Ludwigia is an important broadleaf weed of direct-seeded rice in Asia. Crop interference that relies on shading may have potential as a component of integrated weed management strategies but it requires understanding the extent to which rice can interfere with weed growth and how these weeds may respond. The growth of ludwigia was studied when grown alone and in competition with 4 and 12 rice (cv. IR72) plants. Rice interference reduced ludwigia height, number of branches, and shoot and root biomass. However, ludwigia showed the ability to reduce the effects of rice interference by increasing leaf weight ratio, increasing stem and leaf biomass in the upper half of the plant, and increasing specific stem length. At 11 wk after seeding, for example, ludwigia grown with 12 rice plants had 38% greater leaf weight ratio compared to plants grown alone. When grown with 12 rice plants, the weed had 82% of its leaf biomass in upper half of the plant compared to only 25% in weeds grown alone. The results showed that ludwigia responded to rice interference with a combination of adaptations typical of many weed species. Despite such plasticity, the control of ludwigia may be achieved by dense rice stands and increasing interference.

Greenhouse and field trials were conducted at the Aberdeen Research and Extension Center to determine the effect of hairy nightshade competition on two potato varieties with different growth habits. Greenhouse replacement trials included treatments of three plants total in each pot with potato : hairy nightshade ratios of 3 : 0, 2 : 1, 1 : 2, or 0 : 3. Varieties tested were ‘Russet Burbank’ and ‘Russet Norkotah’. Greenhouse-grown hairy nightshade (cotyledon to one-leaf stage) were transplanted into pots earlier than or at the same time as potato plant emergence. As the number of Russet Burbank plants per pot decreased, potato plant biomass dry weight (average per plant) increased, regardless of hairy nightshade number or transplant time. When hairy nightshade were transplanted before Russet Norkotah emergence, potato plant biomass dry weight per plant was similar, regardless of potato : nightshade ratio. Field trials were conducted with Russet Burbank and Russet Norkotah in 2004 and 2005. At potato emergence, greenhouse-grown hairy nightshade plants (one- to two-leaf) were transplanted in between potato rows at densities of 0, 1, 2, or 3 plants m−1 row, and solid-seeded at approximately 100 plants m−1 row. Hairy nightshade biomass, stem and berry number, and seeds per berry were reduced by competition from Russet Burbank due to the amount and duration of shading, as compared with Russet Norkotah. Russet Burbank U.S. No. 1 and total tuber yields in plots with 1 hairy nightshade plant m−1 row were similar to weed-free control yields, whereas yields in plots with 2, 3, or 100 m−1 row were reduced in comparison. In contrast, Russet Norkotah yields were reduced when only 1 hairy nightshade plant m−1 row was present. Overall, Russet Norkotah were less competitive with hairy nightshade than Russet Burbank in both the greenhouse replacement and field trials.

Information on the germination and viability of turnipweed seeds could be helpful in developing appropriate management strategies for this weed. Therefore, experiments were conducted to investigate the effects of light, storage conditions, duration of storage or burial, and seed type (naked, i.e., fruit wall was removed and encapsulated in siliques) on germination and viability of turnipweed. The naked and encapsulated seeds (fruit) were kept under five different storage conditions, including dry storage at 25 ± 2 C and 3 ± 1 C and outdoor environments (soil depths of 10, 20, and 40 cm). All seeds were retrieved every 2 mo and tested for germination in light and darkness. At each exhumation date, nongerminated seeds were treated with triphenyltetrazolium chloride to test their viability. The germination of seeds liberated from siliques (85%) was markedly greater than that of seeds in intact siliques (20%). The germination response of naked and encapsulated seeds to light varied between storage conditions and through time. Under indoor conditions (room and cold), both seed types had greater germination percentages in dark on most occasions than those in light. On the contrary, the germination of siliques buried at soil depths of 20 or 40 cm was considerably stimulated by light. Under indoor conditions, the percent viability of both seed types only declined marginally, whereas seeds buried in soil showed high rates of mortality. Seeds in intact siliques persisted longer under either indoor or outdoor conditions than naked seeds.

Redstem filaree is a troublesome weed for sugarbeet growers in northern Wyoming and southern Montana. Field studies were conducted in Powell, WY, in 2006 and 2008 to determine the influence of season-long interference of various redstem filaree densities and the duration of interference on sugarbeet. Root and sucrose yield decreased with increasing redstem filaree density. The rectangular hyperbola model with the asymptote (A) bounded at 100% maximum yield reduction characterized the relationship between redstem filaree density and sugarbeet yield reduction. The estimated parameter I (percent yield reduction per unit weed density as density approaches zero) was 5% for root and sucrose yield reduction. Sugarbeet root yield decreased as the duration of redstem filaree interference increased. The critical timing of redstem filaree removal to avoid 5 and 10% root yield reduction was estimated to be 25 and 32 d after sugarbeet emergence, respectively. Redstem filaree interference did not affect the sucrose content percentage. These results demonstrate that redstem filaree is competitive with sugarbeet and should be managed appropriately to reduce negative effects on yield.

Itchgrass is a weed of many crops throughout the tropics and one of the most important grass weeds of rainfed rice. Experiments were conducted in the laboratory and screenhouse to determine the effects of light, alternating day/night temperatures, high temperature pretreatment, water stress, seed burial depth, and rice residue on seed germination and seedling emergence of itchgrass in the Philippines. Two populations were evaluated and the results were consistent for both populations. Germination in the light/dark regime was greater at alternating day/night temperatures of 25/15 C than at 35/25, 30/20, or 20/10 C. Light was not a requirement for germination, but a light/dark regime increased germination by 96%, across temperature and population. A 5-min high temperature pretreatment for 50% inhibition of maximum itchgrass germination ranged from 145 to 151 C with no germination when seeds were exposed to ≥ 180 C. The osmotic potential required for 50% inhibition of maximum germination was −0.6 MPa for itchgrass, although some seeds germinated at −0.8 MPa. Seedling emergence was greatest for seeds placed on the soil surface, and emergence declined with increasing soil burial depth; no seedlings emerged from seeds buried at 10 cm. The addition of rice residue to soil surface in pots at rates equivalent to 4 to 6 Mg ha−1 reduced itchgrass seedling emergence. Since seedling emergence was greatest at shallow depths and germination was stimulated by light, itchgrass may become a problem in systems where soil is cultivated at shallow depths. Knowledge gained in this study could contribute to developing components of integrated weed management strategies for itchgrass.

Horseweed is a common pest in vineyards of the San Joaquin Valley (SJV) of California. Interest in controlling this weed has increased with the recent discovery of a glyphosate-resistant (GR) biotype that has been observed to be more vigorous than a glyphosate-susceptible (GS) biotype in the SJV. However, the impact that either biotype may have on grapevine growth has not been assessed. Therefore, two glasshouse experiments were conducted to characterize the competitiveness of GR and GS horseweed biotypes from the SJV with young grapevines. ‘Syrah’ grapevines grafted to Freedom rootstocks were planted in 8-L plastic pots, alone, or with a single GR or GS horseweed. Additional GR and GS horseweeds were also planted separately in individual pots, and all plants were grown for 14 and 16 wk in 2006 and 2007, respectively. Grapevines grown with either biotype of the weed produced fewer leaves and amassed approximately 20% less dry mass (DM) than vines grown alone. The GR biotype reduced grapevine stem DM and length by 30%, but the GS biotype did not. The GR biotype accumulated more than twice the DM as the GS biotype, whether in competition with grapevine or not. Grapevines reduced the total leaf number of both horseweed biotypes by almost 50% and aboveground DM of GR and GS biotypes by 50 and 75%, respectively. These preliminary findings indicate that competition from horseweed can substantially reduce the growth of young grapevines and that the GR biotype may be more competitive than the GS biotype.

Recent studies have suggested that soybeans express shade avoidance in response to low red : far-red (R : FR) light reflected from neighboring plants and that this response may determine the onset and outcome of crop–weed competition. We tested the hypothesis that the low R : FR ratio would trigger characteristic shade avoidance responses in soybean and that the subsequent phenotype would experience reproductive costs under non–resource-limiting conditions. Soybeans were grown in a fertigation system in field trials conducted in 2007 and 2008 under two light quality treatments: (1) high R : FR ratio (i.e., weed-free) i.e., upward reflected light from a baked clay medium (Turface MVP®), or (2) low R : FR ratio (i.e., weedy) of upward reflected light, from commercial turfgrass. Results of this study indicated that a reduction in the R : FR ratio of light reflected from the surface of turfgrass increased soybean internode elongation, reduced branching, and decreased yield per plant. Shade avoidance also increased the plant-to-plant variability in biomass and yield per plant. Per plant yield losses were, however, more closely associated with reductions in biomass accumulation than population variability as the expression of a shade avoidance response did not influence harvest index. While these results suggest that weed induced shade avoidance decreases soybean per plant yield by reducing branching, it is possible the productivity of a soybean stand as a whole may be buffered against these reduction by a similar, but opposite, expression of plasticity in branching.

Alfalfa is an important forage crop in North America, and it can also be found as a roadside weed in alfalfa-growing regions. Weediness and invasiveness are greatly facilitated by establishment ability, yet little is known about the ability of alfalfa to establish in competitive environments such as roadsides. The primary objective of this study was to estimate the degree of alfalfa establishment without managed cultivation under different seed-dispersal times and disturbance regimes. The study had a split-plot design with two main plots (spring and fall seed dispersal) and five subplots (mowing, soil disturbance, herbicide spray, seedbed, and undisturbed control). The study examined establishment, growth attributes, and reproductive output of alfalfa in response to these treatments. Alfalfa establishment in the undisturbed grass swards ranged between 0.5 and 9.7% (out of the total number of seeds dispersed) across the dispersal times. The density of alfalfa in fall-seeded plots was about 82% lower than in spring-seeded plots. Soil disturbance reduced the density of alfalfa to < 50% of the initial density. Generally, low plant densities were compensated over time by increased numbers of shoots and reproductive units (racemes and pods) per plant. Herbicide application (2,4-D + dicamba) effectively controlled all emerged alfalfa plants, but in some cases, seedling recruitment was observed in the years following herbicide application. Although mowing did not kill alfalfa plants, mowed plants did not produce mature seeds, and as such, mowing may be useful in restricting the reproductive success and population growth of alfalfa. Overall, it is evident that alfalfa is capable of establishing in competitive environments (such as roadside habitats) and rapidly recovering from moderate disturbances. The results of this study have implications for managing roadside alfalfa and for designing novel trait-confinement protocols for alfalfa.

In 2006, Palmer amaranth with confirmed resistance to glyphosate (GLY-R) was not controlled effectively in cotton with pyrithiobac, an acetolactate synthase (ALS)-inhibiting herbicide. Glyphosate at 870 g ae ha−1 or pyrithiobac at 70 g ai ha−1 applied postemergence provided 5 to 28% control of a putative GLY/ALS-R Palmer amaranth biotype in the field. Glyphosate at 6,930 g ha−1 and pyrithiobac at 420 g ha−1 applied alone provided no more than 89 and 65% control 1 to 8 wk after treatment (WAT), respectively. When applied as a tank mixture, glyphosate plus pyrithiobac at 870 + 70 g ha−1 provided between 16 and 41% control; glyphosate plus pyrithiobac at 6,930 + 420 g ha−1 controlled the Palmer amaranth in the field 89 to 95%. Dose-response analyses developed from greenhouse data indicated that the estimated glyphosate rates required to produce 50% injury and reduce plant fresh weights by 50% relative to the nontreated control in a suspected GLY/ALS-R Palmer amaranth biotype were 12 and 14 times greater, respectively, than the estimated values for the susceptible (S) biotype. The predicted pyrithiobac rates required to produce the same responses in the putative resistant population were 151 (50% injury) and 563 times (50% fresh weight reduction) greater than the estimated rates for the S biotype. Field and greenhouse analyses confirm that the Palmer amaranth biotype evaluated in both studies is resistant to glyphosate and an ALS-inhibiting herbicide.

Jointed goatgrass is an exotic species introduced into the western United States from Eurasia. The weed is an agricultural pest infesting winter wheat fields and causing economic loss. Common ancestry between the two species enables interspecific hybridization, thus providing a mechanism for gene flow to occur. This can facilitate the accumulation of novel genes, which could increase the wild species' competitiveness with wheat and its ability to invade novel habitats. Interest in the development of transgenic wheat cultivars has increased the concern for interspecific gene flow. Gene introgression requires recurrent backcrossing to the weedy species after the initial hybridization event. Field experiments were conducted at two locations in Colorado in 2007–2008 and 2008–2009, with jointed goatgrass acting as the sole source of viable pollen for fertilization of transplanted hybrid plants. Backcrossing rates were determined by conducting germination studies on spikes collected from a total of 206 hybrid plants. Pollination by jointed goatgrass led to the production of 463 BC1 plants from seed produced on these 206 hybrid plants. Ninety-five percent confidence intervals estimate the rate of backcrossing at 0.028 to 0.306% and 0.077 to 0.604%, with medians of 0.062 and 0.152%, respectively, at the two locations. The results demonstrate that backcrossing to jointed goatgrass can occur, despite low rates of hybrid fertility. Subsequent backcrossing would make it likely that a wheat gene conferring a selective advantage will introgress into the weedy population. For the U.S. Great Plains, it is possible that transgenic wheat cultivars will be released in the future and determining proper management of these cultivars is necessary to minimize hybridization and advantageous gene introgression into weedy relatives.

Soil water availability is the most important factor limiting crop yield worldwide. Understanding crop and weed transpiration in response to water supply may provide valuable insight into the mechanisms of crop yield loss in water-limited environments. A greenhouse experiment was conducted to quantify corn and velvetleaf transpiration in response to drying soil. Five plants of each species were well watered by adding back the equivalent water loss each day to reach field capacity, and five plants were subjected to drought stress (dry-down) by not replacing lost water. Normalized daily transpiration of dry-down plants was regressed on soil water content expressed as the fraction of transpirable soil water (FTSW). The critical soil water content below which plants begin to close their stomates occurred at FTSWcr = 0.36 ± 0.015 for corn and 0.41 ± 0.018 for velvetleaf. Total water transpired did not differ among species. Velvetleaf also responded to drought by senescing its oldest leaves, whereas corn mainly maintained its leaf area but with rolled leaves during peak drought stress. During a short-term drought, corn is expected to perform better than velvetleaf because it maintains full transpiration to a lower FTSW and does not senesce its leaves. Under severe long-term drought, the species that closes its stomates at greater FTSWcr will conserve water and increase its chances of survival. Moreover, senescing all but the youngest leaves may ensure at least some seed production. Research is needed to evaluate the effects of soil water supply on corn–velvetleaf interference in the field.

Horse purslane, a C4 species, is a branched, prostrate, and annual weed of upland field crops throughout the tropics. Experiments were conducted to determine the influence of various environmental factors on seed germination and seedling emergence of two populations of horse purslane. Seeds were collected from rice fields of the International Rice Research Institute (the IR population) and from sorghum fields of the University of the Philippines (the UP population); the two sites were 5 km apart in Los Baños, Philippines. Germination response of both populations was greater at 30/20 C and35/25 C day/night temperatures than they were at 25/15 C alternating day/night temperatures. Germination of both populations was greater in the light/dark regime than in darkness. In dark, depending on the temperature, seed germination of the UP population ranged from 37 to 62%, whereas seed germination of the IR population was < 20%. Exposure to 5 min at 117 and 119 C for the IR and UP populations, respectively, reduced germination to 50% of maximum germination. Osmotic potential of −0.26 MPa inhibited germination to 50% of the maximum for the UP population, whereas the corresponding value for the IR population was −0.37 MPa. Seeds placed on or near the soil surface had maximum emergence, and emergence declined with increase in seed burial depth. Seedling emergence of the UP and IR populations was 74% and 13%, respectively, for seeds placed on the soil surface. For both populations, no seedlings emerged from a soil burial depth of 6 cm or more. Germination and emergence responses to light and seed burial depth differed between the two populations of horse purslane. Residues on the soil surface of up to 6 Mg ha−1 did not influence seedling emergence of either populations. Knowledge gained in this study could contribute to developing components of integrated weed management strategies for horse purslane.

Previous studies have suggested that the reduction in the root/shoot ratio that accompanies the shade avoidance response may reduce the tolerance of individuals to subsequent nutrient or moisture limitations. In this work, we examined the impact of the shade avoidance response on maize seedling growth and development and the response of these plants to a subsequent abiotic stress. Seedlings were grown in a field fertigation system under two light quality environments, ambient and a low red to far-red ratio, which were designed to simulate weed-free and weedy conditions, respectively. This system also enabled the controlled restriction of water and nutrients, which reduced the relative growth rate of the crop and created a secondary stress. Results of this study indicate that, while the shade avoidance response did reduce the root/shoot ratio in maize, this effect did not reduce plant tolerance to subsequent abiotic stress. Rather, the apparent additivity or synergism of shade avoidance and the secondary stressor on yield loss depended on whether the net effect of these two stressors was sufficiently large to shift the population toward the point where reproductive allometry was broken.

Greenhouse studies were conducted to determine the relative salinity tolerance of foxtail barley and seven desirable pasture grasses. Grass species were reed canarygrass, timothy, altai wildrye, tall fescue, tall wheatgrass, orchardgrass, creeping meadow foxtail, and foxtail barley. Grasses were exposed to increasing electrical conductivity levels of NaCl and CaCl2 salt solution over time. Grass species were compared using a cumulative value of salt exposure (ECdays), which was calculated to account for the electrical conductivity (EC) and the time a plant was exposed at that level of conductivity. Salinity tolerance varied among grass species. Increasing EC significantly reduced plant biomass of all species. All grass species experienced a 50% biomass reduction (GR50) between 271 and 512 ECdays in 2008 and between 297 and 575 ECdays in 2009. Foxtail barley was among the most salt tolerant (GR50 = 512 and 525 ECdays), requiring the highest salt exposure in 2008 and the second-highest exposure in 2009 to reduce biomass 50%. Grass mortality increased with increasing EC levels. Reed canarygrass and timothy were most susceptible to increasing salinity, with 50% mortality (LD50) of both grass species occurring between 983 and 1,186 ECdays. Moderate salinity tolerance was exhibited by orchardgrass, which required 1,977 and 1,844 ECdays; creeping foxtail, which required 1,998 and 2,431 ECdays; and tall fescue, which required 2,501 and > 2,840 ECdays to LD50 in 2008 and 2009, respectively. Foxtail barley, altai wildrye, and tall wheatgrass were most tolerant of salinity and persisted with little mortality occurring at 3,033 and 2,840 ECdays in 2008 and 2009, respectively. All grass species with higher growth rates than foxtail barley and altai wildrye were more susceptible to salinity, with the exception of tall wheatgrass. Growth rates of foxtail barley and altai wildrye were less than they were for other grasses, suggesting that slower growth rates may aid in salinity tolerance.

Resistance to the thiocarbamates has been selected in early watergrass populations within the rice-growing region of California. To elucidate the processes contributing to the spread of resistance among rice fields, we characterized the genetic diversity and differentiation of thiobencarb-resistant (R) and thiobencarb-susceptible (S) populations across the Central Valley using microsatellite markers. A total of 406 individuals from 22 populations were genotyped using seven nuclear microsatellite primer pairs. Three analytical approaches (unshared allele, Shannon–Weaver, and allelic-phenotype statistics) were used to assess genetic diversity and differentiation in the allohexaploid species. Low levels of genetic variation were detected within populations, consistent with other highly selfing species, with S populations tending to be more diverse than R populations. FST values indicated that populations were genetically differentiated and that genetic differentiation was greater among S populations than R populations. Principal coordinate analysis generated two orthogonal axes that explained 88% of the genetic variance among early watergrass populations and differentiated populations by geographical region, which was associated with resistance phenotype. A Mantel test revealed that genetic distances between R populations were positively correlated with the geographical distances separating populations. Taken together, our results suggest that both short- and long-distance seed dispersal, and multiple local and independent evolutionary events, are involved in the spread of thiobencarb-resistant early watergrass across rice fields in the Sacramento Valley. In contrast, resistance was not detected in early watergrass populations in the San Joaquin Valley.

Horseweed has become increasingly common and difficult to control in San Joaquin Valley vineyards, due in part, to the evolution of glyphosate resistance. The development of weed-suppressive vineyard designs in which the trellis design, spacing, and row orientation combine to cast dense shade on the weed canopy zone (WCZ) may reduce weed growth. The relevance of such a system to horseweed, which can grow to be as tall, or taller, than a typical grapevine trellis, is uncertain. Also unknown is whether a glyphosate-resistant (GR) biotype and glyphosate-susceptible (GS) biotype would perform similarly under such conditions. Therefore, we compared the growth and development of two potted horseweed biotypes (GR and GS) in vinerows oriented east–west (EW) and north–south (NS). Rows oriented EW allowed less light penetration to the WCZ than NS rows throughout the study, and horseweed biotypes responded to low light levels by producing leaves with larger specific leaf area and leaf area ratios than those in the NS rows. Also, the leaf, stem, and root dry weight of the horseweed plants in the EW rows was reduced by 30% compared to the horseweed plants in NS rows. Leaf number was also reduced in the horseweed plants in the EW rows, but only for the GS biotype. Row orientation did not affect phenological development or the number of seeds produced by the GR or GS biotypes, but the GR biotype budded, flowered, and set seed approximately 1 wk earlier than the GS biotype. Thus, shade associated with the EW vinerows reduced horseweed growth, but not fecundity, and the GR biotype reached reproductive maturity earlier than the GS biotype.

Weed seed persistence in soil can be influenced by many factors, including crop management. This research was conducted to determine whether organic management systems with higher organic amendments and soil microbial biomass could reduce weed seed persistence compared with conventional management systems. Seeds of smooth pigweed and common lambsquarters were buried in mesh bags in organic and conventional systems of two long-term experiments, the Farming Systems Project at the Beltsville Agricultural Research Center, Maryland, and the Farming Systems Trial at the Rodale Institute, Pennsylvania. Seed viability was determined after retrieval at half-year intervals for 2 yr. Total soil microbial biomass, as measured by phospholipid fatty acid (PLFA) content, was higher in organic systems than in conventional systems at both locations. Over all systems, locations, and experiments, viable seed half-life was relatively consistent with a mean of 1.3 and 1.1 yr and a standard deviation of 0.5 and 0.3 for smooth pigweed and common lambsquarters, respectively. Differences between systems were small and relatively inconsistent. Half-life of smooth pigweed seeds was shorter in the organic than in the conventional system in two of four location-experiments. Half-life of common lambsquarters was shorter in the organic than in the conventional system in one of four location-experiments, but longer in the organic than in the conventional system in two of four location-experiments. There were few correlations between PLFA biomarkers and seed half-lives in three of four location-experiments; however, there were negative correlations up to −0.64 for common lambsquarters and −0.55 for smooth pigweed in the second Rodale experiment. The lack of consistent system effects on seed persistence and the lack of consistent associations between soil microbial biomass and weed seed persistence suggest that soil microorganisms do not have a dominating role in seed mortality. More precise research targeted to identifying specific microbial functions causing seed mortality will be needed to provide a clearer picture of the role of soil microbes in weed seed persistence.

Glyphosate-resistant giant ragweed has become an increasing problem, and the potential spread of these biotypes is a threat to production agriculture and to the long-term utility of glyphosate and glyphosate-resistant crops. The fate of glyphosate resistance in a giant ragweed population is dependent on the fitness of the resistant biotype. Our objective was to determine the fitness of glyphosate-resistant giant ragweed in the absence and presence of glyphosate. We compared the growth and seed production of glyphosate-resistant (GR) and glyphosate-susceptible (GS) giant ragweed under field conditions in the absence of glyphosate. A greenhouse study was also conducted to determine the survival of GR and GS giant ragweed and their open-pollinated progeny from the field study under glyphosate-induced selection pressure. In the absence of glyphosate, GR giant ragweed displayed rapid, early season growth, but 50 d after planting, the biotypes were similar in height, shoot weight, and leaf area. During reproduction, the GR biotype flowered earlier and produced 25% less seed than the GS biotype. In the presence of glyphosate, an outcrossing rate of 31% was detected between GR and GS biotypes because 61% of progeny were resistant to glyphosate at 840 g ae ha−1. A second application 14 d later at 2,520 g ae ha−1 completely removed the GS alleles from the population, whereas several homozygous and heterozygous GR plants survived and produced seed. These results indicate GR will persist in the population when subjected to glyphosate and that glyphosate resistance in giant ragweed has the potential to spread rapidly in our current agricultural ecosystem.

Sheep sorrel is an invasive, creeping perennial weed of lowbush blueberry fields that decreases yields and hinders harvest. Much of the basic phenology of sheep sorrel in blueberry fields is unknown and not documented in peer-reviewed journals. Three levels of fertilizer (0, 20, and 40 kg N ha−1) and two levels of hexazinone (0 and 1.92 kg ai ha−1) were applied to three vegetative-year blueberry fields to determine their effects on root and shoot growth, biomass allocation, and seed production of sheep sorrel plants. Hexazinone efficacy varied widely between sites, but suppressed shoot biomass, achene number and weight, and reproductive biomass, as well as the reproductive : shoot biomass ratio. Fertilizer tended to increase achene number and increased sheep sorrel shoot biomass in the absence of hexazinone, but had no effect on achene weight, root biomass, or reproductive biomass. When fertilizer was applied, sheep sorrel allocated resources to sexual reproduction at the expense of vegetative growth.

Flax is a minor oilseed crop in Canada largely exported to the European Union for use as a source of industrial oil and feed ingredient. While flax could be genetically engineered (GE) to enhance nutritional value, the adoption of transgenic technologies threatens conventional flax market acceptability. Harvest seed loss of GE crops and the persistence of GE crop volunteers in the seed bank are major factors influencing transgene persistence. Ten commercial fields in Alberta, Canada, were sampled after harvesting conventional flax in 2006 and 2007, and flax seed density and viability were determined. Additionally, artificial seed banks were established at two locations in Alberta in 2005 and 2006 to quantify persistence of five conventional flax cultivars with variability in seed coat color (yellow or brown) and α-linolenic acid (ALA, 18:3cisΔ9,13,15) content (3 to 55%) at three soil depths (0, 3, or 10 cm). Harvest methods influenced seed loss and distribution, > 10-fold more seed was distributed beneath windrows than between them. Direct harvested fields had more uniform seed distribution but generally higher seed losses. The maximum yield loss was 44 kg ha−1 or 2.3% of the estimated crop yield. Seed loss and the viability of flax seed were significantly influenced by year, presumably because weather conditions prior to harvest influenced the timing and type of harvest operations. In artificial seed bank studies, seed coat color or ALA content did not influence persistence. Flax seed viability rapidly declined in the year following burial with < 1% remaining midsummer in the year following burial but there were significant differences between years. In three of four locations, there was a trend of longer seed persistence at the deepest burial depth (10 cm). The current study predicts that seed-mediated gene flow may be a significant factor in transgene persistence and a source of adventitious presence.