Competitive crops or cultivars can be an important component of integrated weed management systems. A study was conducted from 2003 to 2006 at four sites across semiarid prairie ecoregions in western Canada to investigate the weed-suppression ability of canola and mustard cultivars. Four open-pollinated canola cultivars, four hybrid canola cultivars, two canola-quality mustard cultivars, two oriental mustard cultivars, and two yellow mustard cultivars were grown in competition with indigenous weed communities. Yellow mustard was best able to suppress weed growth, followed in decreasing order of weed competitiveness by oriental mustard and hybrid canola, open-pollinated canola, and canola-quality mustard. Competitive response of cultivars, assessed by weed biomass suppression, was negatively correlated with time to crop emergence and positively correlated with early-season crop biomass accumulation (prior to bolting) and plant height.

Leafy spurge causes economic loss by reducing both herbage production and use. Herbage use by grazing cattle in various densities of leafy spurge (Euphorbia esula L. #3 EPHES) was evaluated over a 3-yr period in North Dakota. Forage production and disappearance were estimated in four density classes of leafy spurge. Use of cool- and warm-season graminoids, forbs, and leafy spurge was estimated during the middle and the end of each grazing season. Cattle used 20 and 2% of the herbage in the zero and low density infestations, respectively, by mid-season. Moderate and high density infestations were avoided until the milky latex in leafy spurge disappeared in early fall, and herbage availability in zero and low density infestations declined. Herbage use in moderate and high density infestations increased to an average of 46% by the end of the grazing season compared to 61% in zero and low density infestations. An annual herbage loss of at least 35% occurred in pasture infested with 50% density or more of leafy spurge.

Availability of soybean with dicamba resistance will provide an alternative weed management option, but risk of dicamba injury to sensitive crops from off-target movement and spray tank contamination is of concern. Research conducted at multiple locations and years evaluated soybean injury and yield response to POST applications of the diglycolamine salt of dicamba. Dicamba was applied at the two to three trifoliate stage (V3/V4) at 4.4, 8.8, 17.5, 35, 70, 140, and 280 g ae ha−1 (1/128 to 1/2 of the recommended use rate of 560 g ae ha−1). Soybean injury 7 d after application was 20% following dicamba at 4.4 g ha−1 and increased to 89% at 280 g ha−1. At 14 d after application, injury for the same rates increased from 39 to 97%. In a separate study, dicamba was applied at first flower (R1) at 1.1, 2.2, 4.4, 8.8, 17.5, 35, and 70 g ha−1 (1/512 to 1/8 of use rate). Soybean injury 7 d following dicamba application was 19% at 1.1 g ha−1 and increased to 64% at 70 g ha−1. For the same rates of dicamba, injury from 7 to 14 d after application increased no more than 4 percentage points. For dicamba rates in common for the timing studies, soybean injury 14 d after treatment was greatest for application at V3/V4, but the negative effect on mature soybean height and yield was greatest for application at R1. For dicamba at 4.4 g ha−1 (1/128th of use rate), soybean yield was reduced 4% when applied at V3/V4 and 10% when applied at R1. For 17.5 g ha−1 dicamba (1/32 of use rate), yield was reduced 15% at V3/V4 and 36% at R1. Based on yield reductions for 4.4 and 17.5 g ha−1 dicamba, soybean at flowering was around 2.5 times more sensitive compared with vegetative exposure.

Surveys were conducted across the northern Great Plains of Canada in 1996 and 1997 to determine the nature and occurrence of herbicide-resistant (HR) biotypes of wild oat (Avena fatua). The surveys indicated that resistance to acetyl-CoA carboxylase (ACCase) inhibitors (Group 1) occurred most frequently relative to other herbicide groups. Group 1-HR wild oat occurred in over one-half of fields surveyed in each of the three prairie provinces. Of particular concern was the relatively high incidence of multiple-group resistance in wild oat in Saskatchewan and Manitoba. In Saskatchewan, 18% of Group 1-HR populations were also resistant to acetolactate synthase inhibitors (imidazolinones), even though these herbicides were not frequently used. In Manitoba, 27% of fields surveyed had wild oat resistant to herbicides from more than one group. Four populations were resistant to all herbicides registered for use in wheat (Triticum aestivum). Depending on the nature of resistance in wild oat, alternative herbicides available for their control may substantially increase costs to the grower. The cost to growers of managing HR wild oat in Saskatchewan and Manitoba using alternative herbicides is estimated at over $4 million annually. For some HR biotypes, alternative herbicides either are not available or all have the same site of action, which restricts crop or herbicide rotation options and threatens the future sustainability of small-grain annual cropping systems where these infestations occur.

Overreliance on glyphosate can increase the risks of reduced efficacy of the herbicide on weeds and weed resistance, even in regions without glyphosate-resistant crops. That is the reality in Greece, with frequent reports of reduced efficacy of glyphosate against increasingly problematic weeds, such as Conyza spp. The objectives of this study were to determine the occurrence of glyphosate resistance in hairy fleabane populations in central and southern Greece and the effect of weed growth stage on glyphosate efficacy under controlled environmental conditions and to evaluate alternative herbicides in field trials for control of glyphosate-resistant and -susceptible hairy fleabane. Plants from 60 accessions of hairy fleabane, sampled from five prefectures in Greece, were sprayed with glyphosate at 0.36 kg ae ha−1; 15 were classified as potentially resistant. After initial screening, 15 potentially susceptible or resistant accessions were selected and dose–response experiments were conducted. Glyphosate rates required to control some accessions were four to seven times greater than that for control of the reference susceptible accession, AR4. Sensitivity of a resistant hairy fleabane accession to glyphosate was strongly dependent on growth stage, with plants at the seedling stage being most sensitive to the herbicide. A field trial indicated that diquat, glufosinate, or glufosinate + oxyfluorfen controlled glyphosate-resistant or -susceptible hairy fleabane. These herbicides, along with various integrated management strategies, have good potential to manage or slow the spread of glyphosate resistance in this species.

Antagonistic interactions between herbicides or between herbicides and nonherbicides are common and prevent the use of certain mixtures. Although antagonism is detrimental when it reduces weed control, the degree of antagonism is critical to determine whether a mixture is agronomically useful. If antagonsim is not severe, herbicides with different weed spectrums can be used together to complete the weed spectrum. Antagonism can be beneficial when it increases crop safety. In cases where antagonism significantly reduces control, key factors must be identified and managed to decrease antagonism and increase efficacy. These factors, which include the herbicide rate(s), mode of action, plant species, formulation, adjuvants, timing, stage of growth, and environment, often are complex and poorly understood at the whole plant level. The growing dependence on herbicide mixtures has increased the practical importance of understanding herbicide antagonism.

Weed management is an integral part of agriculture; weeds lower both productivity and quality of agricultural products. A combination of mechanical, chemical, biological, and cultural methods is expected to deliver a sustainable weed management program for the next two decades. While chemical methods offer the most cost effective means of weed management, crop selectivity has hampered the use of the best chemicals for weed management. Recent progress in gene technology has facilitated the introduction and expression of genes to confer a wide range of traits to crop plants. Application of this technology has resulted in the development of crop plant genotypes that are resistant to a specific herbicide. This article describes the progress that has been made by our group toward the introduction of glyphosate tolerance to crop plants. Glyphosate [N-(phosphonomethyl)glycine] kills plants due to inhibition of the biosynthesis of aromatic compounds via the shikimate pathway. Our approach for introduction of glyphosate tolerance is based on insertion and expression in plants of a gene encoding a glyphosate-tolerant 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase, a key enzyme of the shikimate pathway. The wild type enzyme present in plants is susceptible to inhibition glyphosate; variants of EPSP synthase have been produced that are less susceptible to inhibition by glyphosate. Expression of genes encoding these variants has been shown to confer glyphosate tolerance to plants. The degree of glyphosate tolerance is related to the tolerance characteristics of the EPSP synthase variant, its substrate activity, targeting to the plastid, and the level of expression of the variant gene. The tissue specificity of expression of the variant EPSP synthase has also been shown to be critical since glyphosate is a systemic herbicide and is translocated to many growing points within the plant. Our studies on glyphosate tolerance have substantially enhanced our understanding of the mode-of-action of glyphosate, the shikimate pathway, and protein sorting within plant cells, as well as developmental and tissue specific expression of genes in plants. Commercial use of glyphosate tolerance technology is expected to affect positively, the weed management arsenal available to the farmers, the sustainability of farm land and groundwater, and promote the use of a “soft” herbicide.

Glyphosate typically controls Palmer amaranth very well. However, glyphosate-resistant (GR) biotypes of this weed are present in several southern states, requiring the development of effective alternatives to glyphosate-only management strategies. Field experiments were conducted in seven North Carolina environments to evaluate control of glyphosate-susceptible (GS) and GR Palmer amaranth in narrow-row soybean by glyphosate and conventional herbicide systems. Conventional systems included either pendimethalin or S-metolachlor applied PRE alone or mixed with flumioxazin, fomesafen, or metribuzin plus chlorimuron followed by fomesafen or no herbicide POST. S-metolachlor was more effective at controlling GR and GS Palmer amaranth than pendimethalin; flumioxazin and fomesafen were generally more effective than metribuzin plus chlorimuron. Fomesafen applied POST following PRE herbicides increased Palmer amaranth control and soybean yield compared with PRE-only herbicide systems. Glyphosate alone applied once POST controlled GS Palmer amaranth 97% late in the season. Glyphosate was more effective than fomesafen plus clethodim applied POST. Control of GS Palmer amaranth when treated with pendimethalin or S-metolachlor plus flumioxazin, fomesafen, or metribuzin plus chlorimuron applied PRE followed by fomesafen POST was equivalent to control achieved by glyphosate applied once POST. In fields with GR Palmer amaranth, greater than 80% late-season control was obtained only with systems of pendimethalin or S-metolachlor plus flumioxazin, fomesafen, or metribuzin plus chlorimuron applied PRE followed by fomesafen POST. Systems of pendimethalin or S-metolachlor plus flumioxazin, fomesafen, or metribuzin plus chlorimuron applied PRE without fomesafen POST controlled GR Palmer amaranth less than 30% late in the season. Systems of pendimethalin or S-metolachlor PRE followed by fomesafen POST controlled GR Palmer amaranth less than 60% late in the season.

A screening procedure has been developed to evaluate the relative loss of pesticides from soils, to assist the Soil Conservation Service in implementing water quality for conservation planning. This screening procedure is a first-tier evaluation of the impact of using a particular pesticide on a specified soil. The screening procedure was developed from evaluating pesticide loss from over 40 thousand runs of the GLEAMS (Groundwater Loading Efects of Agricultural Management Systems) model. The model input data were a combination of soils and pesticides with a wide range of properties. The estimated pesticide losses were categorized into losses by leaching, losses adsorbed on sediment in runoff, and losses in the solution phase of runoff. Algorithms using soil properties were developed to rate soils into four loss potential classes for leaching and three loss potential classes for the two categories of runoff. Also, algorithms using pesticide properties were developed to rate pesticides into four loss potential classes for leaching and three loss potential classes for runoff. The soil and pesticide groupings are combined in a matrix to give an overall loss potential rating. Statistics of the overall loss potential indicate the low loss potential class is pure, that is, it does not contain occurrences that have medium or high losses. The medium loss potential classes does not contain occurrences of high loss, but does contain many occurrences of low loss. The high loss potential class contains incidences of medium and low loss.

About 15 years ago, I heard several speakers saying that our crop yields were “leveling off.” This stimulated me to assemble data on the subject. The result is shown in Table 1, which gives the U.S. average yields for 10-yr periods during this century for nine crops. The increases are spectacular, varying from two- to sevenfold. Factors contributing to these increases differ from crop to crop. To obtain information on this subject, several experts on each crop were consulted, and their conclusions are summarized in the following text. Only those factors contributing to increased yields are covered. However, great improvements in efficiency of production, product quality, and reduction in soil erosion also have occurred as cultivars and production practices have changed. For most of the crops, increases in yields started in the 1940s and have increased dramatically during the rest of the century. We have not attempted to predict the future at this time, but the rate at which yields are increasing does not appear to be slowing.

At least 57 weed species, including both dicots and monocots, have been reported to have biotypes selected for resistance to the triazine herbicides. In addition, at least 47 species have been reported to have biotypes resistant to one or more of 14 other herbicides or herbicide families. These herbicides include the aryloxyphenoxypropionics, bipyridiliums, dinitroanilines, phenoxys, substituted areas, and sulfonylureas, with two or more resistant biotypes each, as well as several other herbicides in which resistance is less well documented. Although evolved resistance presents a serious problem for chemical weed control, it has also offered new potential for transferring herbicide resistance to crop species. Mechanisms of resistance that are due to single or a few genes have become the focus of biotechnology, as the probability of their successful transfer to crop species is high.

Cyanobacteria (blue-green algae) are undesirable in ponds used to raise fish for human food. Management of cyanobacterial communities in aquaculture ponds has been hindered by the small number of herbicides approved for use in food-fish ponds and by the lack of selectivity toward cyanobacteria for those herbicides that are approved for that use. To facilitate development of additional management options, a rapid bioassay utilizing 96-well cell culture plates was developed for screening herbicides and other phytotoxins for selective toxicity toward cyanobacteria. Oscillatoria cf. chalybea and Selenastrum capricornutum were chosen as representatives of cyanobacteria (Cyanophyta) and green algae (Chlorophyta), respectively. In the bioassay, wells of the cell culture plates were inoculated with cyanobacterial or unialgal culture. One of five herbicides (atrazine, diuron, endothall, fluridone, or paraquat) was then added to the wells at various concentrations, and absorbance (650 nm) was measured at 24-h intervals. Growth of treated cultures relative to untreated cultures was used to determine relative toxicity of the herbicide to the two organisms. Paraquat was the most selective of the herbicides tested and was over 10-fold more toxic to O. cf. chalybea than to S. capricornutum. This method was demonstrated to be a rapid, effective, and highly reproducible bioassay to identify selective algicides.

Certified Crop Advisors of Arkansas and members of the Arkansas Crop Consultants Association were surveyed in Fall 2006 through direct mail to assess current weed management practices and needs in rice from both a research and educational perspective. Consultants reported scouting 228.2 of the possible 567 thousand hectares (40%) of rice grown in Arkansas. Pre-emergence herbicides most often recommended were clomazone (93%) and quinclorac (40%). Propanil (55%) and quinclorac (47%) were the two most commonly recommended postemergence herbicides. Thirty-two percent of the consultants often recommend three or more herbicide applications per field. An average of 37% of the fields were believed to have “serious” or “very serious” weed infestations, and fields were scouted for weeds on average 11 times per growing season. Ninety-two percent of the consultants had “moderate” to “high” concerns with herbicide-resistant weeds. The perceived average additional expense associated with managing a resistant weed in rice was $65.60/ha. Propanil-resistant and quinclorac-resistant barnyardgrass were believed to be infesting 24 and 7% of the scouted rice hectares, respectively. Barnyardgrass was the most problematic weed of rice followed by red rice. Northern jointvetch and smartweeds were the two most problematic broadleaf weeds. The number one research need was improved broadleaf weed control. Respondents indicated that research and educational efforts should continue to focus on herbicide performance and development of economical weed control programs.

Studies were conducted in 1997 and 1998 at the Northeast Mississippi Research and Extension Center to investigate the effects of row spacing (76, 38, and 19 cm), soybean population, and three weed management systems on sicklepod growth and seed production. The cultivars ‘Hartz 5088RR’ (glyphosate-tolerant) and ‘Hutcheson’ (a conventional cultivar) were used in two separate studies. The average soybean populations over cultivars and year were 245,000 (low), 481,000 (medium), and 676,000 (high) plants/ha. The three weed management systems were: no (untreated), one, and two herbicide applications. In the glyphosate-tolerant system, one or two postemergence (POST) applications of glyphosate were used, whereas in the conventional system, flumetsulam plus metolachlor preemergence was used alone (single) or followed by chlorimuron POST (sequential). Reducing soybean row spacing from 76 cm, coupled with increased soybean population, reduced sicklepod population up to 80%. Except for Hutcheson in 1998, reducing row spacing and increasing soybean population also reduced sicklepod seed production in both the untreated and the single applications. A single herbicide application reduced sicklepod population up to 68% from untreated plots. However, except for Hartz 5088RR in 1998, the sequential application did not further reduce sicklepod population. In a shading study, partial shading increased sicklepod height but reduced dry weight. However, as shading level increased from 65 to 80 and 95%, height was also reduced.

The objective of this research was to assess the accuracy of remote sensing for detecting weed species in soybean based on two primary criteria: the presence or absence of weeds and the identification of individual weed species. Treatments included weeds (giant foxtail and velvetleaf) grown in monoculture or interseeded with soybean, bare ground, and weed-free soybean. Aerial multispectral digital images were collected at or near soybean canopy closure from two field sites in 2001. Weedy pixels (1.3 m2) were separated from weed-free soybean and bare ground with no more than 11% error, depending on the site. However, the classification of weed species varied between sites. At one site, velvetleaf and giant foxtail were classified with no more than 17% error, when monoculture and interseeded plots were combined. However, classification errors were as high as 39% for velvetleaf and 17% for giant foxtail at the other site. Our results support the idea that remote sensing has potential for weed detection in soybean, particularly when weed management systems do not require differentiation among weed species. Additional research is needed to characterize the effect of weed density or cover and crop–weed phenology on classification accuracies.

This research examined dicamba measurements following an application to soil inside a humidome. The dicamba formulations examined were the diglycolamine (DGA) and diglycolamine plus VaporGrip® (DGA+VG), both applied with glyphosate. Post-application dicamba measurements were related to ambient temperature, with more dicamba detected as the temperature increased. There also appeared to be a minimum temperature of ~15 C at which dicamba decreased to low levels. The addition of glyphosate to dicamba formulations decreased the spray mixture pH and increased the observed dicamba air concentrations. Adding glyphosate to DGA+VG increased detectable dicamba air concentrations by 2.9 to 9.3 times across the temperature ranges examined. Particle drift would not be expected to be a factor in the research, as applications were made remotely before treated soil was transported into the greenhouse. The most probable reason for the increased detection of dicamba at higher temperatures and with mixtures of glyphosate is via volatility.

Concerns about the effect of pesticides on our environment has stimulated questions about the direction of future research in weed science. Knowledge of crop losses due to weeds would be helpful in guiding research objectives by identifying areas of greatest need. A survey was conducted in 1991 to estimate losses due to weeds in 58 commodities in Canada. Data generated included: percent loss in production, quantity loss in production, and loss of production value (monetary value) for each commodity in each province. The estimated average annual losses caused by weeds in the 58 commodities were $984 million, with losses of $372 million in Eastern Canada and $612 million in Western Canada. In Eastern Canada, approximately 50% of the total loss occurred in hay crops and 33% in field crops, whereas in Western Canada, 84% of the total loss occurred in field crops. Loss of potential production due to weeds for each commodity is shown by province.

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.

In greenhouse experiments, Ohio accessions of 22 weed species representing 13 dicot families were screened as alternative hosts of soybean cyst nematode (SCN, Heterodera glycines). Purple deadnettle (Lamium purpureum), henbit (Lamium amplexicaule), field pennycress (Thlaspi arvense), shepherd's-purse (Capsella bursa-pastoris), and a susceptible soybean (Glycine max) cultivar produced SCN population densities of 510, 155, 73, 1, and 366 cysts/450 cm3 soil, respectively, 5 wk after inoculation with eggs from a race 3 SCN population. Purple deadnettle was also a strong host of race 1 SCN and a weak host of race 6 SCN. Average numbers of eggs/cyst among race 3 hosts were highest in purple deadnettle (357), followed by soybean (292), field pennycress (266), henbit (122), and shepherd's-purse (none detected). To our knowledge, henbit is the only SCN host identified here that has been previously identified as a host. The weeds identified as SCN hosts in this study have a winter annual life cycle in Ohio and may serve as sites for SCN reproduction in infested fields during the early or late growing season and when soybean plants are absent.

Field and greenhouse studies were conducted in 1995 and 1996 in Kansas to determine the efficacy of 21 herbicide treatments for control of tumble pigweed, Palmer amaranth, redroot pigweed, and two biotypes of common waterhemp in soybean. In field studies, nine of eleven preemergence treatments controlled all four species 90% or more. However, pendimethalin and trifluralin controlled Palmer amaranth, redroot pigweed, and tumble pigweed less than the other preplant incorporated and preemergence treatments. With the exception of flumiclorac and NAF-75, postemergence treatments controlled 75 to 90% of all four species. A biotype of common waterhemp collected in Iowa was not controlled by acetolactate synthase-inhibiting herbicides. Greenhouse results were similar to field experiments. Results suggest at least 90% control of these Amaranthus species is possible with proper herbicide selection.