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.

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.

Twenty-one grass weeds have evolved resistance to herbicides in Latin America, particularly in rice, soybean, wheat, and orchards. Junglerice, the most widespread and economically important rice weed, evolved resistance to propanil, acetyl-coenzyme A carboxylase (ACCase)-inhibitor herbicides, quinclorac, and imazapyr in Central America, Colombia, and Venezuela. Some junglerice populations are resistant to at least three herbicide modes of action. Other herbicide-resistant (HR) rice weeds are barnyardgrass and gulf cockspur to quinclorac in Brazil, and saramollagrass to ACCase-inhibitor herbicides in Colombia and bispyribac in Venezuela. Populations of weedy rice resistant to imidazolinones are now emerging, most likely originated from gene flow from imidazolinone-resistant rice. Saramollagrass also became resistant to nicosulfuron in corn in Venezuela. Eight species associated with soybean are resistant to ACCase-inhibitor herbicides in Brazil (alexandergrass, goosegrass, and southern crabgrass) and Bolivia (Louisiana cupgrass, itchgrass, sudangrass, and two common wild sorghum species). Four more ACCase-inhibitor–resistant species (hedgehog dogtailgrass, wild oat, rigid ryegrass, and Italian ryegrass) are found in Chile infesting canola and wheat. ACCase-inhibitor–resistant hood canarygrass, littleseed canarygrass, and wild oat are important in wheat in Mexico. Resistance to acetolactate synthase (ALS)-inhibitor herbicides has been reported in itchgrass, goosegrass, and Mexican grass. Italian ryegrass populations resistant to glyphosate have been found in Chile and Brazil. Glyphosate resistance has also evolved in goosegrass in Bolivia and johnsongrass in Argentina. In general, little is done to prevent resistance evolution. An exception is the stewardship programs aiming to prevent gene flow from imidazolinone-resistant rice to weedy rice. Once resistance evolves, HR populations are mostly managed by shifting to herbicides with different modes of action and, in some cases, by slightly modifying agronomic practices. Propanil formulations containing a synergist are used to manage propanil-resistant junglerice. Increased no-till agriculture and planting of glyphosate-resistant crops are likely to select more glyphosate-resistant weeds.

Greenhouse and field experiments were initiated to evaluate rapeseed and mustard species as green manure crops for weed suppression. Under greenhouse conditions incorporating 20 g fresh wt leaf and stem tissue of rapeseed, two white mustards, and brown mustard into 450 g Sharpsburg, silty clay loam soil resulted in significant reductions in weed emergence, biomass, and height. Kochia, shepherd's-purse, and green foxtail emergences were reduced by all green manure crops. Redroot pigweed emergence was reduced by all green manure crops except brown mustard, and velvetleaf emergence was reduced by white mustards only. Kochia and shepherd's-purse fresh weights were reduced by all green manure crops, while redroot pigweed and velvetleaf fresh weights were reduced by brown mustard and white mustard var. Salvo. Green foxtail fresh weight was reduced by all green manure crops except rapeseed. With the exception of shepherd's-purse, no relationship between glucosinolate content of the incorporated green manure and suppression of weed growth was found. Under field conditions, early spring-planted green manure crops reduced early season weed biomass in soybean at one of the two locations. Mustard species as green manure crops reduced total weed biomass in soybean by 40% 4 weeks after emergence (WAE) and 49% 6 WAE. Soybean biomass and yield were sometimes reduced by the incorporation of green manure crops in treatments containing weeds; however, hand-weeded plots with green manure treatments yielded similar to hand-weeded plots without green manure.

A soil sampler, elutriator, and associated sample flushing device were designed and constructed for an intensive study of weed seedbanks. This equipment was used in 1993 to collect and process 4980 soil samples. The sampler was durable, core size was consistent, and sampling was efficient. Cores were approximately 200 cm3 and two people could take 120 cores/h. The elutriator separated weed seeds from 36 of these cores at a time. Washing required 60 to 75 min depending on soil texture. Seeds as small as 0.3 mm in diam were recovered and almost 100% of the seeds were recovered from samples spiked with barnyardgrass, redroot pigweed, velvetleaf, and witchgrass. The flushing device was used to transfer sample contents from strainers of the elutriator to propyltex bags for drying and storing. Equipment like this, plus improved technology for identifying and counting seeds, is needed to make weed seedbank studies more feasible.

Research was conducted for 2 yr at Marianna, AR, to determine whether the fall-planted cover crops rye, wheat, turnip, and a blend of brown and white mustard (Caliente) would aid weed management programs in conservation-tilled, enhanced, glyphosate-resistant cotton. Wheat and rye easily were established both years and turnip and mustard blend stands were better in the second year. The cover crops alone were more suppressive of Palmer amaranth, pitted morningglory, and goosegrass in 2007 than in 2008. Rye was generally superior to wheat in suppressing the three evaluated weeds. Once herbicides were applied, there were seldom differences among cover crops for a particular herbicide program as a result of the highly efficacious herbicide programs. Cotton yields were not affected by wheat, rye, or the mustard blend, but yields were lowest in plots that followed turnip both years, possibly because of allelopathy. Integration of cover crops, especially cereals, into conservation-tilled, glyphosate-resistant cotton aided early-season weed management and could reduce the selection of glyphosate for herbicide resistance.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Corn and soybean were planted in narrow and wide row spacings to determine the effect of glyphosate application timing and row spacing on crop yield. Glyphosate was applied when average weed canopy height reached 5, 10, 15, 23, and 30 cm. Weeds present in these studies included velvetleaf, redroot pigweed, common ragweed, common lambsquarters, jimsonweed, barnyardgrass, fall panicum, giant foxtail, yellow foxtail, green foxtail, and eastern black nightshade. Under highly competitive growing conditions (below normal rainfall and high weed density), corn yield was first reduced when weeds reached 10 and 15 cm in height with corn planted in 38- and 76-cm rows, respectively. Under similar conditions, soybean yield was first reduced when weeds reached 15 and 23 cm with soybean planted in 19- and 38-cm rows, respectively. Yield losses occurred only in the untreated control when soybean was planted in 76-cm rows. When growing conditions were less competitive (adequate rainfall and lower weed density), yield losses occurred only when weeds reached 30 cm or more in corn and only in the untreated control in soybean. Corn and soybean yields were higher when planted in narrow rows in three of 4 yr but were more susceptible to early-season weed interference than corn and soybean in wide rows. Corn yield was affected more by weed interference than was soybean yield. The product of weed height by weed density, as the independent variable, resulted in the best linear fit for both corn and soybean yields. High weed densities increase the risk of yield loss and must be considered when determining the appropriate timing for total postemergence herbicide applications such as glyphosate. Sequential glyphosate applications in corn did not increase yield.