The effect of ammonium sulfate (AMS) on efficacy, absorption, and translocation of glufosinate in various weed species was investigated in greenhouse and field experiments. Adding ammonium sulfate at 20 g L−1 increased the efficacy of glufosinate on Echinochloa crus-galli, Setaria faberi, and Abutilon theophrasti, but not on Amaranthus rudis or Chenopodium album. AMS increased foliar absorption of 14C from 14C-glufosinate to the greatest extent in A. theophrasti and S. faberi and to the least extent in C. album. AMS increased the translocation of 14C out of the treated leaf 24 h after treatment in A. theophrasti and S. faberi, but not in C. album. These results suggest that AMS can increase the efficacy of glufosinate on A. theophrasti and S. faberi by increasing foliar absorption and subsequent translocation of glufosinate.

A small assortment of microbial proteins have the ability to activate defense responses and induce necrosis in plant cells through cell signaling pathways. These proteins are of interest because of their potential use as bioherbicides and inducers of plant resistance in agriculture. A 24-kDa protein (Nep1) was purified from culture filtrates of Fusarium oxysporum, and the effects of this protein on weed leaves were investigated. This protein induced necrosis in detached leaves of Papaver somniferum, Lycopersicon esculentum, Malva neglecta, and Acroptilon repens when taken up through the petiole. The pattern and level of necrosis were dependent on the plant species. Treatment with Nep1 induced the production of ethylene in isolated leaves of various species, and the level of ethylene response was shown to be correlated to the concentration of the protein. Pretreating leaves of P. somniferum, L. esculentum, M. neglecta, and Cardaria draba with 100 µl L−1 ethylene enhanced the protein induction of ethylene biosynthesis in those leaves. Application of Nep1 (200 nM) as a spray to intact plants of Abutilon theophrasti, P. somniferum, Centaurea solstitialis, Centaurea maculosa, and Sonchus oleraceus resulted in extensive necrosis of leaves within 48 h. The results of this research are supplemental to our understanding of the role of specific polypeptides in plant/microbe interactions and demonstrates for the first time that a fungal protein can cause extensive necrosis when applied to weed species as a foliar spray.

Carfentrazone-ethyl absorption, translocation, and metabolism was determined in Glycine max, Zea mays, and Abutilon theophrasti. Glycine max absorbed greater than 90% of applied carfentrazone-ethyl within 2 h after treatment (HAT) when nonionic surfactant (NIS) or crop oil concentrate (COC) were included in the treatment solution. The addition of 28% urea ammonium nitrate (UAN) did not improve carfentrazone-ethyl absorption in G. max, but in Z. mays and A. theophrasti, UAN combined with NIS or COC increased the rate of carfentrazone-ethyl absorption. Carfentrazone absorption in A. theophrasti 2 HAT was 70% when UAN was combined with NIS or COC compared to 40% with NIS or COC alone; however, 24 HAT, absorption with NIS and COC were similar to treatments with UAN. Carfentrazone-ethyl did not translocate from the treated leaf to other plant parts in Z. mays and only small amounts of radiolabeled product were detected in the rest of the shoots of A. theophrasti (5%) and G. max (12%). Herbicide metabolism in Z. mays and G. max was greater than in A. theophrasti. All three species converted carfentrazone-ethyl to its phytotoxic metabolite carfentrazone-chloropropionic acid; therefore, the parent molecule was considered to be the sum of the ethyl ester and its hydrolysis product. Estimated half-lives of carfentrazone in Z. mays, G. max and A. theophrasti were 1, 7, and 40 h, respectively. The rate of carfentrazone metabolism corresponded to plant sensitivity (sensitivity to carfentrazone: Z. mays<G. max<<A. theophrasti); however, rapid absorption and translocation of carfentrazone may reduce the tolerance of G. max.

Additive and replacement series studies were conducted to investigate the effect of Glycine max cultivar canopy characteristics, interference, and weed-free period on the vegetative and reproductive growth of Solanum ptycanthum. Solanum ptycanthum established 0, 2, and 4 wk after planting (WAP) did not reduce seed yield of either Group II (Pioneer 9273) or Group IV (Asgrow 4715) varieties. The Group IV variety intercepted a greater percentage of light throughout the season. However, both varieties reduced growth of S. ptycanthum established 2 and 4 WAP, suggesting no disadvantage of Group II in suppressing S ptycanthum growth. The greenhouse and field replacement series experiments indicated that S. ptycanthum plants experienced greater intraspecific than interspecific competition; that is, S. ptycanthum grew more in mixed species competition than in monoculture. Under field conditions, G. max grew equally well in competition with S. ptycanthum or in monoculture. However, in the greenhouse G. max accumulated more biomass and fruit weight in monoculture vs. competition with S. ptycanthum. Relative yield total values (RYT) in the field supported greater productivity per unit area for mixed species competition when compared to G. max or S. ptycanthum grown in monoculture. Data from both studies suggested that S. ptycanthum did not reduce G. max yield when grown in the field. Regardless of maturity group, a fully developed G. max canopy reduced S. ptycanthum growth.

A model is presented of the spread of a weed from a point source, with dispersal being both unaided and aided by a combine harvester. Four “type” weeds were modeled, chosen to represent species of differing population and dispersal ecologies and based broadly on Avena spp. (wild oats), Raphanus raphanistrum (wild radish), Bromus spp. (bromegrass), and Fumaria spp. (fumitory). Wind-adapted species were excluded. The greatest rate of spread was predicted to be for R. raphanistrum and the least for Fumaria, regardless of whether dispersal by combine harvester was included. Rate of spread was more sensitive to dispersal parameters than to demographic parameters and increased up to 16-fold as soon as any seeds were dispersed by the harvester. Given that uptake by the harvester is a function of phenology of seed production and dispersal, better data on such processes is required for weeds. Because rate of spread is more dependent on dispersal than on demographic factors, greater attention needs to be given to describing dispersal frequency distributions and especially to analysis of their shapes.

Field experiments were conducted in 1996 and 1997 to determine the effect of the rate and time of glyphosate application on weed emergence, survival, biomass, and Glycine max yield in reduced-tillage (RT) and no-tillage (NT) glyphosate-resistant G. max planted in rows spaced 18 (narrow-row) and 76 cm (wide-row). Glyphosate was applied at 0.42, 0.63, and 0.84 kg ae ha−1 at V2, V4, R1, and R4 growth stages. On separate plots, 0.84 kg ha−1 glyphosate was applied at each growth stage with hand weeding. A weed-free check was maintained with preemergence imazethapyr plus metolachlor supplemented with hand weeding, and a nontreated check was included. Weed population density before glyphosate application ranged from 239 to 606 plants m−2 in RT and 33 to 500 plants m−2 in NT systems. Setaria faberi and Chenopodium album were the predominant species. Weed control efficacy and crop yield were influenced more by application time than by glyphosate rate. Glyphosate applied at V2, V4, and R1 gave season-long control of weeds in 18-cm rows. In 76-cm rows, glyphosate applied at V2, V4, and R1 gave almost complete control of weeds, but broadleaf weeds emerged after application at V2. The critical time of weed removal, the time beyond which weed competition reduced G. max yield by 3% or more compared to the weed-free check, was at R1 and V4 in 18-cm RT G. max in 1996 and 1997, respectively, and at V2 in 76-cm RT G. max in both years. The predicted critical time of weed removal in 18- and 76-cm NT G. max was R1 and V4, respectively, in 1996 and R1 in 1997. This research showed that there was variation in the onset of the critical time of weed removal between tillage systems, as well as within tillage systems across years. The results indicate a single glyphosate application can prevent yield loss in narrow-row, glyphosate-resistant G. max under favorable conditions, but application timing becomes more critical in wide rows because the critical period of weed removal occurs earlier. Late-emerging weeds may warrant a second glyphosate application in wide-row G. max.

Abutilon theophrasti is one of the worst agricultural weeds in North America, yet it has not reached that status in California in the 80 yr since it was first reported. The research reported here examined the distribution and modeled climatic requirements of A. theophrasti to determine whether it is likely to spread more widely in the state. Herbaria records and weed literature were surveyed to determine the historical occurrences of A. theophrasti in the state; current distribution was assessed through surveys sent to University of California personnel in each county. Combined results showed 42 counties out of 58 with A. theophrasti present historically or currently. A plot of the cumulative number of counties containing A. theophrasti by decade fit a logistic equation. The maximum rate of spread of this species occurred in 1962 and it is likely that its final distribution by county in California is leveling off and not likely to increase further. The climate-matching/mapping software CLIMEX® was used with observed and estimated parameters of environmental requirements of A. theophrasti to model its current distribution from India through China to Japan. The same model parameters were then used to map its potential distribution in California. Areas where A. theophrasti has been reported were predicted by CLIMEX to be poorly suited for its growth and development without added soil moisture in the form of irrigation. It appears that the Mediterranean climate is a deterrent to the integration of A. theophrasti into California. The climate-matching approach provided a biologically reasonable assessment of potential distribution of A. theophrasti in California. The approach also allowed assessment of the effects of common agricultural practices on potential distribution given the environmental requirements and limitations of A. theophrasti.

Full-count random sampling has been the traditional method of obtaining weed densities. Currently it is the recommended scouting procedure when using HERB, a herbicide selection decision aid. However, alternative methods of scouting that are quicker and more economical need to be investigated. One possibility that has been considered is binomial sampling. Binomial sampling is the procedure by which density is estimated from the number of random quadrats in which the count of individuals is equal to or less than a specified cutoff value. This sampling method has been widely used for insect scouting. There has also been interest in using binomial sampling for weed scouting. However, an economic analysis of this sampling method for weeds has not been performed. In this paper, the results of an economic analysis using simulations with binomial sampling and the HERB model are presented. Full-count sampling was included in the simulations to provide a benchmark for comparison. The comparison was made in terms of economic losses incurred when the estimated weed density obtained from sampling was inaccurate and a herbicide treatment was selected that did not maximize profits. These types of losses are referred to as opportunity losses. The opportunity losses obtained from the simulations indicate that in some situations binomial sampling may be a viable economic alternative to full-count sampling for fields with weed populations that follow a negative binomial distribution, assuming no prior knowledge of weed densities or negative binomial k values.

The objective of this study was to identify some epidemiological conditions that affect the pathogenicity and weed control efficacy of the fungal pathogen Dactylaria higginsii on Cyperus rotundus. In controlled environments, the fungus required a minimum dew period of 12 h and a temperature of 25 C during the dew period to produce severe disease on four- and six-leaf-stage plants inoculated with 106 conidia ml−1. Under these conditions, 75% disease (percent leaf area damaged) and excellent weed control (nearly 100% plant mortality) were achieved. In experiments to test the interaction among dew period temperature, dew period duration, and plant growth stages, younger plants (four- to six-leaf) were more susceptible to D. higginsii than older (eight-leaf-stage) plants. At the dew period duration of 24 h and dew period temperature of 30 C, the number of days to obtain 50% disease severity on four- to six-leaf-stage plants was significantly less (10 d) compared with older plants (16 d). To achieve effective control, D. higginsii should be applied early in the growing season when C. rotundus plants are young and the temperature and dew period requirements are not limiting. The need for a long dew period (>12 h) for infection and disease development may be a limiting factor in this pathosystem. This limitation may be overcome by using inoculum amended with moisture-retaining gels.

Greenhouse, field, and laboratory experiments were conducted in northern Greece during 1996, 1997, and 1998 to study possible metribuzin resistance in Amaranthus retroflexus (redroot pigweed) and Chenopodium album (common lambsquarters) biotypes found in potato fields. The greenhouse experiments indicated that the suspected resistant biotypes (R) of both species were not controlled by metribuzin applied either pre- or postemergence at rates of 245, 490, 980, and 1,960 g ai ha−1 (the higher rate is eight times greater than the rate recommended for weed control in Solanum tuberosum [potato]). However, susceptible biotypes (S) were completely controlled by 245 g ai ha−1. Also, both R- and S-biotypes of either species were effectively controlled by prometryn applied either pre- or postemergence at 1.5 kg ai ha−1. The field trials confirmed that metribuzin applied either pre- or postemergence at rates of 490, 980, and 1,960 g ai ha−1 gave fair or partial control of the R-biotype of C. album, whereas prometryn applied preemergence at 1.5 kg ai ha−1 gave excellent control of this weed. Chlorophyll fluorescence measurements performed in laboratory experiments indicated that photosynthetic electron transport in metribuzin-incubated leaves detached from plants of the R-biotypes was not affected, but it was inhibited in leaves detached from plants of the S-biotypes. Electron transport was inhibited by prometryn in leaves detached from both S- and R-biotypes of either species. These results show clearly that the biotypes of both species developed resistance to metribuzin, but they were not cross-resistant to prometryn.

Herbicide soil/solution distribution coefficients (Kd) are used in mathematical models to predict the movement of herbicides in soil and groundwater. Herbicides bind to various soil constituents to differing degrees. The universal soil colloid that binds most herbicides is organic matter (OM), however clay minerals (CM) and metallic hydrous oxides are more retentive for cationic, phosphoric, and arsenic acid compounds. Weakly basic herbicides bind to both organic and inorganic soil colloids. The soil organic carbon (OC) affinity coefficient (Koc) has become a common parameter for comparing herbicide binding in soil; however, because OM and OC determinations vary greatly between methods and laboratories, Koc values may vary greatly. This proposal discusses this issue and offers suggestions for obtaining the most accurate Kd, Freundlich constant (Kf), and Koc values for herbicides listed in the WSSA Herbicide Handbook and Supplement.

Herbicides applied to soils potentially affect soil microbial activity. Quantity and frequency of glyphosate application have escalated with the advent of glyphosate-tolerant crops. The objective of this study was to determine the effect of increasing glyphosate application rate on soil microbial biomass and activity. The soil used was Weswood silt loam. The isopropylamine salt of glyphosate was added at rates of 47, 94, 140, and 234 µg ai g−1 soil based on an assumed 2-mm glyphosate–soil interaction depth. Glyphosate significantly stimulated soil microbial activity as measured by C and N mineralization but did not affect soil microbial biomass. Cumulative C mineralization, as well as mineralization rate, increased with increasing glyphosate rate. Strong linear relationships between mineralized C and N and the amount of C and N added as glyphosate (r 2 = 0.995, 0.996) and slopes approximating one indicated that glyphosate was the direct cause of the enhanced microbial activity. An increase in C mineralization rate occurred the first day following glyphosate addition and continued for 14 d. Glyphosate appeared to be directly and rapidly degraded by microbes, even at high application rates, without adversely affecting microbial activity.

Interactions between weeds and arthropods occur frequently. This review covers the topic of weed/arthropod interactions, and provides the reader with access to literature in the subject area that is scattered in weed science, entomological, crop production, and ecological journals. We first analyze the current status of weed and arthropod management in the context of multidisciplinary integrated pest management (IPM). The remainder of the review is organized according to the mechanisms driving interactions. The first section deals with interactions driven by trophic relationships, and is subdivided into direct and indirect trophic interactions. Direct trophic interactions occur when pest or beneficial arthropods feed directly on weeds. Indirect trophic interactions occur when arthropod feeding damage to crops impacts weeds through alteration of ecosystem resource availability, or through weeds serving as hosts for alternate prey for beneficial arthropods, or via tritrophic interactions. The second mechanism driving interactions is considered in relation to alteration of the physical habitat by the presence of weeds, such as alteration of temperature within the plant canopy. The third major mechanism driving interactions is based on control tactics for the two types of pests. These are considered from the aspect of direct physical effects, such as tillage, and from the aspect of interactions resulting from the use of pesticides. The latter is divided into direct effects of herbicides and insecticides on non-target pests and beneficials, and on interactions that result from alteration of host plant physiology by pesticides. A conclusion section attempts to place the impact of interactions into an IPM framework, and to indicate where multidisciplinary research involving weed and arthropod management should be focused in the future.