Postemergence herbicide treatments that were most effective for controlling horsenettle (Solarium carolinense L.) were glyphosate [N-(phosphonomethyl)glycine], dicamba (3,6-dichloro-o-anisic acid), silvex [2-(2,4,5-trichlorophenoxy)-propionic acid], and picloram (4-amino-3,5,6-trichloropicolinic acid). All exhibited good to excellent long term control of the horsenettle plant when applied annually for at least 2 yr. Picloram and glyphosate both provided better control of horsenettle when applied at a postbloom or fruiting stage of growth. Dicamba and picloram also were very effective in controlling horsenettle when used as a subsurface layer (SSL) treatment. However, all of these treatments caused visual injury to peanuts (Arachis hypogaea L.). Applications of 2,4-DB [4-(2,4-dichorophenoxy)butyric acid] were effective in causing an 80 to 90% fruiting inhibition of horsenettle plants when applied at a prebloom stage of growth, with no damage to the peanuts. Applications of several dinitroaniline herbicides as SSL treatments exhibited some control of horsenettle with little crop injury. Postemergence treatments which effectively controlled seedling horsenettle plants grown in the greenhouse were glyphosate and dinoseb (2-sec-butyl-4,6-dinitrophenol).

Dissipation of 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) in sprinkler-irrigated fields was faster in a silty clay loam soil of eastern Nebraska than in loam soils of central or western Nebraska. Atrazine dissipated faster under fallow than under corn (Zea mays L.) culture in western Nebraska, but there was no difference in atrazine loss whether in corn or fallow in central or eastern Nebraska. In central and western Nebraska, atrazine persisted in the soil for longer than 1 year. In eastern Nebraska under sprinkler irrigation, most of the atrazine had dissipated before the following growing season. Sprinkler irrigation markedly reduced atrazine longevity in soils studied.

Greenhouse pot experiments were conducted to study the effect of various levels of nitrogen, applied as ammonium nitrate (NH4NO3) and ammonium sulfate [(NH4)2SO4], on hemp broomrape (Orobanche ramosa L.) infestation on tobacco (Nicotiana tabacum L. ‘Jordan 1’) and tomato (Lycopersicon esculentum Mill. ‘Caludia Raf’). Infestation of tobacco and tomato by hemp broomrape was drastically reduced as higher levels of both fertilizers were applied. However, the dry weights of roots and shoots of both crop plants, as well as tomato yield, were reduced when NH4NO3 was applied at 50 g/pot, and when (NH4)2SO4 was applied at higher than 60 g/pot. In a separate pot experiment in the field, hemp broomrape infestation on tomato was drastically reduced, and tomato shoot dry weight, as well as tomato yield, were increased when (NH4)2SO4 was applied at higher than 40 g/pot as 20-g increments repeated at 10-day intervals compared to those of the control.

Research was conducted to determine effects of glyphosate [N-(phosphonomethyl)glycine] on the chlorophyll and carotenoid and on the catalase specific activity in purple nutsedge (Cyperus rotundus L.). The isopropylamine salt of glyphosate caused chlorosis to appear on the center leaves of plants maintained under lighted conditions after treatment. Glyphosate-induced chlorosis was delayed by darkness but occurred when treated plants were placed in the light. Glyphosate caused significant reducations in the chlorophyll and carotenoids content of purple nutsedge leaves. Based on levels in untreated plants, carotenoid content was reduced more rapidly than chlorophyll in glyphosate-treated plants. The catalase specific activity was reduced by glyphosate to about 70% of the level in untreated plants. Interrelationships among the effects of glyphosate on the carotenoid and chlorophyll content and catalase activity were not determined but possible interactions are discussed.

Greenhouse and field studies were conducted to compare root development of jointed goatgrass to winter wheat. Time of seminal root development in jointed goatgrass was similar to that of winter wheat and to root development predicted by a model. The only exception was the epiblast node roots (−1A and −1B), which developed approximately 1 phyllochron later in jointed goatgrass than in winter wheat. At crown nodes, A and B roots developed later in jointed goatgrass than in winter wheat, but development of X and Y roots at these nodes was similar for both species. First-order branching at a given root axis in jointed goatgrass roots began approximately 1 phyllochron after development of that axis, compared to 2.5 phyllochrons for winter wheat and the model. Second- and third-order branching of jointed goatgrass roots began 1 and 0.5 phyllochrons earlier than the respective branching of winter wheat roots. Leaf and tiller development followed the same pattern with time for both species. Maximum distance between crown roots was 1.5 to 3 times greater in winter wheat than in jointed goatgrass. Root length density at the 0- to 10-cm depth was less for jointed goatgrass than for winter wheat, but there were no differences at greater depths. The frequency of branching of first-order laterals was greater for jointed goatgrass than for winter wheat. In the field, the relationship of jointed goatgrass shoot development to accumulated growing degree-days was linear (R2 = 0.97), with a slope and y-intercept similar to winter wheat (R2 = 0.95). These data, showing slight differences in root growth and development between jointed goatgrass and winter wheat, may be used in predictive modeling to better understand the biology and ecology of each species, and may be used in conjunction with other models to establish weed threshold levels and improve selective placement of fertilizers and herbicides to benefit crop development.

Witchweed, [Striga asiatica (L.) Kuntze5 STRLU], an important parasitic weed throughout much of the old world, occurs as a recent (30 yr) introduction in a limited region in North and South Carolina. Reproduction in this population has been shown to occur primarily through self-pollination of flowers. Genetic diversity in two populations from this region was assessed, utilizing starch gel electrophoresis and staining of enzymes. An average of 64 individuals was observed to be monomorphic at an estimated minimum of 32 genetic loci coding 18 enzyme systems. This lack of polymorphism is attributable to the probable founding of the population by one or a few individuals coupled with the predominantly autogamous breeding system.

A method is described that provides an accurate and useful assessment of herbicide performance in small plot experiments. This method is an extension of previous rating systems and provides an assessment of the control of individual species through an estimation of the percentage that each contributes to the total broadleaf or grass weed cover on the whole plot area. It permits data collection on individual weed species but is less time-consuming than counting individual plants and it takes into account the size and vigor of individual weed plants as well as their relative abundance. Further, the system allows tabulation of data from across the entire plot rather than from small areas within the plot. The data can be readily entered into various computerized retrieval systems.

Preplant or preemergence application of 2-chloro-2′, 6′-diethyl-N-(methoxymethyl) acetanilide (alachlor) at 3.36 or 4.48 kg/ha effectively controlled yellow nutsedge (Cyperus esculentus L.) in field and greenhouse studies. Depth of tubers in the soil did not affect response to alachlor. Alachlor at 3.36 kg/ha only controlled yellow nutsedge effectively in soils with less than 6% organic matter. Alachlor at 3.7 × 10-6 M and 1.8 × 10-4 M inhibited growth and killed newly emerging shoots in petri dish cultures. However, alachlor did not inhibit sprouting of yellow nutsedge tubers.

The effect of Jerusalem artichoke (Helianthus tuberosus L. # HELTU) density and duration of interference in soybeans [Glycine max (L.) Merr. ‘Hodgson 78’] was investigated in two studies. Jerusalem artichoke densities of 1, 2, and 4 tubers/m of row reduced soybean seed yield by 31, 59, and 71%, respectively. Soybean height, branches/plant, pods/plant, and seed weight were usually reduced by all three weed densities. Soybean leaf area and relative growth rates were reduced by densities of 2 and 4 tubers/m of crop row, and net assimilation rate (NAR) was reduced by 4 tubers/m of crop row. Jerusalem artichoke interference at 4 tubers/m of row for 4, 6, 8, and 20 weeks (full season) reduced soybean yields 9, 10, 38, and 82%, respectively. Branches/plant, pods/plant, and seeds/plant were reduced after 8 weeks of interference. These results suggest that Jerusalem artichoke is highly competitive with soybean and should be controlled within 6 weeks after planting.

Quadratic response surface analysis was applied to data of seed germination in relation to temperature. Analysis of these data is the comparing of several levels of quantitative factors. In such situations, multiple-comparison procedures, such as Duncan's multiple range test, are inappropriate. The data were the germination percentages of four species at 55 constant or alternating day and night temperature regimes. A quadratic response surface was developed for each species using multiple-regression techniques. Estimated germination values and their confidence limits were derived from the quadratic response surface of each species for each cold-period temperature through a series of warm-period temperatures. Many parameters were synthesized from the response surfaces to evaluate germination of the four species in relation to temperature. These comparisons form a statistically valid basis from which to select the most adapted plant species and predict the best time for planting or applying weed-control treatments.

Several species of Potamogeton and Myriophyllum from New Jersey were collected and analyzed for 11 elements. Wide variations were observed in the inorganic composition of the plants, both within species and among species. In some instances, there appeared to be a relationship between the content of a specific element in a plant sample and the content of that element in the water in which the plants were growing. The nitrogen content of the plants was sufficiently high so that no extra nitrogen would be required to prepare composts from them.

Today, the aim of weed management is to keep the weed community at an acceptable level rather than to keep the crop totally free of weeds. Satisfactory control of weeds may often be obtained when herbicides are used at lower doses than normally recommended. To facilitate the decision of what is an adequate dose in a specific field, the farmer needs support. In 1988 and 1989, a total of 10 field trials in spring cereals were initiated in Sweden with the objective of studying long-term effects of herbicide application according to recommendations from guidelines: the guidelines were developed at the Swedish University of Agricultural Sciences and consisted of printed cards designed for in-field use. Treatments also included a full and a half dose and an untreated control. As an average over the experimental time, i.e., until 1997, the dose used in the guideline treatment varied at different sites between 20 and 70% of a full dose. In 1998, i.e., 1 yr after the last herbicide application, the plant densities of annual weeds in the guideline treatment, the half and the full doses were 51, 57, and 67% lower, respectively, than in the untreated control when averaged over sites. At two and four sites, the half and full doses resulted in significantly lower weed densities than where guidelines had been used. Compared with the control, the full and half doses increased the proportion of difficult-to-control weed species significantly at five and four sites by 21 and 24%, respectively. In the guideline treatment the proportion of difficult-to-control weeds was increased at one site. In 1998, weed counts were higher where guidelines had been used than in the full dose for common lambsquarters and common chickweed at three sites each and for wallflower mustard, catchweed bedstraw, field violet, Galeopsis spp., and Lamium spp. at one site each. At three sites, no significant treatment effects on crop yields were found, whereas yields at the remaining seven sites were higher where guidelines had been used than in other treatments in several years. It is concluded that application of dose rates according to recommendations from guidelines can be a fruitful way to reduce herbicide use.

Palmer amaranth has greatly disrupted agricultural practices in the United States with its rapid growth and rapid evolution of herbicide resistance. This weed species is now suspected in Argentina. To document whether the suspected plant populations are indeed Palmer amaranth, molecular comparisons to known standards were conducted. Additionally, these same plant populations were screened for possible herbicide resistance to several acetolactate synthase (ALS)-inhibiting herbicides. Sequencing data confirmed that suspected populations (A2, A3, A4) were indeed Palmer amaranth. Another population (A1) was tested to determine whether hybridization had occurred between Palmer amaranth and mucronate amaranth the native amaranth species of the region. Tests confirmed that no hybridization had occurred and that A1 was simply a unique phenotype of mucronate amaranth. Each population was screened for resistance to imazapic, nicosulfuron, and diclosulam. All Palmer amaranth populations from Argentina were shown to be resistant to at least one ALS-inhibiting herbicide. The populations were then subjected to further testing to identify the mutation responsible for the observed ALS resistance. All mucronate amaranth populations exhibited a mutation previously documented to confer ALS resistance (S653N). No known resistance-conferring mutations were found in Palmer amaranth.

Weeds that respond more to nitrogen fertilizer than crops may be more competitive under high nitrogen (N) conditions. Therefore, understanding the effects of nitrogen on crop and weed growth and competition is critical. Field experiments were conducted at two locations in 1999 and 2000 to determine the influence of varying levels of N addition on corn and velvetleaf height, leaf area, biomass accumulation, and yield. Nitrogen addition increased corn and velvetleaf height by a maximum of 15 and 68%, respectively. N addition increased corn and velvetleaf maximum leaf area index (LAI) by up to 51 and 90%. Corn and velvetleaf maximum biomass increased by up to 68 and 89% with N addition. Competition from corn had the greatest effect on velvetleaf growth, reducing its biomass by up to 90% compared with monoculture velvetleaf. Corn response to N addition was less than that of velvetleaf, indicating that velvetleaf may be most competitive at high levels of nitrogen and least competitive when nitrogen levels are low. Corn yield declined with increasing velvetleaf interference at all levels of N addition. However, corn yield loss due to velvetleaf interference was similar across N treatments except in one site–year, where yield loss increased with increasing N addition. Corn yield loss due to velvetleaf interference may increase with increasing N supply when velvetleaf emergence and early season growth are similar to that of corn.

Pyrazon (5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone) was absorbed by the roots of both common lambsquarters (Chenopodium album L.) and sugar beets (Beta vulgaris L.) and translocated in an acropetal direction to all parts of the plant. Common lambsquarters plants accumulated greater amounts of 3H-pyrazon per gram of tissue than did sugar beet plants and this was especially true of leaf tissues. Translocation into the leaves of both species occurred equally into mature and developing leaves. Neither basipetal nor acropetal translocation of pyrazon occurred following leaf applications of 3H-pyrazon. Pyrazon accumulated in the leaves of common lambsquarters, but it was metabolized when absorbed into sugar beets. Roots, petioles, and leaf blades of beets rapidly metabolized pyrazon while only roots metabolized pyrazon in common lambsquarters. Selectivity of pyrazon appeared to be associated with the rate of metabolic breakdown occurring in the leaf. Accumulations occurred in the susceptible common lambsquarters plant while metabolism kept pace with uptake in the leaves of the tolerant sugar beet plant.

Smellmelon is becoming a problem weed in southern Texas and Louisiana cotton-producing areas. Experiments evaluating the density and duration of smellmelon competition are necessary for the recommendation of appropriate control practices to minimize yield loss. In 1999 and 2000, field experiments were initiated to determine the density, critical period of competition, and potential control measures for smellmelon in cotton. Although the degree of competition differed between the years, similar trends were observed with respect to smellmelon density. As few as 2 or 3 smellmelon plants 10 m−1 row reduced yield at least 17% each year. When densities were increased to 5 or 10 plants 10 m−1 row, yield reductions increased to at least 34%. Results from the duration of smellmelon competition in 1999 and 2000 indicated that cotton should remain smellmelon free for 1 to 7 wk after planting (WAP) and 2.5 to 6 WAP, respectively. Results from the control studies indicate that smellmelon size at application influences control. Early-season control strategies are necessary to reduce yield losses associated with smellmelon competition.

To study rainfall effects, simulated rainfall was applied to velvetleaf (Abutilon theophrasti Medic.), common lambsquarters (Chenopodium album L.), wild mustard [Brassica kaber (DC.) L. C. Wheeler var. pinnatifida (Stokes) L. C. Wheeler], soybean [Glycine max (L.) Merr. ‘Hodgson’], and redroot pigweed (Amaranthus retroflexus L.) in greenhouse and field studies following foliar applications of the alkanolamine (AKA) salt or the butoxyethanol (BE) ester of 2,4-D [(2,4-dichlorophenoxy)acetic acid] at rates that induced equivalent levels of phytotoxicity. Simulated rainfall less than 1 min after herbicide treatment reduced the phytotoxicity of the AKA salt of 2,4-D to a much greater extent than that of the BE ester with effects ranging from elimination of all injury from the AKA salt to soybeans to no reduction in phytotoxicity of the BE ester to common lambsquarters. The quantity of simulated rainfall required to induce maximum reductions in phytotoxicity of the BE ester ranged from 1 mm on common lambsquarters to 15 mm on velvetleaf. The time interval from 2,4-D treatment until rainfall required to achieve a phytotoxic response level of 80% of that attained without rainfall varied greatly among plant species and herbicide formulations; ranging from less than 1 min for the BE ester on common lambsquarters to more than 24 h for the AKA salt on velvetleaf. The addition of an alkylarylpolyoxyethylene glycol surfactant to 2,4-D spray solutions reduced herbicide rates required to induce equivalent levels of phytotoxicity, increased losses in phytotoxicity of the BE ester caused by rainfall, and reduced the time interval from treatment to rainfall required to attain an equivalent level of phytotoxicity with the AKA salt.

Glyphosate, a widely used preplant herbicide in annual and perennial crops, was introduced to Sri Lanka in 1977. Its use has expanded since 2008 with the phase-out and ban in 2014 of paraquat. In December 2014, glyphosate use in Sri Lanka was regionally restricted. Crop protection and production in the country was severely affected in 2016 and 2017 due to the irrational decisions of the government of Sri Lanka (GoSL). Increased crop production costs due to the absence of effective and economically viable weed control techniques, low crop yields, loss of foreign exchange, and enhanced use of smuggled glyphosate products are the consequences of the glyphosate ban. The ban was imposed without a scientific basis because of sociopolitical pressure. A series of dialogues with the GoSL helped rescind the ban in 2018 for a period of 36 mo, but its use is limited to tea [Camellia sinensis (L.) Kuntz.] and rubber [Hevia brasiliensis (Willd. ex A. Juss.) Müll. Arg.]. In August 2019, the Cabinet of Ministers of the GoSL also decided to allow use of glyphosate to devitalize propagules in the floriculture industry (export oriented) and destroy coconut (Cocos nucifera L.) trees infected by Weligama coconut leaf wilt disease and sugarcane (Saccharum officinarum L.) infected by white leaf disease. However, glyphosate products with the co-formulant polyethoxylated tallow amine are still not permitted in Sri Lanka.

Shortawn foxtail is a competitive annual grass weed widely spread in east, south-central, and southwest China and parts of the Yellow River basin. One shortawn foxtail population (JSQT-1) resistant to fenoxaprop was identified in Jiangyan, Jiangsu province. Whole-plant experiments determined that the resistant population conferred high-level resistance to fenoxaprop (93-fold), clodinafop (21-fold), sethoxydim (107-fold), mesosulfuron (41-fold), and pyroxsulam (12-fold); moderate-level resistance to haloxyfop (8-fold), clethodim (9-fold), and pinoxaden (8-fold), and no resistance to isoproturon. Molecular analyses confirmed that the Ile-1781-Leu mutation was present in the resistant population. A dCAPS marker was used to detect the Ile-1781-Leu mutation. All 97 plants of the resistant population analyzed were homozygous mutants at the 1781 position. Our study established the first case of fenoxaprop resistance in shortawn foxtail, determined cross resistance to other herbicides, and elucidated that the molecular basis of resistance resulted from, at least partly, an Ile to Leu mutation at amino acid position 1781 in the plastid ACCase.

Greenhouse experiments were conducted to quantify resistance levels to the 4-hydroxyphenyl-pyruvate dioxygenase (HPPD)-inhibiting herbicides mesotrione (MES) and isoxaflutole (IFT) in NEB (Nebraska HPPD- and atrazine-resistant) and SIR (Stanford, IL, HPPD- and atrazine-resistant) waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] populations. These populations differ in their field-use histories and resistance levels to MES. Foliar growth responses were compared with ACR (HPPD sensitive; metabolic atrazine-resistant) and SEN (sensitive to HPPD and photosystem II [PSII] inhibitors). A greenhouse dose–response study was conducted with each herbicide at two POST timings: early (EPOST) (5 cm; 4 to 5 true leaves) and POST (10 cm; 8 to 9 true leaves). At the EPOST timing, SIR was 10-fold resistant to IFT and 32-fold resistant to MES, while NEB was 4-fold resistant to IFT and 7-fold resistant to MES when compared with ACR. At the POST timing, SIR was 17-fold resistant to IFT and 21-fold resistant to MES, while NEB was 3-fold resistant to IFT and 7-fold resistant to MES when compared with ACR. Results overall indicated greater fold-resistance levels to MES relative to IFT at each timing. However, POST treatments to SIR showed contrasting effects on resistance levels relative to EPOST. To investigate potential management strategies for resistant A. tuberculatus populations, a POST interaction study was conducted using combinations of metribuzin and either IFT or MES. A metribuzin rate (191 g ai ha−1) causing an approximately 20% biomass reduction was chosen for interaction studies and combined with varying rates of either IFT or MES. Results indicated 52.5 g ai ha−1 of MES combined with metribuzin displayed a synergistic effect on biomass reduction in SIR. However, other combinations of either MES or IFT and metribuzin resulted in additive effects on biomass reduction in both HPPD-resistant populations. These results provide insights into the joint activity between HPPD and PSII inhibitors for controlling metabolism-based, multiple herbicide–resistant A. tuberculatus.