Knowledge of the effects of burial depth and burial duration on seed viability and, consequently, seedbank persistence of Palmer amaranth (Amaranthus palmeri S. Watson) and waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] ecotypes can be used for the development of efficient weed management programs. This is of particular interest, given the great fecundity of both species and, consequently, their high seedbank replenishment potential. Seeds of both species collected from five different locations across the United States were investigated in seven states (sites) with different soil and climatic conditions. Seeds were placed at two depths (0 and 15 cm) for 3 yr. Each year, seeds were retrieved, and seed damage (shrunken, malformed, or broken) plus losses (deteriorated and futile germination) and viability were evaluated. Greater seed damage plus loss averaged across seed origin, burial depth, and year was recorded for lots tested at Illinois (51.3% and 51.8%) followed by Tennessee (40.5% and 45.1%) and Missouri (39.2% and 42%) for A. palmeri and A. tuberculatus, respectively. The site differences for seed persistence were probably due to higher volumetric water content at these sites. Rates of seed demise were directly proportional to burial depth (α=0.001), whereas the percentage of viable seeds recovered after 36 mo on the soil surface ranged from 4.1% to 4.3% compared with 5% to 5.3% at the 15-cm depth for A. palmeri and A. tuberculatus, respectively. Seed viability loss was greater in the seeds placed on the soil surface compared with the buried seeds. The greatest influences on seed viability were burial conditions and time and site-specific soil conditions, more so than geographical location. Thus, management of these weed species should focus on reducing seed shattering, enhancing seed removal from the soil surface, or adjusting tillage systems.

Effects of nonchemical weed control on weed and seedbank demography in ridge-tilled fields were simulated to examine long-term effects on weed population dynamics. Ridge tillage entails active seedbank management, including removal of much of the seedbank from the crop growth zone (ridge) to the interridge zone (furrow) at planting, and other seedbank manipulations. Dynamics of simulated yellow foxtail and velvetleaf populations were sharply sensitive to the rate of seed removal from the ridge. Variation in winter seed survival, ridge plant demography (emergence, survival, and seed production), and seed survival in the furrow after removal from the ridge also had strong effects on simulated dynamics. To prevent rapid population growth in a simulated corn-soybean rotation, very effective control of seedlings emerging with each crop was necessary to offset seed production from seedlings emerging after ridge rebuilding in corn years. Dynamics of both species were highly sensitive to cultivation efficiency. Effective “rescue” weed control must be available to prevent sharp increases in weed seedbanks during years when cultivation is ineffective.

Perception of light by phytochrome is a mechanism that triggers weed seed germination in response to soil disturbance. Photoconversion of phytochrome from the red light absorbing form to the active far-red absorbing form depends on hydration of phytochrome. This research was conducted to determine the soil water threshold for the photoinduction of germination by the brief exposure of light that occurs during soil disturbance, and to determine how this threshold is affected by the fluence of the light stimulus and fluence sensitivity of the seed population. Redroot pigweed seedling emergence and germination response to red light (R) was studied for a range of water potentials. Water potential gradients were established by incubating seeds in soils wetted to various water contents, or in polyethylene glycol 8000 (PEG) solutions. After imposing the light treatments, seeds were returned to a fully hydrated state. Seedling emergence in response to R increased as the volumetric water content (θv) of soils increased. At volumetric water contents of 4.0%, R-induced seedling emergence was inhibited 50% compared to photoinduced seedling emergence at the highest soil water contents tested. Attenuation of photoinduction was more pronounced at low vs. high R fluences in freshly imbibed seeds, but was unaffected in seeds that exhibited enhanced fluence sensitivity. In ecosystems where seasonal soil moisture extremes are prevalent, the photoinduction of seed germination may be limited in dry microsites such as surface crusts or under extreme drought conditions.

Herbicide-resistant populations have evolved only in monoculture and/or monoherbicide conditions at predictable rates for each compound and weed. No populations of triazine-resistant weeds have appeared in corn where rotations of crops and herbicides or herbicide mixtures were used. This is due to the greatly reduced competitive fitness of the resistant individuals, which could be expressed only during rotational cycles, and also to the greater sensitivity of resistant individuals to other herbicides, pests, and control practices (“negative cross-resistance”). The model presented here describes how an understanding of all of these factors can provide strategies to decrease the frequency of the resistant individuals during rotation. Rotations or mixtures may not delay the rate of appearance of resistance to inhibitors of acetolactate synthase (ALS), where the fitness of resistant biotypes is claimed to be near normal. The best way to delay resistance to ALS inhibitors is to use those compounds with less persistence so that the selection pressure will be lowered. Too little is known about the frequency of resistance to other herbicides with target-site resistance–to dinitroanilines, to acetyl CoA carboxylase inhibitors, or to those situations where a single enzyme system confers resistance to a broad spectrum of seemingly unrelated herbicides.

In western Canada, seasonal seedling recruitment has been reported in weedy canola populations, and seed persistence has been linked to the secondary seed dormancy potential of a genotype. Temperature influences secondary seed dormancy induction in this species. In these experiments we (1) investigated the influence of temperature and osmotic potential on secondary seed dormancy induction in canola, (2) related these to seedbank dynamics and seedling recruitment of two canola genotypes with different seed dormancy potentials in the field, and (3) investigated the influence of residue, burial depth, and soil type on seedbank dynamics and seedling recruitment in the field. In the laboratory, rates of seed dormancy induction were positively correlated to increasing temperatures and water stress. The role of temperature was approximately threefold more important to seed dormancy development than was osmotic potential within the tested ranges of these variables. In the field, seasonal seedbank dynamics of canola buried at 10 cm were strongly influenced by a genotype's inherent potential for secondary dormancy. An increase in the ungerminable portion of the seedbank was observed in the high-dormancy genotype as soil temperatures increased during spring. This did not occur in the low-dormancy genotype, resulting in sixfold less seed persistence in this genotype by midsummer, by which time, the total remaining seedbank was ungerminable in both genotypes. At the 1-cm burial depth, most of the seedbank was depleted by midsummer of the year after seedbank establishment because of high seedbank mortality in all treatments. Thus, the seasonal recruitment behavior in canola was primarily a function of seed death in the shallow seedbank and a shift to an ungerminable state in the deep seedbank.

Horseweed is an increasingly problematic weed in soybean because of the frequent occurrence of glyphosate-resistant (GR) biotypes. The objective of this study was to determine the influence of crop rotation, winter wheat cover crops (WWCC), residual nonglyphosate herbicides, and preplant herbicide application timing on the population dynamics of GR horseweed and crop yield. A field study was conducted at a site with a moderate infestation of GR horseweed (approximately 1 plant m−2) with crop rotation (soybean–corn or soybean–soybean) as main plots and management systems as subplots. Management systems were evaluated by quantifying horseweed plant density, seedbank density, and crop yield. Crop rotation did not influence in-field horseweed or seedbank densities at any data census timing. Preplant herbicides applied in the spring were more effective at reducing horseweed plant densities than when applied in the previous fall. Spring-applied, residual herbicide systems were the most effective at reducing season long horseweed densities and protecting crop yield because horseweed in this region behaves primarily as a summer annual weed. Horseweed seedbank densities declined rapidly in the soil by an average of 76% for all systems over the first 10 mo before new seed rain. Despite rapid decline in total seedbank density, seed for GR biotypes remained in the seedbank for at least 2 yr. Therefore, to reduce the presence of GR horseweed biotypes in a local no-till weed flora, integrated weed management (IWM) systems should be developed to reduce total horseweed populations based on the knowledge that seed for GR biotypes are as persistent in the seed bank as glyphosate-sensitive (GS) biotypes.

Horseweed is an increasingly common and problematic weed in no-till soybean production in the eastern cornbelt due to the frequent occurrence of biotypes resistant to glyphosate. The objective of this study was to determine the influence of crop rotation, winter wheat cover crops (WWCC), residual non-glyphosate herbicides, and preplant application timing on the population dynamics of glyphosate-resistant (GR) horseweed and crop yield. A field study was conducted from 2003 to 2007 in a no-till field located at a site that contained a moderate infestation of GR horseweed (approximately 1 plant m−2). The experiment was a split-plot design with crop rotation (soybean–corn or soybean–soybean) as main plots and management systems as subplots. Management systems were evaluated by quantifying in-field horseweed plant density, seedbank density, and crop yield. Horseweed densities were collected at the time of postemergence applications, 1 mo after postemergence (MAP) applications, and at the time of crop harvest or 4 MAP. Viable seedbank densities were also evaluated from soil samples collected in the fall following seed rain. Soybean–corn crop rotation reduced in-field and seedbank horseweed densities vs. continuous soybean in the third and fourth yr of this experiment. Preplant herbicides applied in the spring were more effective at reducing horseweed plant densities than when applied in the previous fall. Spring-applied, residual herbicide systems were the most effective at reducing season-long in-field horseweed densities and protecting crop yields since the growth habit of horseweed in this region is primarily as a summer annual. Management systems also influenced the GR and glyphosate-susceptible (GS) biotype population structure after 4 yr of management. The most dramatic shift was from the initial GR : GS ratio of 3 : 1 to a ratio of 1 : 6 after 4 yr of residual preplant herbicide use followed by non-glyphosate postemergence herbicides.

Sand stockpiles, located near commercial cranberry beds in Massachusetts, New Jersey, Washington, and Wisconsin, were sampled quarterly over a 2-yr period. Samples were collected from the surface and the interior of the piles. Utilizing a simple greenhouse germination bioassay, 74 plant species representing 23 plant families were identified across all locations and samples. Plant density varied by region and by sampling depth; species richness varied by region only. More seedlings germinated from samples taken from the surface of the stockpile compared with interior samples. Almost half of all species detected in either surface or interior samples were represented by only one seedling. Fifty-nine percent of the plant species from Massachusetts and New Jersey samples, and 79% and 96% of the plant species from Wisconsin and Washington, respectively, were considered to be weeds in cranberry production. Only one-third of the identified species had wind-blown seeds; seed dispersal was by other mechanisms in almost half of the species. This survey documented the weed management implications of spreading stockpiled sand on cranberry bogs for horticultural or pest management purposes. To minimize the introduction of additional weed problems, growers and integrated pest management consultants should evaluate the seedbank potential of a stockpile prior to application of the sand to the production area.

Manipulation of cropping systems to improve weed management requires a better understanding of how crop- and soil-related factors affect weed life cycles. Our objective was to assess the impacts of timing of primary tillage and use of legume green manure on giant foxtail demography and soil properties. We measured giant foxtail seed survival and dormancy, seedling emergence and survival, and fecundity, in addition to soil phytotoxicity, chemical properties affecting soil fertility and soil water, in the transition between the wheat and corn phases of a wheat–corn–soybean crop sequence. Postdispersal predation of giant foxtail seeds was measured in all three phases of the crop sequence. Wheat was grown either as a sole crop (W) or underseeded with red clover (R), and residues from this phase were rototilled either in the fall (FT) or in spring (ST). There were strong interactions between Red clover and Tillage timing in their effects on giant foxtail recruitment and fecundity in corn. Giant foxtail seedling emergence was 30% lower, and time to 50% emergence was more than 1 wk later, in the ST/R treatment than in the ST/W, FT/W, and FT/R treatments, which did not differ. However, fecundity of giant foxtail was 200% greater in the ST/R treatment than in the other three treatments because of suppressed early corn growth. The net effect of the ST/R treatment on giant foxtail demography in corn was to greatly increase inputs to the seedbank compared with the ST/W, FT/W, and FT/R treatments. Giant foxtail demography in the wheat phase was also affected by Red clover. There was a 200% increase in daily rates of postdispersal seed predation in the wheat phase of the R treatment compared with the W treatment. High-seed predation in the wheat phase and low fecundity in the corn phase of the FT/R treatment suggest that population growth rate of giant foxtail will be lower in this treatment than in the other treatments. The degree of soil phytotoxicity from red clover residues, the changes in the amount of interference from the corn crop early in the growing season, and the differential suitability of crop residues in the different rotations as habitat for seed predators all contributed to changes in giant foxtail demography. Understanding the effects of cropping system characteristics on entire weed life cycles will facilitate the design of integrated suites of complementary weed management tactics.

Contrasting cover-cropping systems were compared to determine whether fundamental differences in cover-cropping strategies affect weed seed predators and resulting seed predation. We conducted typical “feeding” trials in which 25 seeds of each of six weed species, including velvetleaf, wild mustard, yellow foxtail, common lambsquarters, redroot pigweed, and hairy galinsoga, were placed in the field. Exclosures showed that the majority of seed predation could be attributed to invertebrates: 43% out of a total of 56% seed predation for 11 d in 2002 and 40% out of a total of 58% seed predation for 4 d in 2003. The predominant invertebrate seed predator across all entry points of four cropping systems was a ground-dwelling carabid beetle, Harpalus rufipes, which was more abundant in vegetated treatments, particularly red clover, compared with treatments recently tilled and planted to a fall cover crop. In the absence of vertebrates, H. rufipes activity–density was positively correlated with mean seed predation in 2002 (Spearman ρ = 0.489; P < 0.001) but not in 2003 (Spearman ρ = 0.090; P = 0.504), possibly because of a delay between pitfall trapping and predation assay. The activity–density of invertebrate seed predators measured in these systems and the high level of predation imposed on weed seeds at the soil surface indicate that cover-cropping strategies should consider late-season weed management, which maintains seeds on the soil surface and provides desirable habitat for invertebrate predators.