This paper reviews the biology of two closely related Mediterranean annuals, yellow starthistle and common crupina, which have invaded grassland, shrub steppe, and open woodland habitats in western North America. Despite the similarity of their winter annual life cycle, the two species differ significantly in population dynamics. Common crupina has traits that favor persistence rather than rapid population growth: large, heavy achenes with an after-ripening requirement; lower fecundity but higher germination success; and reproduction regulated by vernalization and photoperiod in addition to thermal time. Persistence traits also foster invasion of undisturbed or less degraded steppe habitats. Yellow starthistle has more ruderal traits: small, light, rapidly germinating achenes; higher fecundity, with greater seedling mortality; and reproduction that is less sensitive to photoperiod and vernalization. These characteristics confer a greater adaptability for rapid spread and colonization of disturbance by yellow starthistle than by common crupina. An understanding of the relative differences in biological characters of each species and in their function in invaded environments is relevant to ecological management of these pest species.

Nomenclature: Common crupina, Crupina vulgaris Cass., CJNVU; yellow starthistle, Centaurea solstitialis L., CENSO.

In 1997, farmers in Ontario, Canada, reported failure of some ALS-inhibiting herbicides to provide adequate control of pigweed species. Growth room experiments were conducted to confirm resistance to ALS inhibitors in populations of Powell amaranth and redroot pigweed. Twenty-two out of 35 collected seed samples were able to grow in the presence of soil-applied imazethapyr or flumetsulam. Dose–response curves were generated for 11 and 9 populations of Powell amaranth and redroot pigweed, respectively, using foliar-applied imazethapyr and thifensulfuron. Resistance to ALS inhibitors was confirmed in nine and five populations of Powell amaranth and redroot pigweed, respectively. Within each species, comparison of the herbicide rate required to reduce plant dry weight 50% (GR₅₀) between the resistant populations and a susceptible population was conducted to obtain resistance factors. For imazethapyr, resistance factors ranged from 4.2 to 3,438 and from 33 to 168 for Powell amaranth and redroot pigweed, respectively. High-level cross-resistance to thifensulfuron was found in two populations of each species, with resistance factors ranging from 270 to 2,416. In both species, populations could be grouped according to their cross-resistance patterns: some populations were resistant to imazethapyr only, whereas others expressed resistance to both imazethapyr and thifensulfuron. The observed patterns of cross-resistance were not correlated with known herbicide exposure history of the fields where these populations originated.

Nomenclature: Flumetsulam; imazethapyr; thifensulfuron; Powell amaranth = green pigweed, Amaranthus powellii S. Wats. AMAPO; redroot pigweed, Amaranthus retroflexus L. AMARE.

Greenhouse experiments were conducted to determine the effects of glufosinate on ammonium accumulation and photosynthetic inhibition in Palmer amaranth. Glufosinate applied at 410 g ha⁻¹ reduced glutamine synthetase activity and enhanced ammonium content 30 min after treatment. Glufosinate application resulted in rapid inhibitions of the photosynthetic rate and stomatal conductance during the first 2 h after treatment (HAT), whereas the ammonium concentration increased over the same time period. Ammonium content 6 HAT was 22 times higher in treated plants than in untreated plants, whereas the photosynthetic rate of treated plants decreased by 63%. At 24 HAT, the ammonium content was 53 times higher in treated plants than in untreated plants; however, no further inhibition of photosynthesis occurred. Photosynthetic inhibition in Palmer amaranth coincided with the rapid accumulation of ammonium and decrease in stomatal conductance shortly after glufosinate application.

Nomenclature: Glufosinate; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA.

The influence of herbicide placement and plant growth stage on the absorption and translocation patterns of ¹⁴C-glyphosate in glyphosate-resistant cotton was investigated. Plants at four growth stages were treated with ¹⁴C-glyphosate on a 5-cm² section of the stem, which simulated a postemergence-directed spray (PDS) application, or on the newest mature leaf, which simulated a postemergence (POST) application. Plants were harvested 3 and 7 d after treatment and divided into the treated leaf or treated stem, mature leaves, immature leaves and buds, stems, roots, fruiting branches (including the foliage on the fruiting branch), squares, and bolls. The PDS versus POST application main effect on absorption was significant. Absorption of ¹⁴C-glyphosate applied to stem tissue was higher in PDS applications than in POST applications. Plants receiving PDS applications absorbed 35% of applied ¹⁴C-glyphosate, whereas those receiving POST applications absorbed 26%, averaged over growth stages at application. Absorption increased from the four-leaf growth stage to the eight-leaf stage in POST applications but reached a plateau at the eight-leaf stage. Plants with PDS applications showed an increase in absorption from the four- to eight- to twelve-leaf stages and reached a plateau at the 12-leaf stage. Translocation of ¹⁴C-glyphosate to roots was greater at all growth stages with PDS treatments than with POST treatments. Herbicide placement did not affect translocation of ¹⁴C-glyphosate to squares and bolls. Squares and bolls retained 0.2 to 3.7% of applied ¹⁴C-glyphosate, depending on growth stage. Separate studies were conducted to investigate the fate of foliar-applied ¹⁴C-glyphosate at the four- or eight-leaf growth stages when harvested at 8- or 10-leaf, 12-leaf, midbloom (8 to 10 nodes above white bloom), and cutout (five nodes above white bloom, physiological maturity) stages. Thirty to 37% of applied ¹⁴C-glyphosate remained in the plant at cutout in four- and eight-leaf treatment stages, respectively. The concentration of ¹⁴C-glyphosate in tissue (Bq g⁻¹ dry weight basis) was greatest in mature leaves and immature leaves and buds in plants treated at the four-leaf stage. Plants treated at the eight-leaf stage and harvested at all growth stages except cutout showed a higher concentration of ¹⁴C-glyphosate in squares than in other plant tissue. Accumulation of ¹⁴C-glyphosate in squares reached a maximum of 43 Bq g⁻¹ dry weight at harvest at the 12-leaf stage. This concentration corresponds to 5.7 times greater accumulation of ¹⁴C-glyphosate in squares than in roots, which may also be metabolic sinks. These data suggest that reproductive tissues such as bolls and squares can accumulate ¹⁴C-glyphosate at higher concentrations than other tissues, especially when the herbicide treatment is applied either POST or PDS during reproductive stages (eight-leaf stage and beyond).

Nomenclature: Glyphosate; cotton, Gossypium hirsutum L. ‘Delta Pine 5415RR’.

All red rice found in commercial rice in the United States has traditionally been classified as Oryza sativa ssp. indica. This assumption was tested by analyzing red rice samples collected from across the southern United States rice belt with 18 simple sequence length polymorphism (SSLP) markers distributed across all 12 chromosomes. The results clearly demonstrate that the traditional classification of red rice is inadequate. Some red rice is closely related to O. sativa ssp. indica cultivated rice. However, other red rice is more closely related to O. sativa ssp. japonica. Most importantly, some red rice samples collected from Arkansas, Louisiana, Mississippi, and Texas form a distinct group that includes a number of Oryza nivara and Oryza rufipogon accessions from the National Small Grains Center. In particular, red rice samples from three states were identified that for all 18 markers are identical to the O. rufipogon accession IRGC 105491. These different classes of red rice are intermingled across the southern U.S. rice belt and within individual production fields. Oryza sativa ssp. indica-like red rice and O. rufipogon-like red rice have been found within a single 9-m² collection site. While the classification of red rice as O. sativa ssp. indica, O. sativa ssp. japonica, or O. rufipogon using DNA markers is generally in agreement with classification based on simple morphological traits, readily observed morphological traits alone are not sufficient to reliably classify red rice. Because red rice is much more diverse than previously assumed, this diversity must be considered when developing red rice management strategies.

Nomenclature: Glufosinate; Oryza nivara L.; Oryza rufipogon L.; Oryza sativa L. ORYSA, red rice; Oryza sativa L., rice.

Acetyl-coenzyme A carboxylase (ACCase) assays and absorption, translocation, and metabolism experiments were conducted to investigate the mechanism(s) responsible for resistance in a johnsongrass biotype that exhibited low levels of resistance to the cyclohexanedione (CHD) herbicide sethoxydim and the aryloxyphenoxypropionate (APP) herbicides quizalofop-P and fluazifop-P. The rate of [¹⁴C]quizalofop-ethyl absorption was significantly higher in the resistant compared to the susceptible biotype 8, 24, and 48 h after treatment (HAT), but by 72 HAT, there was no significant difference in the amount of [¹⁴C]quizalofop-ethyl detected in either biotype. Additionally, little or no differences in the translocation of [¹⁴C]quizalofop-ethyl were observed in the resistant and susceptible biotypes at any time interval after application. In [¹⁴C]quizalofop-ethyl metabolism experiments, similar levels of quizalofop-ethyl and quizalofop metabolites were observed in the resistant and susceptible biotypes 8, 24, 48, and 72 HAT, but slightly higher levels of quizalofop acid were detected in the resistant biotype 48 and 72 HAT. In ACCase assays, the concentrations of quizalofop-P, clethodim, and sethoxydim that inhibited ACCase activity by 50% (I₅₀) were statistically similar in the two biotypes, indicating that the resistant johnsongrass biotype contains an ACCase that is sensitive to the APP and CHD herbicides. In the absence of APP or CHD herbicides, however, the specific activity of ACCase in the resistant biotype was two to three times greater than that of the susceptible biotype. The specific activity of ACCase in the resistant biotype was also significantly greater than that of the susceptible biotype in the presence of all concentrations of quizalofop-P and sethoxydim and in the presence of 0.1, 1, and 10 µM clethodim. These results suggest that resistance to quizalofop-P and sethoxydim is conferred by an overproduction of ACCase in the resistant johnsongrass biotype.

Nomenclature: Clethodim; fluazifop-P; quizalofop-P; sethoxydim; johnsongrass, Sorghum halepense (L.) Pers. SORHA.

A population of common ragweed not controlled by an acetolactate synthase (ALS)-inhibiting herbicide, cloransulam-methyl, was sampled near Dunkirk, IN, the first year of the herbicide's commercialization in 1998. Resistance in the Dunkirk population was confirmed by treating greenhouse-grown seedlings with cloransulam-methyl. ALS activity assays and DNA sequencing were used to identify the resistance mechanism. ALS isolated from plants of the Dunkirk population exhibited an R/S ratio for cloransulam-methyl of >5,000 when compared to ALS activity of populations from Claire City, SD, and V & J Seed Farms. R/S ratios of 4,100 and 110 were observed for two other ALS-inhibiting herbicides, chlorimuron and imazaquin, respectively. DNA sequencing revealed that an inferred leucine for tryptophan substitution at amino acid position 574 in ALS was responsible for the observed herbicide resistance. Additionally, DNA sequencing revealed significant variability among common ragweed ALS alleles. Two fragments of ALS were sequenced from three plants each of the Claire City and Dunkirk populations, totaling 688 nucleotide base pairs, of which 72 were polymorphic.

Nomenclature: Chlorimuron; cloransulam-methyl; imazaquin; common ragweed, Ambrosia artemisiifolia L. AMBEL.

Weed suppression by cover crops grown during the winter fallow period in continuous corn may lead to a reduction in herbicide use. Rye, crimson clover, and subterranean clover cover crops were compared with corn stubble under a conventional management system (CS) that included plowing and use of preemergence residual herbicides and a low-input management system (LIS) that included no-tillage and use of a presowing nonresidual herbicide for three consecutive years (1994–1996). Cover crop and above-ground weed biomass prior to desiccation were not influenced by management system. Cover crop biomass ranged from 1,420 to 5,657 kg ha⁻¹ for rye, from 563 to 4,217 kg ha⁻¹ for crimson clover, and from 563 to 4,248 kg ha⁻¹ for subterranean clover. At crop planting, rye reduced weed biomass from 54 to 99%, crimson clover from 22 to 46% (with a negative value in 1995), and subterranean clover from 21 to 67%. Weed growth suppression was usually higher in years when cover crop biomass was higher. There were no differences in weed suppression by cover crops later in the season (corn in the fourth leaf stage), while total weed density was higher in LIS than CS in 2 of 3 yr. Total weed cover at corn's ‘full dent’ stage ranged from 1 to 7% in CS and from 24 to 47% in LIS. Cover crops influenced weed composition only in years when cover crop growth was high; otherwise their effect was masked by that of the management system. Weed communities showed higher diversity under LIS than under CS. Consistency of associations between weed species and treatments over sampling dates and years was found especially for some of the species associated with LIS. After 3 yr, redroot pigweed, common lambsquarters, and black nightshade were regularly associated with rye-LIS at an early corn growth stage; this may indicate a species shift toward a more troublesome composition.

Nomenclature: Dicamba; glyphosate; nicosulfuron; terbuthylazine; black nightshade, Solanum nigrum L. SOLNI; common lambsquarters, Chenopodium album L. CHEAL; corn, Zea mays L.; crimson clover, Trifolium incarnatum L.; redroot pigweed, Amaranthus retroflexus L. AMARE; rye, Secale cereale L.; subterranean clover, Trifolium subterraneum L.

The goal of a mechanistic model is to determine the outcome of weed–crop interference. An understanding of weed phenology is essential for construction of such models because phenological development is a major factor determining the outcome of weed–crop competition. Growth cabinet studies were conducted to determine the influence of temperature and photoperiod on the phenological development of common lambsquarters. Common lambsquarters is a short-day species adapted to a temperature range of 6.5 to 44.5 C. Phenological development of common lambsquarters grown under a constant temperature of 20 C and an 8-h photoperiod was described in terms of biological days (Bd: chronological days at the optimum photoperiod and temperature). Three development phases of common lambsquarters were described as (1) a juvenile phase of 6.3 Bd, (2) a photoperiod-sensitive inductive phase of 8.2 Bd, and (3) a photoperiod-sensitive postinductive phase of 34.4 Bd. The photoperiod sensitivity of rate of development did not differ among phases of development across the life cycle. Interpretation of constant sensitivity to photoperiod will simplify simulation of weed phenology in mechanistic models.

Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL.

Field margin weeds may contribute to the invasion and persistence of weeds in arable fields. Experimental studies of this hypothesis, however, have been inconclusive. We examined the role of field margin weed populations with a spatially explicit simulation model of Canada thistle population dynamics. We measured the contribution of field margin populations to weed pressure in the field across a wide range of parameter values and compared the weed control value of efforts applied to the field margin to that of similar efforts applied to the field. Under most combinations of parameter values, field margin weeds contributed little to weed pressure in the field, suggesting that controlling field margin weeds may often be of little value. Two conditions appeared to be necessary for field margin weeds to influence weed population dynamics within the field: the presence of unoccupied weed habitat, which increased the importance of dispersal to population growth, and high dispersal rates of field margin weeds relative to field weeds, which increased the relative contribution of field margin weeds to dispersal.

Nomenclature: Canada thistle, Cirsium arvense (L.) Scop. CIRAR.

Dodder (Cuscuta) species are obligate shoot parasites that attach to stems and leaves of plants belonging to diverse families, diverting water, minerals, and metabolites, resulting in suppression of crop plants and yield reduction. Lespedeza dodder has the most diverse and numerous host range among the Cuscuta genus and is a major weed problem in tomato production in certain parts of the world, including California. At present, few resistant varieties of normally susceptible plant species have been developed or identified, and none are available in California. In this study, greenhouse and field studies were conducted to test and characterize the tolerance of commercial hybrid tomato varieties to lespedeza dodder. In greenhouse studies, Heinz varieties ‘9492’, ‘9553’, and ‘9992’ exhibited tolerance to the parasite, compared to the susceptible variety, ‘Halley 3155’. Lespedeza dodder germinated, made contact, twined around tomato stems, and adhered to them, but in most cases, haustoria failed to penetrate into the stem, eventually leading to the death of the parasite. In field studies, lespedeza dodder attachments were 75% less on tolerant varieties, and dodder growth was reduced by more than 70%.

Nomenclature: Lespedeza dodder, Cuscuta pentagona Engelm. CVCPE; tomato, Lycopersicon esculentum L. ‘Heinz 9492’, ‘Heinz 9553’, ‘Heinz 9992’.

An experiment examining the effect of burcucumber emergence date and corn competition on burcucumber fecundity was conducted in 1997 and 1998 in central Pennsylvania. Burcucumber seedlings were transplanted in the field in approximately 10-d intervals starting in late May through mid-August with or without competition from corn. Burcucumber plants grown without competition from corn produced 716 g dry matter and 4,500 seeds plant⁻¹ in 1997 and 607 g dry matter and 1,800 seeds plant⁻¹ in 1998. Biomass was greatest for plants established in late May, whereas seed production was greatest for plants established in mid-June. Although seed numbers were reduced in comparison to the May and June establishment periods, plants established as late as August still produced seed. Burcucumber established in corn produced 96% less dry matter and seed than the plants in a noncompetitive environment in both years of the study. Although the growth and seed production of burcucumber grown in corn was drastically reduced, plants established in mid-July still produced seed.

Nomenclature: Burcucumber, Sicyos angulatus L. SIYAN; corn, Zea mays L.

Trials were carried out to investigate the effects of seed burial depth on seedling emergence rate of 20 weed species. Marked depth-mediated variation in emergence ability of the different species was observed, together with a general pattern of decreasing emergence with increasing soil depth. At 10 cm, only johnsongrass, velvetleaf, catchweed bedstraw, and cutleaf geranium emerged, albeit only in limited numbers. Species most severely inhibited by burial depth were buckhorn plantain, large crabgrass, common purslane, chickweed, and corn spurry, none of which emerged from beyond 6 cm. In all species, depth-mediated inhibition was found to be sigmoidal (polynomial regression). In addition, the number of seedlings and rate of seedling emergence decreased when depth of burial increased. The depth at which the number of emerged seedlings was halved varied by species and ranged from 3.6 cm for common purslane and chickweed to 7 cm for velvetleaf and catchweed bedstraw. Excessive burial depth generally induced dormancy (in roughly 85% of cases) rather than suicide germination. A close inverse relation (second-degree equation) between seed unit weight and depth-mediated inhibition was observed. The physiological involvement of depth inhibition in seed bank ecology is discussed.

Nomenclature: Buckhorn plantain, Plantago lanceolata L. PLALA; catchweed bedstraw, Galium aparine L. GALAP; common chickweed, Stellaria media (L.) Vill. STEME; common purslane, Portulaca oleracea L. POROL; corn spurry, Spergula arvens L. SPRAR; cutleaf geranium, Geranium dissectum L. GERDI; johnsongrass, Sorgum halepense (L.) Pers. SORHA; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; velvetleaf, Abutilon theophrasti Medicus ABUTH.

The adoption of precision technologies that spatially register measurements using global positioning systems (GPS) greatly facilitates conducting large-scale on-farm research by farmers. On-farm experiments that utilize producer equipment include variations in agronomic practices that occur in situations where we want to predict the effect of inputs on yield. The domain of inference for such on-farm studies therefore more closely matches that desired by researchers. To investigate the feasibility of on-farm research using GPS, a study was conducted to evaluate the potential benefit of site-specific weed management. The study utilized producer-maintained field-scale equipment on four Montana farms in dryland spring wheat production. Paired site-specific and whole-field herbicide treatment areas were established in 0.9 to 1.9-ha blocks using consultant weed maps and a geographic information system (GIS). Yield was unaffected by herbicide treatment strategy (site-specific or broadcast). Minimal detectable yield differences were evaluated for the experimental design (0.2 T ha⁻¹). Net returns increased when the percentage of field infested by wild oat decreased. Visual ratings of wild oat density taken at harvest indicated no difference in wild oat control between treatments in two of four site-years. This research suggests that producer-owned equipment can be used to compare treatments, but the accuracy and subsequent power of such comparisons are likely to be low.

Nomenclature: Fenoxaprop; imazamethabenz; tralkoxydim; kochia, Kochia scoparia L. KCHSC; Russian thistle, Salsola iberica Sennen and Pau SASKR; spring wheat, Triticum aestivum L; wild oat, Avena fatua L. AVEFA.

Pyrithiobac control of Palmer amaranth on the Texas Southern High Plains was correlated previously with temperature at the time of application. In the present study, the thermal dependence of pyrithiobac efficacy was used to define a thermal application range (TAR) for postemergence pyrithiobac applications. Several years of temperature data from four cotton-growing regions of the United States were analyzed with respect to the TAR to determine the extent to which temperature limitations could affect pyrithiobac applications. Temperatures outside the TAR occurred in all years and regions analyzed. Analyses of four geographic regions utilizing 4 to 11 yr of data for each region indicated the following percentages of hours inside the TAR: Lubbock, TX, 54 to 94%; Maricopa, AZ, 27 to 33%; Raleigh-Durham, NC, 70 to 97%; and Jackson, MS, 81 to 99%. A detailed analysis of the frequency and duration of the TAR in Lubbock, TX, showed that, periodically, temperatures outside the TAR may limit the efficacy of postemergence pyrithiobac applications for several consecutive days. Finally, the TAR was shown to be useful as a postapplication diagnostic tool for evaluating herbicide applications that resulted in poor efficacy. These results suggest that long-term evaluation of historic temperatures with respect to the TAR for a given herbicide may provide insight into the potential limitations of herbicide efficacy and underscore the potential utility of developing TARs based on field and laboratory analyses of herbicide thermal dependence.

Nomenclature: Pyrithiobac; cotton, Gossypium hirsutum L.; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA.

Field studies were conducted at four locations in North Carolina in 1998 and 1999 to evaluate the use of the Herbicide Application Decision Support System (HADSS™) for weed management in nontransgenic, bromoxynil-resistant, and glyphosate-resistant cotton. Weed management systems included trifluralin preplant incorporated (PPI) plus fluometuron preemergence (PRE) or no soil-applied herbicides. Postemergence (POST) options included bromoxynil, glyphosate, or pyrithiobac early POST (EPOST) followed by (fb) MSMA plus prometryn late postemergence–directed (LAYBY) or herbicide recommendations given by HADSS. Glyphosate-resistant systems provided control equivalent to or better than control provided by bromoxynil-resistant and nontransgenic systems for smooth pigweed, Palmer amaranth, large crabgrass, goosegrass, ivyleaf morningglory, and fall panicum. Trifluralin PPI fb fluometuron PRE fb HADSS POST provided equivalent or higher levels of weed control and yield than trifluralin PPI fb fluometuron PRE fb bromoxynil, glyphosate, or pyrithiobac EPOST fb MSMA plus prometryn LAYBY. The trifluralin PPI fb fluometuron PRE fb HADSS POST systems controlled large crabgrass at Goldsboro and fall panicum better than HADSS POST-only systems in nontransgenic cotton. Cotton yield and net returns in the glyphosate-resistant systems were always equal to or higher than the nontransgenic and bromoxynil-resistant systems. Net returns were higher for the soil-applied fb HADSS POST treatments in 8 of 12 comparisons with HADSS POST systems without soil-applied herbicides. Early-season weed interference reduced cotton lint yields and net returns in POST-only systems.

Nomenclature: Bromoxynil; fluometuron; glyphosate; MSMA; prometryn; pyrithiobac; trifluralin; cotton, Gossypium hirsutum L. ‘Deltapine 51’, ‘BXN 47’, ‘Deltapine 5415RR’; fall panicum, Panicum dichotomiflorum Michx. PANDI; goosegrass, Eleusine indica (L.) Gaertn. ELEIN; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. IPOHE; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; smooth pigweed, Amaranthus hybridus L. AMACH.

Manipulations of the soil environment can affect the growth and competitive ability of annual weeds because of the large influence that soil conditions exert on seedlings early in the growing season. Our objective was to identify soil nitrogen (N) management systems with weed suppression potential. We hypothesized that competition from wild mustard against sweet corn would be weaker when N was supplied by organic sources (organic) or a split application of NH₄NO₃ fertilizer applied at planting and 4 wk thereafter (split) than when NH₄NO₃ fertilizer was applied in a single dose at planting (early). This hypothesis was tested in a 2-yr field experiment conducted in central Maine. Wild mustard's maximal relative growth rate (RGR) was 12% lower (P < 0.05) in 1997 and 1998, and the amount of time needed to achieve maximal RGR was delayed by 0.8 d (P < 0.05) in 1997 in the organic compared to the early treatment. The competitive effect of wild mustard on sweet corn yield was lower in the organic treatment than in the early and split treatments in 1 of 2 yr. In 1997, competition from wild mustard reduced marketable ear yields of sweet corn by 30%, but the magnitude of yield reduction did not differ between the three N addition treatments. In contrast, in 1998, sweet corn yield in the organic treatment was not reduced by weed competition, whereas yield loss in the early and split treatments was 20 and 35%, respectively. The mechanisms underlying selective suppression of weeds, but not crops, by organic N sources require further attention.

Nomenclature: Red clover, Trifolium pratense L., ‘Mammoth’; spring wheat, Triticum aestivum L., ‘Belvedere’; sweet corn, Zea mays L. ‘Clockwork’; wild mustard, Brassica kaber (D.C.) L.C. Wheeler SINAR.

An experiment was conducted at two locations in Georgia and one location in North Carolina during 1994 and 1995 to evaluate weed management systems utilizing pyrithiobac applied preemergence (PRE) in conventional-tillage bromoxynil-resistant cotton. Weed management systems evaluated included different combinations of pyrithiobac PRE, bromoxynil or bromoxynil plus MSMA applied early postemergence (EPOST), bromoxynil applied postemergence (POST), and cyanazine plus MSMA applied late postemergence-directed (LAYBY). Pyrithiobac PRE improved control of Florida beggarweed, pitted morningglory, prickly sida, sicklepod, and spurred anoda compared with systems that did not include pyrithiobac PRE. Averaged across locations, pyrithiobac PRE increased cotton lint yields 330 kg ha⁻¹. Bromoxynil applied EPOST or POST increased weed control and cotton lint yield. Bromoxynil EPOST, POST, or EPOST plus POST did not control sicklepod. The addition of MSMA to bromoxynil EPOST improved sicklepod control. Two applications of bromoxynil controlled more pitted morningglory and sicklepod than one application. Control of all dicotyledonous weeds was increased by cyanazine plus MSMA LAYBY, and this treatment increased yields at all locations. Cotton was not injured by pyrithiobac PRE or by bromoxynil applied EPOST or POST, but temporary visual injury was observed with EPOST treatments of MSMA.

Nomenclature: Bromoxynil; cyanazine; fluometuron; MSMA; pyrithiobac; cotton, Gossypium hirsutum L. ‘BXN 57’; Florida beggarweed, Desmodium tortuosum (Sw.) DC. DEDTO; pitted morningglory, Ipomoea lacunosa L. IPOLA; prickly sida, Sida spinosa L. SIDSP; sicklepod, Senna obtusifolia (L.) Irwin and Barneby CASOB; spurred anoda, Anoda cristata (L.) Schlecht. ANVCR.

Field studies were conducted at Delta Research and Extension Center, Stoneville, MS, in 1996, 1997, and 1998 to assess the effect of tillage systems (conventional tillage and subsoiling) on the environmental fate of imazaquin in a Sharkey clay soil. Imazaquin was applied preemergence at 140 g ai ha⁻¹. Subsoiling in the fall did not affect imazaquin dissipation, total volume of runoff, imazaquin concentration in runoff, or imazaquin concentration in soil, as determined by chemical extraction. A corn root bioassay revealed no differences due to tillage systems in plant-available imazaquin in soil. Imazaquin concentration measured by chemical extraction or bioassay diminished over time, with a half-life ranging from 8 to 25 d. A field bioassay utilizing cotton and corn was conducted in 1997 and 1998 using plots that had received imazaquin the previous year. In 1997, 2 wk after planting, cotton and corn injury ranged from 3 to 15%, whereas no injury was observed in 1998. Injury symptoms declined over time, with no injury 5 wk after planting in either year. Although early-season cotton stunting and slight discoloration of corn was apparent in 1997, imazaquin residues did not affect subsequent vegetative and reproductive growing patterns of cotton or corn. In 1998, corn and cotton height were significantly greater in subsoiled plots compared to conventional tillage.

Nomenclature: Corn, Zea mays L. ‘HyPerformer HS 9773’, ‘Pioneer 3167’; cotton, Gossypium hirsutum L. ‘DPL 50’; soybean, Glycine max (L.) Merr. ‘DPL 3589’.