Common groundsel is an alien annual weed that has become increasingly troublesome in many crops in Ohio. Understanding the periodicity of seedling emergence and longevity of seeds buried in the soil may help growers devise more efficient strategies to control common groundsel. Studies were conducted to determine the dormancy status of common groundsel seeds over 24 mo, and to describe the effect of tillage and fertilizer on the pattern of seedling emergence and the rate of depletion of seeds from the soil seed bank. Common groundsel seeds were collected (June 2000 and 2002) from sites along a 700-km transect from Kentucky to Michigan (39°1′ and 43°36′N, respectively). Seeds were cleaned and placed in nylon mesh bags for burial in a common garden. Every month for the following 24 mo, replicate bags from each location were exhumed. Germination was tested under alternating temperatures of 20 and 10 C, for 14 and 10 h day/night, respectively. Germination response at each sampling date was similar regardless of seed source, but differed for the 2000 to 2002 and 2002 to 2004 experiments. Laboratory germination of seeds buried was initially high (98%) and declined rapidly to about 20% by midwinter. Germination increased to about 60% during the second summer, followed by a slow decline to 40% during winter and another rapid decline before the third summer. The rapid declines in germination were preceded by low soil temperatures (<5 C) and the germination peaks corresponded with periods of high soil temperatures (∼ 20 C). Results suggested that common groundsel follows a cycle of dormancy and nondormancy corresponding to decreases and increases, respectively, in soil temperature. During 2 yr of deep burial in undisturbed soil, 94% of the seeds germinated or died, suggesting that common groundsel seeds may not persist more than a few months in regularly disturbed soils.

Genomics programs in the weed science community have not developed as rapidly as that of other crop, horticultural, forestry, and model plant systems. Development of genomic resources for selected model weeds are expected to enhance our understanding of weed biology, just as they have in other plant systems. In this report, we describe the development, characteristics, and information gained from an expressed sequence tag (EST) database for the perennial weed leafy spurge. ESTs were obtained using a normalized cDNA library prepared from a comprehensive collection of tissues. During the EST characterization process, redundancy was minimized by periodic subtractions of the normalized cDNA library. A sequencing success rate of 88% yielded 45,314 ESTs with an average read length of 671 nucleotides. Using bioinformatic analysis, the leafy spurge EST database was assembled into 23,472 unique sequences representing 19,015 unigenes (10,293 clusters and 8,722 singletons). Blast similarity searches to the GenBank nonredundant protein database identified 18,186 total matches, of which 14,205 were nonredundant. These data indicate that 77.4% of the 23,472 unique sequences and 74.7% of the 19,015 unigenes are similar to other known proteins. Further bioinformatics analysis indicated that 2,950, or 15.5%, of the unigenes have previously not been identified suggesting that some may be novel to leafy spurge. Functional classifications assigned to leafy spurge unique sequences using Munich Information Center for Protein or Gene Ontology were proportional to functional classifications for genes of arabidopsis, with the exception of unclassified or unknowns and transposable elements which were significantly reduced in leafy spurge. Although these EST resources have been developed for the purpose of constructing high-density leafy spurge microarrays, they are already providing valuable information related to sugar metabolism, cell cycle regulation, dormancy, terpenoid secondary metabolism, and flowering.

Feral rye is an agricultural and ruderal weed of the western United States. We investigated how it has phenotypically diverged from its cultivated ancestor, domesticated cereal rye, and across its range since the introduction of its progenitor. Vegetative growth, flowering phenology, and reproductive characters of feral populations from across a 13° range in latitude in the northwestern United States were compared to that of rye cultivars under both vernalized (cold-treated) and nonvernalized conditions. Feral populations as a whole had smaller seeds, thinner culms, and a delay in flowering relative to cultivars, regardless of cold treatment. Vernalized feral populations from northern latitudes (northern California and eastern Washington) produced more, but smaller leaves and more tillers than both vernalized rye cultivars and southern California feral populations. Northern feral populations also flowered significantly later, irrespective of vernalization treatment. We conclude that feral rye is phenotypically distinct from domesticated cereal rye and that feral populations have diverged regionally from one another. Reproductive isolation from domesticated rye, due both to the loss in popularity of the crop and to phenological shifts in feral rye relative to cultivars, may be contributing to the rapid evolution of this weed away from its domesticated ancestor in less than 120 yr since its introduction.

Concern over the consequences of increased weed seed inputs to the soil seed bank during the transition period from conventional to organic production is one obstacle to grower adoption of reduced input and nonchemical weed management strategies. An 11-yr study was established in southwest Montana to investigate the effect of a single pulse of wild oat seeds on subsequent seed bank dynamics. In 1993, wild oat seeds were sown at five densities (0, 20, 80, 320, and 800 seeds m−2) in eight wheat–small grain cropping systems that differed in the number of crops in rotation and fallow periods. Wild oat seed banks were measured each spring from 1994 to 2004 in half of the cropping systems and from 2001 to 2004 in all eight systems. In 1994, seed bank densities in response to the pulse were as much as 11 times higher than controls that received no seeds in 1993. By 1996, after mechanical fallowing of all cropping systems, wild oat seed bank densities were not significantly different from densities in control plots regardless of the size of the initial seed pulse and remained so through 2004. These data suggest that increases in wild oat seed inputs during the organic transition period will have relatively few long-term agronomic effects on the dynamics of wild oat seed banks in these systems. In addition, wild oat seed banks may be constrained by factors other than cropping sequence when herbicides are not used, such as possible density-dependent regulation as a result of increased soil pathogen attack and seed predation.

The objectives of this research were to characterize the extent of intraspecifc variation in seed characteristics of Powell amaranth and to evaluate whether such variation was correlated with crop rotation history of the collection sites. We compared characteristics of seeds originating from dairy farms with a corn–alfalfa crop rotation history with seeds originating from farms with a history of intensive vegetable production. We hypothesized that (1) multiple years of perennial alfalfa would select for greater seed dormancy and longevity in seeds of the summer annual Powell amaranth, (2) earlier spring planting dates of corn and alfalfa compared with most vegetable crops would select for earlier emergence, and (3) greater competition and lower soil moisture in the nonirrigated corn–alfalfa rotation would select for greater seed size. Seeds from 10 to 20 plants from each of 10 farms from each habitat were collected in the fall of 2002 and 2003 in central New York. To control for maternal effects on seed dormancy, a second generation of seeds was produced from plants grown under common greenhouse conditions. Germination in petri dishes was greater for second-generation seeds from vegetable farms (46%) than for those from dairy farms (32%). Total emergence following overwinter burial in the field was greater for seeds originating from dairy farms (62%) compared with those from vegetable farms (52%). Neither seed weight nor the rate of emergence varied by habitat of origin. Our results suggest that perennial alfalfa in dairy rotations may have selected for greater dormancy and longevity of Powell amaranth seeds. The large intraspecific variation in seed characteristics observed, underscores the importance of considering multiple populations when making comparisons of germination characteristics across biotyes (e.g., resistant vs. susceptible) or species, or when developing and interpreting models of weed emergence or weed population dynamics.

Experiments were conducted in 2002 and 2003 to determine the influence of soil thermal amplitude on common cocklebur emergence. Additionally, common cocklebur achenes were collected in fall of 2003 and of 2004 to assess changes in light and temperature requirements for germination over a 12-mo period under field conditions. Common cocklebur germination in response to the light environment and to constant and fluctuating temperatures were evaluated under controlled conditions following achene retrieval from the field. There was a linear inverse relationship between shade intensity and soil thermal amplitude, which explained 77% of the variability in emergence in the field over 2 yr. Emergence decreased as soil thermal amplitude declined, with 95% shading resulting in a 72 to 88% reduction in emergence. Neither red nor far-red light had much effect on germination, and burial did not induce a light requirement. Germination of achenes retrieved from the soil surface or buried in soil generally was not affected by red or far-red light, and the achenes did not acquire a red-light requirement following burial. Daily exposure to natural greenhouse light at 24 to 30 C was essential for germination immediately following achene maturation, whereas no germination occurred in darkness. A thermal fluctuation of 15 C increased germination percentages over those at constant temperatures regardless of time after maturation or retrieval-depth of achenes. The mean fluctuating temperature over all sample dates that was generally optimal for germination was 25 C (17.5/32.5 C low/high temperatures; 15 C daily fluctuation) in April or May and July or August in both years. Germination was generally optimum at constant temperatures of 35 or 40 C. The higher mean temperature requirement for germination at constant temperatures than at fluctuating temperatures likely contributes to reduced emergence in spring and summer months, when earlier emerging weed species or crops have already become established, and the thermal fluctuation requirement for germination reduces the likelihood of emergence in an environment where light availability is diminished.

A field experiment was conducted in Urbana, IL, from 1997 to 2000 to evaluate the effect that crop, tillage, and soil depth have on common waterhemp seed-bank persistence. A heavy field infestation of common waterhemp (approximately 410 plants m−2) was allowed to set seed in 1996 and was not allowed to go to seed after 1996. In 1997, 1998, 1999, and 2000, the percentage of the original common waterhemp seed bank that remained was 39, 28, 10, and 0.004%, respectively, averaged over tillage treatments. Initially, germination and emergence of common waterhemp was greater in no-till systems. Consequently, the number of remaining seeds was greater in the till treatments compared with no-till in the top 0 to 6 cm of the soil profile. This reduction was in part explained by the higher germination and emergence of common waterhemp in the no-tillage treatments. Tillage increased the seed-bank persistence of common waterhemp in the top 0 to 2 cm of the soil profile in 1997 and the top 0 to 6 cm in 1998. Crop had no effect on common waterhemp emergence or seed-bank persistence. In 2001, > 10% of the seed germinated that was buried 6 to 20 cm deep compared with 3% for seed 0 to 2 cm deep.

Greenhouse and growth chamber studies were conducted to determine the effect of drought, flooding, and cold stress on the efficacy of glyphosate for velvetleaf control, and the interaction between these stresses and adjuvant and posttreatment temperature. Glyphosate activity on velvetleaf decreased when plants were stressed with drought ≥ flooding > cold. Leaf blades of environmentally stressed velvetleaf angled downward, which increased tolerance to glyphosate but was not as great a cause of tolerance as the stress effects. Glyphosate applied to 6- and 12-leaf velvetleaf was two and eight times more phytotoxic on nonstressed compared with drought-stressed plants, respectively. Glyphosate was most effective on nonstressed plants, followed by plants recovering from stress, and least effective on plants still under stress. None of the adjuvants completely overcame the adverse affects of stress on glyphosate efficacy; use of a nonionic surfactant and ammonium sulfate resulted in a 9–13 percentage point improvement in control of stressed plants compared with glyphosate applied without an adjuvant. Low temperatures (5 or 12 C) maintained for 48 h after herbicide treatment enhanced glyphosate phytotoxicity to stressed and nonstressed velvetleaf. Glyphosate at a low rate stressed velvetleaf, which made them more tolerant to subsequent glyphosate application compared with velvetleaf not pretreated with glyphosate.

Due to known variation in canopy properties among sweet corn hybrids, weed suppressive ability (WSA), the crop's ability to reduce weed fitness, may not be uniform among hybrids. This hypothesis was tested using a range of wild proso millet densities subjected to four canopy treatments (three hybrids + weedy monoculture) under irrigated conditions in Washington and primarily rainfed conditions in Illinois. Parameter estimates for responses of weed growth and seed rain to wild proso millet density were used to quantify variation in WSA among hybrids. The same parameter estimates were used in a correlation analysis to identify associations between weed response and sweet corn canopy properties. Weed suppressive ability, as measured by wild proso millet shoot biomass and seed rain, varied among canopy treatments. Hybrid GH2547 was 25 to 31% more suppressive of wild proso millet than hybrid Spirit in Washington and 70 to 91% more suppressive in Illinois. Weed fitness was negatively correlated with leaf area index (LAI) after crop anthesis (−0.48 to −0.63), intercepted photosynthetically active radiation (PAR) at one of two harvest times (−0.51 to −0.56), and LAI at the 120- to 150-cm height (−0.51 to −0.55). Information on WSA may be useful in breeding programs; however, even near-term use of this knowledge offers modest but cumulative improvements to weed management systems in sweet corn.

Experiments were conducted to determine the influence of soybean row width and glyphosate application timing on survival, biomass production, and fecundity of three sicklepod cohorts along with soybean seed yield. The first cohort comprised sicklepod plants that emerged from soybean planting through the V3 stage of soybean (two fully developed trifoliate leaves plus the unifoliate leaves; cohort 1). The second cohort comprised plants that emerged between the V3 to V6 stages of soybean (five fully developed trifoliate leaves plus the unifoliate leaves; cohort 2), and the third cohort emerged after the V6 stage through the R2 stage of soybean (full bloom; cohort 3). Glyphosate was applied at V3; V6; V3 and V6; and V3, V6, and R2 in rows 19 and 97 cm wide, and a nontreated control was included for comparison in each row width. Survival of cohort 1 in 2004 in glyphosate-treated plots occurred only after a single glyphosate application at V3 or V6 in wide rows. Narrowing the soybean row width reduced sicklepod survival throughout the growing season, even without glyphosate. Total biomass production from all cohorts averaged over years was 1,602 g m−2 in wide rows compared with 648 g m−2 in narrow rows. Cohort 1 accounted for 70 and 77% of the total sicklepod biomass in wide and narrow rows, respectively. Cohort 2 contributed 29% of the total sicklepod biomass in wide rows and 22% in narrow rows. Cohort 3 produced minimal biomass, contributing no more than 1% of the total sicklepod biomass. Sicklepod emerging after V6 failed to produce seed in 2004, and no sicklepod seed were produced in 2005 by plants emerging after V3. Averaged over years, sicklepod from cohort 1 in nontreated controls produced 3,695 seed m−2 in narrow rows compared with 6,685 seed m−2 in wide rows. Nontreated sicklepod from cohort 2 in 2004 produced 510 seed m−2 in narrow rows compared with 1,640 seed m−2 in wide rows. Soybean yields were similar among all glyphosate applications averaged over years and row widths, ranging from 3,340 to 3,700 kg ha−1 compared with 1,290 kg ha−1 without glyphosate (61 to 65% yield loss).

An experiment was conducted in controlled environments to evaluate the effects of relative humidity (RH) and soil moisture (SM) on fluroxypyr efficacy on kochia and Palmer amaranth. Plants were grown in growth chambers with constant RH of 35 or 90 ± 5% and 28/23 C day/night temperature. Within each growth chamber, plants were grown in SM regimes of either −20 or −40 kPa. When plants were 8 to 10 cm tall, fluroxypyr was applied at 26, 52, 78, or 104 g ae ha−1; a nontreated control was included. At 21 days after treatment, control of both species increased with increasing fluroxypyr rate. Kochia control was not affected by RH, but control was greater when plants were grown in moist soil (−20 kPa) than in dry soil (−40 kPa). Conversely, Palmer amaranth control was greater when plants were grown at 90% RH than at 35% RH, but control did not differ between moist and dry soils. This study showed that the influence of environmental factors on fluroxypyr efficacy on kochia and Palmer amaranth is species dependent.

Laboratory and greenhouse studies were conducted to determine the effect of temperature, solution pH, water stress, and planting depth on cutleaf eveningprimrose germination and emergence. Field studies were conducted to measure growth parameters of cutleaf eveningprimrose throughout the fall season. When treated with constant temperature, cutleaf eveningprimrose germinated over a range of 15 to 32 C, with the optimum germination occurring at 24 C. Onset, rate, and total germination were greatest in an alternating 20/35 C temperature regime. Germination decreased as solution pH increased, with greatest germination occurring at solution pH of 4. Germination decreased when cutleaf eveningprimrose seed was subjected to increased water stress. Emergence was optimum when seed were buried at depths of 0.5 cm. Germination decreased with increasing burial depth, and no seed emerged from a depth of 10 cm. Cutleaf eveningprimrose control was maximized when 2,4-D was applied in mixture with glyphosate or paraquat. These data suggest that cutleaf eveningprimrose can germinate and gain biomass from early March to late October. These attributes could contribute to poor control before cotton planting if preplant control applications are delayed after early March.

Houndstongue is a troublesome weed of pasture, rangeland, and open forest in British Columbia, Canada. Recently, a root-feeding weevil was released in Canada that successfully controls houndstongue patches, but it has been difficult to propagate this weevil in sufficient numbers for widespread release. The goal of the current study was to develop methods for growing houndstongue as a crop in a farm-field setting for weevil propagation. Field experiments were conducted to determine optimum seeding dates, depths, and rates for houndstongue. The effects of straw-residue cover and nitrogen-application rates were also examined. More than 90% of the seed used was viable, and about 50% of the planted seed emerged. The most consistent plant densities occurred when houndstongue was seeded in October and had a winter and early spring moist chilling period to break seed dormancy. For fall seeded houndstongue, plants emerged equally well from 2- and 5-cm depths. Houndstongue is moderately responsive to nitrogen fertilizer but usually did not benefit from additional straw cover on the soil before emergence. Houndstongue plants also survived in drought conditions. In conclusion, this weed can be consistently grown as a crop for the propagation of a root-feeding weevil for houndstongue control.