The first step in a satisfactory forage-legume program on the farm is the establishment of the legume. Success or failure here can mean the difference between a continued efficient rotation system and a financial loss. Many farmers have been reluctant to plow up old seedings at the proper time because of the cost of new seedings and the possibilities of seeding failures. Also, new seedings may appear successful in the seeding year only to be lost during the first winter. This paper is a report of 3 years' study at Ithaca, New York, on the use of herbicides applied pre-emergence to aid in the establishment of forage legumes without a companion grain crop.

The first case of evolved protoporphyrinogen oxidase (PPO)-inhibitor resistance was observed in 2001 in common waterhemp [Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif]. This resistance in A. tuberculatus is most commonly conferred by deletion of the amino acid glycine at the 210th position (ΔGly-210) of the PPO enzyme (PPO2) encoded by PPX2. In a field in Kentucky in 2015, inadequate control of Amaranthus plants was observed following application of a PPO inhibitor. Morphological observations indicated that survivors included both A. tuberculatus and Palmer amaranth (Amaranthus palmeri S. Watson). Research was conducted to confirm species identities and resistance and then to determine whether resistance evolved independently in the two species or via hybridization. Results from a quantitative PCR assay based on the ribosomal internal transcribed spacer confirmed that both A. tuberculatus and A. palmeri coexisted in the field. The mutation conferring ΔGly-210 in PPO2 was identified in both species; phylogenetic analysis of a region of PPX2, however, indicated that the mutation evolved independently in the two species. Genotyping of greenhouse-grown plants that survived lactofen indicated that all A. tuberculatus survivors, but only a third of A. palmeri survivors, contained the ΔGly-210 mutation. Consequently, A. palmeri plants were evaluated for the presence of an arginine to glycine or methionine substitution at position 128 of PPO2 (Arg-128-Gly and Arg-128-Met). The Arg-128-Gly substitution was found to account for resistance that was not accounted for by the ΔGly-210 mutation in plants from the A. palmeri population. Results from this study provide a modern-day example of both parallel and convergent evolution occurring within a single field.

Field and greenhouse experiments were conducted from 2013 to 2015 at the University of Wyoming to evaluate the response of Beta vulgaris (L.) to reflected-light quality. Large-pail field studies included a factorial arrangement of three varieties of B. vulgaris (sugar beet, table beet, and Swiss chard) and reflected-light treatments (using either colored plastic mulch, grass, or bare-soil controls). Greenhouse studies included sugar beet as influenced by either grass or soil surroundings. In all studies, grass was grown in separate containers from B. vulgaris, so there was no root interaction. Grass was clipped regularly to prevent shading and competition for sunlight. Reflected light from different-colored plastic mulches (red, blue, green, black, clear) did not affect B. vulgaris growth. However, reflected light from the grass reduced the number of leaves in all B. vulgaris varieties such that there were 10 to 14 fewer leaves in B. vulgaris surrounded by grass compared with the soil treatment at 90 d after planting in the field study. Shade avoidance cues from surrounding grass reduced B. vulgaris total leaf area by 49% to 66%, leaf biomass by 21% to 30%, and root biomass by 70% to 72%. Similar results were observed in greenhouse experiments, where the grass treatment reduced sugar beet leaf biomass by 48% to 57% and root biomass by 35% to 64%. Shade avoidance cues have the potential to significantly reduce B. vulgaris yield, even in the absence of direct resource competition from weeds.

Weed management in tobacco (Nicotiana tabacum L.) is accomplished primarily with soil-residual herbicides, cultivation, and hand removal. Management practices that reduce weed emergence, like reduced tillage and cover crop mulches, may improve weed management efficacy. Depending on cover-cropping goals, growers face trade-offs in species selection and management priorities—producing weed-suppressive mulches may lead to transplanting difficulties and soil-residual herbicide interception. Managing more complex cover crop mixtures may result in different challenges. We established on-farm trials across 4 site-years to study impacts of cover crop composition [wheat (Triticum aestivum L.) monoculture or mixture], termination treatment (early or late chemical termination or removing aboveground biomass), and soil-residual herbicides on weed density and biomass. The cover crop mixture contained cereal rye (Secale cereale L.), crimson clover (Trifolium incarnatum L.), and hairy vetch (Vicia villosa Roth.), with canola (Brassica napus L.) at 1 site-year. The mixture typically produced more biomass than monoculture wheat, although composition had few impacts on weed density or biomass. With residual herbicides, termination treatment had few impacts on weed density, suggesting that residues did not adversely affect herbicide efficacy. Without residual herbicides, early-season weed density was often higher following the late-terminated cover crop compared with other termination treatments, though midseason weed density was typically lower. When termination treatment affected final weed biomass, it was lower following late termination, with one exception—crop establishment was reduced at 1 site-year, leading to reduced weed–crop competition and greater weed biomass. Our results suggest that growers can use mixtures and, if well-timed to a rainfall event for incorporation, still effectively use soil-residual herbicides to maintain adequate weed control in tobacco regardless of how the cover crop is managed. Later termination, resulting in more residue, may lead to less weed biomass accumulation in the absence of herbicide use.

The level of glyphosate resistance in kochia [Bassia scoparia (L.) A. J. Scott] was reported to be due to an increase in 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene copy number. A field study was conducted near Manhattan, KS, in 2014 and 2015 to evaluate the relationship between EPSPS gene copy number and growth and fecundity variables of B. scoparia individuals within suspected glyphosate-resistant (GR) populations from western Kansas. Initial assays of EPSPS gene copy and in vivo shikimate accumulation showed that B. scoparia populations from Finney (FN-R), Scott (SC-R), and Thomas (TH-R) counties were segregating for glyphosate resistance, with some individuals still being glyphosate susceptible (GS). A target-neighborhood competition approach was used to evaluate the competitive response of individual target plants with relatively low (classified as GS) and high (classified as GR) EPSPS gene copy number within the populations. There was no relationship observed between EPSPS gene copy number and vegetative or fecundity variables. There was no differential competitive response of target plant biomass to increasing neighbor density between individuals with low and high EPSPS gene copy number within each population. Lack of associated vegetative growth and fecundity cost to the increased EPSPS gene copy in the GR B. scoparia plants suggests that the plants are likely to persist in field populations, except when effective weed management strategies are adopted that would prevent their growth and seed production.

Field studies were conducted on a Taloka silt loam (Mollic Albaqualf) to determine the effects of residual levels of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] on rice (Oryza sativa L.). Yield reductions could be overcome by increasing rice seeding rate, by decreasing seeding depth, or by applying the safeners CGA-43089 [α-(cyanomethoxyimino)-benzacetonitrile] or CGA-92194 [α-(1,3-dioxolan-2-yl-methoximino)-benzacetonitrile] to the rice seed. Greenhouse studies with the safeners and reduced seeding depths indicated that CGA-92194 was better than CGA-43089 for protecting rice from metolachlor injury, but reduced seeding depths did not reduce metolachlor injury.

Canada thistle poses a particular threat to organic producers in temperate agriculture due to its ability to reproduce through an extensive system of underground roots. The Canada thistle life cycle, growth, and development are seasonally affected, and exploiting this biology may be useful for weed management. The objective of this study was to evaluate smother crop mixtures, seeded at different times, for Canada thistle control. Field trials were established in 2009 and 2010 to evaluate the ability of smother crop mixtures to suppress Canada thistle growth and development. Canada thistle aboveground biomass was suppressed 50% in 2009 and 87% in 2010 by the sorghum–sudangrass mixture, averaged over planting times. The oat mixture suppressed annual weed biomass more than 58% in 2009 and 67% in 2010 in all planting dates. Percent cover of Canada thistle was affected by crop mixture in 2009 and 2010, with sorghum–sudangrass being the most suppressive. The sorghum–sudangrass mixture was more suppressive of Canada thistle, probably because it included soybean and sunflower, all high-biomass, competitive crops. Planting date affected smother crop suppression of Canada thistle growth, but the effect was not consistent between 2009 and 2010 due to differences in weather conditions.

The adsorption-desorption equilibria of chlorbromuron [3-(4-bromo-3-chlorophenyl)-1-methoxy-1-methylurea] in soil conformed to the Freundlich equation. Values of the constants, k and n, ranged from 4.2 to 12.3 and from 1.2 to 2.5, respectively, in eight soils. No significant linear correlation was detected between the Freundlich k values and soil texture, organic-matter content, pH, or water-holding capacity. Degradation studies were conducted using 14C-carbonyl-radiolabeled chlorbromuron in soil incubated at 18 or 32 C, with and without prior autoclaving, and with and without glucose enrichment. Degradation was enhanced by the higher temperature and glucose amendment, and it was retarded by autoclaving. Thin-layer chromatography indicated no accumulation of the monomethyl or monomethoxy derivative of chlorbromuron in any of the treatments. Chlorbromuron degraded rapidly in the incubation studies. Rapid loss of phytotoxicity was also noted in a greenhouse study with cucumber (cucumis sativus L. ‘Explorer’) bioassay. Phytotoxicity from 4 ppmw chlorbromuron had diminished to a very low level after 12 weeks.

A laboratory procedure for evaluating the effect of pesticides on nitrification in soil proved to be simple to perform, reproducible, and offers a procedure for rapid screening of a large number of chemicals in a short period. The recovery of added nitrate by extracting with distilled water was essentially 100% complete. The conversion of added ammonium to nitrate by the soil microorganisms was nearly complete after the 2-week incubation period. Nitrification in soil treated with several different herbicides and insecticides was determined by our procedure. A nitrification inhibitor, N-Serve3 (2-chloro-6-trichloromethyl pyridine) was included as a standard. None of the herbicides or insecticides inhibited nitrification and the N-Serve completely inhibited nitrification during the 2-week incubation.

Conflicting reports on the dissipation of ie isopropyl N-phenylcarbamate (IPC) and isopropyl N-(3-chlorophenyl) carbamate (CIPC) prompted the present investigation on the dissipation and biological activity of these two herbicides. The dissipation patterns of IPC and CIPC were similar. Under the field and laboratory conditions described, more than 90% of both herbicides was gone in 4 weeks. CIPC was biologically more active than IPC on all plants tested, and both herbicides were nearly 10 times more toxic to buckwheat (Fagopyrum esculentum Moench.) in quartz sand as in a silt loam soil. Although both herbicides were present in equal amounts throughout their dissipation, CIPC appears to persist longer in field applications, as it is more toxic to most plants than IPC.

Selected paraffinic and naphthenic phytobland oils used as adjuvants markedly enhanced the foliar uptake and acropetal translocation of ring-labeled atrazine [2-chloro-4-(ethylamino)-6-isopropylamino-s-triazine] by corn (Zea mays L. ‘VPI 646′), when compared with atrazine-14 C applied with nonionic polysorbate surfactant or in aqueous 25% methanol alone. Generally, 14C uptake increased with increasing concentration (1.25 to 20.0%) of a 100-sec viscosity paraffinic oil and with time (up to 32 hr) after application. No consistent additional promotion of atrazine-14 C uptake was derived from the use of emulsifier concentrations above 2.0% in the oil of a 10% oil-in-water emulsion. With a few exceptions, paraffinic and naphthenic oils of comparable viscosities were about equally effective as penetrant aids for atrazine. For both the paraffinic and naphthenic series, the 100-sec viscosity oils appeared to be slightly better penetrant aids than comparable 70, 150, and 200-sec oils.

Ethofumesate [(±)-2-ethoxy-2,3-dihydro-3,3-dimethyl-5-benzofuranyl methanesulfonate] applied postemergence in November at rates from 1.4 to 4.5 kg/ha eliminated all annual bluegrass (Poa annua L.) that had survived application of diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] in fall carbon-planted perennial ryegrass (Lolium perenne L.) seed fields. Ethofumesate also eliminated all annual bluegrass in established perennial ryegrass seed fields when applied in late October at rates from 1.1 to 3.4 kg/ha or when applied as sequential treatments with atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] at 1.4 kg/ha or with simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] at 2.2 kg/ha. In most experiments, perennial ryegrass seed yields were not adversely affected by applications of ethofumesate alone or by sequential treatments with diuron, atrazine, or simazine.

Uptake of 2,4,5-T-1-14C [(2,4,5-trichlorophenoxy)-acetic acid] by immersion of honey mesquite [Prosopis juliflora (Swartz) DC var. glandulosa (Torr.) Cockerell] leaflets rapidly diminished as pH was increased from 3.5 to 9.5. Uptake diminished less rapidly as pH increased when 10-μl droplets were applied to leaflets that were kept moist. Uptake was equivalent from solutions of pH 3.5, 5.5, and 7.5; but severely reduced at pH 9.5, when leaflets were droplet treated and let dry. Uptake under dry conditions from pH 7.5 and 9.5 droplets containing 1 M NH4Cl was equivalent to uptake from pH 3.5 and 5.5 droplets lacking NH4Cl. NH4Cl had no enhancing effect on uptake at any pH when leaflets were immersed or droplet-treated and maintained moist. Low concentrations of urea had no enhancing effect on uptake at pH 9.5 by droplet-treated leaflets that were allowed to dry. Urea concentrations above 0.1 M inhibited uptake.

Soil-applied imazaquin or chlorimuron plus metribuzin reduced ivyleaf morningglory photosynthetic rate, leaf area, biomass, and seed production per plant compared to the nontreated control. Photosynthetic rate of ivyleaf morningglory was reduced a minimum of 53%, and total leaf area was reduced at least 67% with either herbicide, regardless of ivyleaf morningglory densities. Ivyleaf morningglory biomass and seed production were reduced at least 62% with either herbicide across all ivyleaf morningglory densities. Soybean leaf area, photosynthetic rate, and biomass were not affected by herbicide treatment or ivyleaf morningglory densities of 2, 4, 6, or 8 per m of row.

A better understanding of the influence of various crop and weed management practices on spatiotemporal dynamics of weeds could improve the design of integrated weed management systems. We examined the influence of 18- and 76-cm soybean row spacings on emergence pattern and spatial aggregation of giant foxtail, common lambsquarters, and velvetleaf seedling cohorts. In addition, we characterized the soil seedbank and determined the quantitative and spatial relationship between the seedbank and seedling populations. Viable seeds of about 10 weed species and twice as many species of seedlings were identified in the weed community. Giant foxtail and common lambsquarters were the predominant species in the seedling and seedbank population, respectively, each accounting for 60 to 70% of the total weed species density. Emergence of giant foxtail, common lambsquarters, and velvetleaf depleted 12 to 33%, < 2% and 12 to 49% of the seedbank in the upper 10 cm of the soil profile. Peak time and periodicity of weed emergence was not influenced by soybean row spacing, and peak time of emergence of giant foxtail, common lambsquarters, and velvetleaf occurred 3 to 4, 3 to 6, and 3 to 9 weeks after soybean planting (WAP), respectively. Magnitude of giant foxtail emergence 5, 6, and 9 WAP was 98, 96, and 76% greater in 76- than in 18-cm row soybeans only when the population of 76-cm row soybeans was 57% lower than the 18-cm soybeans in 1997. Giant foxtail and common lambsquarters seeds in the seedbank were aggregated in 1996 and 1997 according to the Taylor power law (TPL) and the negative binomial distribution (NBD). The TPL and the NBD were similar in describing the spatial aggregation of giant foxtail and common lambsquarters but not some velvetleaf seedling cohorts. The spatial aggregation of seedlings varied among cohorts for different weed species and was likely due to species-specific biological characteristics that influence seed dispersal, germination, and seedling emergence. Within a 1.5-ha area, aggregation declined with decreasing density. Within a 24-m2 area, the level of aggregation of all weed species decreased as seedling densities increased. These results indicated that soybean row spacing influenced neither weed emergence pattern nor weed spatial aggregation; thus, several management decisions can be similar in 18- and 76-cm row soybeans.

Germination and longevity of weedy and three domestic okra [Abelmoschus esculentus (L.) Moench. # ABMES] cultivars were investigated. Hard seed coats are the major reason for okra seed dormancy. Hard seeds of weedy okra, induced to germinate by scarification of seed coats, produced plants yielding 94 to 99% hard seeds. Of these, 40% remained hard but viable after overwintering in the soil at 5 cm deep. Nonhard seeds of weedy okra produced plants that yielded 95 to 99% hard seeds, but only 20% remained hard after overwintering in the soil at 5 cm deep. The ‘White Velvet’ cultivar of okra produced a few hard seeds, but none survived longer than 3 months in the soil. The other domestic cultivars, ‘Dwarf Green Long Pod’ and ‘Clemson Spineless', produced no hard seeds. None of these seeds survived over winter in the soil. Some seeds of White Velvet became slightly harder during dry storage based on time in concentrated H2SO4 necessary to induce 80% germination. Although the hard-seed trait existed in the population of this domestic cultivar, it is unlikely that the seeds would overwinter. Seeds that overwinter in a dry condition may be the exception. Because a high percentage of hard seeds overwinter and germinate the following spring, weedy okra has the potential to become a persistent problem.

Seven herbicides and two herbicide combinations were applied once either preplant or postplant to control perennial grasses and forbs as planting-site preparation for 2-yr-old seedlings of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) and Douglas fir [Pseudotsuga menziesii var. glauca (Beissn.) Franco] at locations in Washington and Oregon. Hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H, 3H)-dione] applied at 2.2 kg/ha provided most effective and lasting herbaceous weed control in both states. A combination of dalapon (2,2-dichloropropionic acid) and atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] applied at 9.0 + 4.5 kg/ha also proved similarly effective. In Washington, each conifer species responded with exponential growth increases over a 6-yr period. For ponderosa pine and Douglas fir, respectively, hexazinone treatment increased tree height by 58 and 70%, stem diameter by 70 and 69%, and stem-volume yield by 387 and 650% as compared to untreated checks. Corresponding gains for the two conifers associated with dalapon + atrazine were 73 and 54% in height, 63 and 46% in diameter, and 421 and 349% in yield.

Broadcast applications of hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione] pellets and foliar sprays were tested at four rates for hardwood control and safety to loblolly pine (Pinus taeda L.) at each of eight study locations differing in soil characteristics. Reduction in the number of hardwoods in the stand (hardwood density reduction) was greater with the pellet on soils with more than 60% sand, while the liquid formulation was most efficacious for finely textured soils. Hardwood density reduction with the pellet was negatively correlated with percent silt, clay, soil organic matter, and cation exchange capacity, and positively correlated with percent sand. With foliar sprays, hardwood density reduction was positively correlated with hexazinone rate and negatively correlated with soil pH. Pine mortality was positively correlated to percent sand with the pellet and negatively correlated to soil pH with broadcast sprays. Regression models incorporating pine height, herbicide rate, soil texture, cation exchange capacity, soil organic matter, and acidity could explain up to 78% of the variation in hardwood density change and 77% of the variation in pine mortality. Selective control of hardwoods in young loblolly pine stands is a function of hexazinone rate, formulation, and various soil factors.

Combinations of herbicides applied preemergence and postemergence, with and without cultivation, were evaluated for their effectiveness for controlling weeds in soybeans [Glycine max (L.) Merr. ‘Williams’]. Annual weeds were controlled best when a sequential treatment of a mixture of alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] + linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] was applied preemergence, and glyphosate [N-(phosphonomethyl)glycine] was applied postemergence, followed by one cultivation. The largest soybean yields were obtained when alachlor + linuron applied preemergence was followed by a cultivation, bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] applied postemergence, bentazon applied postemergence and a cultivation, or glyphosate applied postemergence and a cultivation. Regression analysis of the soybean yield vs. weed control data for all treatments indicated that soybean yields were suppressed an average of 180 kg/ha when glyphosate was applied postemergence. A cultivation in addition to the preemergence and postemergence treatments improved both weed control and soybean yield.

Yields of cotton (Gossypium hirsutum L. ‘Stoneville 213′) did not differ with various cultivation practices on two soils in Alabama over a 3 to 5-yr period. Cultivation treatments resulted in cotton yields equivalent to but not greater than those resulting from weed removal without soil stirring. However, lack of any weed control resulted in more than 90% yield reduction on Decatur clay loam soil based on a 5-yr average and 95% yield reduction on Hartsells fine sandy loam soil, based on a 3-yr average.