Prickly lettuce is an annual weed that germinates in both the fall and the spring. It is often found in no-till soybeans and winter wheat in Ontario, Canada, as well as along the edges of fields. Field studies were conducted from 2001 to 2004 to estimate crop yield losses, and to characterize the phenology and seed production of prickly lettuce in relation to time of emergence. Prickly lettuce had a large impact on soybean yield, with losses of 60 to 80% at densities of 50 plants m−2 or more. Prickly lettuce density estimated to cause a 10% soybean yield loss varied from 0.2 plants m−2 in 2002 to 1.2 plants m−2 in 2003 and 2004. In winter wheat, prickly lettuce at densities up to 200 plants m−2 caused no detectable yield loss in this study. Plants that emerged in the fall generally were larger, flowered earlier. and produced more seeds than those emerging in spring, but size and fecundity were strongly density-dependent. The number of flowers produced per plant could be estimated from the height of the main stem. Seed production per plant ranged from 2,200 to 67,000 in soybeans, and up to 87,000 in a noncrop area at the edge of the field. Winter wheat harvest interrupted prickly lettuce flowering, and only about 25 to 30% of the plants present in the wheat crop survived harvest and flowered in untreated stubble. These plants produced less than 4,000 seeds per plant. Postharvest control with glyphosate, mowing, or cultivation prevented prickly lettuce seed production in wheat stubble. This study suggests that prickly lettuce populations could build up quickly in continuous no-till soybeans, but rotation with winter wheat and control of plants at the edge of the field would help to limit population growth.

The rates of degradation of 14C-benefin (N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine) and 14C-DCPA (dimethyl tetrachloroterephthalate) were significantly faster in thatch than in soil from a Kentucky bluegrass (Poa pratensis L.) turf. These results suggest that herbicides applied preemergence in turf will persist for shorter periods of time because of the carbon-enriched medium (thatch) and that higher rates or more frequent applications may be required to maintain concentrations at effective levels.

Common cocklebur root and shoot interferences were investigated as factors causing soybean yield reductions in the field. Significant decreases in soybean yield resulted from common cocklebur shoot interference when the two were grown together with the common cocklebur roots confined in plastic bags. Soybean yield decreased further when common cocklebur roots were not confined, but this decrease resulted, in part, from increased common cocklebur shoot growth. Wire mesh cylinders of varying diameters, placed in gaps in the soybean rows and designed to spatially restrict soybean shoot growth, reduced yield more than did gaps alone. When similar cylinders lined with black plastic were placed next to the soybean rows to restrict both within-canopy light and space for soybean shoot growth, soybean yield was reduced most (44%) with concomitant changes in stem height, stem diameter, lodging, and branches, nodes, and pods/plant. Interference of common cocklebur with soybeans resulted primarily from shoot interference, and competition for light within the soybean canopy implicated as the major factor causing the soybean yield reduction.

Root and shoot extracts of 3-week-old sudangrass [Sorghum sudanense (Piper) Stapf, var. Piper], grain sorghum [Sorghum bicolor (L.) Moench], and corn (Zea mays L.) plants degraded 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) in order: shoot > root and corn ≫ sorghum = sudangrass. In 3-week-old detopped plants, the rate of atrazine exudation was 14 times greater in sudangrass and sorghum than in corn when grown in Keith sandy loam containing 0.5 ppmw 14C-atrazine. Extraction and analysis of plant shoots revealed that 7 to 8% of the 14C was present as atrazine in sudangrass and sorghum whereas no atrazine was found in corn. In 14C tracer studies, thin-layer chromatography showed that sudangrass and sorghum metabolized atrazine by a pathway which differed from the pathway in corn. Sudangrass and sorghum metabolized atrazine primarily to 2-chloro-4-amino-6-(isopropylamino)-s-triazine and 2-chloro-4-amino-6-(ethylamino)-s-triazine which are only partially detoxified compounds. Corn metabolized atrazine to 2-hydroxy-4-(ethylamino)-6-(isopropylamino)-s-triazine (hydroxyatrazine) which is non-phytotoxic.

As a first step toward developing a genomics-based research program to study growth and development of underground adventitious shoot buds of leafy spurge, we initiated a leafy spurge expressed sequence tag (EST) database. From the approximately 2,000 clones randomly isolated from a cDNA library made from a population containing growth-induced underground adventitious shoot buds, we have obtained ESTs for 1,105 cDNAs. Approximately 29% of the leafy spurge EST database consists of expressed genes of unknown identity (hypothetical proteins), and 10% represents ribosomal proteins. The remaining 60% of the database is composed of expressed genes that show BLASTX sequence identity scores of ≥ 80 with known GenBank accessions. Clones showing sequence identity to a Histone H3, a gibberellic acid-responsive gene, Tubulin, and a light-harvesting chlorophyll a/b-binding protein were shown to be differentially expressed in underground adventitious shoot buds of leafy spurge after breaking of dormancy. RNA encoding a putative cyclin-dependent protein kinase (CDK)-activating kinase, a gene associated with cell division, and Scarecrow-like 7, a gene involved in GA signaling, were present at similar levels in dormant and growth-induced underground adventitious shoot buds. These data show how even a small EST database can be used to develop a genomics-based research program that will help us identify genes responsive to or involved in the mechanisms controlling underground adventitious shoot bud growth and development.

Laboratory studies were conducted to measure the relative volatility of triallate [S-(2,3,3-trichloroallyl) diisopropylthiocarbamate] from soils and to determine the effect of surfactants on triallate mobility. Significant volatilization occurred from continuously moist soils under constant air exposure but triallate did not volatilize more rapidly than herbicides which do not respond to incorporation. Volatilization rates from Ray silt soil fell in the order of propachlor (2-chloro-N-isopropylacetanilide) > triallate > alachlor [2-chloro-2′, 6′-diethyl-N-(methoxymethyl)acetanilide] and from Drummer silty clay loam were propachlor > alachlor > triallate. In suspension studies soils reversibly adsorbed 93 to 98% of the triallate in aqueous solution. With soil at field moisture capacity, the concentration of triallate in soil solution was 13 ppbw without surfactant and 14 ppbw with 3 parts emulsifier blend to 1 part of triallate (w/w). With this level of emulsifier 92% of triallate applied to the soil surface remained in the upper 3 cm of soil following a 15.2-cm simulated rainfall. Without surfactant 99% remained in the upper 3 cm of soil. Lower triallate mobility in moist soils observed in this study coupled with the strong dependence of triallate bioactivity as reported by others strongly suggests that deactivation by dry surface soils rather than volatility is primarily responsible for the triallate response to incorporation.

We have come together to discuss the way in which weeds and water work together to influence yields in agriculture. In the small allotment of time available to us this morning a review of basic knowledge about this relationship would be so general that it could help no one. I believe it would be more appropriate, and more helpful for a group so large, if I tell you of some of the hard realities which come to bear on food production across the world because of this relationship of weeds with moisture. These stories will have to do with such practical matters as the expansion of the world's most important crop, and the difference between good and bad crops on the two sides of the same river under identical climatic conditions in Middle Asia.

The effects of a gall mite (Aceria malherbae) and sublethal doses of either 2,4-DB or glyphosate on field bindweed growth were evaluated under laboratory conditions. Mite feeding reduced field bindweed shoot biomass 37 to 48% and root biomass 46 to 50%. 2,4-DB at 0.07 to 0.14 kg ae ha−1 or glyphosate at 0.14 to 0.28 kg ai ha−1 reduced field bindweed root biomass 25 to 52%. Combining A. malherbae feeding with either 2,4-DB or glyphosate application reduced root biomass of field bindweed plants more than mites or either herbicide alone. Live A. malherbae were present on field bindweed 3 wk after treatment with either herbicide. Combination of A. malherbae with sublethal herbicide doses may allow for field bindweed suppression while reducing potential herbicide injury to crops and maintaining A. malherbae populations.

Isolated plant cells (Zinnia elegans Jacq. leaf tissue) and protoplasts (immature tomato fruit, Lycopersicon esculentum Mill.) were obtained by hydrolysis of pectic substances in the middle lamella and the primary cell wall, respectively. Both types of cells were stabilized in an inorganic salt solution and used to study absorption and distribution of four 14C-herbicides. Intracellular distribution within the protoplast was determined by ultracentrifugation in a discontinuous Ficoll gradient. Marked absorption of p-nitrophenyl-α,α,α-trifluoro-2-nitro-p-tolyl ether (fluorodifen) and α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) by the isolated cells was observed; absorption increased with time. Trifluralin was retained within the cell more strongly than fluorodifen. Similar absorption patterns of fluorodifen and trifluralin by protoplasts were observed. Trifluralin was incorporated to a greater extent than fluorodifen by the cells and protoplasts. Trifluralin was distributed evenly between the mitochondrial and the chloroplasts-nuclei fractions; whereas fluorodifen was concentrated in the chloroplast-nuclei fraction. No significant absorption of 1,1-dimethyl-3-(α,α,α-trifluoro-m-tolyl)urea fluometuron) or 3-(4-bromo-3-chlorophenyl)-1-methoxy-1-methylurea (chlorbromuron) was detected by either the isolated cells or protoplasts.

A single dominant mutation conferring resistance to aryloxyphenoxypropionate (AOPP) and cyclohexanedione (CHD) herbicides was incorporated into a quantitative model for the population development of wild oat. The model was used to predict the times required to develop field resistance in a number of different scenarios. Field resistance was defined as a threshold of four plants m−2 surviving herbicide treatment, and in most scenarios, a very large proportion of these plants were resistant. The model predicts that plow cultivation could delay the development of resistance relative to tine cultivation. With an initial seed bank of 100 seeds m−2 and annual use of AOPP/CHD herbicides, which kill 90% of susceptible but no resistant plants, field resistance develops in 15 yr with annual tine cultivation 10 cm deep but only after 23 yr with annual plowing 20 cm deep. The model predicts that herbicide rotation can dramatically increase the times required for field resistance to develop in a tine cultivation system. With annual use of AOPP/CHD herbicides, field resistance develops in 15 yr, whereas using alternative modes of action 1 in 2 yr delays field resistance to 28 yr. The model predicts that resistance can be delayed for at least 66 yr if three herbicides, each with a different mode of action, are rotated and each herbicide causes 90% mortality. The model predictions on the number of years required for field resistance to develop are not highly sensitive to the initial density of the seed bank (range modeled = 102 to 104), the mutation rate for resistance (10−4 to 10−7 per generation), the rate of outcrossing (0.1 to 100%) or the herbicide kill rate (80 to 95%).

Knowing the distribution of weed seedlings in farmer-managed fields could help researchers develop reliable distribution maps for site-specific weed management. With a knowledge of the spatial arrangement of a weed population, cost effective sampling programs and management strategies can be designed, so inputs can be selected and applied to specific field areas where management is warranted. In 1997 and 1998, weeds were sampled at 612 to 682 sites in two center pivot irrigated corn fields (71 and 53 ha) in eastern Colorado. Weeds were enumerated when corn reached the two-leaf, four-leaf, and physiological maturity stages in a 76.2- by 76.2-m grid, a random-directed grid where sites were established at intervals of 76.2 m, and a star configuration based on a 7.62- by 7.62-m grid within three 23,225 m2 areas. Directional correlograms were calculated for 0, 30, 60, 90, 120, and 150° from the crop row. Fifteen weed species were observed across fields. Spatial dependence occurred in 7 of the 93 samples (a collection of sampling units for a particular weed species that was detected within a field at a particular sampling time and year) for populations of field sandbur, pigweed species, nightshade species, and common lambsquarters. Correlogram analysis indicated that 18 to 72% of the variation in sample density was a result of spatial dependence over a geographic distance not exceeding 5 to 363 m among the examined data. Because of the lack of spatial correlation for weed seedling distributions in these eastern Colorado corn fields, interpolated density maps should be based on grid sizes (separation distances) less than 7.62 m for weed seedling infestations.

A model of competition for light between peanut and three broadleaf weed species has been developed to run with the PNUTGRO model. The model simulates shading of the peanut canopy by reducing the total daily PAR received by the peanuts in a manner that realistically represents timing and quantity of light capture by the weeds. Data were collected in nursery plots of Florida beggarweed, sicklepod, and wild poinsettia in 1989, 1990, and 1991. These data provided the values for the critical parameters: maximum attenuation of PAR by the weed, time when the weed overtops the peanut canopy, time when maximum attenuation is reached, and the distance of influence of the weed. Florida beggarweed overtopped the peanut canopy 52 DAP, and reduced PAR reaching the peanuts 45% by 73 DAP. Sicklepod overtopped the peanut canopy 42 DAP and reached an attenuation of 41% 79 DAP. Wild poinsettia overtopped the peanut canopy 44 DAP, and had an attenuation value of 39% 85 DAP. The distances of influence were 162, 150, and 192 cm for Florida beggarweed, sicklepod, and wild poinsettia, respectively. Observed yield losses in the distance of influence were 26, 27, and 22%, respectively. The model predictions accounted for at least 90% of the yield losses observed in field studies. The model also proved capable of simulating competitive differences between morphologically and phenologically different populations of Florida beggarweed. Simulation models will play an important role in reducing the expenditure of time and resources required to document yield losses due to weeds in peanuts.

In a special symposium on weed biology presented at the 1975 annual meeting of the Weed Science Society of America, Parka (37) reviewed the trend of published papers in WEED SCIENCE related to weed biology over the 20-yr period, 1952 to 1972. His data indicated a steady increase from 9.2 to 17.2% of published papers directly related to weed growth and development. My recent assessment of similarly classified papers in 1978, 1979, and 1980 showed a remarkable increase to 21.9, 24.3, and 27.2%, respectively. Further, the more recent papers on weed growth and development also include crop-effect data, i.e., the association of weed growth with crop competition.

Postemergence control of broadleaf weeds in canola with DPX A7881 was studied under controlled-environment and field conditions. DPX A7881 controlled weeds of the Cruciferae family such as wild mustard, field pennycress, and flixweed, which are not controlled with existing herbicides used for weed control in canola. Weed control was better when DPX A7881 was applied at the two-leaf than at the six-leaf stage of the weeds. At the two-leaf stage, control of flixweed was achieved with 15 g ai/ha of DPX A7881; 20 to 30 g ai/ha was required for control of wild mustard and field pennycress. Control of redroot pigweed required 30 g ai/ha. Common lambsquarters was not controlled in this study with DPX A7881 up to 60 g ai/ha. Control of weeds with DPX A7881 increased canola yields in this study without altering the important quality components: oil content, 1000-kernel weight, and green seed content. DPX A7881 provides a postemergence alternative to existing herbicides for broadleaf weed control in canola.

The transition period to certified organic production can present a significant weed management challenge for growers. Organic certification requires that prohibited fertilizers and pesticides must not have been used for 36 mo before harvest of the first organic crop. Understanding how organic management practices and initial weed seed-bank densities affect weed population dynamics during the transition period may improve weed management efficacy and adoption of organic practices. We examined how tillage systems (full or reduced) and cover crop species planted during the first transition year (rye or a mixture of timothy and red clover) affect the seedling densities of three common annual weed species, common lambsquarters, velvetleaf, and foxtail spp., during the 3-yr transition period. Weed seeds were applied in a one-time pulse at the beginning of the study at three densities, low, medium, and high (60, 460, and 2,100 seeds m−2, respectively), and cumulative seedling densities of each species were assessed annually. Treatment factors had variable and species-specific effects on weed seedling densities. In general, the full-tillage system, with an initial cover crop of timothy and red clover, resulted in the lowest density of weed seedlings following seed-bank augmentation. There was little consistent association between the initial densities of applied weed seeds in the weed seed bank at the start of the transition and weed seedling densities at the end of the transition period. This suggests that when multiple crop and weed cultural management practices are employed during the organic transition period, initial failures in weed management may not necessarily lead to persistent and intractable annual weed species management problems following organic certification.

Cultivation timings were used in conjunction with postemergence herbicides to determine their effect on sicklepod control in soybean. Treatments using cultivation alone resulted in less than 70% sicklepod control. A postemergence application of chlorimuron following metribuzin applied postemergence was usually more effective in controlling sicklepod with cultivation 7 days after application than with cultivation 3 days before or at application, or with no cultivation. Cultivation at any timing in conjunction with a postemergence application of imazaquin following imazaquin preemergence improved sicklepod control compared to imazaquin without cultivation. Sicklepod control with chlorimuron postemergence followed by cultivation 7 days after application was as good as or better than two sequential applications of chlorimuron with no cultivation. Imazaquin or metribuzin preemergence followed only by cultivation provided less than 80% control in two of three experiments.

Granule, and in some cases, tablet and ball formulations of bromacil (5-bromo-3-sec-butyl-6-methyluracil), dicamba (3,6-dichloro-o-anisic acid), picloram (4-amino-3,5,6-trichloropicolinic acid), karbutilate [tert-butylcarbamic acid ester with 3(m-hydroxyphenyl)-1,1-dimethylurea], and tebuthiuron {N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N′-dimethyl urea} were applied to three areas within 30 km of College Station in the Claypan Resource area of Texas. Woody species growing in the area included blackjack oak (Quercus marilandica Muenchh.), post oak (Quercus stellata Wangenh.), yaupon (Ilex vomitoria Ait.), winged elm (Ulmus alata Michx.), white ash (Fraxinus americana L.), mockernut hickory (Carya tomentosa Nutt.), and tree huckleberry (Vaccinium arboreum Marsh.). Granules were applied either broadcast or in rows at several intervals. Tebuthiuron was the most effective herbicide on oaks, winged elm, and white ash. Tebuthiuron and picloram were equally and most effective on yaupon, mockernut hickory, and tree huckleberry. Tebuthiuron + picloram at 2.2 + 2.2 kg/ha was the most effective herbicide treatment on huckleberry. However, a mixture of tebuthiuron + picloram (1:1 w/w) was usually no more effective than the same rate of tebuthiuron in the mixture applied alone. Tablet and ball formulations of karbutilate and tablet formulations of tebuthiuron were generally equally as effective as granules. Applications of picloram and tebuthiuron granules in rows 1.8, 3.0, 4.6, or 6.1 m apart gave control equal to broadcast applications except for picloram on yaupon. On yaupon all row spacing treatments of picloram were less effective than the broadcast treatment, whereas with tebuthiuron the 6.1-m spacing was least effective. Picloram and tebuthiuron granules applied in rows, approximately at 1.8-m intervals with a tractor, were as effective as granules applied by hand in straight rows.

Individual droplets of 14C-2,4-D dimethylamine (DMA) were applied to oriental mustard seedlings to determine the effect of droplet size and herbicide concentration on absorption and translocation 1 or 3 d after application. Absorption of 14C-2,4-D was generally not affected by droplet size which ranged from 198 to 2760 μm. However, the percentage of absorbed 14C-2,4-D that was translocated away from the treated leaf increased as droplet size decreased. Absorption of 14C-2,4-D either increased slightly or was not affected by increased herbicide concentration, but translocation of absorbed 14C-2,4-D was reduced as herbicide concentration increased. Absorption and translocation of 14C-2,4-D DMA were reduced when applied after a drop of 21.3 mM commercially formulated 2,4-D had been applied at the same location on the leaf. These results indicate that large amounts of 2,4-D, either in the form of large droplets or a more concentrated herbicide solution, inhibit translocation of 2,4-D from oriental mustard leaves.

Seeds of dyers woad (Isatis tinctoria L.) readily germinated at temperatures from 3 to 25 C. Dyers woad seeds in their fruits, from which they do not normally dehisce, had greatly depressed germination compared to that of threshed seeds. The presence of dyers woad fruits in the same Petri dish markedly depressed germination and/or root elongation of threshed dyers woad seeds and the seeds of some competing broadleaf and grass species. The depression was especially evident on seeds of other species of Cruciferae. Similar reductions in germination or root elongation were obtained when water used to soak dyers woad fruits was used to wet germination substrates.

Redstem is an important weed in California water-seeded rice fields because of its aquatic habit, wide distribution, interference with harvest, and resistance to the herbicide bensulfuron. Our objective was to understand the mechanisms of competition for light between rice and redstem, with the goal of improving redstem control. A replicated greenhouse experiment was done in 1993 and 1994. Rice was water-seeded at a rate of 400 seeds m−2, and redstem was seeded simultaneously at approximate densities of 0, 50, and 100 seeds m−2 in continuously flooded 0.77 m2 basins. Plants were harvested once at final harvest in 1993 and twice in 1994, with an additional nondestructive sampling 34 days after seeding (DAS). Despite slower early growth, redstem height exceeded rice height about 45 DAS. At the midseason harvest in 1994 (56 DAS), no effects of redstem competition on any rice response variables were detected. However, at final harvest (110 and 118 DAS, 1993 and 1994, respectively) redstem competition at both treatment densities reduced rice tiller density, panicle density, shoot drymass, and grain drymass. Redstem competition reduced rice growth only after penetrating the canopy. Shade cast by redstem through rice maturity decreased shoot and grain production and increased tiller mortality. Lodging caused by redstem further affected rice growth. Season-long competition from redstem at mean densities of 67 and 110 plants m−2 reduced rough rice yields 31 and 39%, respectively, making redstem the most competitive broadleaved rice weed yet studied. Improved understanding of rice-redstem interactions indicates that using alternative herbicides to bensulfuron is unlikely to increase yield losses to redstem, and that control may be improved by increasing rice plant densities or slightly delaying early season chemical control. Because these strategies are mechanistic, they may also be useful for controlling other rice weeds with growth patterns similar to redstem.