Tolerance to glufosinate has been bioengineered into cotton through the expression of a gene encoding the enzyme phosphinothricin acetyl transferase (PAT). Studies were conducted to determine thermal limitations on herbicide efficacy in bioengineered cotton. The 50% inhibition (I50) of glufosinate of the target-site enzyme glutamine synthetase was thermally dependent with the lowest values between 25 and 35 C. Larger values of I50 were measured above and below the 25 to 35 C range. The apparent Michaelis constant KM of the enzyme PAT was relatively stable from 15 to 30 C and increased more rapidly from 30 to 45 C. The two components in combination suggest the aggregate tolerance to glufosinate would not be thermally limited between 15 and 45 C. The thermal dependence of the aggregate tolerance in cotton suggests that glufosinate would not damage the crop over a range of temperatures. This prediction is in agreement with the results of field studies carried out over a number of years, which showed the glufosinate-tolerant cotton to be undamaged by glufosinate over a wide range of temperatures.

Differential sensitivity to paraquat was observed between cucumber cultivars and leaf age. Physiological responses to paraquat, including antioxidative enzyme activity, were investigated in leaf age classes of cucumber to identify mechanisms of paraquat tolerance. Leaf injury for ‘Naeseosamcheok’, ‘Daehandadagi’, ‘Baekgwangdadagi’, ‘Sangrokheukjinju’, and ‘Eunseongbaekdadagi’ cultivars was less than that of six other cultivars tested, averaged over leaf age and herbicide rate. The level of foliar injury caused by paraquat was Leaf 1 > 2 > 3 > 4 in seven of 11 cultivars used, where 4 was the youngest leaf. There was a positive correlation between leaf age and its relative susceptibility to paraquat, regardless of growth stage. Lipid peroxidation was less in the youngest leaf (Leaf 4) than in the older leaves at all herbicide concentrations. The youngest leaf had higher values for apparent photosynthesis than the oldest leaf. Differential leaf response to paraquat was partially correlated with the change in superoxide dismutase, ascorbate peroxidant, and glutathione reductase activities in treated leaves. Enzyme activity of ascorbate peroxidase (APX) was higher in Leaf 4 than in Leaves 1, 2, or 3 in untreated plants and after exposure to paraquat. APX isozymes were more abundant in treated than in untreated leaves and produced in higher amounts in younger than in older leaves. Application of ascorbate and glutathione before paraquat treatment protected cucumber leaves from paraquat injury.

Laboratory studies were conducted to examine the leaf surface, epicuticular wax content, and spray droplet behavior on common lambsquarters, common purslane, and velvetleaf. Adaxial and abaxial leaf surfaces were examined using scanning electron microscopy, and leaf wax was extracted and quantified for all three weed species. The spread of 1-μl droplets of distilled water, primisulfuron solution (without surfactant), primisulfuron solution with a nonionic low foam wetter/spreader adjuvant (0.25% v/v), and with an organosilicone wetting agent (0.1% v/v) was determined on the adaxial leaf surfaces of each of the weed species. Glands and trichomes were present on both the adaxial and abaxial leaf surfaces of velvetleaf. Common purslane had neither glands nor trichomes on either side of the leaf. Common lambsquarters did not have any glands or trichomes, but it had globular bladder hairs on both adaxial and abaxial leaf surfaces. Stomata were present on both adaxial and abaxial leaf surfaces in all three weed species. Common purslane had a much lower number of stomata per unit area of leaf as compared with velvetleaf or common lambsquarters. Common lambsquarters had the highest epicuticular wax content on the leaf surface (274.5 μg cm−2), followed by common purslane (153.4 μg cm−2) and velvetleaf (7.4 μg cm−2). There were no significant variations in the spread of the 1-μl droplet of distilled water and primisulfuron (without adjuvant) among the species. Spread of primisulfuron droplets with surfactant was highest on the leaf surface of velvetleaf that had the lowest wax content. Droplet spread was greatest with organosilicone surfactant followed by the nonionic surfactant.

Discoloration of bermudagrass often results from application of MSMA herbicide used to control southern crabgrass and other weeds. However, when products containing iron sulfate (FeSO4) are tank-mixed with MSMA, this discoloration is reduced. Experiments investigated the effect of tank-mixing organic arsenical herbicides with FeSO4 or a chelated iron source (Sprint 330) in terms of southern crabgrass control and injury to bermudagrass. Tank-mixing MSMA with FeSO4 reduced bermudagrass injury. However, southern crabgrass control was also reduced by at least 50% with the addition of ≥0.38 kg Fe2+ ha−1. Neither antagonism nor safening of bermudagrass was observed when the chelated Fe2+ source was used. Applying FeSO4 as a separate treatment 1 to 4 d before or after MSMA application did not reduce visual burmudagrass injury 1 wk after treatment. Solution pH and FeSO4 concentration controlled the extent of complexation and level of antagonism observed in the field; inorganic Fe2+ reacted with MSMA to form a complex having reduced herbicidal activity. Potentiometric and spectrophotometric investigations found that methylarsonate, the parent acid of MSMA and other organic arsenical herbicides, reacts with inorganic Fe2+ to form a stable 1:1 Fe2+-methylarsonic acid chelate having two points of metal coordination and a stability constant log10 (β) = 2.77 ± 0.04. Tank-mixing MSMA with FeSO4 to protect against bermudagrass injury negates the benefit of applying the herbicide for weed control, and therefore is not a recommendable practice for turf managers.

The role of ethylene in kochia injury following the application of dicamba and other auxinic herbicides is not known. Experiments were conducted to evaluate kochia response to ethylene generated within plant tissue and to quantify ethylene emission from kochia following dicamba application to determine if ethylene production could be used to differentiate between dicamba-susceptible (S) and dicamba-resistant (R) kochia accessions. Dicamba at 0.28 kg ha−1 and fluroxypyr at 0.28 kg ha−1 caused severe stem and petiole twisting within 6 h of application, with S kochia exhibiting a greater level of epinasty than R kochia and fluroxypyr causing more epinasty than dicamba. Fluroxypyr provided similar control of S and R kochia, but dicamba provided less control of R kochia than S kochia. Ethylene evolution in S kochia from ethephon at 1.1 kg ha−1 and dicamba at 0.28 kg ha−1 was similar, approximately 70 ng (g fresh weight)−1 h−1. Elevated ethylene concentration from as much as 2 kg ha−1 ethephon did not produce epinastic symptoms or other plant response in S or R kochia. Ethylene evolution following dicamba application increased with time up to 48 h after application and with increasing dicamba rate up to 0.56 kg ha−1, which were the end points of the research. Ethylene evolution from the S accession was 2.5 times that of the R accession averaged across dicamba rates and harvest times. The greatest difference in ethylene evolution between S and R accessions occurred 24 h after treatment at 0.14 kg ha−1 dicamba, with S kochia producing ethylene at 470%, compared with R kochia.

Weed competitiveness can be quantified with the concept of competitive index (CI), a relative scale of weed competitiveness. Field studies were conducted in 2002 and 2003 in northeastern and southeastern Nebraska to evaluate the influence of soybean row spacing and relative weed emergence time on the competitiveness of major weed species in soybean. Ten weed species were seeded in soybean spaced 19 and 76 cm apart at the planting, emergence, and first trifoliate leaf stages of soybean. Total weed dry matter (TDM), weed plant volume, and percent soybean yield loss were arbitrarily selected as a base for determining the CI for each weed species. Soybean yield loss was the least variable parameter used to quantify weed competitiveness and rank their CIs. In general, weeds grown with soybean planted in 19-cm rows produced less TDM, plant volume, and reduced soybean yield less than weed species grown in 76-cm rows. Later-emerging weeds produced less TDM, plant volume, and reduced soybean yield less than the early-emerging ones. In general, broadleaf species were more competitive than grass weed species. Common sunflower was the most competitive weed species in this study.

Conservation tillage systems, such as no-tillage, are ecologically advantageous because they reduce soil erosion; however, they rely heavily on herbicide use. Our goal was to determine how weed communities of no-tillage systems are affected when the system is modified to reduce herbicide use through a combination of banded herbicides and interrow cultivation. To this end, we conducted a 9-yr study in a no-tillage corn–soybean–winter wheat rotation. All management systems had a preplant application of glyphosate, followed by either broadcast PRE herbicides (conventional no-tillage), interrow cultivation with banded PRE herbicides, or interrow cultivation alone. Aboveground weed densities were assessed each year and data were grouped into early (1991 to 1993) and late (1996 to 1998) time periods. Over time, weed communities became more distinct, showing a strong response to management and crop. In the early years, weed communities separated more in response to management than crop. In the late years, this was reversed. Weed communities in systems with interrow cultivation were more diverse than those in conventional no-tillage. The response to weed management system and crop was species specific. For example, the abundance of yellow foxtail was higher when interrow cultivation was employed, but abundance was equal in all crops. Dandelion was more abundant in conventional no-tillage of corn and soybean; however, it was equally abundant in all management systems in wheat. Seed bank species richness increased over time and was highest in systems with interrow cultivation. Herbicide use can be reduced in a modified no-tillage corn–soybean–wheat rotation by incorporating interrow cultivation, with or without banded herbicides, into the management plan. The weed community trajectory changes, and the weed community becomes more diverse. A more diverse weed community will not necessarily alter how we manage weeds.

Signals from both leaves and apical or axillary meristems of leafy spurge are known to inhibit root bud growth. To test the hypothesis that carbohydrates and growth regulators affect root bud growth, decapitated leafy spurge plants were hydroponically treated with glucose, sucrose, gibberellic acid (GA), abscisic acid (ABA), 1-naphthaleneacetic acid (NAA), 6-benzylaminopurine (BA), and a GA biosynthesis inhibitor, paclobutrazol. Both glucose and sucrose caused suppression of root bud growth at concentrations of 30 mM. The inhibitory effect of sucrose was counteracted by GA at 15 μM. In contrast, BA, ABA, NAA, and paclobutrazol inhibited root bud growth at concentrations as low as 1, 2, 1, and 16 μM, respectively. Sugar and starch levels were also determined in root buds at various times after decapitation. Buds of intact plants contained the highest level of sucrose compared with buds harvested 1, 3, and 5 d after decapitation. To determine how seasonal changes affect root bud dormancy, growth from root buds of field-grown plants was monitored for several years. Root buds of field-grown leafy spurge had the highest level of innate dormancy from October to November, which persisted until a prolonged period of freezing occurred in November or early December. Our data support the hypothesis that carbohydrates may be involved in regulating dormancy status in root buds of leafy spurge.

In a 6-yr study on four farms (36 fields) in Ontario, Canada, we tested the effects of tillage (moldboard, chisel plow, no tillage) and crop rotations (continuous corn, corn-soybean, corn-soybean-winter wheat) on emerged and seedbank weed species diversity and density. Aside from the imposed experimental treatments, all other management was generally consistent among farms. Tillage had the largest effect on weed diversity and density. No tillage promoted the highest weed species diversity, chisel plow was intermediate, and moldboard plow resulted in the lowest species diversity. These results are consistent with ecological succession theory. The increase in weed species diversity resulted from 20 species being associated with no tillage systems, 15 of which were winter annuals, biennials, or perennials. Emerged weed density was affected only by tillage. Over 6 yr, seedbank declined in no-tillage systems from 41,000 to 8,000 seeds m−3. Crop yields were not affected by tillage or crop rotation. In practical terms, reduced tillage in combination with a good crop rotation may reduce weed density and expenditures on weed management.

Pitted morningglory is an adaptable species with an indigenous range encompassing the southern Midwest and southeast United States. In 2000 through 2002, 64 pitted morningglory accessions from 11 states were grown in Fayetteville, AR, to compare their morphology in a common environment to document potential morphological variation and to determine whether variation proves the existence of pitted morningglory morphological ecotypes. Accessions were evaluated for leaf size and vine length 8 wk after emergence (WAE), date of flower initiation, flower color, leaf pubescence 12 WAE, capsule and sepal pubescence, sepal length and width, plant weight, and seed number at physiological maturity. Morphological variables were standardized and analyzed with cluster analysis to differentiate the morphological variation among accessions. Documented variation was best described by eight clusters. Four clusters distinguished themselves morphologically. Accessions within these distinct clusters were originally from Arkansas, Delaware, Kentucky, Louisiana, Mississippi, and Missouri and were documented with leaf size, vine length, and day of flower initiation generally increasing with decreasing latitude. The other four clusters were nondistinct because most variables differed very little, but characteristics such as capsule pubescence separated these clusters. Accessions within these nondistinct clusters originated from Arkansas, Georgia, Louisiana, North Carolina, Oklahoma, and Tennessee. Leaf shapes of arrow, heart, an arrow and heart mixture, and heart with pointed projections and white or purple flower colors were documented. Documented differences indicate the existence of pitted morningglory morphological ecotypes.

Age distributions of sulfur cinquefoil populations were determined on sites that were historically grazed, cultivated, and mechanically disturbed. From 12 sites, a total of 279 reproductively active plants were collected and aged by using herbchronology (counting rings in the secondary root xylem of the root crown) to (1) estimate the age structure of the populations, (2) relate plant size and flower production to plant age, and (3) examine the relation of population age structure to environmental variables and disturbance history. Results indicated that the mean age for all sampled plants was 3.5 (± 1.74 SD) yr and ranged from 1 to 10 yr. Age was not related to number of flowers, plant size (number of stems per plant or plant height), or site disturbance type but was positively correlated with site elevation (P < 0.001). The pooled age distribution from all 12 sites was right-skewed with fewer old plants than young plants. We conclude that sulfur cinquefoil plants sampled in northeast Oregon are able to colonize, establish, and reproduce at disturbed sites rapidly. We suggest that herbchronology may be a useful technique to improve understanding of invasion biology and ecology for invasive plant species that form annual rings.

Weed management systems in carrot are limited in part by a lack of fundamental understanding of crop–weed interactions. Irrigated field studies were conducted to quantify the effect of volunteer potato density and duration of interference on carrot yield and to determine relationships among weed density, duration of weed growth, and volunteer potato tuber production. A season-long volunteer potato density of 0.06 plants m−2 produced from 150 to 230 g tubers m−2 and resulted in an estimated 5% crop yield loss. At two volunteer potato plants m−2, the same level of crop loss was estimated with a duration of interference of 430 growing degree days (GDD), a time at which the weed had already produced 130 g tubers m−2. Volunteer potato height at the time of weed removal predicted carrot yield loss (R 2 = 0.77) and may be useful for timing of management strategies such as hand weeding. Functional relationships describing carrot–volunteer potato interactions provide simple information that is useful for developing weed management recommendations for carrot, a crop that relies on multiple tactics for managing weeds, and rotational crops that are negatively affected by persistence of volunteer potato.

Field experiments were established in 1998 and 1999 to evaluate the effect of giant smutgrass competition and hexazinone application on bahiagrass forage yield. The experimental design was a split-plot, with low (< 20% groundcover), medium (20 to 70% groundcover), and high (> 70% groundcover) giant smutgrass density as the main plot factors and hexazinone application or no hexazinone application as the subplot factors. In 1998, without hexazinone, bahiagrass biomass accumulation was 1,164 kg ha−1 mo−1 under low giant smutgrass infestation but 590 and 154 kg ha−1 mo−1 under medium and high giant smutgrass densities, respectively. From harvests occurring 1 yr after hexazinone application, bahiagrass yield in the weed-free area was similar to that growing under low giant smutgrass density. However, as giant smutgrass density increased to moderate or high levels, bahiagrass yield was reduced relative to the weed free. Giant smutgrass biomass accumulation was also measured over time. Giant smutgrass biomass, in both years, increased dramatically in the late summer months at the medium and high densities but not at the low density. It was concluded that bahiagrass was competing with the giant smutgrass at low density and depressed late season growth but was not capable of doing so at higher infestation levels. A rapid increase in late-season giant smutgrass growth was partially explained by the fact that bahiagrass is a short day plant that begins to flower in mid- to late summer, and aboveground biomass production decreases in late summer. This shift in carbon allocation in bahiagrass would exert less competition on giant smutgrass and thus partially be responsible for the late season increase in giant smutgrass growth. Economic analysis performed on these data illustrated that a net loss of $15.20 per stocking unit (cow–calf pairs) would be realized if hexazinone were used to control low densities of giant smutgrass. However, a net gain of $29.28 and $55.75 per stocking unit was observed if hexazinone was used to control giant smutgrass that had reached medium or high levels of infestation, respectively. It was concluded from these data that giant smutgrass should not be controlled until densities reach approximately 35% infestation.

The effects of crop rotation and management system on annual variability in weed communities and crop yields were assessed in a 4-yr study in Michigan. Variability of the weed community and corn yields were assessed using the coefficient of variation (CV) and a multivariate dissimilarity index (Bray-Curtis) that accounted for changes in both weed species abundance and composition. The treatments included two rotations: continuous corn and a corn–corn–soybean–wheat rotation, and two management systems: conventional (CONV) and organic-based (ORG). Weed biomass was significantly higher in the ORG system; however, there was no effect of crop rotation on weed biomass or number of weed species in a treatment (species richness). Annual variability in weed community composition and structure was affected by both crop rotation and management system and was highest in the ORG rotation. In contrast to the weed community, variability in corn yield was highest in the least-diverse cropping system (CONV monoculture), despite that system having a more constant weed community. Corn yield in the ORG rotation was not significantly different from that in the CONV monoculture. Results of this study suggest that management aimed at increasing cropping system diversity may have additional effects on weed communities and crop yields beyond those commonly reported, and these may have important implications for the development of more efficient and sustainable weed and crop management practices.

Catchweed bedstraw and wild mustard each produce two populations per year: a winter population (WP) in June, and a summer population (SP) in September. Experiments were conducted to determine whether the WP and SP differ in seed mass and seasonal germination. Seeds of both weeds were buried at 0, 5, 10, and 20 cm in cultivated fields, and retrieved at monthly intervals for 24 mo for germination tests in the laboratory. Additionally, seedling emergence from seeds buried at 0, 5, and 10 cm in the field was evaluated for 1 yr. Seeds from the WP were heavier than those from the SP for both species. Germination of exhumed seeds was affected by burial depth and by seed population. It was highest for seeds that remained on the soil surface and declined with increasing depth of burial. The WP of catchweed bedstraw produced two germination peaks per year, whereas the SP and all populations of wild mustard had only one peak. The WP of both weeds germinated earlier than the SP. Seedling emergence for both species in the field was greater for the WP than for the SP. Increasing soil depth reduced seedling emergence of both the WP and SP of wild mustard and affected only the WP of catchweed bedstraw. We conclude that the WP and SP of catchweed bedstraw and wild mustard seeds used in this study differed in seed mass, seasonal germination, and seedling emergence. The ability of a WP to produce large seeds that germinate early and have two germination peaks per year could make these populations a serious problem in cropping systems.

A 2-yr field study was conducted to compare growth characteristics of 24 Palmer amaranth accessions collected from across the indigenous range of the species in the United States. Variation in growth and development of Palmer amaranth was noted among accessions based on leaf area ratio (LAR), specific leaf area (SLA), net assimilation rate (NAR), and stem leaf ratio (SLR), but only SLR varied across harvest intervals among accessions. Accessions collected across the range of Palmer amaranth in the United States displayed variation in growth and development based on differences in LAR, SLA, NAR, and SLR. Observed differences among accessions indicate the existence of Palmer amaranth ecotypes.

London rocket is a common winter annual weed in southern New Mexico that can host beet curly top virus and its insect vector, the beet leafhopper. Experiments were conducted in southern New Mexico to determine if London rocket could serve as a host for overwintering beet leafhopper. Field experiments were carried out from 2002 to 2003 and from 2003 to 2004 to compare the impact of three London rocket planting dates on plant emergence and life history and leafhopper survival. Emergence was highest in October-planted London rocket, low in January/February plantings, and did not occur for August plantings. The life cycle was 185 d and 125 d for October- and January-planted London rocket, respectively, and growth of the plant (including height and rosette base diameter) was greater for London rocket that was planted in October. October-planted London rocket survived from late October through mid to late April, the period of time needed to serve as an overwintering host for beet leafhoppers. Caging the plants to assess beet leafhopper survival did not affect rate of plant growth, but it reduced the time to flowering for October-planted London rocket and increased the height and weight of plants. Beet leafhoppers were able to survive for approximately 2 mo in early winter or spring on caged London rocket plants.

Knowledge of how plants will partition their new biomass will aid in understanding competition between crops and weeds. This study determined if the amount of biomass partitioned to the root versus the shoot can be predicted from tissue carbon [C] and nitrogen [N] concentrations and the daily gain in C (GC) and N (GN) for each unit shoot and root biomass, respectively. Pots measuring 28 cm diameter and 60 cm deep were embedded in the ground, and each contained one plant of either corn or velvetleaf. Each plant received one of three nitrogen treatments: 0, 1, or 3 g of nitrogen applied as ammonium nitrate in 2001 and 0, 2, or 6 g of nitrogen in 2002. Measurements of total above- and belowground biomass and tissue [C] and [N] were made at 10 different sample dates during the growing season. Fraction of biomass partitioned to roots (Pr) was predicted from [C], [N], GC, and GN. Accurate prediction of the fraction of biomass partitioned to roots versus shoots was evaluated by comparing observed and predicted Pr across all treatments. The coordination model has potential as a reliable tool for predicting plant biomass partitioning. Normalized error values were close to zero for corn in 2001 and 2002 and for velvetleaf in 2001, indicating that biomass partitioning was correctly predicted.

Integrated weed management (IWM) decision strategies in herbicide-resistant canola-production systems were assessed for net returns and relative risk. Data from two field experiments conducted during 1998 to 2000 at two locations in Alberta, Canada, were evaluated. A herbicide-based experiment included combinations of herbicide system (glufosinate-, glyphosate-, and imazethapyr-resistant canola varieties), herbicide rate (50 and 100% of recommended dose), and time of weed removal (two-, four-, and six-leaf stages of canola). A seed-based experiment included canola variety (hybrid and open-pollinated), seeding rate (100, 150, and 200 seeds m−2), and time of weed removal (two-, four-, and six-leaf stages of canola). For the herbicide-based experiment, strategies with glyphosate were profitable at Lacombe, but both imazethapyr and glyphosate strategies were profitable at Lethbridge. Weed control at the four-leaf stage was at least as profitable as the two-leaf stage at both sites. For the seed-based experiment, the hybrid was more profitable than the open-pollinated cultivar, seed rates of 100 and 150 seeds m−2 were more profitable than 200 seeds m−2, and weed control at the two- and four-leaf stages was more profitable than at the six-leaf stage. When risk of returns and statistical significance was considered, several strategies were included in the risk-efficient set for risk-averse and risk-neutral attitudes at each location. However, the glyphosate-resistant cultivar, the 50% herbicide rate, and weed control at four-leaf stage were more frequent in the risk-efficient IWM strategy set. The open-pollinated cultivar, 200 seeds m−2 rate, and weed control at the six-leaf stage were less frequent in the set. The risk-efficient sets of IWM strategies were consistent across a range of canola prices.

Postdispersal weed seed predation by animals during the summer fallow period may lead to a reduction in the number of weeds that grow in the following winter cropping season. In this study, we investigated the patterns of weed seed removal, the influence of crop residue cover on seed removal, the types of granivores present and their seed preferences in a 16-ha postharvest cropping field in Western Australia during the summer months over 2 yr. Seed removal from caches was extremely variable (from 0 to 100%). Removal rates were generally highest along the edges of the field near bordering vegetation and lowest in the center of the field and within the bordering vegetation. However, there were many deviations from this general pattern. There was no change in rates of predation with different levels of residue cover. Ants or other small invertebrates were found to remove the most seeds. However, seed removal by other animals, such as rodents, was also evident. Annual ryegrass seeds were preferred over wild oat seeds, followed by wild radish pod segments. Seed harvesting was lowest in late January, peaked in February, and decreased in March. Results from this study suggest seed harvesters could reduce the number of surface seeds in the field, reducing the weed seed bank. Management options that increase the activity of the seed harvesters may lead to less variability in seed predation and could, therefore, be incorporated into an integrated weed management program.