and made direct comparisons between the data from numerous trials. Unweeded plots had the largest biomass of all species as expected. Contrasting the results for the standard herbicide and the mechanical control treatments in a separate analysis, we could rank species from those best controlled by the herbicide (several annuals) to those best controlled by the mechanical treatment (a perennial grass and a climbing annual).

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.

Glyphosate-resistant crop species have increased in number over the past decade as growers eagerly adopt this simple and effective weed management technology. Glyphosate-resistant wheat cultivars are being developed and may soon be available to growers. The objective of this paper is to discuss the pest management implications of glyphosate-resistant wheat in the western United States, a region stretching from the Great Plains to the Pacific Ocean that produces more than 80% of the nation's wheat crop. The benefits of glyphosate-resistant wheat include: (1) improved weed control, particularly of difficult-to-control weeds, such as winter annual grasses belonging to the Aegilops, Avena, Bromus, Lolium, Poa, Secale, and Setaria genera; (2) an ability to control weeds resistant to currently available wheat herbicides; (3) an extended application window for control of late-emerging weeds; and (4) improved crop safety. Although these benefits are not to be minimized, they need to be considered in the light of the concerns surrounding this new technology in wheat. These concerns are about (1) the lack of an equally effective and affordable herbicide to control glyphosate-resistant volunteer wheat, which may increase wheat diseases such as wheat streak mosaic and Rhizoctonia root rot; (2) the possibility that overreliance on glyphosate will lead to species shifts, with unknown consequences for weed management in wheat; and (3) the use of multiple glyphosate-resistant crops in rotation with glyphosate-resistant wheat, which could rapidly increase glyphosate-resistant weeds, thereby limiting the future utility of glyphosate. If, or when, glyphosate-resistant wheat becomes commercially available, it will require careful management to sustain its usefulness. We have proposed several areas of research that we feel are critical to help develop sound management guidelines for deployment and use of this new weed management technology in wheat. These include (1) developing effective “green bridge” management strategies, i.e., using cultural and chemical approaches to control plants that sustain insect vector populations between wheat crop periods; (2) predicting potential weed species shifts resulting from the use of glyphosate-resistant wheat; and (3) developing management systems that include herbicide-resistant wheat on a rotational basis and rotating the use of glyphosate with other weed management strategies in the fallow period to minimize the potential development of glyphosate-resistant weeds or weed communities.

Weed management decision aids have proliferated in recent years, but none of them have been rigorously compared with actual farmer weed management on farm fields. This research compares the Michigan WEEDSIM/GWM bioeconomic model and the CORNHERB and SOYHERB herbicide selection models with farmer weed management in Michigan. In 19 site-years of research in corn and soybean during 1996 to 1997, we found that crop yield, weed control costs, and gross margin over weed control costs (profitability) with the computerized decision aids were not statistically superior to the farmer treatments, even at a one-sided threshold of P = 0.10. In corn the gross margin of the farmer treatment ranked highest in both years. In soybean the gross margin of the SOYHERB treatment ranked highest in 1996 and that of the WEEDSIM/GWM treatment was highest in 1997. Overall, the farmer treatment had the highest gross margin 5½ times, the CORNHERB–SOYHERB treatment 6 times, and the WEEDSIM/GWM treatment 7½ times (where ties were divided equally to give 1/2 to each). However, none of these rank differences corresponded to a statistically significant gross margin gain over the farmer treatment.

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.