Jointed goatgrass, downy brome, and horseweed are increasingly troublesome winter annual weeds during fallow periods in conservation-tillage systems in the southern Great Plains. These experiments determined the optimum weed size, vigor, and minimum herbicide rate required for 95% or better control of these weeds on fallow land. Jointed goatgrass and downy brome were controlled best when plants were 10 cm or less tall and growing vigorously at time of treatment. Horseweed was controlled best when plants were 30 cm tall and growing vigorously. Based on local retail and application costs and assuming optimum conditions for control, the two most economical herbicide treatments that controlled each weed 95% or better were: jointed goatgrass, clethodim at 250 g ai/ha and glyphosate + 2,4-D at 249 + 479 g ae/ha; downy brome, quizalofop at 18 g ai/ha and glyphosate + 2,4-D at 582 + 950 g ae/ha; and horseweed, 2,4-D at 560 g ae/ha and metsulfuron at 5 g ai/ha.

Field, greenhouse, and laboratory studies were conducted to examine the potential for antagonism of postemergence graminicides when tank-mixed with cloransulam and to determine the role of herbicide absorption and translocation in observed antagonistic responses. Cloransulam antagonized annual grass control with aryloxyphenoxypropionate herbicides fluazifop-P, quizalofop, and the prepackaged formulation of fluazifop-P plus fenoxaprop. Cloransulam did not affect annual grass control with the cyclohexandiones clethodim and sethoxydim. In the greenhouse, increasing the rate of the graminicides was a more effective strategy for overcoming antagonism for quizalofop than for fluazifop-P or fluazifop-P plus fenoxaprop, and success was species dependent. Annual grass control with clethodim, sethoxydim, and glyphosate was not adversely affected by tank mixtures with cloransulam. Control of large rhizome johnsongrass was initially reduced when cloransulam was mixed with sethoxydim, fluazifop-P plus fenoxaprop, or quizalofop. By 6 wk after treatment, control of rhizome johnsongrass was antagonized only when cloransulam was mixed with sethoxydim. Rainfall within 1.5 h of application reduced johnsongrass control with glyphosate and sethoxydim but did not affect activity of the other herbicides. Absorption of 14C-fluazifop-P and 14C-quizalofop into broadleaf signalgrass was not affected by cloransulam 6 or 24 h after treatment. Translocation of 14C-fluazifop-P to broadleaf signalgrass shoot tissue above and below the treated leaf was decreased when fluazifop-P was combined with cloransulam. Translocation of quizalofop was not affected by cloransulam.

Weeds are a major constraint to rice production in labor-limited, upland rice-based systems in West Africa. The effects of weeding regimes and rice cultivars on weed growth and rice yield were investigated at two upland locations (Abomey-Calavi and Niaouli) in the degraded coastal savanna zone of Benin in 2005 and 2006 with below-average rainfall. Four weeding regimes (hoe weeding at 21 d after sowing [DAS], delayed hoe weeding at 31 DAS, hoe weeding at 21 and 42 DAS, and a no weeding control) were the main plot treatments. Cultivars comprising three interspecific upland rice cultivars (NERICA 1, NERICA 2, and NERICA 7) and the parents (Oryza sativa WAB56-104 and O. glaberrima CG14) were tested in subplots. The most dominant weed species identified were Jamaican crabgrass, Mariscus, and silver spinach. Rice yield was generally low because of drought stress; none of the experiments had a higher mean yield than 1,400 kg ha−1 across cultivars. Across cultivars, the best weeding regimes in terms of weed control and rice yields were single weeding at 31 DAS (W31) and double weeding at 21 and 42 DAS (W21+42). Under these weeding regimes, WAB56-104 out-yielded the three NERICA cultivars. CG14 showed the strongest weed suppressive ability (WSA) in Abomey-Calavi but did not have strong WSA in Niaouli because of lower biomass accumulation. WSA of WAB56-104 was similar to that of the three NERICA cultivars. Single weeding at 31 DAS, together with the use of cultivars with good adaptation to unfavorable rice growing conditions, would increase land and labor productivity of upland rice-based systems in West Africa.

A 2-yr study was conducted near Mead, NE, to determine the interaction of six corn (Zea mays) hybrids having different heights and leaf angles and four herbicide rates on velvetleaf (Abutilon theophrasti) and green foxtail (Setaria viridis) biomass rating and seed production. Corn hybrid grain yields averaged over herbicide rates differed by up to 1,100 kg/ha in 1993 and 3,000 kg/ha in 1994, yet similar grain yield reductions occurred for all hybrids as herbicide rate decreased. Corn hybrids differed in suppression of weed growth and seed production, but suppression differences among hybrids were not consistent across years. Plots with the very tall, erect-leaf hybrid had 7.4 velvetleaf plants/m2, whereas other hybrids had 8.6 to 10.1 plants/m2. Plots with the very tall, erect-leaf hybrid also produced the lowest quantity of velvetleaf seeds of 23,100 seeds/m2 versus 25,100 to 30,700 seeds/m2 for plots with other hybrids. Erect-leaf hybrids suppressed weeds more effectively than horizontal-leaf hybrids. Corn hybrids had less effect on early- Fand late-season weed densities and biomass ratings and on seed production than herbicide application. Selection among current corn hybrids as a tool in integrated weed management will have a small or inconsistent influence on weed suppression.

A 2-yr field experiment was conducted at Fayetteville and Stuttgart, AR, to predict soybean yield loss as a function of barnyardgrass density and total dry weight. Predicted soybean yield reductions were 0 to 78% for original densities of 1 to 500 barnyardgrass plants per m of row. Soybean yield reductions were linear from 0 to 150 barnyardgrass plants per m of row with an average yield reduction of 0.25% for each plant per m of row. Soybean yield was reduced 10, 25, and 50% from original barnyardgrass densities of 42, 110, and 250 plants m of row. Predictions of soybean yield reduction from barnyardgrass dry weight and original density were similar. traspecific interference of barnyardgrass occurred for all variables tested, with individual barnyardgrass plants becoming less competitive as density increased.

In field experiments at three locations, wheat row spacing, seeding rate, and herbicide treatment affected Italian ryegrass control, wheat yield, dockage in the grain, and net returns. Diclofop at 560 or 840 g ai/ha controlled Italian ryegrass better than chlorsulfuron at 18 or 26 g ai/ha. Net returns were increased at all locations by diclofop POST at either rate and at two locations by chlorsulfuron PRE at either rate. Although increasing the wheat seeding rate reduced dockage at two of three locations, net returns were maximized by herbicide application alone without increased seeding rates or reduced row spacing.

The “micro-rate” application, a POST combination of desmedipham plus phenmedipham at 0.045 + 0.045 kg ai/ha (desphen) or desmedipham plus phenmedipham plus ethofumesate (1:1: 1 ratio) (desphenetho) at 0.09 kg ai/ha plus triflusulfuron at 0.004 kg ai/ha plus clopyralid at 0.026 kg ae/ha plus 1.5% methylated seed oil received registration in 1998 and 2000 in North Dakota and Michigan, respectively. Herbicide rates are reduced by 80%, compared to standard-split applications, and growers typically apply the micro-rate three to five times to very small weeds that are 1 cm or less in height. In standard-split applications, growers make two sequential applications, the first when weeds are 1.5 cm tall and the sequential application usually 10 to 14 d later. Research was conducted in small plots and large grower plots in 2001 and 2002 to determine the effect of PRE herbicides on weed control and sugarbeet injury from micro-rates compared to standard-split POST herbicide applications. Sugarbeet populations were reduced in the cycloate treatment compared to all other PRE and the no-PRE treatment in 2001 and in the S-metolachlor compared to the ethofumesate treatment in 2002. Sugarbeet injury was 6% or less from POST-only treatments in 2001. Control of common lambsquarters and Amaranthus spp. by desphen and desphenetho treatments was similar. Sugarbeet injury in 2002 was 29 to 43% from POST-only treatments. The standard-split of desphenetho was more injurious than the standard-split of desphen. Common lambsquarters control was greater in both the standard-split and micro-rate of desphenetho compared to the standard-split of desphen in 2002. However, sugarbeet populations and recoverable white sucrose per hectare did not differ among POST herbicide treatments in either year. No herbicide program provided 100% control of all weeds in both years. In the seven large production fields, PRE herbicide treatments did not reduce sugarbeet populations or recoverable sucrose per hectare compared to the no-PRE control. Weed control from four POST micro-rate applications only was similar to weed control in instances in which PRE herbicides were applied prior to the POST micro-rate applications.

The involvement of allelopathy in various crop–weed competition studies has been suggested by several authors, but its significance has been demonstrated with varying success. It is extremely difficult to unambiguously demonstrate allelopathy in nature because of the complexity of plant interference and its relationship to soil chemistry. However, with an increased understanding of the chemical processes occurring in the agroecosystem, genetic mapping of quantitative traits, and the ability to identify allelochemicals, an effort should now be directed toward understanding the mechanisms for allelopathy, as well as trying to optimize an allelopathic effect to produce more competitive crops. The approach used for rice allelopathy research can be used as a general framework for understanding how genetically encoded traits affect the competitive ability of plants. This framework requires the involvement of a range of scientists from multidisciplinary research areas with the overall objective of optimizing competitive ability in crops. Such research efforts could reduce dependency on herbicides and thus increase the sustainability of weed management practices. This paper aims to illustrate the importance of allelopathy for crop competitive ability and to identify a framework suitable for result-oriented collaborative research toward breeding for competitive ability in crops.

The suitability of a bioherbicide as a component of an integrated weed management program not only relies on its field efficacy, but also on its compatibility with other pest control measures that may be employed during the cropping season. The effects of selected pesticides applied according to label rates on Dactylaria higginsii, a biological control agent for purple nutsedge, were determined using mycelial growth on pesticide-amended potato dextrose agar (PDA) and conidial germination as indicators of pesticide sensitivity. Among the pesticides tested, the herbicides oxyfluorfen and sethoxydim and the fungicides fosetyl-Al and thiophanate methyl inhibited D. higginsii mycelial growth and reduced or completely inhibited conidial germination; the herbicide diuron, the fungicides metalaxyl and copper hydroxide, and the insecticide cyromazine reduced mycelial growth but did not reduce conidial germination. The miticide dicofol reduced mycelial growth and completely inhibited conidial germination while the herbicide imazapyr had no adverse effect on either the mycelial growth or conidial germination of D. higginsii.

“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.

Field studies were conducted for three years in each of two successive experiments and analyzed to determine net return from stale seedbed plantings of soybean. Treatments in the first experiment were early May and late May/early June plantings in seedbeds that had combinations of tillage at various times between harvest and planting, incorporation of trifluralin, or no preplant tillage. Treatments in the second experiment were irrigated and nonirrigated stale or no-till seedbed plantings with weed control treatments that included POST cultivation plus combinations of preplant foliar-applied (PFA) glyphosate, PRE metribuzin, and POST herbicides. Results indicate that 1) early planting of soybean resulting from use of a stale seedbed will result in higher net returns when irrigation is used, 2) use of a wheat cover crop does not justify the expense, 3) soybean planted in a no-till seedbed will result in net return similar to that from plantings in fall-tilled, spring-tilled, or prepared seedbeds, 4) use of only a PFA herbicide plus POST cultivation in a stale seedbed planting where competitive weeds such as common cocklebur appear later will result in a significant reduction in net returns, and 5) use of PRE herbicides in conjunction with PFA herbicide and POST cultivation in a stale seedbed planting will result in the highest net return with the least input.

A survey determined the views of individuals in seven groups comprising the weed science community in the corn belt (primarily in Illinois) for importance of 8 environmental and 14 crop production issues and 16 weed species in setting weed science research priorities for the next 3 to 5 yr. The survey also considered if funding of research to solve these environmental and production issues should be from the private or public sector. Velvetleaf, foxtail species, and common lambsquarters were considered the top three weed species by all respondents, and each of these weeds was among the five most important weeds within each of the seven survey groups. Improving ground and surface water quality were the foremost environmental issues for all respondents, but soybean growers listed herbicide carryover as their top environmental concern. Reducing herbicide residues in food and developing sustainable practices were given low preference by all groups. Sustainable growers rated reducing herbicide carryover and minimizing applicator exposure as their lowest priorities. Among all respondents, the top three production issues were improved weed control in conservation tillage, more economical weed control, and improved integrated control strategies. Studying the biology/life cycles of weeds was the third highest production priority of University and U.S. Department of Agriculture (USDA) weed scientists, but was the last choice when averaged over the aggregate survey group. Developing strategies for resistant weeds and herbicide-resistant crops were chemical dealers top two priorities. Industry representatives gave the former subject their highest rating and the latter their lowest rating. Crop consultants seemed to want decision aids, as they chose assessing weed loss/thresholds and developing weed control/economic models among their top three production issues. Both corn and soybean growers desired more economical weed control as a first choice, while sustainable growers wanted improved cultural control strategies. Corn and soybean growers ranked developing new herbicides among their top three choices, but this issue was the lowest choice of the sustainable growers. University, USDA, and industrial weed scientists suggested that their own organizations conduct the research on their highest priorities issues.

Antagonism of quizalofop-P efficacy on annual grasses by bromoxynil has been noted in both the field and greenhouse. Laboratory experiments were conducted on yellow foxtail (Setaria glauca) to determine the effect of mixing bromoxynil with the ethyl ester of quizalofop-P on absorption, translocation, and metabolism of 14C-quizalofop-P Applying bromoxynil in mixture with quizalofop-P reduced 14C-label absorption from 63 to 51%, 73 to 52%, 77 to 68%, and 90 to 80% at 4, 8, 24, and 96 h after treatment, respectively. Translocation of 14C-label from the treated leaf into the shoot above or shoot below was unaffected by bromoxynil. However, translocation into the roots was reduced from 0.9 to 0.4% and 1.0 to 0.5% at 4 and 8 h after treatment, respectively, but differences were not noted at 0.5, 1, 24, and 96 h after treatment. Bromoxynil increased deesterification of quizalofop-P-ethyl into quizalofop-P acid from 45 to 60% in the shoot above the treated leaf. However, bromoxynil did not affect metabolism of quizalofop-P in the treated leaf or shoot below the treated leaf. These results suggest that antagonism of quizalofop-P activity by bromoxynil is primarily due to decreased absorption of quizalofop-P, whereas effects on translocation and metabolism were minor.

Look at a field of almost any grain and the top of the crop will be almost as tabletop flat as the ground below. Modern genetics produce almost perfectly uniform crop varieties and hybrids with identical, “cookie-cutter” individual plants. But one crop stubbornly refuses to cooperate with breeders. Look at a field of mature hybrid grain sorghum [Sorghum bicolor (L.) Moench ssp. bicolor] and you will see rugged individualists sticking up above the fruited plain of sameness like grain elevators towering over a small Midwestern town. These “off type” sorghum plants can be mutants, or the result of the random outcrossing of parent plants in seed production fields from the pollen of volunteer plants from a previous year's crop, nearby grain, or forage sorghums. They may also come from crosses with weedy types growing nearby such as johnsongrass [Sorghum halepense (L.) Pers.] or the dreaded shattercane [Sorghum bicolor (L.) Moench ssp. drummondii (Nees ex Steud.) de Wet ex Davidse] (Clark and Rosenow 1992). Just what is shattercane? Where did this “black sheep” of the Sorghum clan come from? And why is it such a widespread weed today?

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.

Field studies were conducted from 1998 to 2000 to evaluate the effects of the trimethylsulfonium (Tms) salt of glyphosate on glyphosate-resistant soybean at Belleville, IL. Glyphosate-Tms and glyphosate-isopropylamine (Ipa) at 1,120, 1,680, 2,240, 3,360, and 4,480 g ai/ha were applied at the V4 and R1 growth stages of glyphosate-resistant soybean. Glyphosate-Tms and glyphosate-Ipa caused greater chlorosis when applied at the R1 growth stage when compared with the V4 growth stage, and chlorosis increased with rate. Chlorosis ranged from 0 to 20% depending on the year. In 1998, glyphosate-Ipa at 2,240 and 3,360 g/ha applied at the R1 growth stage caused 4 to 5% more chlorosis than glyphosate-Tms at the same rates. In addition to chlorosis, glyphosate-Tms caused bleaching (white speckling) of soybean leaves, with bleaching increasing as glyphosate-Tms rate increased. Glyphosate-Ipa caused no bleaching, regardless of rate. Glyphosate-Tms and -Ipa caused no visible height reduction at 14 and 28 d after treatment in any year. In 1998 and 1999, glyphosate-Tms and -Ipa, at the highest rate applied at the R1 growth stage, increased days to maturity of soybean. Despite the injury and delay in maturity caused by glyphosate-Tms and -Ipa, there was no difference in grain yield across years because of glyphosate salt, rate, or application timing.

Experiments were conducted for two years at one location and one year at a second location to determine the effects of paraquat on a range of peanut genotypes. Paraquat caused yield reductions of most Valencia-, Spanish-, and runner-market types as compared with hand-weeded checks. Virginia-market types were less affected by paraquat treatment. Most grade characteristics were adversely affected by paraquat. Because the paraquat treatments increased the proportion of other kernels, the herbicide may be affecting yield and grade by delaying maturity.

The effects of chlorimuron, imazaquin, imazethapyr, nicosulfuron, primisulfuron, and thifensulfuron were evaluated on a population of smooth pigweed in Painter, VA with no history of treatment with acetolactate synthase (ALS)-inhibitor herbicides. Imazethapyr and nicosulfuron gave the greatest smooth pigweed control, and subsequently were used in field and greenhouse studies to investigate susceptibility of smooth pigweed and livid amaranth populations to ALS-inhibitor herbicides. Approximately 5 million smooth pigweed plants from Painter were treated with imazethapyr or nicosulfuron from 1992 to 1994 and no ALS-inhibitor-resistant plants were identified. In the greenhouse, the response of smooth pigweed from Painter, VA, Marion, MD, and Oak Hall, VA and livid amaranth from Warren County, NJ to imazaquin or imazethapyr and nicosulfuron was investigated. Smooth pigweed from Marion and Oak Hall and livid amaranth from NJ had histories of treatment with ALS-inhibitors. Painter smooth pigweed control was 81 to 97% by imazethapyr and nicosulfuron while control of the Marion and Oak Hall populations was 3 and 18% by imazaquin at 560 and 1120 g ai/ha, respectively, and control by nicosulfuron at 35 g ai/ha was 50 to 73%. Control of livid amaranth from Warren County, NJ was 8 to 15% by imazethapyr at 560 g ai/ha, and was 30 to 58% by nicosulfuron at 35 g/ha.

Managing glyphosate-resistant (GR) horseweed in no-till cotton continues to be a serious challenge for midsouthern producers. Field studies were conducted in 2008 and 2009 to evaluate spring burndown applications of saflufenacil on GR horseweed prior to planting cotton. Saflufenacil controlled GR horseweed at least 94% up to 7 d before planting (DBP) without causing significant cotton injury. Saflufenacil applied at 7 or 14 DBP controlled GR horseweed while still providing residual control until planting. Moreover, saflufenacil, on silt loam soil evaluated in this study, showed no more injury than dicamba applied 7 or more DBP. Results indicated that saflufenacil is an option in cotton for controlling GR horseweed much closer to cotton planting than 42 DBP (current saflufenacil label). At 25 g ha−1, which is the standard labeled rate in cotton, saflufenacil provided > 90% control of GR horseweed. Saflufenacil as a GR horseweed burndown, could replace the current dicamba standard every other year to reduce the probability of horseweed developing resistance to dicamba or salflufenacil.

Studies were conducted at Clayton, Lewiston, and Rocky Mount, NC, to evaluate weed and cotton response to herbicide systems in bromoxynil-resistant Stoneville BXN 47 cotton. Herbicide systems that included clomazone preemergence (PRE) controlled broadleaf signalgrass, common lambsquarters, common ragweed, goosegrass, large crabgrass, and prickly sida greater than 88%. Inconsistent Palmer amaranth control was observed with the addition of fomesafen PRE to clomazone PRE and pendimethalin preplant-incorporated (PPI) herbicide systems. Addition of bromoxynil early postemergence (EPOST) to clomazone and pendimethalin systems increased ivyleaf morningglory control to greater than 84% and provided higher yields than did the same systems without bromoxynil. Bromoxynil EPOST followed by (fb) cyanazine + MSMA late postemergence directed (LAYBY) improved weed control in clomazone and pendimethalin systems. Clomazone PRE and pendimethalin PPI with or without fomesafen PRE fb bromoxynil EPOST fb LAYBY herbicides controlled weeds and yielded equivalent to the standard herbicide system of pendimethalin PPI fb fluometuron PRE fb bromoxynil EPOST fb LAYBY.