A simulation model of rice-barnyardgrass competition for light was used for two management applications. First, simulations using 47 weather data sets from four locations in Asia were conducted to evaluate the influence of weather variation on single year economic threshold densities of barnyardgrass. Second, rapid leaf area expansion and leaf area index were evaluated as potential indicators of improved rice competitiveness and tolerance to barnyardgrass. Influence of weather variation on single year economic thresholds was small under the assumption that competition was for light only. Increasing early leaf area expansion rate reduced simulated barnyardgrass seed production and increased single year economic thresholds, suggesting that the use of competitive rice cultivars may reduce the need for chemical weed control. The model predicted that rice leaf area index 70 to 75 d after planting was a good indicator of early leaf area expansion rate.

Glyphosate is the most used herbicide worldwide, which has contributed to concerns about its environmental impact. Compared with most other herbicides, glyphosate has a half-life in soil and water that is relatively short (averaging about 30 d in temperate climates), mostly due to microbial degradation. Its primary microbial product, aminomethylphosphonic acid, is slightly more persistent than glyphosate. In soil, glyphosate is virtually biologically inactive due to its strong binding to soil components. Glyphosate does not bioaccumulate in organisms, largely due to its high water solubility. Glyphosate-resistant crops have greatly facilitated reduced-tillage agriculture, thereby reducing soil loss, soil compaction, carbon dioxide emissions, and fossil fuel use. Agricultural economists have projected that loss of glyphosate would result in increased cropping area, some gained by deforestation, and an increase in environmental impact quotient of weed management. Some drift doses of glyphosate to non-target plants can cause increased plant growth (hormesis) and/or increased susceptibility to plant pathogens, although these non-target effects are not well documented. The preponderance of evidence confirms that glyphosate does not harm plants by interfering with mineral nutrition and that it has no agriculturally significant effects on soil microbiota. Glyphosate has a lower environmental impact quotient than most synthetic herbicide alternatives.

Under greenhouse conditions, postemergence herbicide effectiveness was greater when barnyardgrass [Echinochloa crus-galli (L.) Beauv.] plants were 5-or 10-cm tall as compared to 15-or 20-cm tall. Absorption of 14C-glyphosate [N-(phosphonomethyl)glycine] decreased from 58% of applied to 47% as plant height increased from 5 to 15 cm, and an additional reduction (to 32% of applied at 5 cm) occurred when terbutryn [2-(tert-butylamino)-4-(ethylamino)-6-(methylthio)-s-triazine] was added to glyphosate. Translocation of 14C-glyphosate was reduced in older plants or when terbutryn was added to glyphosate. Water stress studies included soil moisture levels of 10, 20, 30, and 40% moisture (PW). Herbicide treatments were less effective at soil moisture levels below field capacity (30 PW). When soil moisture was 10 PW (-37 bar) absorption of 14C-glyphosate was greatly reduced in plants treated at both 7.5-and 15-cm heights with or without terbutryn. At 10 PW only 15 to 20% of applied 14C was absorbed whereas at 40 PW, absorption was over 45% and 62% with and without terbutryn, respectively. The addition of terbutryn to glyphosate resulted in decreased translocation for all soil moisture levels except the 20 PW treatment at the 15-cm height. Translocation of 14C was greatly reduced for plants in soils at 10 PW for both herbicide treatments.

Phytotoxicity of glyphosate (N-(phosphonomethyl) glycine), applied at 0, 0.56, 1.12, 1.68, 2.24 and 4.49 kg ai/ha to uniform, naturally growing quackgrass, [Agropyron repens (L.) Beauv.] plants, was studied with the electron microscope. Visible damage (yellowing of the leaves) to the plants was observed at the 2.24 and 4.49 kg ai/ha dosage rates within 72 hr. Similar damage became evident 120 hr after treatment at the 0.56 to 1.68 dosages. Leaf discs (1 mm in diameter) were harvested at 24, 48, 96, and 192 hr and prepared for electron microscopy by standard techniques. Cellular damage could be detected at all dosage rates as early as 24 hr after treatment. The type of damage observed was partial to complete disruption of the chloroplast envelope, and swelling of the rough endoplasmic reticulum (RER) with a subsequent formation of vesicles. With loss of integrity of the envelope, the chloroplast became completely disrupted with increased time. Other organelles within the cell were also destroyed.

The distribution of sicklepod (Cassia obtusifolia L.) and coffee senna (Cassia occidentalis L.) in the United States is similar and both species are increasing as problems within their range. Sicklepod is a problem in 11 southern states and increasing in 12 while coffee senna is a problem in 7 states and increasing in 8 states. Both species germinated over a wide range of temperature. Germination characteristics were similar in the range of 15 to 36 C. Germination of coffee senna was drastically reduced at 39 C by Rhizopus sp. Seedling growth was similar for both species and was maximum from 30 to 36 C. Both species have a hard seedcoat. Abrasion for 5, 10, or 15 s, puncturing with a needle, and sulfuric acid were all effective in initiating germination. Both species emerged from a soil depth of 12.5 cm; however, emergence of sicklepod was more rapid than coffee senna with 63% of the sicklepod seedlings emerging from 2.5 cm after only 3 days.

Concentrations of DIBOA [2,4-dihydroxy-1,4-(2H)-benzoxazine-3-one] and BOA [2-(3H)-benzoxazolinone], described previously as major allelochemicals in Secale cereale (rye), were determined in eight field-grown cultivars, harvested at booting, using high-performance liquid chromatography (HPLC). Allelochemicals were also quantified in greenhouse-grown cultivar ‘Bates’ harvested 30, 45, 60, and 75 days after planting (DAP). The total production of DIBOA and BOA from field-grown S. cereale ranged from 137 to 1,469 μg g−1 dry tissue among the eight cultivars. ‘Bonel’ cultivar had the highest hydroxamic acid (HA) content and ‘Pastar’ the lowest. Bonel also showed the highest activity on Eleusine indica (goosegrass) and Pastar the least, in culture plate bioassays using aqueous extracts. HA content in shoot tissue varied with S. cereale maturity. The greatest level of HA in greenhouse-grown Bates was obtained 60 DAP compared to 30 DAP.

Field persistence of imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} applied preemergence as high as 16 times the 0.14 kg ai/ha recommended rate was studied at two locations in Arkansas in 1984 and 1985. Rapid dissipation of these high rates occurred on the Taloka silt loam under hot, dry field conditions in 1984 when no rainfall occurred for 2 weeks after application. In 1985, when furrow irrigation was applied 7 days after application, imazaquin phytotoxicity was greater and dissipation was delayed. Imazaquin persistence and soil adsorption were greater on Sharkey silty clay than on Taloka silt loam. Following 3- and 8-cm irrigation, 84 and 78%, respectively, of the 14C-labeled imazaquin remained in the surface 5 cm, but in plots covered between rainfalls and receiving 8 cm rain, 74% leached out of this top layer. Since nonincorporated imazaquin dissipated rapidly under hot, dry field conditions, carryover into the next growing season or leaching below the plow layer would not be expected. However, in cool, wet conditions immediately after application, the weakly adsorbed imazaquin molecule was readily bioavailable and mobile in the soil profile.

Wild oat (Avena fatua L.) competition increased the losses in yield of both wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) with increased densities of seedlings. At Fargo in 1965, wild oat densities of 70 and 160 seedlings/sq yd reduced the wheat yield 22.1% and 39.1%, respectively, compared to the wild oat-free check. Similar wild oat densities reduced the barley yield 6.5% and 25.9%, respectively. The addition of nitrogen and phosphorus fertilizer reduced the yield loss caused by wild oat 2 out of 3 years of the experiment. Although a considerable yield reduction occurred in barley and wheat, percent protein and seed size were not influenced noticeably.

Removal of nitrogen and phosphorus by waterhyacinth [Eichhornia crassipes (Mart.) Solms] in static and flowing water was investigated. Milligrams of phosphorus absorbed per plant per day in static water averaged 1.1, 2.1, 3.1, and 1.6 in 10, 25, and 50% Hoagland's solution and in sewage effluent, respectively, while in flowing water the values were 1.7, 2.5, and 3.3 for 10, 25, and 50% Hoagland's solution. Milligrams of nitrogen absorbed per plant per day from these same solutions averaged 5.3, 11.4, 19.8, and 6.6 from static water and 9.9, 18.4, and 20.8 from flowing water. Transpiration per plant per day averaged approximately 175 mi in static water and 225 ml in flowing water. One hectare of waterhyacinth plants under optimum conditions could absorb the average daily nitrogen and phosphorus waste production of over 800 people.

In field tests, common cocklebur (Xanthium pensylvanicum Wallr.) was controlled at the one- and two-leaf stages with 0.3 kg/ha acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid}. Four-leaf common cocklebur required 1.1 kg/ha for comparable control. Acifluorfen at 0.6 and 0.8 kg/ha controlled 96 and 90% of entireleaf morningglory [Ipomoea hederacea (L.) Jacq. var. integriuscula] atone- and two-leaf stages, respectively, but control was reduced as plants aged. Dark applications (2100 h) were more effective than those at sun-up (0600 h) or midday (1200 h) in control of hemp sesbania [Sesbania exaltata (Raf.) Cory], pitted morningglory (Ipomoea lacunosa L.), and smooth pigweed (Amaranthus hybridus L.). Hour of application had no significant influence on control of common cocklebur or prickly sida (Sida spinosa L.). An increase in surfactant (80% alkyl aryl polyoxyethylene glycols) concentration enhanced acifluorfen control of entireleaf morningglory and Texas gourd [Cucurbita texana (A.) Gray] at the 0.3-kg/ha rate applied 2 weeks after emergence. Soybean [Glycine max (L.) Merr.] injury was increased with the 1.1-kg/ha rate when surfactant concentration was increased from 0.5 to 0.75%. Increasing the spray volume increased pitted morningglory control but decreased hemp sesbania control. In the growth chamber, better control of pitted morningglory and common cocklebur was obtained at high (35 C day, 26 C night) than at low temperatures (27 C day, 18 C night) when plants were treated 1 week after emergence.

For most effective weed control 2–chloro–4,6–bis(ethylamino)–s–triazine (simazine) must be applied to the soil. As a foliar spray it is much less effective. Simazine penetrates into leaves of some plants, but apparently the cuticle is a barrier to foliar absorption of this herbicide. It is absorbed rapidly by roots and is transported upward to the aerial plant parts.

Downy brome (Bromus tectorum L. # BROTE), also known as cheatgrass, downy chess, broncograss, Mormon oats, and junegrass, was introduced into the United States from Europe, apparently during the middle of the nineteenth century (11, 21). According to Mack (23), downy brome entered British Columbia, Washington, and Utah around 1890; and by 1928 it had reached its present range, occupying much of the perennial grassland in Washington, Idaho, Oregon, Nevada, Utah, and British Columbia. Today, downy brome is a widespread weed throughout most of Canada, Mexico, and the United States, except for the southeastern United States (5, 17). Some consider downy brome to be an important forage because it provides most of the early spring grazing for livestock in western United States rangeland (21). However, it is also considered a troublesome weed in rangeland (31), winter wheat (Triticum aestivum L.) (27), several other crops (29), and noncropland (32).

A method is described to produce inoculum of Alternaria cassiae Jurair and Khan for biocontrol studies. Approximately 8 g of a conidial preparation containing 1 × 108 conidia/g were produced per liter of growth medium. In greenhouse studies the effectiveness of A. cassiae as a biocontrol agent for sicklepod (Cassia obtusifolia L.) was affected by dew-period temperature, dew-period duration, inoculum concentration, and the stage of growth of sicklepod seedlings at the time of inoculation. Optimum environmental conditions included at least 8 h of free moisture at 20 to 30C. Spray mixtures containing approximately 5 × 104 or more conidia/ml gave maximum control when sicklepod seedlings were sprayed to runoff. Sicklepod seedlings in the cotyledon to first-leaf stage were most susceptible to the pathogen.

Improved fallow systems are needed to reduce the negative effects of increased soil erosion and reduced soil quality. Field experiments were conducted to determine weed suppression attained with yellow sweetclover grown as a green manure fallow replacement crop. Yellow sweetclover was undersown in field pea, flax, or Indian mustard and then killed in June of the following fallow year. Living yellow sweetclover competed strongly with weeds during the first fall and spring of fallow. Weed biomass accounted for <1 to 12% of the total plant biomass when yellow sweetclover was terminated in June. Yellow sweetclover residues remaining after termination of growth continued to provide excellent weed suppression. Weed densities in April before planting the succeeding wheat crop were 75 to 97% lower in yellow sweetclover than in untreated fallow treatments. Yellow sweetclover controlled the perennial weeds dandelion and perennial sowthistle, as well as the annuals kochia, flixweed, Russian thistle, and downy brome. Weed suppression was similar whether yellow sweetclover was harvested as hay or its residues were incorporated or left on the soil surface, suggesting that a portion of the weed suppression effect may be due to allelopathic compounds being released from decomposing yellow sweetclover. Results will be used to develop more sustainable agronomic practices in regions where fallow is still widely employed.

Late-season weed escapes are often ignored because they rarely cause crop yield penalty. Traditional weed management recommendations are based on the economic threshold (ET) approach, wherein management is required if the predicted current-season yield loss is greater than the cost of control interventions. While ET-based weed management can reduce current-season production costs and promote farmland biodiversity, it does not consider the long-term biological and economic consequences associated with late-season weed seed production. An important concern is that late-season weed seed production will replenish the soil seedbank, ensuring future weed problems. In the context of herbicide resistance evolution, allowing late-season weed seed production can be problematic because the probabilities of occurrence of resistant mutants rise with increases in seed production. A key component of herbicide resistance mitigation and management is preventing seed production and buildup of the soil seedbank. Late-season weed management efforts constitute additional expenses to growers, which cannot be recouped in that growing season, but any such investment must be weighed against the perceived long-term benefits. It appears that management of late-season weed escapes is valuable in a number of situations, and the degree to which management interventions should be employed can be case-specific. Adoption of economic optimum thresholds (EOTs), which can be established using bio-economic models, will be useful for making management decisions for late-season weed escapes. In systems vulnerable to herbicide resistance evolution, bio-economic resistance thresholds (BERTs) will be appropriate and bio-economic resistance models (BERMs) will be helpful for establishing such thresholds for specific production scenarios. Management considerations for late-season weed escapes are discussed, and knowledge gaps for future research are identified.

Atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] retention by standing and flat wheat (Triticum aestivum L.) stubble was determined over time. Immediately following an application of 1.7 kg/ha atrazine, approximately 60% of the herbicide had been intercepted by the stubble and 40% was found in the underlying soil. After 3 weeks and 50 mm of rainfall, atrazine on standing and flat stubble had decreased by 90 and 63%, respectively, while atrazine in the soil increased nearly twofold. No atrazine was found in stubble 9 weeks after application, and only 17% of that originally applied remained in the upper 4 cm of the soil. Successive alternate-day applications of 12.5, 25, or 50 mm water on 14C-atrazine retention by stubble was determined. Atrazine loss from stubble was greatest during the first water application, but there was no linear relationship between water application rate and retention of the herbicide. Atrazine loss from stubble decreased significantly following the second water application in all treatments. The amount of water applied did not influence atrazine loss following the third washing. Subsequent loss after the fourth, fifth, sixth, and seventh washings was not significant at any water application volume. After the seventh washing, atrazine residues were greatest on stubble which had received the least amount of water.

Laboratory and field experiments were conducted to assess the allelopathic potential of buckwheat. In the field, buckwheat demonstrated strong inhibitory activity by suppressing weeds. In laboratory studies, aqueous and organic solvent extracts of the aerial parts of common buckwheat inhibited the root and shoot growth of lettuce seedlings. The chloroform and ethyl acetate extracts showed maximum activity, and plants grown in the presence of the ethyl acetate extract showed severe root browning. The allelopathic constituents of the ethyl acetate phase were isolated and identified as gallic acid and (+)-catechin by nuclear magnetic resonance spectroscopy. Gallic acid and (+)-catechin were present in the upper part of buckwheat at concentrations of 0.02 and 0.01%, of fresh weight, respectively. Gallic acid was found to be selectively and strongly inhibitory to root and shoot growth of tested plants at 100 and 10 μg ml−1. (+)-Catechin, however, inhibited plant growth to a lesser extent. These results suggest that buckwheat may have allelopathic potential and that when used as a ground cover crop or green manure may produce inhibitors, which could suppress weeds.

Radiolabeled (14C) 2-[4-(3-chloro-5-trifluoromethyl-2-pyridinyloxy)phenoxy] propionic acid (Compound C); its methyl-,n-butyl-, and ethoxyethyl-esters; and fluazifop-butyl {(±)-butyl 2-[4-[(5-(trifluoromethyl)-2-pyridinyl)oxy] phenoxy] propionate} were applied to the primary shoots of young rhizomatous quackgrass [Agropyron repens (L.) Beauv. # AGRRE]. Plants were sampled from 0.5 to 24 days after treatment (DAT) and analyzed for radiochemical. All treatments caused phytotoxic symptoms in primary shoots, rhizomes, and tillers and significantly reduced growth of primary shoots and rhizomes. Treatment with compound C or its methyl and butyl esters eliminated regrowth from all rhizomes excised from treated plants. The ethoxyethyl-ester and fluazifop-butyl controlled regrowth from rhizomes from 50% of the plants and substantially reduced shoot regrowth from the remainder. Rhizomes that did produce shoots contained significantly less radiochemical than those from which no regrowth occurred. At 24 DAT, a maximum of only 1% of each radiochemical applied was translocated to the first tiller and rhizomes and had been sufficient to cause marked phytotoxic symptoms. Soon after application (1 DAT), the major metabolites in the treated leaves and remainder of the plant for all compounds were the free acid and substantial amounts of polar conjugates that were hydrolyzable to the free acids. Radiochromatography of extracts from rhizomes and first tillers from all treatments gave similar chemical profiles, with the free acids and their conjugates as the predominant components present. These results provide evidence that esters of the pyridinyloxyphenoxypropionic acids are rapidly hydrolyzed after absorption by quackgrass; the free acids are then translocated and are the active form of these herbicides.

Common waterhemp is a significant weed problem in Midwestern cropping systems partly because of its potential for multiple emergence events during the growing season. The effects of shade and time of emergence on this weed have not been characterized. In the field, common waterhemp vegetative and reproductive growth were evaluated under different irradiance levels at two emergence times. In full sunlight a common waterhemp plant emerging in late May produced 720 g of biomass and over one million seeds, and a plant emerging in late June produced 350 g of biomass and over 730,000 seeds. Plant biomass and seed production were lower as irradiance levels were decreased to 40, 68, and 99% shade. Mortality was high for common waterhemp grown in 99% shade; however, surviving plants produced some viable seed. Common waterhemp plants grown under reduced irradiance had higher leaf area ratios and lower relative growth rates.

Weed seeds initiate most weed invasions of arable fields, yet there is relatively little information on the value of managing weed seed banks. Matrix population models were used to examine the relative importance of managing weed seed banks, in relation to other life stages, for four model weed species with varying life histories. Simulations for giant foxtail and common lambsquarters, summer annual weeds of arable fields; garlic mustard, an obligate biennial invasive weed of temperate forests; and Canada thistle, a perennial weed of pastures and arable fields, were run under conditions of varying population density and efficacy of seedling control. The models were subjected to elasticity analysis to determine what happened to weed populations when different life stages were targeted. Losses from the dormant seed bank were most important for summer annual weeds, of intermediate importance for biennial weeds, and of low importance for perennial weeds. More effort is needed to develop weed seed-bank management techniques for summer annual weed species as part of integrated weed management systems.