The antidotes dichlormid, MON-4660, CGA-154281, R-29148, and MON-13900 were tested in the greenhouse to protect Zea mays L. (corn) against RPA 201772 injury. High rates of RPA 201772 injured four Z. mays hybrids > 30%. R-29148 and MON-13900 were the most effective of the five antidotes evaluated. R-29148 applied at rates ⩾ 45 g ha−1 provided excellent protection against RPA 201772 injury and also prevented injury to Z. mays from diketonitrile, the active metabolite of RPA 201772. In laboratory studies, R-29148 did not alter absorption of 14C-RPA 201772 from soil; however, R-29148 significantly enhanced the rate of RPA 201772 metabolism and inactivation in Z. mays. The mixed function oxidase inhibitor piperonyl butoxide (PBO) increased RPA 201772 injury on all hybrids. These results demonstrate that Z. mays tolerance to RPA 201772 can be enhanced with the use of antidotes such as R-29148 and MON-13900, that R-29148 protects Z. mays from RPA 201772 and diketonitrile by the enhancement of metabolism, and that oxidative reactions may be involved in the metabolism of RPA 201772 in Z. mays.

Puccinia obtegens (Link) Tul., an autoecious rust pathogen, is a potential biological control agent of Canada thistle [Cirsium arvense (L.) Scop.]. Ten ecotypes of Canada thistle were inoculated with uredospores of P. obtegens and sporulation was observed on all ecotypes. Infection types varied among and within ecotypes, indicating that host-resistance is one factor limiting rust infection. No correlation was found between Canada thistle susceptibility to the rust and host plant ecotype classification, stomatal density, amount of leaf pubescence, or spore germinability on leaf surfaces.

Artificial shading studies indicated that competition for photosynthetically active radiation (PAR) will limit tuber production in purple nutsedge. There were no data available to test whether there is a relationship between incident PAR underneath crop canopies and tuber production of this weed. In this study, the effect of crop competition on net reproductive rates of purple nutsedge tubers was measured under field conditions. Purple nutsedge plants were grown in association with bush beans, maize, maize and beans intercropped, sweet potato, pole beans, and bell pepper in a 2-yr field study in Costa Rica. Measurements were taken on the number of tubers produced during the growing season of each crop, and PAR transmittance was monitored weekly for the duration of the respective cropping cycles. Data on transmittance and incident solar radiation were used to calculate the daily average amount of PAR, available 15 cm above the soil surface. Regressions indicated that average incident PAR accounted for 95% of the variation in net reproductive rates. Average incident PAR also allowed a more precise competitive ranking of crops than either average or minimum transmittance. Bush beans had consistently the lowest average incident PAR values and therefore ranked as the most competitive crop in both years. Our data suggest that no net increase in tuber populations occurs if average incident PAR is below 2.7 MJ m−2 d−1. Differences in the duration of the cropping cycle accounted only for a small proportion of the overall variation in net reproductive rates of purple nutsedge tubers. Information on the competitive ranking of different crops can be used to design crop rotations that could reduce reliance on herbicides on small farms in Costa Rica.

Triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy]acetic acid}, 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid], and 3,6-dichloropicolinic acid were applied to the foliage of juvenile crop plants at 0.002, 0.009, 0.03, 0.14, and 0.56 kg/ha. Corn (Zea mays L.), oat (Avena sativa L.), wheat (Triticum aestivum L.), grain sorghum [Sorghum bicolor (L.) Moench], and kleingrass (Panicum coloratum L.) were generally more tolerant to the herbicides than were peanuts (Arachis hypogaea L.), cotton (Gossypium hirsutum L.), cucumber (Cucumis sativus L.), and soybean (Glycine max [L.] Merr.). Triclopyr was usually more phytotoxic to corn, oat, grain sorghum, and kleingrass than either 2,4,5-T or 3,6-dichloropicolinic acid at 0.14 and 0.56 kg/ha, but few differences occurred among herbicides at lower rates. Kleingrass was not affected at any rate of 3,6-dichloropicolinic acid. Wheat tolerated most rates of all three herbicides. At 0.56 kg/ha, triclopyr and 3,6-dichloropicolinic acid caused greater injury to peanuts than did 2,4,5-T; whereas, 2,4,5-T and triclopyr were more damaging to cotton and cucumber than 3,6-dichloropicolinic acid. The three herbicides at 0.14 and 0.56 kg/ha killed soybeans. Soybean injury varied from none to severe at 0.002 to 0.03 kg/ha, depending upon species investigated, but many plants showed morphological symptoms typical of the auxin-type herbicides.

Round-leaved mallow interference in spring wheat and flax was investigated under field conditions in southern Manitoba. Additionally, the growth of mallow alone and in a wheat crop was compared. In six of seven trials, wheat yield losses due to mallow interference were not significant. In a seventh trial, where mallow densities of up to 237 plants m−2 were recorded, wheat yields were reduced up to 15%. In contrast, significant flax yield losses occurred in each of three trials where maximum mallow densities ranged from 21 to 52 plants m−2. From the regression equations it was calculated that 20 mallow plants m−2 caused flax yield losses of 33, 9, and 10% in 1987, 1988, and 1989, respectively. Periodic sampling of mallow plants growing alone and in wheat, and regression analysis of these data using a logistic function as a biological model of plant growth indicated that mallow growth was severely suppressed by wheat By the eighth week after emergence, the lamina area and shoot dry matter of mallow plants growing in wheat was less than 3% of plants growing alone. Similarly, in wheat, mallow produced less than 1% of the seed produced by plants growing alone. This extreme suppression of mallow growth corresponded with an 80 to 90% decrease in photosynthetically active radiation penetrating the wheat canopy for approximately six continuous weeks, beginning 4 wk after crop emergence.

Studies were conducted to evaluate the effect of wild oat (Avena fatua L. # AVEFA) interference in lentils (Lens culinaris Medik). An infestation of 32 and 65 wild oats/m2 maintained up to 5 weeks in the field did not reduce lentil grain yield. However, 32 wild oats/m2 reduced yields 32% when allowed to remain for 7 weeks and 49% if they remained until harvest time (11 weeks). Sixty-five wild oats/m2 reduced grain yield 42 and 61% for the same time periods, respectively. In the growth chamber, 69 wild oats/m2 reduced lentil plant dry weight 29% if allowed to remain for 3 weeks, 61% for 5 weeks, and 72% for 7 weeks (harvest time). The field data suggest that wild oat control measures may be delayed for several weeks after lentil emergence without reducing crop yield.

The movement and adsorption of soil-applied 14C-labeled 2,4-bis(isopropylamino)-6-(methylthio-s-triazine (prometryne) and 1,1-dimethyl-3-(a,a,a-trifluoro-m-tolyl)urea (fluometuron) in the presence of 0.01 and 0.5 N CaCl2 were studied. The soils were Ca++-saturated Eufaula loamy fine sand and Norge loam. In equilibrium adsorption studies, fluometuron adsorption was decreased and prometryne adsorption was increased by increasing the CaCl2 concentration from 0.01 to 0.5 N. The mobility of prometryne in the two water-saturated soils was decreased by an increase in CaCl2 concentration. Fluometuron mobility was unchanged by the two CaCl2 concentrations in Eufaula, but was greater in Norge at the higher CaCl2 concentration. The adsorption of each herbicide in the flowing system was less than that predicted by the distribution coefficient. Differences in CaCl2 concentration do not appear to influence the mobility of the two herbicides sufficiently at the soil-water flow rate used in this study to require changes in current field application practices.

Weed management systems in carrot are limited in part by a lack of fundamental understanding of crop–weed interactions. Irrigated field studies were conducted to quantify the effect of volunteer potato density and duration of interference on carrot yield and to determine relationships among weed density, duration of weed growth, and volunteer potato tuber production. A season-long volunteer potato density of 0.06 plants m−2 produced from 150 to 230 g tubers m−2 and resulted in an estimated 5% crop yield loss. At two volunteer potato plants m−2, the same level of crop loss was estimated with a duration of interference of 430 growing degree days (GDD), a time at which the weed had already produced 130 g tubers m−2. Volunteer potato height at the time of weed removal predicted carrot yield loss (R 2 = 0.77) and may be useful for timing of management strategies such as hand weeding. Functional relationships describing carrot–volunteer potato interactions provide simple information that is useful for developing weed management recommendations for carrot, a crop that relies on multiple tactics for managing weeds, and rotational crops that are negatively affected by persistence of volunteer potato.

Growth of an agricultural weed population and a coastal nonweed population of poorjoe was compared in a competition experiment with soybean and when grown alone. Mean aboveground biomass production of the weed population was roughly twice that of the nonweed population, whether grown alone or with soybean. A path-coefficient model of growth was fitted to the data in order to compare the populations with respect to five independent measurements of weed growth: establishment rate, early and later growth rates, growth form, and final growth rate. These measurements indicated plant growth over successive intervals of the growing season by statistically equating the size of all plants at the beginning of an interval of growth using analysis of covariance. In general, growth measurements were positively associated with aboveground biomass production. The weed population of poorjoe had a greater establishment rate and a greater early aboveground growth rate compared to the nonweed population; however, the final growth rates of the two populations were similar whether soybean was present or absent. Genetic changes leading to earlier establishment and faster early growth, but not increasing tolerance of soybean and intraspecific competition, may have occurred in the adaptation of poorjoe to an agricultural ecosystem.

The effect of spray carrier volume on the antagonism of sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} by bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] was investigated under greenhouse and field conditions. In the laboratory, the spray droplet overlap between components of a sequential spray application increased from 17 to 50% as carrier volume was increased from 94 to 374 L/ha, respectively. Carrier volume had a significant influence on treatments of sethoxydim alone and sethoxydim sequentially applied with bentazon, but not on the tank mixture of the two herbicides. Carrier volumes of 94 and 187 L/ha used to deliver sethoxydim alone and the sethoxydim/bentazon sequential treatment were superior to the use of 374 L/ha for the control of large crabgrass [Digitaria sanquinalis (L.) Scop. # DIGSA], fall panicum (Panicum dichotomiflorum Michx. # PANDI), and goosegrass (Eleusine indica Gaertn. # ELEIN). There was no difference in grass control between sethoxydim alone and the sequential treatment except at carrier volumes of 187 L/ha or more with which the sequential treatment was less effective. Rates of 0.06 and 0.11 kg ai/ha of sethoxydim were influenced more by changes in carrier volume than 0.22 kg/ha in both the alone and sequential treatments.

The competitiveness of 10 sorghum [Sorghum bicolor (L.) Moench] hybrids to weeds and their tolerances to commonly used sorghum herbicides were studied across Nebraska. The three best weed competitors in this study were sorghum hybrids ‘RS 671′, ‘RS 690′, and ‘AKS 663′; whereas the three poorest weed competitors were ‘AKS 614′, ‘NB 505′, and ‘RS 633′. Sorghum hybrids showing a significant yield reduction from a 2× rate of 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) were ‘SD 441′, ‘SD 451′, and ‘NB 505′. ‘SD 441 also suffered a significant yield reduction from a normal rate of 2-chloro-N-isopropylacetanilide (propachlor) applied preemergence plus a 4× rate of (2,4-dichlorophenoxy)acetic acid (2,4-D) applied postemergence. With the above exceptions, sorghum hybrids generally showed considerable tolerance to 2× rates of herbicides commonly used on sorghum.

Soil applications of α,α,α,-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin), O,O-diethyl S-((ethylthio)methyl)-phosphorodithioate (phorate) and their combinations did not affect cotton (Gossypium hirsutum L.) germination. Plants grown on soil treated with trifluralin were shorter than plants on soil treated with phorate. High rates of phorate reduced shoot weights. Root weights of phorate-treated plants were higher than those of trifluralin-treated plants but lower than those of the controls. In culture solution and soil, trifluralin inhibited lateral roots of cotton. In the presence of phorate, trifluralin-treated plants produced more lateral roots than plants treated with trifluralin alone. Other organophosphorus compounds, when used in combination with trifluralin, did not increase the number of lateral roots.

Live root material of leafy spurge (Euphorbia spp.) was collected from 38 locations across northern United States, southern Canada, and from 1 location in Austria. These materials were established in a nursery at Lincoln, Nebraska along with E. agraria M. Bieb. and E. cyparissias L. # EPHCY. Nineteen vegetative and reproductive characters, taken from various taxonomic keys, were measured on plants established for a period of 3 yr. Data were compared with previous taxonomic keys of Euphorbia species. A key developed by A. Radcliffe-Smith was used to separate the nursery entries. Leaf width measurements were the most useful taxonomic criteria. Five morphologically separable taxa were observed: E. × pseudovirgata (Schur) Soó, E. esula L. # EPHES, E. uralensis Fisch. ex Link., E. agraria M. Bieb., and E. cyparissias L.

In a Crowley silt loam, between 30 kPa, continuous flood, and alternate flood laboratory treatments at 25 C, dissipation of incorporated oxadiazon [2-tert-butyl-4(2,4-dichloro-5-isopropoxyphenyl)-Δ2-1,3,4-oxadiazolin-5-one] varied little, with an average of 59% remaining after 20 weeks. In the greenhouse, subsurface application of oxadiazon reduced its phytotoxicity but increased its persistence up to four times more than with surface-applied oxadiazon. In the field, 50% of the surface-applied oxadiazon dissipated from the soil within 6 to 11 days when the soil was flush irrigated and then flooded, compared to 15 to 17 days when the soil was irrigated but not flooded in two rice (Oryza sativa L.) management systems. Oxadiazon dissipation in the field was greater during the first 2 or 3 weeks after application than in the laboratory study. This is explained at least partially by the lack of herbicide incorporation in the field.

Management practices and cropping systems that serve as integrated weed management practices, and at the same time can contribute to improved soil quality, will be important for the sustainability of agricultural production systems. The objective of this study was to assess weed species population density under contrasting tillage (conventional tillage [CT] and no tillage [NT]), residue burning (burn and no burn), and residue level (low and high) treatments after 5 and 6 yr of consistent management in a wheat–soybean double-crop production system. A field experiment was conducted from fall 2001 to fall 2007 in the Mississippi River Delta region of eastern Arkansas on a Calloway silt–loam. Weed assessments were conducted twice during the soybean growing season, before (early season) and after herbicide application (late season) in 2006 and 2007. Total weed density was greater under CT (513 plants m−2) than under NT (340 plants m−2) early in the growing season in 2006, but was greater under NT than CT late in the season in 2007, suggesting that the effectiveness of glyphosate on total weeds differs between CT and NT. Averaged across residue levels, grass species density was greatest in the NT–burn (68 to 167 plants m−2) combination and lowest in the NT–no-burn (41 to 63 plants m−2) early in the growing season in both years. Broadleaf density was greater early (200 to 349 plants m−2) than late (18 to 20 plants m−2) in the growing season under both CT and NT in 2006, but in 2007 broadleaf density did not differ by tillage treatment between seasons. Perennial weed density was greater in the burn (99 plants m−2) than in the no-burn (59 plants m−2) treatment in 2006. No tillage, no burning, and a high residue level appeared to contribute to the suppression of most weed species without reducing herbicide efficiency.

Toadflax invasion into natural areas has prompted interest in weed management via biological control. The most promising biological control agent currently available for the control of Dalmatian toadflax is Mecinus janthinus, a stem-boring weevil that has been shown to significantly reduce toadflax populations. Some land managers, however, are reluctant to release approved weed biological control agents based on concerns about possible nontarget impacts. Few postrelease examinations of biocontrol impact and host specificity have been performed, despite the call for such information. This study examined the host specificity of Mecinus janthinus, postrelease, in relation to Colorado sites to provide information to managers about its relative safety as a weed biological control agent. This study employed three components: (1) greenhouse choice and no-choice experiments; (2) no-choice caged field experiments; and (3) release-site evaluation of nontarget use of native plant species where this weevil has been released and has established. Both greenhouse and field experiments failed to demonstrate nontarget use of native plant species by M. janthinus in the region where it was studied, even in no-choice starvation tests. We found no evidence of nontarget herbivory on native plants growing at toadflax sites where M. janthinus was well established. These results support the continued use of M. janthinus as a low-risk biological control agent for the management of toadflax in the Rocky Mountain Front Range.

Early spring spray applications of the potassium salt of 4-amino-3,5,6-trichloropicolinic acid (picloram) and certain formulations of 1:1 mixtures of picloram plus (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T) controlled yaupon (Ilex vomitoria Ait.). Ester formulations of picloram or picloram plus 2,4,5-T were ineffective. Combinations of surfactant and oil: water carriers usually did not improve herbicide performance over water carriers. Granular picloram was superior to sprays when applied as soil treatments at equal rates and controlled yaupon at most dates of application. Post oak (Quercus stellata Wangenh.) and blackjack oak (Quercus marilandica Muenchh.) were controlled more effectively by picloram or picloram plus 2,4,5-T sprays on the foliage than by granular picloram as a soil treatment. Picloram granules usually killed winged elm (Ulmus alata Michx.) regardless of date of application.

Field pennycress (Thlaspi arvense L.) was seeded in rows at approximately 10-day intervals from June 1 to August 30, 1963, and from July 12 to August 31, 1966, in subarctic, south-central Alaska (61.5° N.). Plants from June 20 and earlier seedings produced mature seed as summer annuals; those from seedings during the first 3 weeks of July produced elongated stems and green pods, while those from later seedings produced unelongated, basal rosettes. Seedlings that emerged during July or later were potential winter annuals. No study plants survived the 1963-64 winter. The 1966 plantings survived the 1966-67 winter as follows: July 12, 50%, July 20, 54%, August 1, 93%, and August 12, 23, and 31, 100%. Morphology, time of flowering, and seed pod production of plants surviving the winter also were influenced by planting date the previous growing season. To prevent field pennycress from producing seed as a summer annual, plants that emerge before the middle of July must be eradicated. Plants that emerge during July or later should be destroyed as seedlings or prior to seed production time the following year.

The aryloxyphenoxypropionate and cyclohexanedione herbicides, which inhibit acetyl-coenzyme A carboxylase (EC 6.4.1.2), have also been hypothesized to act at specific sites on the plasmalemma. An impermeant sulfhydryl binding agent was reported to block the diclofop acid-induced depolarization of the membrane potential (Em) in rigid ryegrass. A correlation between the antagonistic interaction with auxin herbicides both in the field and in the Em response, and the repolarization of Em in herbicide-resistant rigid ryegrass following removal of diclofop acid also provide support for this hypothesis. However, similar membrane responses in resistant grasses and broadleaf species suggest that the membrane response may not be important in the phytotoxic activity of the postemergence graminicides under field conditions. In addition, an antagonistic interaction was not observed in roots of susceptible grasses exposed to combinations of diclofop-methyl and 2,4-D. Furthermore, the repolarization of the Em in diclofop-resistant rigid ryegrass was correlated to differential acidification of the external solution and an increase in the protonated form of diclofop acid, rather than a site-specific interaction at the plasmalemma. Although the membrane response is probably not involved in herbicide phytotoxicity in agricultural systems, a higher extracellular pH in the resistant biotypes of rigid ryegrass may inhibit the movement of these weak-acid herbicides across the plasmalemma, and possibly contribute to increased herbicide tolerance.

Variation in primary dormancy among populations of black nightshade and hairy nightshade collected on two dates was investigated. In addition, emergence characteristics of both species were studied during 2 yr with several populations and soil disturbance regimes. Results revealed a variation in the level of primary dormancy among populations and collection dates in black nightshade. However, fresh seeds of hairy nightshade showed no or negligible germinability, indicating a deeper level of dormancy. Emergence was higher in black nightshade than in hairy nightshade in both years, but with significant differences among populations. In addition, various categories regarding the soil temperature requirements for seedling emergence were found within seed batches. This enables the species to extend their emergence timing and therefore escape from climatic extremes or weed control operations. This information can be used to improve weed management strategies by timing weed control measures to larger seedling flushes.