Many conditions need to be satisfied for gene flow to occur between a transgenic crop and its weedy relatives. Flowering overlap is one essential requirement for hybrid formation. Hybridization can occur between canola and its wild relative, wild radish. We studied the effects of wild radish plant density and date of emergence, canola (glyphosate resistant) planting dates, presence of other weeds, and presence of a wheat crop on the synchrony of flowering between wild radish and canola (as a crop and volunteer). Four field experiments were conducted from 2000 to 2002 in St-David de Lévis, Québec. Flowering periods of wild radish emerging after glyphosate application overlapped with early-, intermediate-, and late-seeded canola 14, 26, and 55%, respectively, of the total flowering time. Flowering periods of early-emerging wild radish and canola volunteers in uncropped treatments overlapped from mid-June until the end of July, ranging from 26 to 81% of the total flowering time. Flowering periods of wild radish and canola volunteers emerging synchronously on May 30 or June 5 as weeds in wheat overlapped 88 and 42%, respectively, of their total flowering time. For later emergence dates, few flowers or seeds were produced by both species because of wheat competition. Wild radish density in canola and wild radish and canola volunteer densities in wheat did not affect the mean flowering dates of wild radish or canola. Increasing wild radish density in uncropped plots (pure or weedy stands) hastened wild radish flowering. Our results show that if hybridization is to happen, it will be most likely with uncontrolled early-emerging weeds in crops or on roadsides, field margins, and uncultivated areas, stressing the need to control the early flush of weeds, weedy relatives, and crop volunteers in noncrop areas.

Nomenclature: Glyphosate; wild radish, Raphanus raphanistrum L. RAPRA; canola, Brassica napus L. ‘Hyola 357 RR’; wheat, Triticum aestivum L.

A 2-yr experiment assessed the potential for using soil degree days (DD) to predict cumulative weed emergence. Emerged weeds, by species, were monitored every 2 wk in undisturbed plots. Soil DD were calculated at each location using a base temperature of 9 C. Weed emergence was fit with logistic regression for common ragweed, common lambsquarters, velvetleaf, giant foxtail, yellow foxtail, large crabgrass, smooth pigweed, and eastern black nightshade. Coefficients of determination for the logistic models fit to the field data ranged between 0.90 and 0.95 for the eight weed species. Common ragweed and common lambsquarters were among the earliest species to emerge, reaching 10% emergence before 150 DD. Velvetleaf, giant foxtail, and yellow foxtail were next, completing 10% emergence by 180 DD. The last weeds to emerge were large crabgrass, smooth pigweed, and eastern black nightshade, which emerged after 280 DD. The developed models were verified by predicting cumulative weed emergence in adjacent plots. The coefficients of determination for the model verification plots ranged from 0.66 to 0.99 and averaged 0.90 across all eight weed species. These results suggest that soil DD are good predictors for weed emergence. Forecasting weed emergence will help growers make better crop and weed management decisions.

Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; common ragweed, Ambrosia artemisiifolia L. AMBEL; eastern black nightshade, Solanum ptycanthum Dun. SOLPT; giant foxtail, Setaria faberi Herrm. SETFA; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; smooth pigweed, Amaranthus hybridus L. AMACH; velvetleaf, Abutilon theophrasti Medikus ABUTH; yellow foxtail, Setaria glauca (L.) Beauv. SETLU.

Surviving rigid ryegrass plants were collected from a cropping field at Pindar, Western Australia (population WALR 50), after inadequate control by glyphosate applied at the normal field rate. Plants were grown to maturity in pots and seeds were collected. Glyphosate dose–response experiments with known susceptible and resistant control populations confirmed the resistant status of the WALR 50 population. The glyphosate rate resulting in 5% mortality (LD₅₀) and GR₅₀ (the glyphosate rate required to reduce mean growth of individuals to 50% of the untreated control) values for this population were 1,069 and 217 g ae ha⁻¹, respectively, corresponding to R:S ratios of 3.4 and 1.9 for mortality and growth. In addition, a novel root growth–based assay of glyphosate resistance was developed and validated, giving a root growth GR₅₀ R:S ratio of 3.4. A resistance profile was established by assessing population-level survival of WALR 50 after applications at recommended rates of a range of herbicides commonly used for rigid ryegrass control in Australia. High levels of resistance to the acetolactate synthase (ALS)–inhibiting sulfonylurea herbicides chlorsulfuron and sulfometuron, moderate resistance to the acetyl coenzyme A carboxylase (ACCase)–inhibiting herbicide diclofop, and low levels of resistance to the imidazilinone herbicide imazethapyr were found. More detailed dose–response experiments confirmed resistance to chlorsulfuron, sulfometuron, and diclofop. In vitro enzyme-inhibition studies demonstrated that ALS resistance in WALR 50 is due to an insensitive target enzyme and that ACCase resistance is due to a nontarget site–based mechanism. WALR 50 is the first glyphosate-resistant weed population with confirmed resistance to ACCase- and ALS-inhibiting herbicides.

Nomenclature: Chlorsulfuron; diclofop; glyphosate; imazethapyr; sulfometuron; rigid ryegrass, Lolium rigidum Gaud. LOLRI.

California's interior grasslands have undergone dramatic changes during the last two centuries. Changes in land-use patterns and plant introductions after European contact and settlement resulted in the conversion of perennial-dominated grasslands to exotic annual grasses. More recently, the annual grasslands have been heavily invaded by the deeply rooted late-maturing forb yellow starthistle. This series of invasions and conversions has changed the community structure and phenology of the grasslands. We hypothesized that these changes have resulted in significant differences in soil water–use patterns in the grasslands. We studied soil water depletion and recharge patterns of three grassland community types dominated by perennial grasses, annual grasses, or yellow starthistle with contrasting phenology and rooting depths for 4 yr. Soil moisture measurements were taken every month from March to December in 1998, 1999, and 2000 and every other month in 2001. Measurements were taken with a neutron probe at depths of 30 to 150 cm at 30-cm intervals. The results indicate that the yellow starthistle community maintained a significantly drier soil profile than the annual grass community. The perennial grass community maintained an intermediate soil water content that was not significantly different from either of the other two communities. Significant time by community and depth by community interactions indicated that the yellow starthistle community continued depleting soil moisture later into the season and at deeper depths than the other grass communities. This study demonstrates the effect of plant invasion on soil water recharge and depletion patterns in California grasslands.

Nomenclature: Yellow starthistle, Centaurea solstitialis L. CENSO.

Comparing distributions among fields, species, and management practices will help us understand the spatial dynamics of weed seed banks, but analyzing observational data requires nontraditional statistical methods. We used cluster analysis and classification and regression tree analysis (CART) to investigate factors that influence spatial distributions of seed banks. CART is a method for developing predictive models, but it is also used to explain variation in a response variable from a set of possible explanatory variables. With cluster analysis, we identified patterns of variation with direction of the distance over which seed bank density was correlated (range of spatial dependence) with single-species seed banks in corn. Then we predicted patterns of the seed banks with CART using field and species characteristics and seed bank density as explanatory variables. Patterns differed by magnitude of variation in the range of spatial dependence (strength of anisotropy) and direction of the maximum range. Density and type of irrigation explained the most variation in pattern. Long ranges were associated with large seed banks and stronger anisotropy with furrow than center pivot irrigation. Pattern was also explained by seed size and longevity, characteristics for natural dispersal, species, soil texture, and whether the weed was a grass or broadleaf. Significance of these factors depended on density or type of irrigation, and some patterns were predicted for more than one combination of factors. Dispersal was identified as a primary process of spatial dynamics and pattern varied for seed spread by tillage, wind, or natural dispersal. However, demographic characteristics and density were more important in this research than in previous research. Impact of these factors may have been clearer because interactions were modeled. Lack of data will be the greatest obstacle to using comparative studies and CART to understand the spatial dynamics of weed seed banks.

Nomenclature: Corn, Zea mays L.

Yellow starthistle is an invasive plant of canyon grasslands in north-central Idaho. The distribution of yellow starthistle is associated with general landscape characteristics that include land use and specific terrain-related features such as elevation, slope, and aspect. Slope and aspect can be considered as indicators of plant community composition and distribution. Hence, these variables may be incorporated into prediction models to estimate the likelihood of yellow starthistle occurrence because plant communities differ in susceptibility to invasion. An empirically derived nonlinear model based on landscape characteristics has previously been developed to predict the likelihood of yellow starthistle occurrence in north-central Idaho. Although the model was used to predict the invasion potential of yellow starthistle into new areas, it could also be used as auxiliary data for classifying this weed species in remotely sensed imagery. To accomplish this, the predicted values from the model are regarded as prior information on the presence of yellow starthistle. A Bayesian image classification algorithm using this prior information is then applied to a corresponding set of remotely sensed data. This results in a map indicating the posterior probabilities of yellow starthistle occurrence given the landscape characteristics. This technique is demonstrated and is shown to reduce omissional error rates by 50% when the landscape characteristics are incorporated into the classification process.

Nomenclature: Yellow starthistle, Centaurea solstitialis L. CENSO.

Field studies were conducted in 2002 and 2003 to determine whether canopy formation influences pitted morningglory, common cocklebur, and sicklepod emergence in surface-tilled soybean. Each weed species was broadcast seeded before planting soybean in 19- and 97-cm-wide rows. Weed emergence beneath soybean was monitored after soybean emergence and compared with weed emergence in the absence of soybean (bareground treatment). Magnitude of daily diurnal soil temperature fluctuations diminished after soybean canopy formation, and light interception by soybean was positively related to the reduction in soil temperature. Canopy formation (50% light interception) occurred 16 to 17 d later in wide compared with narrow rows in both years. The red/far-red ratio of light available to seed on or near the soil surface was reduced from as much as 1.2 in full sunlight to less than 0.1 in the presence of a dense soybean canopy. Pitted morningglory emergence was not influenced by soybean canopy formation, whereas common cocklebur and sicklepod emergence were reduced as much as 33 and 68%, respectively. Although common cocklebur and sicklepod emergence diminished after soybean canopy formation, a small portion of the seedbank of both species emerged beneath the canopy. This research indicates that in a tilled system, emergence of some weed species is diminished by presence of an overlying canopy, but emergence does not completely cease with canopy formation. Late-season emergence of sicklepod and common cocklebur beneath a soybean canopy may contribute to replenishment of the soil seedbank, especially if these late-emerging cohorts are capable of surviving until the light environment is favorable for seed production.

Nomenclature: Common cocklebur, Xanthium strumarium L. XANST; pitted morningglory, Ipomoea lacunosa L. IPOLA; sicklepod, Senna obtusifolia (L.) Irwin and Barneby CASOB; soybean, Glycine max (L.) Merr. ‘Northrup King S73-Z5’, ‘Delta and Pineland 6880’.

Cogongrass is a noxious perennial grass that has invaded many countries in the tropical and subtropical regions of the world. Its management has been a significant challenge because of large rhizome and bud reserves in the soil. The emergence pattern of this weed under field conditions has received little attention. Field trials were conducted in 2002 and 2003 in the humid forest zone of southeastern Nigeria to model shoot emergence. The experiment had four treatments: (1) count and tag crop-free cogongrass shoots, (2) count and suppress crop-free cogongrass shoots with paraquat, (3) count and cut crop-free cogongrass shoots, and (4) count and cut cogongrass shoots in cultivated corn. The rationale for these treatments was to determine the effect of different monitoring techniques on shoot emergence of cogongrass. The development of the model was based on hydrothermal time, which was calculated from soil moisture and soil temperature at a 2-cm depth. A Weibull function was fitted to cumulative percent shoot emergence values of Treatment 4 and hydrothermal time. The model closely fit the observed pattern of cogongrass shoot emergence (r² = 0.95, n = 36). It also predicted shoot emergence satisfactorily in six treatments (r² > 0.85, P < 0.001, n = 7 in each treatment) that simulated farmers' practices in southwestern Nigeria. This is the first model developed for cogongrass shoot emergence based on hydrothermal time under field observations. The model should facilitate further analyses of cogongrass emergence patterns and the timing of its management.

Nomenclature: Paraquat; cogongrass, Imperata cylindrica (L.) Beauv. IMPCY; corn (maize), Zea mays L.

The effect of hydration (priming) treatment on dormancy release in annual ryegrass seeds from two populations was investigated. Hydration duration, number, and timing with respect to after-ripening were compared in an experiment involving 15 treatment regimens for 12 wk. Seeds were hydrated at 100% relative humidity for 0, 2, or 10 d at Weeks 1, 6, or 12 of after-ripening. Dormancy status was assessed after each hydration treatment by measuring seed germination at 12-hourly alternating 25/15 C (light/dark) periods using seeds directly from the hydration treatment and seeds subjected to 4 d postpriming desiccation. Seeds exposed to one or more hydration events during the 12 wk were less dormant than seeds that remained dry throughout after-ripening. The longer hydration of 10 d promoted greater dormancy loss than either a 2-d hydration or no hydration. For the seed lot that was most dormant at the start of the experiment, two or three rather than one hydration event or a hydration event earlier rather than later during after-ripening promoted greater dormancy release. These effects were not significant for the less-dormant seed lot. For both seed lots, the effect of a single hydration for 2 d at Week 1 or 6 of after-ripening was not manifested until the test at Week 12 of the experiment, suggesting that the hydration events alter the rate of dormancy release during subsequent after-ripening. A hydrothermal priming time model, usually used for modeling the effect of priming on germination rate of nondormant seeds, was successfully applied to dormancy release resulting from the hydration treatments.

Nomenclature: Annual ryegrass, Lolium rigidum Gaud.

Multiple weed species in the field combine to cause yield losses and can be described using one of several empirical models. Field studies were conducted to compare observed corn yield loss caused by common sunflower and shattercane populations with predicted yield losses modeled using a multiple species rectangular hyperbola model, an additive model, or the yield loss model in the decision support system, WeedSOFT, and to derive competitive indices for common sunflower and shattercane. Common sunflower and shattercane emerged with corn and selected densities established in field experiments at Scandia and Rossville, KS, between 2000 and 2002. The multiple species rectangular hyperbola model fit pooled data from three of five location–years with a predicted maximum corn yield loss of 60%. Initial slope parameter estimate for common sunflower was 49.2 and 4.2% for shattercane. A ratio of these estimates indicated that common sunflower was 11 times more competitive than shattercane. When common sunflower was assigned a competitive index (CI) value of 10, shattercane CI was 0.9. Predicted yield losses modeled for separate common sunflower or shattercane populations were additive when compared with observed yield losses caused by low-density mixed populations of common sunflower (0 to 0.5 plants m⁻²) and shattercane (0 to 4 plants m⁻²). However, a ratio of estimates of these models indicated that common sunflower was only four times as competitive as shattercane, with a CI of 2.5 for shattercane. The yield loss model in WeedSOFT underpredicted the same corn losses by 7.5%. Clearly, both the CI for shattercane and the yield loss model in WeedSOFT need to be reevaluated, and the multiple species rectangular hyperbola model is proposed.

Nomenclature: Common sunflower, Helianthus annuus L. HELAN; shattercane, Sorghum bicolor (L.) Moench SORVU; corn, Zea mays L. ‘Asgrow 770RR’, ‘Asgrow RX740RR’, ‘Garst 8349RR’, ‘Midland 7B05RR’.

The phylogenetic relationships within two major locoweed genera, Astragalus and Oxytropis, and among varieties of woolly loco found in New Mexico were analyzed by comparing their chloroplast rpoC1 and rpoC2 gene sequences. Nucleic acids from locoweed species and varieties collected from different geographical locations in New Mexico were amplified using specific primer sets and subjected to restriction fragment analyses. Identity of the amplicons was confirmed by determining the 5′-end sequences from pea and woolly loco var. matthewsii. The amplified sequences from all samples were digested with 16 different restriction enzymes. Presence or absence of individual restriction fragments was scored as binary characters and used to develop a similarity coefficient matrix for cladistic analyses to determine the phylogenetic relationships. The target sequence was conserved, yielding 7% polymorphic data. Oxytropis species were monophyletic and, as expected, formed a clade distinct from Astragalus. The average similarity coefficient among woolly loco varieties was very high (0.9733), but the varieties still separated into three different clades. The phylogenetic relationship among woolly loco varieties coincided with their geographic distribution but was unrelated to insect feeding preference.

Nomenclature: Woolly loco, Astragalus mollissimus Torr. ASAML; pea, Pisum sativum L. ‘Alaska’.

Field, laboratory, and greenhouse experiments were conducted to determine the seed production potential and effect of environmental factors on germination, emergence, and survival of texasweed. Texasweed produced an average of 893 seed per plant, and 90% were viable. Seed exhibited dormancy, and prechilling did not release dormancy. Percent germination ranged from 56% for seed subjected to no prechilling to 1% for seed prechilled at 5 C for 140 d. Seed remained viable during extended prechilling conditions, with 80% of seed viable after 140 d of prechilling. Texasweed seed germinated over a range of 20 to 40 C, with optimum germination (54%) occurring with a fluctuating 40/30 C temperature regime. Seed germinated with fluctuating 12-h light/dark and constant dark conditions. Texasweed seed germinated over a broad range of pH, osmotic potential, and salt concentrations. Seed germination was 31 to 62% over a pH range from 4 to 10. Germination of texasweed ranged from 9 to 56% as osmotic potential decreased from − 0.8 MPa to 0 (distilled water). Germination was greater than 52% at less than 40 mM NaCl concentrations and lowest (27%) at 160 mM NaCl. Texasweed seedlings emerged from soil depths as deep as 7.5 cm (7% emergence), but emergence was > 67% for seed placed on the soil surface or at a 1-cm depth. Texasweed seed did not germinate under saturated or flooded conditions, but seed survived flooding and germinated (23 to 25%) after flood removal. Texasweed seedlings 2.5 to 15 cm tall were not affected by emersion in 10-cm-deep flood for up to 14 d. These results suggest that texasweed seed is capable of germinating and surviving in a variety of climatic and edaphic conditions, and that flooding is not a viable management option for emerged plants of texasweed.

Nomenclature: Texasweed, Caperonia palustris (L.) St. Hil. CNPPA.

Jointed goatgrass is a troublesome weed in winter wheat with selective control only possible with a herbicide-resistant crop. Even with herbicide-resistant wheat, cultural control is still an important part of jointed goatgrass management. A study was conducted in 1998 and 2000 to determine whether using larger sized seed of a tall wheat variety at an increased seeding rate would reduce the effect of jointed goatgrass on winter wheat. Wheat seed size, seeding rate, and variety height had no effect on jointed goatgrass plant density. Tall (∼130 cm) wheat reduced mature jointed goatgrass biomass 46 and 16% compared with short (∼100 cm) wheat in years 1 and 2 of the experiment, respectively. Spikelet biomass was also reduced approximately 70 and 30% in the same respective years. One thousand–spikelet weight of jointed goatgrass was reduced 37 and 7% in years 1 and 2, respectively, when grown in competition with taller compared with shorter wheat. Moreover, dockage was 80 and 30% less in years 1 and 2, respectively, when grown in competition with taller than shorter wheat. Mature jointed goatgrass height was similar regardless of the competitive wheat height. However, jointed goatgrass was as much as 18% taller than the short wheat and 15% shorter than the tall wheat. Seeding rate had the most consistent effect on wheat yield. Wheat seed yield was about 10% greater with 60 than 40 seed m⁻¹ of row when competing with jointed goatgrass. Results of this study indicate that growers could use a tall winter wheat variety to improve crop competition against jointed goatgrass. Results also indicate that plant breeders should consider plant height because herbicide-resistant varieties are developed for the integrated management of jointed goatgrass.

Nomenclature: Jointed goatgrass, Aegilops cylindrica Host. AEGCY; winter wheat, Triticum aestivum L. ‘Nugaines’.

Common ragweed is a native annual that colonizes disturbed habitats including agricultural fields and roadsides. It is especially abundant along roadways receiving regular applications of deicing salt. Anecdotal evidence has suggested that the emergence of common ragweed seedlings often occurs before the emergence of other roadside species and at salinity concentrations as high as 400 mM L⁻¹, a level that can be found in roadside soils in early spring. However, the extent of this tolerance to salinity in common ragweed populations has not been quantified. The objective of this study was to assess the germination behavior of common ragweed seeds collected from three roadside and two agricultural populations across a salinity gradient. Seed germination of these five populations was monitored daily for 21 d across a sodium chloride gradient [0, 100, 200, 300, and 400 mM L⁻¹] under controlled conditions. Seeds from roadside populations showed consistently greater total germination and rate of germination than seeds from agricultural populations. Germination differences were most evident at the 300 and 400 mM L⁻¹ salinity concentrations. Average germination at the 400 mM L⁻¹ sodium chloride concentration was 31% for two roadside populations and only 3% for two agricultural populations. Germination of seeds placed in distilled water after the 21-d salinity exposure treatments (i.e., recovery rates) was also greater for the roadside vs. agricultural populations. Findings indicate that the germination behavior of common ragweed seeds to salinity for roadside populations may be locally adaptive and allows common ragweed to emerge relatively early in spring thus providing a competitive advantage over later emerging roadside plants.

Nomenclature: Common ragweed, Ambrosia artemisiifolia L., AMBEL.

Research during the past several decades on jointed goatgrass management has focused on individual cultural practices rather than on multi- or interdisciplinary components. Field studies were conducted at Hays, KS, from 1997 to 2003 to evaluate the interaction of crop rotation, fallow weed management, and winter wheat variety on jointed goatgrass density. Extending a wheat–fallow (W–F) rotation to include grain sorghum or grain sorghum and sunflower reduced jointed goatgrass populations more than other cultural practices tested. Fallow treatments were equal in most years, but mechanical fallow resulted in increased jointed goatgrass emergence compared with chemical fallow under drought conditions. Winter wheat cultivars had little effect on jointed goatgrass populations. However, taller, more competitive varieties are favorable for jointed goatgrass control in an integrated management program. No specific combination of crop rotation, fallow weed management, and wheat variety consistently reduced jointed goatgrass density more than other combinations during multiple years.

Nomenclature: Jointed goatgrass, Aegilops cylindrica Host AEGCY; grain sorghum, Sorghum bicolor (L.) Moench ‘DK-35’; sunflower, Helianthus annuus L. ‘Triumph 562’; winter wheat, Triticum aestivum L. ‘Ike’, ‘TAM 107’, ‘Jagger’, ‘Vista’.

Few studies have examined the combined effect of herbicide-induced stress and arthropod herbivory to reduce weed fitness. The purpose of this study was to quantify the effect of arthropod herbivory on the herbicide dose–response of a perennial weed. Fluroxypyr dose–response bioassays using volunteer potato were conducted in the presence and absence of Colorado potato beetle (CPB) herbivory. Logistic model parameter estimates for leaf area, shoot biomass, tuber number, and tuber biomass were often lower with herbivory, compared with no herbivory. Greater variance of parameter estimates within herbivory plots was attributed largely to differential feeding because CPB density was not manipulated in the field. Results from short-season field studies (1,000 growing degree days [GDD] after postemergence [POST] herbicide application) indicated that herbivory had the most effect on potato during a period that coincided with high CPB density and optimal temperatures for CPB development. Season-long bioassays (> 3,100 GDD after POST) revealed that addition of herbivory reduced herbicide use 65 to > 85%, compared with the dose needed to achieve the same reduction in tuber production in the absence of herbivory. Integrated weed management systems targeting volunteer potato are more effective when fluroxypyr applications are made before periods of high herbivory. Moreover, this article describes an experimental approach contributing to optimization of combined effects of arthropod herbivory and reduced herbicide doses.

Nomenclature: Fluroxypyr; volunteer potato, Solanum tuberosum L.

Research in irrigated and nonirrigated corn production systems was conducted to evaluate the effect of leaf architecture of corn hybrids on weed management. The corn hybrids used in each study were ‘Pioneer 3394’ (upright leaf) and ‘Pioneer 3260’ (horizontal leaf). In the irrigated study, residual weed control treatments included two rates of prepackaged mixtures of metolachlor plus atrazine, encapsulated acetochlor plus atrazine, nonencapsulated acetochlor plus atrazine, or a tank mixture of simazine and metolachlor plus atrazine. In the irrigated experiments, horizontal leaf architecture reduced weed density (all three sites in 1 of 2 yr), weed biomass (five of six sites), solar radiation reaching the ground (all six sites), and weed seed production (one site each year) compared with upright leaf architecture. Weed density and weed biomass did not differ between herbicide rates or acetochlor formulation at any site. Corn hybrid was significant for yield at only one site. Reduced weed biomass did not translate into yield differences. The nonirrigated study evaluated two factors at four sites over 2 yr: leaf architecture (upright or horizontal leaf) and weed management program (preemergence residual and postemergence no residual) at two application rates. Neither weed density nor weed biomass was reduced because of corn leaf architecture or herbicide rates in the nonirrigated study. No interaction was detected in either irrigated or nonirrigated studies between leaf architecture and herbicide treatments, indicating that these factors are independent of one another. On the basis of these studies, it appears that horizontal leaf architecture of corn hybrids can assist in integrated weed management in irrigated corn production but may not be beneficial when corn is grown under drought-prone conditions.

Nomenclature: Acetochlor; atrazine; metolachlor; simazine; corn, Zea mays L. ‘Pioneer 3260’, ‘Pioneer 3394’.

Variation in crop–weed interference relationships has been shown for a number of crop–weed mixtures and may have an important influence on weed management decision-making. Field experiments were conducted at seven locations over 2 yr to evaluate variation in common lambsquarters interference in field corn and whether a single set of model parameters could be used to estimate corn grain yield loss throughout the northcentral United States. Two coefficients (I and A) of a rectangular hyperbola were estimated for each data set using nonlinear regression analysis. The I coefficient represents corn yield loss as weed density approaches zero, and A represents maximum percent yield loss. Estimates of both coefficients varied between years at Wisconsin, and I varied between years at Michigan. When locations with similar sample variances were combined, estimates of both I and A varied. Common lambsquarters interference caused the greatest corn yield reduction in Michigan (100%) and had the least effect in Minnesota, Nebraska, and Indiana (0% yield loss). Variation in I and A parameters resulted in variation in estimates of a single-year economic threshold (0.32 to 4.17 plants m⁻¹ of row). Results of this study fail to support the use of a common yield loss–weed density function for all locations.

Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; corn, Zea mays L. ‘DK404SR’, ‘DK493SR’, ‘DK592SR’, ‘Asgrow RX602SR’.

Agronomic research on the effects of nitrogen fertilizer and weed control in corn has focused primarily on maintaining or increasing yield. Few studies have examined the effect of nitrogen (N) fertilizer rate or weed competition (or both) on whole plant growth and development. The objectives of this research were to determine how N influences the growth and development of corn and to explore how green foxtail density affects this relationship. Field experiments were conducted on a sandy low organic matter soil from 1999 to 2001. The experiment was designed as a factorial with N rate ranging from 0 to 200 kg N ha⁻¹ and targeted green foxtail density ranging from 0 to 300 plants m⁻². Under weed-free conditions, a higher rate of N fertilizer increased corn leaf and grain N content, leaf area index (LAI), plant height, and aboveground dry matter (DM) production, including kernel weight. However, in the presence of green foxtail, corn leaf N content, LAI, growth rate, plant height, and aboveground DM were reduced at each N level. Despite having significant main effects, there was no interaction between N rate and green foxtail density. Results indicate that in corn grown on a coarse-textured soil with low organic matter, the additional stress brought about by the presence of green foxtail exacerbated the effect of low N rates on corn growth and development. More intensive weed management may be required in corn if N fertilizer rates are reduced.

Nomenclature: Green foxtail, Setaria viridis (L.) Beauv. SETVI; corn, Zea mays L. ‘Pioneer 3905’.

This study indicates that induced disease resistance might be useful to control branched broomrape. Strains of the rhizosphere bacteria Pseudomonas spp. (Proradix®), salicylic acid derivates (Bion®), and extracts of the algae Ascophyllum nodosum L. (Goemar Fruton Spezial®) can decrease branched broomrape infection to 80%. Results suggest that agents working as elicitors for resistance in other plant–pathogen interactions by induced systemic resistance or by systemic acquired resistance also could reduce branched broomrape infestation. These findings suggest that activation of immune responses before infection of plants could be an innovative control method for parasitic weeds.

Nomenclature: Bion®, benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester; branched broomrape, Orobanche ramosa L. ORARA.