Wild oat, a troublesome weed in small grain cereals, infests about 11 million ha of cropland in the United States. Diversifying cereal production with alternative crops, such as yellow mustard and canola, provides flexible cropping systems, decreases production risks, and may allow more effective weed suppression. A greenhouse study was conducted to assess the competitive ability of yellow mustard and canola with wild oat in 1999 and 2000, using replacement series interference experiments to relate the results to plant development stages. Yellow mustard, regardless of its proportion in mixture, reduced aboveground biomass of wild oat 33 to 66%, leaf and tiller number 34 to 36%, and panicle production 58% compared with wild oat in monoculture. Canola did not affect wild oat biomass in mixtures. Yellow mustard per plant biomass in 2000 and inflorescence production in 1999 decreased 30 and 20% with increased density of yellow mustard in mixtures. Yellow mustard biomass was not affected by the addition of wild oat to the mixture, indicating the greater importance of intraspecific competition between yellow mustard relative to interspecific competition with wild oat. Canola per plant biomass was affected more by interspecific competition with wild oat than by intraspecific competition. A second greenhouse experiment was conducted to compare plant height and biomass accumulation by the three species over 7 wk. Yellow mustard had the greatest biomass accumulation and plant elongation rate, followed by canola and wild oat. The greater competitive ability of yellow mustard with wild oat, compared with canola, is likely associated with the rapid growth and canopy elevation of yellow mustard.

Growth chamber experiments evaluated the influence of ambient temperature and soil moisture on cotton and velvetleaf response to pyrithiobac. Additional studies determined the basis for observed plant responses to 14C pyrithiobac. Cotton injury from six times the normal dosage was < 20% at 2 wk for all temperatures and soil moistures. Pyrithiobac injured velvetleaf less at lower soil moistures. Both species absorbed more 14C-pyrithiobac at 30/28 or 35/33 C than at 25/23 C. Cotton absorbed more herbicide than velvetleaf at all temperatures and soil moistures. Velvetleaf translocated < 16% of absorbed 14C out of the treated leaf while cotton translocated < 3% of absorbed material. At warmer temperatures, velvetleaf translocated less 14C when soil was dry (–1.0 MPa) than when plants were watered to field capacity (–0.03 MPa). This decreased absorption and translocation may affect pyrithiobac activity on velvetleaf growing in dry soil. Translocation differences did not fully explain whole plant effects. The metabolism difference may account for cotton tolerance.

Experiments were conducted in growth chambers by growing oats (Avena sativa L. ‘Ansi’) in water culture and soil with admixed TCA (trichloroacetic acid), pyrazon [5-amino-4-chloro-2-phenyl-3-(2H)-pyridazinone], and two mixtures (pyrazon: less TCA; 1:25 and 1:50). The sigmoid log-dose relationship for dry-matter yield was fitted with a four-parameter logistic equation. Generally, the mixtures appeared to be less active than would be expected under an additive dose model. Around GR50 the departure of joint action of the mixtures from the reference model was almost similar in the two growth media. At higher dose levels the mixtures approached the reference model, and the bioactivity of one mixture (1:25) in the water culture experiment even showed a higher activity than would be expected from the reference model.

Understanding the environmental variables influencing the phenological development of weeds is essential for simulation model development. Temperature and photoperiod are important variables governing the phenological development of weeds. Growth cabinet studies were conducted to characterize the phenological development of Setaria viridis in response to variations in temperature and photoperiod and to determine the duration of the juvenile phase and the effect of temperature and photoperiod on reproductive development. Setaria viridis was adapted to a temperature range from 6.5 to 47 C. Phenological development of S. viridis was described accurately in terms of thermal days (cumulative day degrees above a base temperature) and biological days (Bd: chronological days at the optimum temperature and photoperiod). Four developmental phases of S. viridis were described: (1) a juvenile (photoperiod insensitive) phase of 2.6 Bd; (2) a photoperiod-sensitive inductive phase of 2.2 Bd; (3) a photoperiod-sensitive postinductive phase of 6.0 Bd; and (4) a photoperiod-insensitive inductive phase of 10.9 Bd. Photoperiod sensitivity of S. viridis did not differ with stage of development when expressed as a rate. Interpretation of constant sensitivity to photoperiod will simplify simulation of weed phenology in mechanistic models.

Rush skeletonweed (Chondrilla juncea L. CHOJU) infestations occur along the eastern seaboard and in several western states of the United States. This Eurasian species was inadvertently introduced prior to 1870, with established stands first reported in Maryland and West Virginia (16). These infestations (16) were assessed as lacking aggressive characteristics and posed little threat as a problem weed. Although rush skeletonweed was discovered in the Pacific Northwest as early as 1938, the species was not recognized as a potential weed problem until nearly three decades later (27). Subsequent surveys revealed that infestations occupied over 2.3 million ha in California, Idaho, Oregon, and Washington (6). Attempts to generate support for an organized control program in Idaho were met with little enthusiasm during the 1960's.

A 3-yr study was conducted to compare the performance of several herbicides applied alone and in mixtures under three flooding conditions in dry-seeded rice (Oryza sativa L. ‘Labelle’). When the soil remained moist during the 2-week unflooded periods (1979 and 1981), weed emergence was slowed and no advantage with respect to enhanced weed control and increased rice yields could be demonstrated over that of the standard sequential propanil (3′,4′-dichloropropionanilide) application. However, in 1980 when drought stress may have reduced initial postemergence activity and weeds emerged during the 14-day unflooded periods, differences in weed control between herbicides were obtained. Results of this study also indicate that yield losses from weed competition resulted primarily from early season competition, and that the principal advantage of the residual herbicides in dry-seeded rice may be to eliminate the need for retreatment with propanil when flooding is delayed or fields are drained.

Clethodim resistance was identified in 12 rigid ryegrass populations from winter cropping regions in four different states of Australia. Clethodim had failed to provide effective control of these populations in the field and resistance was suspected. Dose–response experiments confirmed resistance to clethodim and butroxydim in all populations. During 2012, the LD50 of resistant populations ranged from 10.2 to 89.3 g ha−1, making them 3 to 34–fold more resistant to clethodim than the susceptible population. Similarly, GR50 of resistant population varied from 8 to 37.1 g ha−1, which is 3 to 13.9–fold higher than the susceptible population. In 2013, clethodim-resistant populations were 7.8 to 35.3–fold more resistant to clethodim than the susceptible population. The higher resistance factor in 2013, especially in moderately resistant populations, could have been associated with lower ambient temperatures during the winter of 2013. These resistant populations had also evolved cross-resistance to butroxydim. The resistant populations required 1.3 to 6.6–fold higher butroxydim dose to achieve 50% mortality and 3 to 27–fold more butroxydim for 50% biomass reduction compared to the standard susceptible population. Sequencing of the target-site ACCase gene identified five known ACCase substitutions (isoleucine-1781-leucine, isoleucine-2041-asparagine, aspartate-2078-glycine, and cysteine-2088-arginine, and glycine-2096-alanine) in these populations. In nine populations, multiple ACCase mutations were present in different individuals. Furthermore, two alleles with different mutations were present in a single plant of rigid ryegrass in two populations.

The problem in determining the economic feasibility of mechanical, chemical, and combination weed control methods in corn (Zea mays L.) is a complex decision involving many factors. The effects of cost, yield, timeliness, and alternative uses of labor are the factors considered in comparing various methods of weed control in this study. Costs of weed control operations range from $2.84/A for two cultivations to $10.45/A for the most intensive, mechanical-chemical operation studied. Most common weed control methods have costs of approximately $3 to $4/A. Yields range from 83 bu/A for two cultivations to 96 bu/A with the highest cost chemical-mechanical weed control methods. All yields used in this study are supported by agronomic studies and are indexed on the basis of 100 bu/A for weed-free conditions. The consideration of both yields and costs does affect the selection of a weed control method. Considering only these two factors, mechanical methods have a slight advantage over chemical weed control methods. Penalizing mechanical methods because of delay or timeliness characteristics gives chemical methods an advantage by as much as $3.00/A. Alternative uses of labor, even though a common argument in selecting chemical or mechanical methods, is a very small item and has only moderate effects on the changes in net income.

Purple nutsedge (Cyperus rotundus L.) plants were treated in the greenhouse and field with glyphosate [N-(phosphonomethyl)glycine]. Fresh weight of leaves, sprouts per original tuber, and sprouts per new tuber were reduced by 4 kg/ha glyphosate in the greenhouse. In the field, glyphosate at 2 and 4 kg/ha was compared to several herbicides for nutsedge control in repeated applications over an 8-month period. After several applications, glyphosate reduced the number of plants per unit area. After rotovation (working the soil thoroughly to a depth of 15 cm) and reapplication, glyphosate and MSMA (monosodium methanearsonate) plots had fewer plants and tubers per unit area than controls.

The importance of including crop density in studies of weed-wheat competition and in making estimates of crop losses due to weeds was demonstrated. Wheat density in adjacent quadrats of 1 m2 varied by an average of 25 plants in the farm fields and research plots examined. Differences between adjacent quadrats were in the same range for fields sown with different implements. Yield per plant, dry weight per plant, and tillering per plant decreased with increasing wheat density. For weed-crop competition studies in small-grain crops, the yields of weed-free and weedy quadrats are normally compared to estimate crop loss caused by competition. It was demonstrated that such estimates can be highly distorted if crop density is not taken into account. Current procedures used widely in agronomic competition studies are inadequate to properly define the relationship between crops and weeds. A more accurate method for performing such investigations is described in which both weed and crop abundance are determined. These variables are related to crop loss using multiple regression.

The effects of adjuvants and relative humidity (RH) on absorption, translocation, and metabolism of the methyl ester of 14C-haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy] propanoic acid} in corn (Zea mays L. ‘B73 X Mo17’ hybrid) were evaluated. Addition of 1.0% (v/v) petroleum oil concentrate (POC) to the treatment solution resulted in greater foliar absorption and translocation of 14C than addition of 1.0% (v/v) soybean oil concentrate (SBOC), 0.1% (v/v) oxysorbic (20 POE) (polyoxyethylene sorbitan monolaurate) (OXY), or no adjuvant (NONE). The least amount of 14C absorption occurred in the treatment containing OXY. Absorption and translocation of radioactivity were significantly greater at 70% RH than at 30% RH. Thin-layer chromatography revealed that most of the 14C recovered from treated plants was in haloxyfop-methyl 5 h after treatment (HAT). The remaining 14C recovered was haloxyfop and an unidentified polar metabolite. The average percentage of 14C-haloxyfop in the nonabsorbed fraction was 5, 39, 7, and 7% for treatments containing NONE, OXY, POC, and SBOC, respectively. The ratio of haloxyfop-methyl to haloxyfop and the percentage of polar metabolite in the absorbed 14C fraction was not different among adjuvant treatments or between levels of RH.

A bioeconomic weed management model was tested as a decision aid for weed control in corn at Rosemount, MN, from 1991 to 1994. The model makes recommendations for preemergence control tactics based on the weed seed content of the soil and postemergence decisions based on weed seedling densities. Weed control, corn yield, herbicide active ingredient applied, and economic return with model-generated treatments were compared to standard herbicide and mechanical control treatments. Effects of these treatments on weed populations and soybean yield the following year were also determined. In most cases, the model-generated treatments controlled weeds as well as the standard herbicide treatment. The quantity of herbicide active ingredient applied decreased 27% with the seed bank model and 68% with the seedling model relative to the standard herbicide treatment. However, the frequency of herbicide application was not reduced. In 1 yr, seed bank model treatments did not control weeds as well as the standard herbicide or seedling model treatments. Corn yields reflected differences in weed control. Net economic return to weed control was not increased by using model-generated control recommendations. Weed control treatments the previous year affected weed density in the following soybean crop. In 2 of 3 yr, these differences did not after weed control or soybean yield. Although tactics differed, the bioeconomic model generally resulted in weed control and corn yield similar to the standard herbicide. The model was responsive to differing weed populations, but did not greatly after economic returns under the weed species and densities in this research.

Sicklepod (Cassia obtusifolia L. ♯3 CASOB) seed were obtained from 10 locations in nine southern states, and plants were grown in a nursery for 3 yr. In field experiments, plant height, seed per pod, number of seed per plant, and number of pods per plant varied among seed sources and years. The vegetative growth phase, as measured from emergence to the initiation of the first floral buds, was approximately 30 days shorter for plants grown from the Tennessee source than for those grown from seed collected in Alachua, FL, in all 3 yr. Hand-harvested seed from plants grown from the 10 seed sources that were planted in 1982 were planted beside the original source sicklepod seed in 1983, and both flowered at approximately the same time, indicating self-fertilization. Percent germination of hand-harvested sicklepod seed from the 10 sources ranged from 2 to 11% for 1981 and 3 to 23% for 1982.

Corn (Zea mays L. ‘Pioneer 3732′) showed little to no injury following the postemergence-directed application of sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio) propyl]-3-hydroxy-2-cyclohexen-1-one} plus crop oil concentrate (COC) at 56 g/ha plus 1.25% (v/v) at nine locations across Midwestern U.S. in 1984 and 1985. Little corn injury also occurred for the postemergence-directed application of sethoxydim plus COC at 110 g/ha plus 1.25% (v/v) at most locations in both years. Considerable variation in tolerance was seen across locations for over-the-top applications of sethoxydim at all rates tested and for the directed application at 220 g/ha. Although corn at most locations showed no yield reduction with the over-the-top application of sethoxydim plus COC at 56 g/ha plus 1.25% (v/v), a 70% yield reduction occurred in one location in one year. For an over-the-top application of sethoxydim plus COC at 110 g/ha plus 1.25% (v/v), yields ranged from 3 to 95% of the untreated check in 1984, and from 3 to 88% in 1985. Stand reductions from an over-the-top application of sethoxydim plus COC at 220 g/ha plus 1.25% (v/v) ranged from 0 to 99%. A significant negative correlation was found between yield of corn treated over the top with sethoxydim and precipitation on the day of application and in the week following application. Air temperature on the day of application was positively correlated with corn injury from over-the-top and directed sethoxydim applications, but no correlation existed between percent relative humidity and corn injury. Open pan evaporation and solar radiation before and after application were not correlated with corn injury from sethoxydim.

A ‘penncross' bentgrass (Agrostis palustris Huds.) golf-type green was treated in the spring with seven herbicides at three rates each for three consecutive annual applications to assess their phytotoxicity. Herbicides which caused slight foliage injury the first year were tri-calcium arsenate, dimethyl tetrachloroterephthalate (DCPA), and polychlorodicyclopentadiene isomers (bandane); moderate injury occurred with 2,6-di-tert-butyl-p-tolyl methylcarbamate (terbutol) and N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine (benefin). No injury was observed with O,O-diisopropyl phosphorodithioate S-ester with N-(2-mercaptoethyl)benzenesulfonamide (bensulide) and 1-(2-methylcyclohexyl)-3-phenylurea (siduron). For the second and third year bandane was reapplied at half and tri-calcium arsenate at one fourth the original rate. Loss of bentgrass sod the second year was slight with terbutol and bandane and moderate with benefin and DCPA. No sod loss occurred with tri-calcium arsenate, siduron, and bensulide. Sod loss the third year was slight with tri-calcium arsenate, moderate with siduron and DCPA, and severe with bensulide, benefin, bandane, and terbutol. A persisting sod loss the fourth season, an observation year, was nil with tri-calcium arsenate; slight with siduron, benefin, and DCPA; slight to moderate with bensulide; moderate with bandane; and severe with terbutol.

Following foliar application of 14C-bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-(4)3H-one 2,2-dioxide], a promising selective postemergence herbicide for the control of broadleaf weeds in soybean [Glycine max (L.) Merr.] and navy bean (Phaseolus vulgaris L.), the movement of the 14C-label was primarily acropetal in both species, with some basipetal movement. Metabolism was more rapid in the trifoliate navy bean leaf than in the unifoliate navy bean leaf and the trifoliate soybean leaf. Prior treatment with several other herbicides did not decrease metabolism of bentazon in navy bean. The metabolites in navy bean and soybean appeared similar. Four apparent 14C-conjugates were isolated from soybean plants harvested at full maturity.

One to 5.6 μM DCPA (dimethyl tetrachloroterephthalate) inhibited oat (Avena sativa L. ‘Victory’) root growth within 12 to 18 h. Treated roots were severely stunted and swollen. An analysis of cell division in roots treated with DCPA revealed a disruption of normal mitosis after prophase. Metaphase, anaphase, and telophase division figures were absent 8 to 10 h after treatment with 5.6 μM DCPA. In contrast, a 24-h treatment with 5.6 μM DCPA was necessary to eliminate prophase division figures. The number of aberrant division figures increased concomitantly with the reduction in normal division figures. The predominant type of aberrant division figure was a condensed prophase. When the aberrant division cycle was completed and cells entered interphase, the dispersed chromosomes coalesced to form large, polymorphic nuclei and, occasionally, micronuclei. Approximately 60% of the outer four tiers of cells in roots treated with 5.6 μM DCPA developed abnormal cell walls. These data suggest that DCPA causes root growth inhibition by disrupting several processes involving organized microtubules.

The mode of action of two experimental pyridinyloxyphenoxy propionate herbicides, haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy] phenoxy] propanoic acid} and CGA-82725 {2-propynyl-2-[4-[(3,5-dichloro-2-pyridinyl)oxy] phenoxy] propanoic acid} was studied. Oat (Avena sativa L.) roots showed a rate-dependent sensitivity to both herbicides. Consequently, a series of assays was performed to study cell division, cell cycle dynamics, and nucleic acid and protein synthesis in oat root tips in order to characterize growth inhibition. Both herbicides inhibited cell division, apparently by inhibiting protein synthesis in the G2 stage of interphase. Supporting evidence for this assumption comes from the following observations: a) There was almost complete absence of leucine incorporation within 1 h of exposure to 2.7 × 10−4 M of either herbicide; b) the number of 3H-labeled dividing cells was reduced to about 20% of the control within 4 h, and essentially to zero within 8 h, after exposure to 2.7 × 10−4 M of herbicide; c) the number of unlabeled cells was reduced to less than 10% of the control within 8 h of exposure to 2.7 × 10−4 M of herbicide; d) during this 8-h period, the number of 3H-labeled cells in interphase which had been exposed to herbicide greatly exceeded the control; e) thymidine incorporation into DNA was inhibited but was not proportional to reduction in cell division; f) uridine incorporation was enhanced during the first 8 h of exposure before a reduction was evident at 12 h; and g) an oat coleoptile elongation bioassay showed that cell enlargement was not inhibited at herbicide concentrations less than 2.7 × 10−4 M.

Membranes define the outer boundary of the living protoplast and the internal compartmentation of plant cells. From a structural point of view, membranes consist of a lipid bilayer and proteins essential for functions such as solute transport, signal transduction, and numerous metabolic reactions. While membranes can represent a significant barrier to the free movement of many solutes, those with sufficient lipid solubility may move across membranes by dissolving into the lipid bilayer. However, selective membrane transport is generally observed for hydrophilic solutes such as mineral nutrients and cell metabolites. Such selective transport requires an input of metabolic energy, and in plants this occurs via the production of proton electrochemical gradients across the membrane by substrate- (ex. ATP, PPi) driven proton pumps. Selective solute transport is then mediated by membrane-associated secondary transport systems which utilize the proton electrochemical gradient to drive the transport process. This review of membrane structure and transport system function provides a background for a further examination of herbicide interactions with plant membranes.

Isopropyl m-chlorocarbanilate (chlorpropham) at 6 lb/A, the standard treatment, usually controlled dodder (Cuscuta spp.) completely for 3 to 5 weeks. Of 27 other herbicides compared with chlorpropham, seven controlled dodder completely for longer periods (6 to 18 weeks), and nine for comparable or slightly shorter periods. Eleven controlled dodder only partially or completely for a period shorter than that from chlorpropham. The period of dodder control from chlorpropham was doubled when p-chlorophenyl N-methylcarbamate at 1.5 lb/A was included with it.