Foliar absorption of carboxyl-labeled 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) by mesquite [Prosopis juliflora var. glandulosa (Torr.) Cockerel] seedlings was continuous throughout a 72-hr period. Approximately 50% of the 2,4,5-T applied to a single leaf was absorbed when 5 to 100 μg were applied. No significant differences were found in the amounts of 2,4,5-T absorbed at 70 and 85 F, but an increase occurred at 100 F after 72 hr. Only slight differences in absorption were found at different humidity levels. Translocation was primarily basipetal from the point of application at 70 F, both acropetal and basipetal at 85 F, and only a short distance acropetal at 100 F. The quantities of 2,4,5-T translocated into untreated tissues at 100 F were less than at 70 and 85 F. The highest concentrations of 2,4,5-T were found in tissues with highest soluble sugar concentrations. From 3 to 27% of the 2,4,5-T absorbed by mesquite leaves was subsequently detected in untreated stem, leaf, and root tissues.

Approximately 80% of the 2,4,5-T absorbed by mesquite leaves was metabolized after 24 hr. Moving the seedlings to a different environment at the time of treatment increased the rate of 2,4,5-T metabolism. Metabolism of 2,4,5-T was completely inhibited at 50 F and a lower rate of metabolism was noted at 100 than at 70 and 85 F. No important differences in metabolism were found at different humidity levels.

The dynamics of the host–parasite relationship between tomato cv. Brigade and Egyptian broomrape is temperature-related. This relationship was utilized for the development of an equation on the basis of thermal time (as measured by growing degree days, GDD, C) to predict the parasitism dynamics of Egyptian broomrape in tomato. To obtain a reliable prediction from thermal time values, studies based on a wide range of temperatures are essential. Four temperature-regime treatments and five levels of infestation with Egyptian broomrape seeds were tested in a multiclimate greenhouse (phytotron) and a temperature-controlled greenhouse, respectively. The day/night temperature regimes were 20/12 C, 23/15 C, 26/18 C, and 29/21 C and the infestation levels were 0 (noninfested control), 1, 5, 10, and 25 mg of Egyptian broomrape seeds per liter of soil. As expected, increasing temperature or infestation levels resulted in faster appearance and higher rate of attachments, respectively. The relation between development of attachments and GDD was described as a three-parameter logistic curve. In both temperature-regime and infestation-level experiments, the development of attachments began 200 GDD after planting and the maximal number of attachments was recorded 800 GDD after planting. A significant reduction in the aboveground biomass of the tomato plants due to increased Egyptian broomrape biomass was recorded only for the 26/18 C and 29/21 C day/night treatments and the three highest infestation levels (5, 10, and 25 mg L−1 soil). The ability to predict the start of parasitism can be used to develop a climate-based system for Egyptian broomrape control with herbicides.

The allelochemicals 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) and 6-methoxy-benzoxazolin-2-one (MBOA) in wheat are considered to have a role in plant defense against weeds. This study explored the effect of proximity to two weeds, wild oat and flixweed, on DIMBOA/MBOA production in wheat seedlings under hydroponic culture to identify whether the breeding of modern wheat varieties with higher concentrations of these compounds could ensure plant-mediated weed control. MBOA was detected and was noted to exert a significant response; its exudation by some wheat seedlings was significantly increased irrespective of whether the roots were in contact with or separate from those of the weeds. The weeds were a source of biotic stress to wheat when grown in proximity to it, and the stress resulted in production of higher levels of MBOA in wheat seedlings, although the concentration varied with the wheat cultivar. Therefore, the synthesis and exudation of DIMBOA/MBOA in wheat seedlings appears to be an active metabolic process influenced by the environment, particularly the presence of weeds.

Greenhouse studies were initiated to determine the efficacy of four herbicide antidotes as protectants of corn (Zea mays L., ‘DeKalb XL72AA’ and ‘DeKalb XL67′) against injury from preemergence or early postemergence applications of the herbicides chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide}, fluazifop-butyl {butyl-2-[4-[5-(trifluoromethyl-2-pyridinyl)oxy] phenoxy] propanoate}, or BAS 9052 OH {2-[1-(ethoxyimino)-butyl]-5-[2-(ethylthio)-propyl]-3-hydroxy-2-cyclohexene-1-one}. NA (1,8-naphthalic anhydride) was the most effective of the four antidotes in protecting corn against injury from preemergence applications of chlorsulfuron. CGA-43089 [α-(cyanomethoxy)-imino-benzeneacetonitrile] and CGA-92194 [α-(1,3-dioxolan-2-yl-methoxy)-imino-benzeneacetonitrile] partially protected corn from chlorsulfuron injury, whereas R-25788 (N,N-diallyl-2,2-dichloroacetamide] was the least effective. R-25788 offered partial to good protection to corn against injury from preemergence applications of fluazifop-butyl and BAS 9052 OH. NA, or CGA-43089, and CGA-92194 partially protected corn from fluazifop-butyl injury but their efficacy against BAS 9052 OH injury to corn was limited. In general, all four antidotes offered better protection to the thiocarbamate-tolerant ‘XL72AA’ hybrid than to the thiocarbamate-susceptible ‘XL67’ hybrid. With the exception of NA, which partially protected corn from postemergence-applied chlorsulfuron, none of the four herbicide antidotes was effective in protecting corn against injury from postemergence applications of any of the three herbicides.

As a weed, wheat has recently gained greater profile. Determining wheat persistence in cropping systems will facilitate the development of effective volunteer wheat management strategies. In October of 2000, glyphosate-resistant (GR) spring wheat seeds were scattered on plots at eight western Canada sites. From 2001 to 2003, the plots were seeded to a canola–barley–field-pea rotation or a fallow–barley–fallow rotation, with five seeding systems involving seeding dates and soil disturbance levels, and monitored for wheat plant density. Herbicides and tillage (in fallow systems) were used to ensure that no wheat plants produced seed. Seeding systems with greater levels of soil disturbance usually had greater wheat densities. Volunteer wheat densities at 2 (2002) and 3 (2003) yr after seed dispersal were close to zero but still detectable at most locations. At the end of 2003, viable wheat seeds were not detected in the soil seed bank at any location. The majority of wheat seedlings were recruited in the year following seed dispersal (2001) at the in-crop, prespray (PRES) interval. At the PRES interval in 2001, across all locations and treatments, wheat density averaged 2.6 plants m−2. At the preplanting interval (PREP), overall wheat density averaged only 0.2 plants m−2. By restricting density data to include only continuous cropping, low-disturbance direct-seeding (LDS) systems, the latter mean dropped below 0.1 plants m−2. Only at one site were preplanting GR wheat densities sufficient (4.2 plants m−2) to justify a preseeding herbicide treatment in addition to glyphosate in LDS systems. Overall volunteer wheat recruitment at all spring and summer intervals in the continuous cropping rotation in 2001 was 1.7% (3.3 plants m−2). Despite the fact that volunteer wheat has become more common in the central and northern Great Plains, there is little evidence from this study to suggest that its persistence will be a major agronomic problem.

In field studies combinations of herbicides applied preplant, preemergence, or postemergence were compared for control of prickly sida (Sida spinosa L.) in cotton (Gossypium hirsutum L. ‘Stoneville 7A’). All plots received an application of α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) incorporated into the soil prior to planting. Additional preemergence applications increased control of prickly sida 94% with 1,1-dimethyl-3-α,α,α-trifluoro-m-tolyl)urea (fluometuron) and 53% with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron). Seven postemergence treatments evaluated were at least partially effective in controlling prickly sida. However, trifluralin applied preplant followed by fluometuron applied preemergence gave the most effective prickly sida control in combination with all post-emergence treatments. Prickly sida populations of 130 plants per square meter competed with cotton all season and reduced yield 41 and 37% in 1969 and 1970, respectively. Prickly sida did not appear to affect cotton grade or staple length.

A 3-yr study was initiated in 1982 to determine the effects of herbicides and crop rotations on large crabgrass [Digitaria sanguinalis (L.) Scop. # DIGSA] and broadleaf signalgrass [Brachiaria platyphylla (Griseb.) Nash # BRAPP] population dynamics. Regardless of the crop rotation sequence, broadleaf signalgrass immediately became the predominant weed where standard herbicide programs were used. Large crabgrass became the predominant species after two growing seasons if no herbicides were applied. Domination by large crabgrass appeared to be due to greater seed production. The domination by broadleaf signalgrass in plots treated with herbicides was attributed to its tolerance to the primary grass herbicide alachlor [2-chloro-N-(2,6-diethylphenyl)-N-methoxymethyl)acetamide]. Broadleaf signalgrass emergence from soil treated with 2.2 kg ai/ha was not statistically different from that in untreated soil, while large crabgrass and fall panicum [Panicum dichotomiflorum (L.) Michx. # PANDI] emergence was significantly reduced at the same rate.

Spotted knapweed is an invasive mycorrhizal weed prevalent in the Pacific Northwest of the United States. Little is known about the effects of spotted knapweed or its management methods on soil quality and soil structure. This study compared soils from spotted knapweed–infested areas with areas where spotted knapweed is being managed using several herbicides and mechanical treatments. We measured concentrations of glomalin, a glycoprotein produced by arbuscular mycorrhizal fungi (AMF), that is correlated with soil aggregate stability, AMF hyphal length, and percent water-stable aggregates (WSA) in soils from managed and unmanaged areas. Areas with high knapweed density (unmanaged areas) generally had lower glomalin concentrations and AMF hyphal lengths compared with areas receiving chemical and combined mechanical–chemical management treatments. Total glomalin was significantly negatively correlated with percent knapweed cover. However, WSA was high (70 to 80%) in soils from all management treatments and was not affected by knapweed cover. Our results suggest that spotted knapweed does not have negative effects on soil quality from our study site, likely because of the high aggregate stability of the soils in the area. However, Centaurea maculosa may have negative effects on soil quality in soils with lower aggregate stability.

The response of pepper varieties to Egyptian broomrape and nodding broomrape was characterized in comparison with tomatoes grown in pots and in aqueous solution, according to two criteria: 1. germination of the parasite's seeds in the presence of the host roots; and 2. the number of parasites per host plant. More than 50% of Egyptian broomrape seeds germinated in the presence of all pepper variety roots except the sweet pepper Maor. However, only a few attached and developed. The two paprika varieties Lehava and Shani were hosts to more parasites than the sweet pepper varieties, and showed as many as 15 parasites per host plant. In contrast, tomato roots induced less then 10% seed germination but were highly susceptible to the parasite (30 parasites per host plant). Damage caused to pepper foliage and fruit was not significant, compared to the heavy damage caused to tomato. Pepper roots stimulated germination of 22 to 26% of nodding broomrape seeds but without the formation of parasitic attachments, whereas tomato roots stimulated germination of less than 10% of the seeds but the plants were then heavily damaged by the parasite. Interplanting of tomato with pepper in nutrient solution did not change the amount of Egyptian broomrape parasites on pepper, hut a fourfold increase in the number of nodding broomrape was observed on tomato roots, compared to the number of parasites on tomato roots when planted without pepper.

Soybean yield loss from weed interference depends upon weed density and competitiveness of crop and weed species in response to environment. Soil water availability and nitrogen fertility were evaluated for their effect on competitiveness of individual species in field experiments. Early-season temperatures in 1995, which were cool compared to 1994, slowed hemp sesbania growth without affecting soybean growth. This resulted in negligible competition with soybean by hemp sesbania at densities of 3 or 6 plants m−2. In 1994, hemp sesbania grew above the soybean canopy, decreasing soybean light interception 29 to 68%, and reducing soybean yield 30 to 48%. Fertilizer nitrogen increased soybean competitiveness, as indicated by biomass production, only in irrigated plots with hemp sesbania at 3 m−2, but did not affect soybean yield. Apparently, competition for light is a primary cause of soybean yield loss from hemp sesbania infestations. In growth chamber studies, simulating temperatures from the field, hemp sesbania growth was stimulated more by warm temperatures than was soybean. Hemp sesbania and soybean dry weights increased 4.4- and 2.7-fold, respectively, at 30/20 C day/night temperatures compared to 25/15 C.

Isolated plant cells from leaf tissue of soybean [Glycine max (L.) Merr. ‘Harosoy’] and wild onion (Allium canadense L.) were used to study the effects of four surfactants on cell permeability as measured by loss of intracellular material and on photosynthetic 14CO2 fixation. Surfactant-induced alteration in cell permeability could not be related to surfactant-ionogenic class but could be related to reported phytotoxicity. Surfactants that altered cell permeability also inhibited photosynthetic 14CO2 fixation, indicating that surfactant effects were not restricted to the outer-cell membrane. Isolated cells from the monocotyledonous wild onion were more susceptible to surfactant-induced alterations in cell permeability than were cells from the dicotyledonous soybean. Efflux of intracellular material was modified by Ca++ and Mg++ to a greater extent for soybean than for wild onion. Integrity of the isolated cells, before and after surfactant treatment, was verified by microscopic observation.

Weeds are alternative hosts for plant-parasitic nematodes and have long been recognized for their ability to maintain nematode populations targeted for suppression by various management strategies. The impact of weeds as alternative hosts depends largely on nematode feeding behavior, which is determined by the level of host specialization required for the parasite to feed successfully. In general, the more specialized feeding adaptations are associated with greater crop damage, more diverse nematode management options, and greater negative impact from weeds. Besides serving as alternative hosts, certain weeds can protect nematodes from pesticides and the environment, provide nematode suppression through antagonism, contribute to changes in future nematode biotic potential, or exert indirect effects through competition with crops or by the effects of weed control strategies on nematode populations. Shrinking nematicide options and increasing environmental concerns are making integrated pest management (IPM) a necessity for nematode management in many crops. A prominent similarity between most major weeds and plant-parasitic nematodes is that both are place-bound organisms that are passively dispersed. Weed–nematode interactions in agricultural production systems may be more intricate and complex than the simple function of weeds as alternative hosts. Their relationship may represent a normal adaptation resulting from the limited mobility of both groups of organisms and the obligate parasitism of phytophagous nematodes. The challenge that faces weed scientists and nematologists is to identify effective, compatible IPM strategies that address weed and nematode management collectively.

POST weed harrowing and other cultivation methods to control weeds in early crop growth stages may result in crop damage due to low selectivity between crop and weeds. Crop tolerance to cultivation plays an important role but it has not been clearly defined and analyzed. We introduce a procedure for analyzing crop tolerance on the basis of digital image analysis. Crop tolerance is defined as the ability of the crop to avoid yield loss from cultivation in the absence of weeds, and it has two components: resistance and recovery. Resistance is the ability of the crop to resist soil covering and recovery is the ability to recover from it. Soil covering is the percentage of the crop that has been buried because of cultivation. We analyzed data from six field experiments, four experiments with species of small grains, barley, oat, wheat, and triticale, and two experiments with barley cultivars with different abilities to suppress weeds. The order of species' tolerance to weed harrowing was triticale > wheat > barley > oat and the differences were mainly caused by different abilities to recover from soil covering. At 25% soil covering, grain yield loss in triticale was 0.5%, in wheat 2.5%, in barley 3.7%, and in oat 6.5%. Tolerance, resistance, and recovery, however, were influenced by year, especially for oat and barley. There was no evidence of differences between barley cultivars in terms of tolerance indicating that differences among species are more important than differences among cultivars. Selectivity analysis made it possible to calculate the crop yield loss due to crop damage associated with a certain percentage of weed control. In triticale, 80% weed control was associated with 22% crop soil cover on average, which reduced grain yield 0.4% on average in the absence of weeds. Corresponding values for wheat, barley, and oat were 23, 21, and 20% crop soil cover and 2.3, 3.6, and 5.1% grain yield loss.

Red beets (Beta vulgaris L., var. Detroit Dark Red) converted the herbicide 5-amino 4-chloro-2-phenyl-3(2H)-pyridazinone (pyrazon) to N-2-chloro-4-phenyl-3-(2H)-pyridazinone glucosamine. Isolation was accomplished by water, benzene, and alcohol extraction of foliage followed by purification through molecular sieve columns. Identification was established by infrared spectroscopy, and thin layer and gas liquid chromatography.

Under greenhouse conditions activated carbon, applied as a slurry at the time of seeding, protected Italian ryegrass (Lolium multiflorum Lam.) seedlings from the toxicity of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) applied preemergence. Plants growing from seeds planted 1.3 cm deep received less protection from diuron than plants growing from seeds planted 0.6, 1.9, or 2.5 cm deep. Sandy loam soil required at least three times more activated carbon than a clay loam soil to obtain the same protection. Activated carbon applied at 130 kg/ha in a 2.5-cm band on the soil surface directly over the seeds provided adequate protection on a clay loam soil. On a sandy loam soil the 130-kg/ha rate was not adequate. Wetting agents added to aid in carbon wetting reduced the protective effect of activated carbon when added at 0.9 or 2.7% (v/v) but had only a slight effect on protection when added at 0.3% (v/v). The soil moisture level at the time of carbon application and the amount of subsequent irrigation did not influence the protection obtained with activated carbon.

Ground beetles are postdispersal weed seed predators, yet their role in consuming buried seeds is not well studied. We conducted greenhouse experiments to investigate how seed burial affects consumption of weed seeds (volunteer canola) by adult ground beetles (Coleoptera: Carabidae). Seed burial depth influenced seed consumption rates as demonstrated by a significant interaction between seed burial depth, carabid species, and gender of the carabid tested. We observed higher seed consumption by females of all species, and greater consumption of seeds scattered on the soil surface compared with seeds buried at any depth. However, there was evidence of seed consumption at all depths. Adults of Pterostichus melanarius (Illiger) and Harpalus affinis (Schrank) consumed more buried seeds than did those of Amara littoralis Mannerheim. Agricultural practices, such as tillage, bury seeds at different depths and based on the results of this study, these practices may reduce seed consumption by carabids. Soil conservation practices that reduce tillage (conservation or zero tillage) will favor greater weed seed predation due, in part, to the high availability of seeds at the soil surface or at shallow soil depths.

The moisture content of soils is frequently reported to have an important effect upon the phytotoxicity of herbicides. Yet, specific experimental results establishing such relationships are, for the most part, lacking. Most moisture studies with herbicides have been designed to evaluate the effect of applied water on leaching and its indirect effect on volatilization and breakdown by exposure to sunlight. Some studies have shown the effect of moisture content on the rate of herbicide breakdown. Research on the fate of herbicides in soils has been summarized by Hill et al. and Ogle and Warren. The experiments reported here were designed to establish the effect of soil-moisture content per se upon the response of cotton to 3–(3,4–dichlorophenyl)–1,1-dimethylurea (diuron), isopropyl–N–(3–chlorophenyl) carbamate (CIPC) and 4,6–dinitro ortho secondary butylphenol (DNBP) as the alkanolamine salts.

Over 20 experiments comparing glyphosate [N-(phosphonomethyl)glycine], dicamba (3,6-dichloro-2-methoxybenzoic acid), fosamine [ethyl hydrogen (aminocarbonyl)phosphonate], and 2,4-D [(2,4-dichlorophenoxy) acetic acid] for control of field bindweed (Convolvulus arvensis L. # CONAR) were conducted from 1976 to 1982 at various times of the year and different stages of weed growth. In three of the studies, 1.7, 3.4, and 5.0 kg ae/ha of glyphosate gave 54, 72, and 80% control, respectively, 1 yr after application. Control with glyphosate at 3.4, 2,4-D at 1.1, dicamba at 1.1, and fosamine at 13.7 kg ae/ha in the 20 studies was 71, 55, 57, and 73%, respectively, 1 yr after application. Glyphosate, 2,4-D, and fosamine gave good control at any time of the year if weed growth was lush. Dicamba gave good control anytime if growth was good and in the fall regardless of growing conditions. Control with mixtures of dicamba and picloram, picloram and 2,4-D, or glyphosate and picloram was higher than with the other herbicides. Dicamba at 1.1 kg/ha, applied after August caused some injury to wheat planted the same fall. Herbicide combinations with picloram at 0.28 kg/ha applied after June injured wheat planted in the fall.

CGA 82725 {2-propynyl [2-[4-[(3,5-dichloro-2-pyridinyl)oxy)] phenoxy] propanoate}, haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl] oxy]phenoxy] propanoic acid}, sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one}, and fluazifop {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} were applied postemergence to Texas panicum (Panicum texanum Buckl. # PANTE), large crabgrass [Digitaria sanguinalis (L.) Scop. # DIGSA], and broadleaf signalgrass [Brachiaria platyphylla (Griseb.)Nash. # BRAPP] in peanut (Arachis hypogaea L. ‘Florunner’). Fluazifop applied at 280 and 410 g ai/ha, sethoxydim at 340 g ai/ha, haloxyfop at 140 g ai/ha, and CGA 82725 at 280 g ai/ha usually gave better control when applied to annual grasses in the two- to four-leaf stage than when applied at the six- to eight-leaf stage. Higher rates of application were required to provide acceptable weed control at the later stage of growth. Peanut yields were usually higher following the early applications, indicating that timing of application is important in obtaining improved yields.

Canada thistle is a noxious weed that occurs in a wide range of habitats and is difficult to control because of its extensive root system and prolific seed production. Here, we focused on estimating the level of genetic diversity between populations in North Dakota as a first step in examining diversity across North America. Two types of genetic marker, intersimple sequence repeats (ISSRs) and microsatellites were used. Both marker types resulted in polymorphic alleles suitable for assessing diversity. Analysis of molecular variance (AMOVA), molecular diversity analyses, and cluster analysis were conducted. Highly significant variation was detected between populations (P < 0.01). The greatest variance recovered was between individuals within populations. Gene flow among populations in the Northern Great Plains was indicated by the presence of shared alleles between the North Dakota and Minnesota populations and in cluster formation. Multiple introductions and continued gene flow between populations has led to the continued success of Canada thistle as an invasive plant in North America.