Weeds compete with crops for light, soil water, and nutrients. Nitrogen (N) is the primary limiting soil nutrient. Forecasting the effects of N on growth, development, and interplant competition requires accurate prediction of N uptake and distribution within plants. Field studies were conducted in 1999 and 2000 to determine the effects of variable N addition on monoculture corn and velvetleaf N uptake, the relationship between plant N concentration ([N]) and total biomass, the fraction of N partitioned to leaves, and predicted N uptake and leaf N content. Cumulative N uptake of both species was generally greater in 2000 than in 1999 and tended to increase with increasing N addition. Corn and velvetleaf [N] declined with increasing biomass in both years in a predictable manner. The fraction of N partitioned to corn and velvetleaf leaves varied with thermal time from emergence but was not influenced by year, N addition, or weed density. With the use of the [N]–biomass relationship to forecast N demand, cumulative corn N uptake was accurately predicted for three of four treatments in 1999 but was underpredicted in 2000. Velvetleaf N uptake was accurately predicted in all treatments in both years. Leaf N content (NL, g N m−2 leaf) was predicted by the fraction of N partitioned to leaves, predicted N uptake, and observed leaf area index for each species. Average deviations between predicted and observed corn NL were < 88 and 12% of the observed values in 1999 and 2000, respectively. Velvetleaf NL was less well predicted, with average deviations ranging from 39 to 248% of the observed values. Results of this research indicate that N uptake in corn and velvetleaf was driven primarily by biomass accumulation. Overall, the approaches outlined in this paper provide reasonable predictions of corn and velvetleaf N uptake and distribution in aboveground tissues.

Halauxifen-methyl is a new synthetic auxin herbicide for control of broadleaf weeds, including preplant applications for corn (Zea mays L.) or soybean [Glycine max (L.) Merr.]. The objective of this study was to investigate the efficacy of halauxifen-methyl in comparison to the current auxin standards, 2,4-D and dicamba, on glyphosate-resistant (GR) horseweed (Erigeron canadensis L.) at different plant heights. In field experiments, a foliar application of halauxifen-methyl at the recommended use rate of 5 g ae ha−1 resulted in 81% control. Dicamba applied at 280 g ae ha−1 provided a comparable level of efficacy of 80%, while 2,4-D at 560 g ae ha−1 resulted in 49% control. The addition of glyphosate improved GR E. canadensis control with 2,4-D more than with halauxifen-methyl or dicamba, possibly due to the higher level of control observed with halauxifen-methyl or dicamba alone. Even though applied at 50 to 100 times lower application rates, the efficacy of halauxifen-methyl on E. canadensis was similar to dicamba and greater than 2,4-D. Thus, halauxifen-methyl should be an effective tool for management of GR E. canadensis before planting both conventional and herbicide-resistant soybean varieties, and it precludes the extended preplant application interval required for dicamba in some soybean management systems.

An understanding of environmental factors governing patchy weed distribution in fields could prove to be a valuable tool in weed management. The objectives of this research were to investigate the relationships between weed distribution patterns and environmental properties in two Mississippi soybean fields and to construct models based on those relationships to predict weed distribution. Two months before planting, fields were soil sampled on a 60- by 60-m coordinate grid, and samples were analyzed for calcium, magnesium, potassium, sodium, phosphorus, zinc, cation exchange capacity, percent organic matter, and soil pH. The relative elevation of each sample location was also recorded. Approximately 8 wk after planting, weed populations were estimated on a 30- by 30-m grid over the soil sample grid. Punctual kriging was used to estimate environmental values at each weed sample location. Discriminant analysis techniques were used to evaluate the associations between environmental characteristics on weed population densities of sample areas within each field. Generally, as sicklepod and pitted morningglory infestations increased, the prediction accuracy of the discriminant functions also increased; however, horsenettle infestations were not closely correlated to the environmental properties. Discriminant functions reasonably predicted presence or absence of sicklepod and pitted morningglory within the field. However, validation of the functions across years within the same field and with data collected from the other field resulted in poor classification of all species infestations. Prediction of weed infestations with environmental properties was specific for each field, year, and species.

The absorption and translocation of 14C in 14C-ring-labelled 4-(2,4-dichlorophenoxy)butyric acid (2,4-DB) at 1 and 7 days after foliar treatment of soybean [Glycine max (L.) Merr.] and cocklebur (Xanthium sp.) were investigated at 3, 5, 7, and 11 weeks after planting by means of radioautography and standard radioassay methods. Partitioning of radioactivity varied with plant age and with treatment time. Although a species difference was not observed in the total uptake of 2,4-DB, the rate of penetration of the herbicide was greater in cocklebur than in soybean. The 2,4-DB was retained in the treated leaf and was not translocated to other plant parts. Translocated radioactivity occurred principally in actively growing parts of both species, but only trace quantities were detected in soybean. No radioactivity was detected in the lower leaves of either plant. Plant response to 2,4-DB may be partially explained on the basis of differential penetration and translocation.

Most farmers now rely on herbicides and, to a lesser extent, cultivation to control weeds in Glycine max in the Midwest. However, the general public is concerned that widely used herbicides will contaminate surface and groundwater. Alternative ways to control weeds in field crops are needed to reduce or prevent herbicide contamination of surface and groundwater. A new between-row-mowing weed management system that consists of band-applied herbicides over crop rows + two or more between-row mowings was tested in G. max over 6 yr in Missouri. Mowing weeds close to the soil surface two or more times between crop rows killed or suppressed annual grass and broadleaf weeds, chiefly Setaria faberi, Ambrosia artemisiifolia, and Amaranthus spp., if properly timed. Shading by crop canopy closure contributed to weed suppression in this weed management system. G. max yield also could not be distinguished from weed-free check plots and was greater than the weedy check plots. Herbicide use was reduced 50% by banding because only 50% of the field area was sprayed. The between-row-mowing weed management system may have use in environmentally sensitive areas to help reduce soil erosion or water contamination by herbicides.

The objective of this study was to test the potential for using a single-entry procedure to revegetate Russian knapweed– and green rabbitbrush–infested rangeland. I hypothesized that simultaneously applying an herbicide and seeding in the fall would produce the highest establishment and growth of desired species. For Russian knapweed, treatments included three seeding rates (zero, low, and high) and two herbicides (with and without clopyralid plus 2,4-D) applied in either the spring (2003) or fall (2004). The seed mixture included pubescent wheatgrass, Siberian wheatgrass, and alfalfa. Grasses were seeded on October 21, 2003 (fall dormant), and April 5, 2004 (spring), using a no-till rangeland drill. The seeding rates consisted of 3.4, 6.8, and 9.3 (low) or 5.0, 9.0, and 12.3 (high) kg ha−1 of alfalfa, Siberian wheatgrass, and pubescent wheatgrass, respectively. For green rabbitbrush, treatments included two seeding levels (13.5 kg ha−1 of Siberian wheatgrass and not seeded) and three herbicides (clopyralid, dicamba, and 2,4-D) and a control applied in July. Clopyralid plus 2,4-D (0.21 plus 1.12 kg ha−1) and dicamba at 2.1 kg ha−1 gave 61 and 66% control of Russian knapweed and green rabbitbrush, respectively. Herbicides interacted with seeding to provide the highest density of seeded species on the Russian knapweed site. These data support the hypothesis that simultaneously applying an herbicide and seeding in the fall would produce the highest establishment and growth of desired species. Conversely, only seeding affected Siberian wheatgrass establishment on the rabbitbrush sites. It may be reasonable to seed Siberian wheatgrass without controlling rabbitbrush if forage production is the primary objective.

Bioassays using cell cultures and callus tissues of leafy spurge were devised to evaluate the potential of rhizobacteria as biocontrol agents. Rhizobacteria isolated from roots of leafy spurge seedlings were screened in suspension-cultured leafy spurge cells. Cell viability was assessed using the Evan's blue bioassay 48 h after bacterial inoculation. Among the 30 isolates tested, LS102 and LS105 consistently caused intensive cell death determined by measuring the A630 of the inoculated cell cultures. Cell death was 2.5 to 3 times higher in cultures inoculated with LS105 and LS102, respectively, than in the control. Population levels of the two isolates within cell cultures and callus tissues of leafy spurge increased during the first 48 h. Leafy spurge callus tissues were inoculated with rhizobacteria either directly or by using the Host Pathogen Interaction System (HPIS). The latter exposes calli to bacteria without any physical contact. LS102 caused cellular leakage and eventually death of the callus tissue. Callus growth was reduced by about 30 to 70% when exposed to LS102 and LS105, respectively. Results suggest that these two isolates may affect leafy spurge at the cellular level by different mechanisms. A screening method based on cell cultures and callus tissues offers a good and rapid technique for detecting deleterious rhizobacteria with potential as biocontrol agents for leafy spurge.

The inclusion of competitive crop cultivars in crop rotations is an important integrated weed management (IWM) tool. However, competitiveness is often not considered a priority for breeding or cultivar selection by growers. Field pea (Pisum sativum L.) is often considered a poor competitor with weeds, but it is not known whether competitiveness varies among semileafless cultivars. The objectives of this study were to determine if semileafless field pea cultivars vary in their ability to compete and/or withstand competition, as well as to identify aboveground trait(s) that may be associated with increased competitive ability. Field experiments were conducted in 2012 and 2013 at three locations in western Canada. Fourteen semileafless field pea cultivars were included in the study representing four different market classes. Cultivars were grown either in the presence or absence of model weeds (wheat and canola), and competitive ability of the cultivars was determined based on their ability to withstand competition (AWC) and their ability to compete (AC). Crop yield, weed biomass and weed fecundity varied among sites but not years. Cultivars exhibited inconsistent differences in competitive ability, although cv. Reward consistently exhibited the lowest AC and AWC. None of the traits measured in this study correlated highly with competitive ability. However, the highest-yielding cultivars generally were those that had the highest AC, whereas cultivars that ranked highest for AWC were associated with lower weed fecundity. Ranking the competitive ability of field pea cultivars could be an important IWM tool for growers and agronomists.

The objectives of this research were to characterize the extent of intraspecifc variation in seed characteristics of Powell amaranth and to evaluate whether such variation was correlated with crop rotation history of the collection sites. We compared characteristics of seeds originating from dairy farms with a corn–alfalfa crop rotation history with seeds originating from farms with a history of intensive vegetable production. We hypothesized that (1) multiple years of perennial alfalfa would select for greater seed dormancy and longevity in seeds of the summer annual Powell amaranth, (2) earlier spring planting dates of corn and alfalfa compared with most vegetable crops would select for earlier emergence, and (3) greater competition and lower soil moisture in the nonirrigated corn–alfalfa rotation would select for greater seed size. Seeds from 10 to 20 plants from each of 10 farms from each habitat were collected in the fall of 2002 and 2003 in central New York. To control for maternal effects on seed dormancy, a second generation of seeds was produced from plants grown under common greenhouse conditions. Germination in petri dishes was greater for second-generation seeds from vegetable farms (46%) than for those from dairy farms (32%). Total emergence following overwinter burial in the field was greater for seeds originating from dairy farms (62%) compared with those from vegetable farms (52%). Neither seed weight nor the rate of emergence varied by habitat of origin. Our results suggest that perennial alfalfa in dairy rotations may have selected for greater dormancy and longevity of Powell amaranth seeds. The large intraspecific variation in seed characteristics observed, underscores the importance of considering multiple populations when making comparisons of germination characteristics across biotyes (e.g., resistant vs. susceptible) or species, or when developing and interpreting models of weed emergence or weed population dynamics.

Levels of MSMA (monosodium methanearsonate) that simulated drift (0.2, 0.39, and 0.78 kg/ha) were applied over-the-top to rice (Oryza sativa L. ‘Vista’, ‘Starbonnet’, or ‘Labelle’) after flooding at four developmental stages between early tillering and late jointing. The MSMA rates used were equivalent to, or lower than, the 0.78-kg/ha rate that would be applied as an over-the-top application to cotton (Gossypium hirsutum L.) for early season weed control. Over-the-top applications of MSMA did not affect heading and maturity dates of filled grains regardless of the rate or application timing. Reductions occurred, however, in plant height, number of panicles, and yield. In addition, the percentage of erect panicles showing typical “straighthead” symptoms increased. Rice sensitivity to MSMA depended on MSMA rate and stage of growth with injury being most severe as rice approached the reproductive stages of development. Because aerial applications of MSMA to cotton are normally made before rice reaches the reproductive stage, injury resulting from MSMA drift should be minimal.

Postemergence applications of atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl) oxirane] and atrazine, and tridiphane and cyanazine {2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl] amino]-2-methylpropanenitrile} resulted in 90% or better control of giant foxtail (Setaria faberi Herrm. # SETFA) less than 2.5 cm in height. Delaying the application of tridiphane plus atrazine or atrazine alone reduced the control of giant foxtail in 1984, control of giant foxtail over 2.5 cm in height was significantly greater with a mixture of tridiphane and atrazine (0.4 and 1.7 kg ai/ha) than with atrazine alone (2.2 kg/ha) or tridiphane plus cyanazine (0.6 and 1.8 kg/ha). Two successive applications of tridiphane and atrazine combinations controlled giant foxtail 8 to 16 cm tall (four to five leaves). Cultivation following applications of tridiphane plus atrazine improved giant foxtail control but was less effective than a second application of atrazine or tridiphane plus atrazine. Giant foxtail control in greenhouse experiments was similar to control obtained in the field. Tridiphane plus atrazine controlled 70% of giant foxtail in the four- to five-leaf stage, while atrazine applied alone resulted in only 23% control, indicating a synergistic effect.

Greenhouse and field experiments were conducted to evaluate the efficacy of chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} on Russian thistle (Salsola iberica Sennen and Pau # SASKR) in a spring wheat (Triticum aestivum L.) summer-fallow rotation. In the greenhouse, when chlorsulfuron was applied to Russian thistle at three stages of growth, shoot dry weight was reduced similarly with either 4.4 g ai/ha plus surfactant or 140 g ai/ha plus surfactant. In the field, postharvest control (1981) of Russian thistle with 17.5 to 140 g/ha of chlorsulfuron ranged from 35 to 60% without surfactant and from 76 to 93% with surfactant. During the summer-fallow year (1982), Russian thistle treated in the preflowering stage of growth was not effectively controlled with 17.5 or 35 g/ha. In contrast, residual control from postharvest treatments 1 yr after application ranged from 90 to 100%. Chlorsulfuron applied in the crop (1983) controlled Russian thistle throughout the growing season. All treatments of chlorsulfuron applied in 1981, 1982, and 1983 increased wheat yield at least 75%.

Freshly harvested smallflower morningglory [Jacquemontia tamnifolia (L.) Griseb. # IAQTA] seed germinated best after 25 to 60 s of scarification in an electric scarifier equipped with 100 to 120 grit emery cloth. Optimum temperature for germination was 25C. At 10 and 45C no germination occurred for up to 7 days. Eight and 12 days after planting in silt loam soil there was 85 and 98% emergence of smallflower morningglory from depths of 0.5 to 1 cm, whereas no emergence was observed from depths of 6 cm or greater 12 days after seeding.

Butylate [S-ethyl bis(2-methylpropyl)carbamothioate] persistence was compared in three soils that were treated annually with four thiocarbamate herbicides in 1981 and 1982. Butylate degraded faster in soils treated two consecutive years with butylate + dichlormid (2,2-dichloro-N,N-di-2-prophenylacetamide) than in soils treated for 2 yr with EPTC (S-ethyl dipropyl carbamothioate) + dichlormid, cycloate (S-ethyl cyclohexylethylcarbamothioate), vernolate (S-propyl dipropylcarbamothioate) + dichlormid, or from previously untreated soils. Butylate + dichlormid applications in 1981 and 1982 enhanced butylate degradation in 1983 at all locations and reduced weed control at two locations. Previously untreated soil or soil previously treated with two annual applications of butylate + dichlormid was treated with 6 ppmw butylate, incubated for 14 days, analyzed for residual butylate, and bioassayed for herbicide activity. Bioassay indicated higher herbicidal activity than was predicted by the residual butylate concentration. A herbicidally active metabolite may be produced during the soil degradation of butylate.

We studied changes in the concentration of 4-amino-3,5,6-trichloropicolinic acid (picloram) with time in roots, stems, and leaves of 20-day-old seedlings of huisache (Acacia farnesiana (L.) Willd.) and honey mesquite (Prosopis juliflora (Swartz) DC. var. glandulosa (Torr.) Cockerell). Exposing root systems to aqueous solutions of picloram (1.0 ppm huisache and 10.0 ppm honey mesquite) for 24 hr killed approximately 60% of the treated plants. In honey mesquite, picloram was redistributed and eventually lost over a 5-day period, whereas neither redistribution nor loss occurred in huisache.

The importance of managing weeds in seminatural habitats that are adjacent to farm fields is unclear. Weedy-margin vegetation may harbor pests or pathogens and may ALSo serve as source populations for ongoing immigration of weeds into the field. It is ALSo possible, however, that margin vegetation provides habitat for organisms that consume weed seeds or suppress the likelihood of pest or pathogen outbreak. We examined the nature of margin habitat using spatial-scaling of Weed-Species richness as an ecological assay. In 2003, we recorded the occurrence of weedy species along the perimeters of 63 fields in Wisconsin. The fields were distributed within six counties that differed in topography, geological history, local climate, and soil type and which spanned the range of variability in the agricultural landscape. We identified seven habitats that differed in geology and land use. The relationship between species richness and margin class was estimated using an analog of the power law. Additionally, we investigated broadscale correlates of habitat heterogeneity at the field level, using a modeling strategy that included additional explanatory factors logically connected to plant diversity. Using a model-confrontation approach, the survey supported the inclusion of two topographical diversity indices, elevation gradient and a field-shape index, into our model. Our broadscale survey provides information on one of a suite of important considerations needed to make decisions about the importance of managing weeds in field margins.

Atrazine has been the most widely used herbicide in North American processing sweet corn for decades; however, increased restrictions in recent years have reduced or eliminated atrazine use in certain production areas. The objective of this study was to identify the best stakeholder-derived weed management alternatives to atrazine in processing sweet corn. In field trials throughout the major production areas of processing sweet corn, including three states over 4 yr, 12 atrazine-free weed management treatments were compared to three standard atrazine-containing treatments and a weed-free check. Treatments varied with respect to herbicide mode of action, herbicide application timing, and interrow cultivation. All treatments included a PRE application of dimethenamid. No single weed species occurred across all sites; however, weeds observed in two or more sites included common lambsquarters, giant ragweed, morningglory species, velvetleaf, and wild-proso millet. Standard treatments containing both atrazine and mesotrione POST provided the most efficacious weed control among treatments and resulted in crop yields comparable to the weed-free check, thus demonstrating the value of atrazine in sweet corn production systems. Timely interrow cultivation in atrazine-free treatments did not consistently improve weed control. Only two atrazine-free treatments consistently resulted in weed control and crop yield comparable to standard treatments with atrazine POST: treatments with tembotrione POST either with or without interrow cultivation. Additional atrazine-free treatments with topramezone applied POST worked well in Oregon where small-seeded weed species were prevalent. This work demonstrates that certain atrazine-free weed management systems, based on input from the sweet corn growers and processors who would adopt this technology, are comparable in performance to standard atrazine-containing weed management systems.

A phytotoxin that inhibited seedling root growth was isolated and purified from rhizomes of quackgrass [Agropyron repens (L.) Beauv.]. Aqueous solutions of the phytotoxin (0.1%, w/v) significantly inhibited the seedling root growth of corn (Zea mays L. ‘New Jersey 8’), oat (Avena sativa L. ‘Clintland 60’), cucumber (Cucumis sativus L. ‘Straight Eight’), and alfalfa (Medicago sativa L. ‘Vernal’). Growth inhibition was not due to extremes of pH or osmotic potential of the inhibitor solutions. The phytotoxin was isolated and purified from a methanol:water extract of dried and ground rhizomes by adsorption and elution from charcoal. A golden-brown, crystalline substance was obtained that was soluble in water, partly soluble in semipolar solvents, and insoluble in nonpolar solvents. Thin layer chromatography (TLC) and high pressure liquid chromatography (HPLC) suggested that a single compound was present. Elemental analysis showed that it contained 42.17% carbon, 5.06% hydrogen, 38.28% oxygen, 4.02% nitrogen, 2.03% sulfur, 0.03% chlorine, and 0.03% phosphorus. The compound was tentatively identified as a glycoside with a molecular weight of 460. Its ultraviolet (UV) absorption spectrum showed an intense maximum at 307 nm.

The establishment of common milkweed in corn, soybean, and wheat and rotations of corn-soybean and soybean-wheat was determined in field studies at Rosemount and Morris, MN. Common milkweed seedling density 115 d after initial seedling emergence was 500, 15,000, and 31,300 seedlings ha−1 in corn, soybean, and wheat, respectively, in 1990; 500, 7,300, and 15,600 in 1991 at Rosemount and 2,900, 26,500, and 37,000 in 1990; and 2,200, 6,700, and 8,100 in 1991 at Morris in the year of seedling establishment. Common milkweed seedling defoliation by mon arch butterfly larva was 1,31, and 0% at Rosemount and 20, 78, and 0% at Morris for corn, soybean, and wheat, respectively. In the 2nd and 3rd rotational years, corn-soybean rotations had fewer shoots ha−1 115 d after shoot emergence than soybean wheat rotations. Seedling densities 115 d after initial seedling emergence were greater in corn-soybean rotations where soybean rather than corn was the rotational crop and in soybean-wheat rotations where wheat rather than soybean was the rotational crop immediately following common milkweed seeding. Conventional tillage treatments had lower common milkweed shoot density than conservation tillage in all years of all rotations except in the initial count, when soybean was the first rotational crop in which shoot density was greater in conventional than in conservation tillage treatments. Cyanazine, imazethapyr, or bromoxynil applications in corn, soybean, or wheat rotational crops, respectively, reduced common milkweed shoot density.

Strategies for controlling invasive plants in northern Australian ecosystems must be designed to use limited resources. This paper examines the potential contribution of ecology to develop management strategies for Indian jujube, an invasive shrub in northern Australian tropical woodlands. Its fleshy fruits are dispersed by a variety of birds and mammals. Domestic cattle are the major dispersal vector, whose movements can be controlled at critical times to minimize the risk of new infestations. Containment of Indian jujube must include avoiding cattle movement from infested to weed-free paddocks and from infested to weed-free areas. The species has a capacity to sprout following topkill due to fire or mechanical treatment. This means that current options for effectively treating established plants are limited to the application of herbicides and the more expensive mechanical techniques. In single paddocks, concentrating limited resources on large reproductive plants may be more effective than concentrating on plants spatially peripheral to an infestation. At the property level, the first priority should be to prevent weed-free paddocks from becoming infested, the second should be to minimize increases in lightly infested paddocks, and the third to treat heavily infested paddocks that are major potential sources of new infestations. At the regional level, distributional outliers become important; their control may significantly reduce the length of the front from which local range expansion will occur.