Henbit is increasing in abundance in western Canada, and control recommendations are largely limited to herbicides. Increased knowledge of henbit biology may allow the development of more integrated control programs. A controlled environment study was conducted to determine the combined effect of various soil temperature and soil water levels on the emergence of henbit. Henbit emerged at soil temperatures ranging from 5 to 25 C, but the highest emergence of 81 to 83% occurred at 15 to 20 C. Henbit emergence declined as soil water content decreased. The interaction of cool and dry soils caused the greatest inhibition of henbit emergence. At progressively lower soil water levels of −0.03, −0.28, −0.53, −0.78, −1.03, and −1.53 MPa, henbit emergence was 78, 61, 64, 40, 38, and 11% at 10 C, respectively. Rate of henbit emergence was affected less by soil water than by soil temperature. A decrease in soil water content from −0.03 to −1.53 MPa increased the time to reach 50% emergence (ET50) by 1 to 5 d, whereas a decrease in temperature from 25 to 5 C increased the time to reach ET50 by 13 to 16 d. The implications of these results in terms of improved management of henbit are discussed.

Wild oat is the most important weed in Argentine barley and one of the worst weeds in wheat and barley worldwide. During 1997 and 1998, field experiments were performed to determine the response of wild oat to N fertilizer at planting and at early tillering at 55 kg ha−1 with and without the use of diclofop-methyl. Seedling emergence and survival rate, individual fecundity, and preharvest seed dispersal rate of wild oat were assessed. The effect on the individual growth of wild oat was also studied. N fertilizer increased both wild oat seedling survival rate and fecundity, especially when it was applied at early tillering. However, the effect of N fertilization was negligible with diclofop-methyl treatment. In addition, the application of N fertilizer reduced the seed dispersal rate immediately before crop harvest. Quantification of the effects of each agronomic practice, such as N fertilization, is useful to develop simulation models that predict the population dynamics. When N was applied at sowing, the annual growth rate of the wild oat population was almost 25% lower than when applied at early tillering.

The critical period for weed control (CPWC) is a period in the crop growth cycle during which weeds must be controlled to prevent yield losses. Knowing the CPWC is useful in making decisions on the need for and timing of weed control and in achieving efficient herbicide use from both biological and economic perspectives. An increase in the use of herbicide-tolerant crops, especially soybean resistant to glyphosate, has stimulated interest in the concept of CPWC. Recently, several studies examined this concept in glyphosate-resistant corn and soybean across the midwestern United States. However, these studies presented various methods for data analysis and reported CPWC on the basis of a variety of crop- or weed-related parameters. The objectives of this study are (1) to provide a review of the concept and studies of the CPWC, (2) to suggest a common method to standardize the process of data analysis, and (3) to invite additional discussions for further debate on the subject. Wide adoption of the suggested method of data analysis will allow easier comparison of the results among sites and between researchers.

Wild radish is a prevalent annual weed throughout the cropping regions of southern Australia. Field experiments were conducted at Wagga Wagga, New South Wales, in 1998 and 1999 to determine the effect of various densities and emergence times of wild radish on yield and quality of canola and on wild radish seed production. As few as 4 wild radish m−2 emerging with canola reduced canola yield 9 to 11%, whereas 64 wild radish m−2 reduced canola yield 77 to 91%. Wild radish interference in canola was greatly affected by its time of emergence relative to canola. At 64 wild radish m−2, canola yield was reduced 77, 54, 33, and 19% in 1998 and 91, 65, 56, and 19% in 1999 when wild radish emerged 0, 2, 4, and 7 wk after canola, respectively. Wild radish that emerged 10 wk after canola did not reduce canola yield. Maximum wild radish seed production ranged from 24,183 to 32,167 seed m−2 when they emerged with canola at high densities. Wild radish that emerged later than canola produced much less seed, but some seed production still occurred in one of the 2 yr when it emerged as late as 10 wk after canola. Wild radish did not directly reduce canola quality in either year, but if wild radish seed were not separated from canola seed, the amount of erucic acid and glucosinolates was increased above marketable levels in some cases. The results of this study will be used to advise growers on wild radish control in canola and will aid the development of a multiyear management strategy for this troublesome weed in annual cropping systems.

Annual bluegrass control was reduced following 7 yr of continuous fall application of dinitroaniline (DNA) herbicides. Annual bluegrass control was < 40% on two fairways in year eight following prodiamine applied at 1.1 kg ai ha−1. In dose–response studies conducted in growth chambers, this annual bluegrass population exhibited 105-fold resistance in shoot growth to prodiamine compared with a known susceptible population. A 6.4-fold resistance to prodiamine was found when comparing annual bluegrass root growth to the known susceptible biotype. Spring-applied oxadiazon did not affect shoot or root growth between annual bluegrass biotypes. Equivalent levels of control were attained with pronamide. The presence of DNA-resistant annual bluegrass, in addition to previously confirmed triazine-resistant biotypes on North Carolina golf courses, indicates a need for resistance management strategies to be integrated into golf turf management practices.

In citrus, weedy grasses compete for moisture, nutrients, and light and can inhibit the growth of young trees and delay fruit production. These weeds are difficult to control, either because of their tolerance to available herbicides or due to growth habits that enable them to resist other control practices. Control of seven such weedy grasses (southern sandbur, large crabgrass, crowfootgrass, guineagrass, Texas panicum, johnsongrass, and yellow foxtail) with a mixture of three fungal pathogens, termed the multiple-pathogen strategy, was field tested in 1996 and 1998. Three fungi indigenous to Florida, Drechslera gigantea, Exserohilum longirostratum, and E. rostratum, isolated from large crabgrass, crowfootgrass, and johnsongrass respectively, were used. Two separate field studies were conducted: one study with seven grasses transplanted and grown within each plot (grass mixture field trial) and another study on a population of guineagrass alone present in a naturally infested field (guineagrass field trial). The objectives of this study were to (1) evaluate the field performance of D. gigantea, E. longirostratum, and E. rostratum individually and in a mixture to control the seven transplanted weedy grasses (grass mixture) and a population of guineagrass in a naturally infested field, respectively, and (2) compare the effectiveness of three carriers (water, Metamucil®, and an invert emulsion) on the bioherbicidal efficacy under field conditions. The fungi were applied as foliar sprays, each pathogen alone or in a mixture of the three fungi (1:1:1, v/v/v, for a total of 5 × 105 spores ml –1) in water, 0.5% aqueous Metamucil®, or an emulsion containing Sunspray® 6E. During the 14-wk experimental period, one or two additional sprays of all treatments were applied. Disease severity was recorded weekly for 4 to 6 wk after the initial spray (WAI). Maximum disease severities were obtained in emulsion-inoculum treatments, and were higher than those in the water-inoculum and the Metamucil-inoculum treatments. The pathogen mixture was equally effective as the individual pathogens in controlling the weeds tested. In the 1996 trial, 6 WAI, disease severity on grasses inoculated with D. gigantea spore suspensions in emulsion ranged from 78 to 100%, with E. longirostratum 90 to 100%, E. rostratum 79 to 100%, and the mixture 74 to 100%. In the 1998 trial, 4 WAI, disease severity on grasses inoculated with D. gigantea spore suspensions in emulsion ranged from 45 to 98%, with E. longirostratum 45 to 98%, E. rostratum 34 to 98%, and the mixture 32 to 98.5%. Thus, it was possible to manage all seven weedy grasses under field conditions using an emulsion-based inoculum preparation with the individual pathogens as well as the mixture of pathogens. The same three fungal pathogens were field tested for their ability to manage populations of guineagrass in a naturally infested field. The experimental design and treatments were identical to the field testing with the seven transplanted grasses. Two applications of an emulsion-based inoculum preparation of each pathogen or the mixture of pathogens effectively controlled guineagrass for up to 10 wk, with no regrowth.

Control of the annual weed, common groundsel, may be troublesome because of insufficient control by herbicides. Biological control based on stimulating epidemics of the rust fungus Puccinia lagenophorae on common groundsel populations may be an alternative to herbicides if the rust fungus can be integrated with the general use of herbicides against other weeds. Formulations of monolinuron, metoxuron, and pendimethalin were selected for the study. The integration of the rust fungus with each of the three herbicides was evaluated. A three-step procedure was followed to evaluate the integration of the rust fungus and these formulated herbicides. The effect of the selected herbicides on common groundsel was tested in the first step. Only formulated monolinuron completely controlled common groundsel under controlled conditions, indicating that biological control is not required in situations where monolinuron is employed for weed control. The effect of metoxuron and pendimethalin on the rust fungus was tested under controlled conditions in the second step. Formulated metoxuron was not compatible with the rust fungus, indicating that biological control cannot be employed in situations where metoxuron is applied. The effect of pendimethalin on P. lagenophorae epidemics was evaluated using an epidemiological model in the third step. Formulated pendimethalin had no detectable effects on P. lagenophorae epidemics. We concluded that use of P. lagenophorae epidemics for common groundsel control is complementary to application of formulated pendimethalin against other weeds.

Genetic diversity of kochia was determined using 45 polymorphic intersimple sequence repeat markers. Comparisons of genetic diversity within and among populations were made after structuring 453 kochia plants into samples representing 13 populations on the basis of the collection site and into two sample groups on the basis of the cropping history and geographic region of the sites. Mean Nei's gene diversity (h) for the 13 populations was 0.35, and total diversity (H T) was 0.35, indicating that genetic diversity in kochia is very high. There was a greater proportion of diversity within (H S = 0.31) populations than among (G ST = 0.09) populations. A hierarchical analysis of molecular variance showed that 90% of the variation occurred within populations (P = 0.0001). Of the remaining variance, 4.5% was attributed to differences between the two groups (P = 0.0001) and 5.5% to differences among the 13 populations within the groups (P = 0.0001). Gene flow (Nm) among populations was 2.4. The high level of gene diversity and large proportion of within-population diversity in kochia suggest that, despite generations of herbicide selection, kochia maintains high genetic diversity through substantial levels of gene flow within and among populations.

Studies were conducted to determine the effect of interference between Pennsylvania smartweed and cotton on plant growth and productivity. Pennsylvania smartweed remained shorter than cotton until at least 80 d after cotton planting. However, Pennsylvania smartweed produced considerable dry biomass by cotton harvest. Pennsylvania smartweed biomass per plant was not affected by weed density when grown with cotton. When grown alone, Pennsylvania smartweed produced 1,640 and 2,060 g dry biomass plant−1 depending on the year. This biomass was over four times greater than the biomass produced by plants grown with cotton. Cotton lint yield decreased between 1.3 and 1.1 kg ha−1 with each gram increase in weed dry biomass per meter of row. The relationship between Pennsylvania smartweed density and cotton percent yield loss was described by the hyperbolic function. The estimated coefficients a (maximum yield loss as density approaches infinity) and i (yield loss per unit density as density approaches zero) were 102 ± 23 and 51 ± 12, respectively, in 1998 and 53 ± 1 and 98 ± 5, respectively, in 2000. Pennsylvania smartweed achene production was also described by the hyperbolic function. Estimated achene production at 1 plant m−1 cotton row was 18,000 and 26,000 achenes m−2 in 1998 and 2000, respectively.

Gene flow via pollen and inheritance of acetyl-coenzyme A carboxylase (ACCase) inhibitor resistance in giant foxtail have not been previously characterized. Therefore, experiments were conducted to quantify outcrossing rates and flowering periods of giant foxtail accessions resistant or susceptible to ACCase inhibitors and to determine the inheritance pattern of gene(s) that confer resistance. In greenhouse experiments, outcrossing rates between resistant and susceptible parents ranged from 0.24 to 0.73%, as gauged by the response of F1 plants to fluazifop-P. Resistant plants and susceptible plants flowered at similar times, with peak flowering occurring 63 to 67 d after planting. During this period, flower initiation was nearly twice as great for resistant plants as for susceptible plants. In inheritance experiments F2 plants segregated in a 1:2:1 ratio of resistant–intermediate–susceptible phenotypes after exposure to fluazifop-P, indicating that resistance was associated with a single nuclear, incompletely dominant allele. The very low rates of outcrossing among giant foxtail plants in the greenhouse indicate that gene flow of resistance traits in the field is likely to occur primarily by seed immigration rather than by pollen movement. However, gene flow of herbicide resistance traits via pollen provides a mechanism in which multiple resistance may develop among giant foxtail populations that are resistant to other classes of herbicides.

Field studies were conducted at four locations in North Carolina in 1998 and 1999 to evaluate a computer program, Herbicide Application Decision Support System (HADSS™), for weed management in peanut (Arachis hypogaea). Weed management systems included metolachlor or ethalfluralin preplant-incorporated (PPI) used alone or in combination with diclosulam preemergence (PRE) or flumioxazin PRE. These herbicide combinations were used alone, followed by (fb) postemergence (POST) herbicides recommended by HADSS™ or fb a standard POST program of paraquat plus bentazon early postemergence (EPOST) fb acifluorfen plus bentazon POST. The standard POST herbicide system and HADSS™ POST recommendations were also used without soil-applied herbicides. Ethalfluralin PPI alone controlled large crabgrass (Digitaria sanguinalis) better than metolachlor PPI. Combinations of metolachlor or ethalfluralin PPI with either diclosulam or flumioxazin PRE provided equivalent control of all weeds evaluated except yellow nutsedge (Cyperus esculentus). The addition of diclosulam or flumioxazin PRE to systems containing metolachlor or ethalfluralin PPI always improved control of ivyleaf morningglory (Ipomoea hederacea) and yellow nutsedge and improved yield and net returns in 15 of 16 comparisons where no POST herbicides were used. For systems that used diclosulam or flumioxazin PRE, the HADSS™ POST and standard POST herbicide systems improved yield in 4 of 12 and 2 of 12 comparisons, respectively, compared with similar systems that did not use diclosulam or flumioxazin. However, in systems using either HADSS™ POST or the standard POST system, yield was always improved when compared with metolachlor or ethalfluralin PPI alone. HADSS™ POST provided equal or higher weed control, peanut yield, and net returns when compared with the standard POST herbicide system.

Common lambsquarters is an annual weed of many important crops. Ascochyta caulina is a plant pathogenic fungus that causes necrotic lesions on the leaves and stems of common lambsquarters. The objective of the present study was to estimate the effect of plant N supply on the biocontrol activity of A. caulina isolates against common lambsquarters. In greenhouse experiments replicated groups of common lambsquarters plants raised with different N supplies were sprayed with various isolates and concentrations of A. caulina 3 wk after planting. Height, number of leaves, total leaf area, fresh and dry weight, and tissue N concentration of common lambsquarters 4 wk after emergence increased significantly with increasing N supply. Disease development was positively related to increasing plant tissue N and also to increasing spore concentration. Fungal spore concentration also had a positive effect on the plant tissue N percentage. Ascochyta caulina isolate W90-1 caused a greater dry weight reduction in common lambsquarters than isolates I-001 and NW-6 did.

A 9-yr (1990–1998) study was conducted at Woodstock, ON, Canada, to evaluate weed densities, crop yields, and gross returns in a modified no-tillage (no primary tillage) corn–soybean–winter wheat rotation under three weed management treatments: (1) minimum, preplant application of glyphosate followed by mechanical control; (2) integrated weed management (IWM), preplant application of glyphosate followed by band application of preemergence herbicides plus mechanical control; and (3) conventional, preplant application of glyphosate followed by broadcast application of preemergence herbicides in corn and soybean. In wheat the minimum and IWM treatments had no additional weed control measures other than the preplant application of glyphosate, whereas the conventional treatment had a broadcast application of a postemergence herbicide. Weed densities were assessed each year, (except in 1990) once during the growing season in corn and soybean and immediately after crop harvest in wheat. Adjusted gross return was calculated as the gross revenue minus the unique costs for weed control for each of the treatments. Weed densities were greater in the minimum treatment compared with the IWM or conventional treatment in all crops. Weed densities in the IWM and conventional treatments did not differ. There was no apparent “buildup” of weed density with time in the rotation resulting from weed escapes. Hence, these data challenge current thinking that weed densities increase with time if weed escapes are allowed to go to seed. Corn and soybean yields in the IWM and conventional treatments did not differ. However, the minimum treatment had the lowest corn and soybean yields. Winter wheat yield was not affected by the treatments. All weed management treatments provided similar gross returns for each crop and for the rotation. Thus, the minimum treatment consisting of glyphosate applied preplant followed by shallow interrow tillage appeared to be a viable option, especially if practiced in a farming system capable of ensuring adequate timing of cultivation operations.

Dormancy is a state of shifted physiological activities with cessation of growth. It occurs in seeds and vegetative propagules and enables plants to survive in adverse growing conditions. Traditional studies on dormancy-related problems have mostly focused on hormone changes along with environmental factors that have achieved great insight on these processes at the physiological level. The molecular nature and cellular basis of signals that carry out the processes of dormancy or dormancy breaking are largely unknown. Recent advances in plant genetics and genomics have provided assorted ways to investigate questions concerning dormancy. Various approaches such as developing genetic maps with DNA-based markers, e.g., amplified fragment length polymorphism (AFLP) and restriction fragment length polymorphism (RFLP), analyzing mutant lines, conducting quantitative trait loci (QTL) analysis, two-dimensional polyacrylamide gel electrophoresis, differential display, microarray, have been performed to resolve different issues related to dormancy. The phenotypic variation in dormant seeds or buds is continuous instead of discrete, and thus QTL analysis is desirable to identify the association between genetically determined phenotypes and specific genetic markers (RFLPs). Some aspects of QTL will be introduced. DNA microarray is a recently developed technology that is used to detect and quantitate large numbers of differences in gene expression simultaneously. We have used the DNA microarray technology to study underground bud dormancy and growth in leafy spurge (Euphorbia esula L.). The principle and versatility of DNA microarray will be introduced, and the strategy for applying this technology will be discussed.

The use of winter cover crops is beneficial to agriculture. We have tried to design a cover crop that self-destructs in response to an environmental cue, thereby eliminating the use of herbicides and tillage to remove the cover crop in late spring. Here, this novel concept is tested in a model system. The onset of summer brings with it elevated temperatures. Using this as the environmental cue, a self-destruction cassette was designed and tested in tobacco. A heat-shock–responsive promoter was used to direct expression of the ribonuclease Barnase. Because Barnase is extremely toxic to cells, it was necessary to coexpress its inhibitor, Barstar, whose expression was under the control of the CaMV 35S promoter. The wild-type and two mutated Barnase genes, one missense and one translation attenuated, were tested. Our results indicated that the translation-attenuated version of the Barnase gene was most effective in causing heat-shock–regulated plant death. Analysis of the T2 progeny of a transgenic plant carrying this Barnase mutant showed that the Barnase gene expression was sixfold higher in heat-shock–treated plants compared with untreated plants. This level of Barnase gene expression was sufficient to kill transgenic plants.

Field studies were conducted at two North Carolina locations to determine the effect of interference between pale smartweed and cotton on plant growth and productivity. Pale smartweed remained shorter than cotton until at least 70 d after cotton planting. However, pale smartweed grew over twice as tall as cotton and produced considerable dry biomass by cotton harvest. Pale smartweed biomass per plant was not affected by weed density up to 3.5 plants m−1 of row when grown with cotton. Cotton competition reduced pale smartweed dry biomass per plant at least 400%. The relationship between pale smartweed and cotton percent yield loss was described by the rectangular hyperbola model with the asymptote (coefficient a) constrained to 100% maximum yield loss. The estimated coefficient i (yield loss per unit density as density approaches zero) was 29 ± 4 and 23 ± 4 in 1998 and 2000, respectively. Pale smartweed achene production was also described by the hyperbolic function. Estimated achene production of smartweed at 1 plant m−1 cotton row was 63,000 and 25,000 achenes m−2 in 1998 and 2000, respectively.

Crop rotations, particularly those that include legumes, often result in improved soil quality and crop yields. A study was conducted to confirm the presence and persistence of the residual effects of crop rotation and weed management on a test crop and weeds in three tillage systems (moldboard plow [MP]; chisel plow [CP]; no tillage [NT]). Rotation (spring barley monoculture and spring barley–red clover rotation) and weed management (intensive, moderate, minimum) treatments, initiated in 1987, were terminated, and a test crop of spring wheat was grown in 1995 and 1996. Tillage treatments were maintained throughout. Multivariate analysis showed that weed communities were more affected by treatment termination in the rotation NT treatment with minimum weed management than in all other treatments. The former treatment was dominated by perennial broadleaf weeds but sustained adequate wheat yields (3.3 Mg ha−1) compared with the monoculture (1.0 Mg ha−1) one year after termination. Weed communities in CP and MP plots were less affected by treatment termination. Yet, changes in herbicide use at termination caused the virtual elimination of quackgrass from tilled plots and allowed field pennycress to become ubiquitous across treatments. Residual effects from crop rotation were more important than those from weed management in increasing wheat yields in tilled systems. Differences in wheat yield in NT systems 2 yr after treatment termination were attributed to residual effects from previous weed management rather than from crop rotation. Beneficial effects of crop rotation and weed management may persist for 2 yr but will vary according to tillage system.

The environmental effect of weed control systems has stimulated research into new practices for weed control, such as selective herbicide application methods on weed-infested crop areas. This research used the color co-occurrence method (CCM) texture analysis to determine the effects of plant maturity on the accuracy of weed species classification of digitized images. Two different experimental combinations of weed species and maturity level were examined. The weed species evaluated were ivyleaf morningglory, giant foxtail, large crabgrass, and velvetleaf, with soil image sets added to each experiment. One study examined classification accuracies for two weed species at three maturity levels, and the second study examined four weed species at two maturity levels. For each species-maturity level combination, 40 digital images were collected from a manually seeded outdoor plant bed. Digitized images were transformed from the red–green–blue (RGB) color format into hue–saturation–intensity (HSI) format to generate CCM texture feature data. Stepwise variable reduction procedures were used to select texture variables with the greatest discriminant capacity. Then discriminant analysis was used to determine the classification accuracy for the two different experiments. When using HSI texture statistics, discriminant analysis correctly classified weed species within and across maturity levels with an accuracy above 97% for both experimental groups. These image processing algorithms demonstrate potential use for weed scouting, weed infestation mapping, and weed control applications using site-specific farming technology.

Knowledge of how reduction in the rate of herbicide application or rotation of their mode of action influences weed growth will provide insight into how successful these practices will be in an integrated weed management program. Field experiments were conducted in 1996 and 1997 to quantify velvetleaf growth response to three postemergence herbicides, each with a different mode of action. A monoculture of velvetleaf was treated with halosulfuron, dicamba, and flumiclorac at 0, 0.10, 0.25, 0.50, 0.75, and 1.0 × the labeled rate for weed control in corn. Percent plant mortality increased with rate of application; the greatest mortality occurred in flumiclorac treatments in 1996 and in halosulfuron and flumiclorac treatments in 1997. Growth rate temporarily decreased as application rate increased. Maximum height decreased as rate of application increased, with the dicamba treatment resulting in the greatest (27%) reduction. Early-season leaf area index decreased with increasing rate of application, the greatest reduction occurring with halosulfuron (1997) and flumiclorac (1996 and 1997) treatments. The number of leaves produced per plant was temporarily reduced by all treatments, but treatment with dicamba later resulted in larger numbers of small leaves. The number of velvetleaf seed capsules produced per surviving plant was not reduced by any treatment, but the number of capsules per square meter was reduced by the 0.5 × rate of flumiclorac (1996) and the 0.5- and 1.0 × rates of halosulfuron (1997). Research is needed to evaluate whether the temporary suspension of velvetleaf growth after herbicide treatment is sufficient to prohibit crop yield reduction and velvetleaf capsule production.