North African knapweed (Centaurea diluta Aiton) is an annual weed that is widespread in southern Spain and is of increasing concern in dryland cropping systems. Despite its expanding range in Spain, there is limited information on the emergence timing and pattern of this species, knowledge of which is critical for developing more timely and effective management strategies. Therefore, there is a need to develop simple and reliable models to predict the timing and emergence of this annual weed under dryland conditions. A multi-location field experiment was established across Spain in 2016 to 2017 to assess the emergence of C. diluta. At each of 11 locations, seeds were sown in the fall, and emergence was recorded. Overall emergence averaged 39% in the first year across all sites and 11% in the second year. In both years, the main emergence flush occurred at the beginning of the growing season. A three-parameter Weibull function best described seedling emergence of C. diluta. Emergence models were developed based on thermal time (TT) and hydrothermal time (HTT) and showed high predictability, as evidenced by root mean-square error prediction values of 10.8 and 10.7, respectively. Three cardinal points were established for TT and HHT at 0.5, 10, and 35 C for base, optimal, and ceiling temperatures, respectively, while base water potential was estimated at −0.5 MPa.

Field experiments were conducted in 1986 and 1987 to determine the effects of shoot and root interference of common cocklebur and soybean. Plants were grown in porous membrane envelopes. Common cocklebur plants had a longer vegetative growth period than soybean and were twice as tall as soybean at maturity. Soybean root and shoot dry weight and seed yield were reduced by shoot and whole plant (root and shoot) interference of common cocklebur, with whole plant interference giving the greatest reduction. Common cocklebur growth was affected little by soybean interference. Common cocklebur shoot interference alone reduced total soybean seed weight by 48%, the amount caused by common cocklebur whole plant interference.

Soybean varieties ‘Ripley’ (semidwarf determinate group IV) and ‘Douglas' (indeterminate group IV) were compared in an interference experiment with common cocklebur. A path analysis model was used to independently estimate interference effects of soybean density on common cocklebur growth and stem diameter, and interference responses of soybean density and seed production to common cocklebur size. Path analysis identified an early period of interference between soybean and common cocklebur, from 40 to 62 d after planting, and a late period, from 63 to 145 d after planting. Late interference measures were independent of early measures, since effects of early interference were statistically removed from estimates of late interference. The semidwarf determinate variety Ripley had early interference effects on common cocklebur 5.3 and 9.5 times greater than those of Douglas in 1987 and 1988, respectively. The Douglas variety had a stronger late interference effect on common cocklebur than Ripley. Soybean varieties were similar in response to common cocklebur interference. Path analysis of crop-weed interference may be useful in breeding crop varieties that suppress weed growth and thus require fewer weed control inputs.

The development of problematic weed populations is a concern in western Canadian fields where canola and pea are grown in a 4-yr sequence with spring cereal grains. Weed densities were examined at a site near Melfort, Saskatchewan, Canada, from 1994 to 1997 in seven zero-till managed crop rotations. Four rotations that included canola, pea, or flax in at least 3 of 4 yr (HBF: high broadleaf–crop frequency) were compared with three rotations that included broadleaf crops grown in 2 of 4 yr (LBF: low broadleaf–crop frequency). Spring wheat and barley were the cereal crops in rotation. Residual (postherbicide application) weed density for each weed species in a given year was summed across all phases for each rotation to reflect the overall weed infestation. Four annual broadleaf weed species were most abundant in 1996 and a second group of three species, having a variety of reproductive strategies, became progressively less abundant as the study progressed. The difference between the HBF and LBF rotations for the density of these species varied and was most prominent in years when environmental conditions were conducive for their growth. More frequent applications of ethafluralin, with its residual weed control, best explained why wild oat and catchweed bedstraw generally were less abundant in the HBF rotations. Of particular interest was the 8 plants m−-2 greater density of dandelion and perennial sowthistle in the HBF vs. LBF rotations in the last year of the study. It is thought that the limited herbicide options for the control of these species could present a future problem if they continued to develop in the HBF rotations. Differences in herbicide use between the HBF and LBF rotations were considered the primary factor controlling the rotation effects on weed density.

Combined effects of herbicides, herbicide rates, and cultivation for weed control in soybeans [Glycine max (L.) Merr. ‘Williams’] were studied using full and one-half rates of either chloramben (3-amino-2,5-dichlorobenzoic acid) or alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] + linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] with no, one, and two cultivations. The full rate was 2.2 kg/ha for alachlor and chloramben and 0.70 kg/ha for linuron. Alachlor + linuron was better than chloramben, and the full rate was better than the one-half rate. One or two cultivations were better than no cultivation. Cultivations were more effective when used with alachlor + linuron than when used with chloramben or when used alone. Alachlor + linuron at the full rate with one or two cultivations produced the best soybean yields. A one-half rate of alachlor + linuron with one or two cultivations yielded the same as a full rate of alachlor + linuron alone or with one cultivation, chloramben at one-half rate with two cultivations, and chloramben at a full rate with two cultivations. Cultivations can be effective by increasing weed control and yields when herbicide rate or effectiveness has been reduced. Use of a cultivation in addition to the preemergence herbicides used in this study is necessary for improved weed control and yields.

In a greenhouse experiment, dosages of 2-methoxy-3,6-dichlorobenzoic acid [dicamba] and 2-methoxy-3,5,6-trichlorobenzoic acid [tricamba] required initially to reduce fresh weights of snap beans (Phaseolus vulgaris L.) 50% (hereinafter referred to as ED50 values) increased as clay and organic matter increased in five soils. In all soils, 2,3,6-trichlorobenzoic acid [2,3,6-TBA] was about equally toxic, but ED50 values for 2,3,6-trichlorophenylacetic acid [fenac] and 2-methoxy-3,6-dichlorophenylacetic acid [hereinafter referred to as methoxy fenac] decreased as clay and organic matter increased. Under experimental conditions of alternately moist and dry soil, phytotoxicity of dicamba, tricamba, and methoxy fenac decreased with time, but that of 2,3,6-TBA and fenac remained approximately the same for about 22 months. When soils were maintained moist throughout the incubation period, 3-amino-2,5-dichlorobenzoic acid [amiben] was least persistent, dicamba intermediate, and 2,3,6-TBA and fenac most; under these conditions, phytotoxic effects of 2,3,6-TBA and fenac decreased with time. In soil-enrichment studies, fenac, methoxy fenac, dicamba, and 2,3,6-TBA, in comparison with 2,4-dichlorophenoxyacetic acid [2,4-D], appeared very resistant to microbial degradation.

“Selective” herbicidal properties of 4–chloro–2–butynyl N–(3–chlorophenyl)carbamate, barban, when applied postemergence for the control of wild oats in cereal grains, have been observed in preliminary trials by a number of workers. Their results suggested that the degree of control was a function of several factors including dosage, volume of diluent, fertility of the soil and the stage of growth of both the weed and crop plants at the time of spraying. Differences in tolerance between grain crops and between varieties within a crop were also indicated.

The effects of MSMA on the development of straighthead, yields of rice, and hormonal balance in rice plants were observed in a greenhouse study. MSMA was applied to soil at 0, 6, and 9 kg ai/ha. At 6 kg/ha some straighthead and a 45% yield reduction occurred. At 9 kg/ha straighthead was evident, and yield was reduced by 85%. Analyses were made for ethylene with gas chromatography and for indoleacetic acid (IAA) with high performance liquid chromatography. Concentrations of ethylene and total arsenic detected in the tissue from rice leaves grown in MSMA-treated soil were higher at every growth stage when compared to plants grown in untreated soil. Both treated and untreated plants showed marked increases of these compounds during the reproductive stage and a decrease prior to panicle emergence. Significant changes occurred in IAA levels with the growth stage of treated and untreated samples, but MSMA had no obvious effect on IAA levels at any of the stages examined in this study.

The research worker who attempts the application of herbicides to a small area in such a way as to simulate general practice, is faced with a unique problem. The ultimate objective is to apply the chemical to as small an area as is feasible but in such a way that it will correctly represent what the activity on both weeds and crop plants would have been had the application been made on an acreage basis. Small plots are desirable if land or labor are limited, if expensive crops in farmers’ fields are being used, and particularly when the chemicals may be highly toxic, or the activity is unknown.

The effect of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] and alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] combinations on various physiological processes was studied in order to establish a basis for the synergistic effect of this mixture on Japanese millet [Echinochloa crus-galli (L.) Beauv. var. frumentacea L.]. In contrast to atrazine, alachlor had no effect on the Hill reaction activity of isolated Japanese millet chloroplasts. When used in combination with atrazine, alachlor had no influence on the inhibitory effect of atrazine on the Hill reaction. Atrazine and alachlor combinations reduced chloroplast protein and severely inhibited chloroplast protein synthesis relative to protein synthesis by other particulate fractions. This inhibition appears to be the basis for the synergistic effect of this herbicide mixture on Japanese millet. Although the mechanism of action of the herbicide mixture is not understood, it does not seem to involve inhibition of atrazine detoxification.

Field and laboratory investigations were conducted from 1983 through 1985 to evaluate morphological and environmental influences on the performance of imazaquin on 26 weed species common to the Mississippi Delta. Imazaquin applied preplant incorporated or preemergence at 140 g ai ha–1 controlled 16 species but controlled only five species at 140 g ha–1 foliar applied. Contour graphs accurately predicted imazaquin postemergence rates necessary to control a given weed species at a given time after emergence. Imazaquin applied preemergence with a sequential postemergence application controlled entireleaf morningglory and sicklepod better than a single soil application. Imazaquin at 140 g ha–1 applied preemergence required at least 50 mm of supplemental water following application to control sicklepod and was more phytotoxic to sicklepod grown at 25/10 C day/night temperatures than those grown at 32/22 C.

Effective outreach is critical to achieving success in managing herbicide-resistant weeds. Interdisciplinary collaboration is needed to adapt information delivery and to engage communities to address the herbicide-resistance problem. Weed scientists must partner with the production community to adapt herbicide-resistance practices for local needs, to work collaboratively with state and regional stakeholders to create effective resistance-management practices, and to provide an overarching national message as to the causes of, and solutions to, resistance.

Silverleaf nightshade (Solanum elaeagnifolium Cav.) is an invasive species that has successfully spread outside its native range to become a noxious weed in 21 states in the United States and 42 countries worldwide. The successful establishment of S. elaeagnifolium outside its native habitat indicates its innate ability to adapt to a multitude of environments. Phenotypic plasticity and/or genetic adaptation have been identified as key mechanisms underlying the adaptive success of invasive species. Whereas phenotypic plasticity allows a species to buffer changes in the environment by altering its phenotypic attributes within the short term, genetic adaptation is responsible for the longer-term adaptability of plants to heterogeneous environments and is dependent on the amount of genetic variation present in the species. In this study, we screened DNA markers that are specific to tomato (Solanum lycopersicum L.) and Solanum lycopersicoides Dunal for their interspecific transferability to S. elaeagnifolium and determined the applicability of the transferable DNA markers in assessing the extent of genetic variation in populations from Lubbock, Littlefield, and Blackwell, TX. Of the 187 markers screened, 78 successfully amplified targets in S. elaeagnifolium, indicating the evolutionary conservation of marker loci across S. lycopersicum, S. lycopersicoides, and S. elaeagnifolium, despite their genetic divergence millions of years ago. Genotyping of S. elaeagnifolium populations using 50 DNA markers that consistently amplified clear bands in more than 60% of the plants identified nine polymorphic markers with 0.014 to 0.621 polymorphism information content. Genetic diversity analysis by DNA marker profiling established genetic variation among populations and within individuals of different populations. Unweighted paired group method with arithmetic mean analysis grouped the plants into six clusters that are generally defined by selection pressures unique to each collection site. Results of the study indicate the capacity of S. elaeagnifolium for genetic differentiation in response to variable selection pressures within the same geographic region.

Ammannia multiflora Roxb. is a dominant broadleaf weed that is a serious problem in southern China rice fields, and acetolactate synthase (ALS)-inhibiting herbicides have been used for its control for more than 20 years. Excessive reliance on ALS-inhibiting herbicides has led to herbicide resistance in A. multiflora. In this study, 10 A. multiflora populations from the Jiangsu Province of China were collected, and the resistance levels and target site–resistance mechanisms to ALS-inhibiting herbicides bensulfuron-methyl and penoxsulam were investigated. The dose–response assays showed that eight populations evolved resistance to bensulfuron-methyl (9.1- to 90.9-fold) and penoxsulam (5.0- to 103.1-fold). Amplification of ALS genes indicated that there were three ALS genes (AmALS1, AmALS2, and AmALS3) in A. multiflora. Sequence analysis revealed amino acid mutations at Pro-197 in either AmALS1 (Pro-197-Ala, Pro-197-Ser, and Pro-197-His) or AmALS2 (Pro-197-Ser and Pro-197-Arg) in resistant populations, and no mutations were found in AmALS3. Moreover, two independent mutations (Pro-197-Ala in AmALS1 and Pro-197-Ser in AmALS2 or Pro-197-Ala in AmALS1 and Pro-197-Arg in AmALS2) coexisted in two resistant populations, respectively. In addition, the auxin mimic herbicides MCPA and florpyrauxifen-benzyl, the photosystem II inhibitor bentazon, and the protoporphyrinogen oxidase inhibitor carfentrazone-ethyl can effectively control the resistant A. multiflora populations. Our study demonstrates the wide prevalence of ALS inhibitor–resistant A. multiflora populations in Jiangsu Province and the diversity of Pro-197 mutations in ALS genes and provides alternative herbicide options for controlling resistant A. multiflora populations.

Poppy (also common poppy or corn poppy; Papaver rhoeas L., PAPRH) is one of the most harmful weeds in winter cereals. Knowing the precise and accurate location of weeds is essential for developing effective site-specific weed management (SSWM) for optimized herbicide use. Among the available tools for weed mapping, deep learning (DL) is used for its accuracy and ability to work in complex scenarios. Crops represent intricate situations for weed detection, as crop residues, occlusion of weeds, or spectral similarities between crop and weed seedlings are frequent. Timely discrimination of weeds is needed, because postemergence herbicides are used just when weeds and crops are at an early growth stage. This study addressed P. rhoeas early detection in wheat (Triticum spp.) by comparing the performance of six DL-based object-detection models focused on the “You Only Look Once” (YOLO) architecture (v3 to v5) using proximal RGB images to train the models. The models were assessed using open-source software, and evaluation offered a range of results for quality of recognition of P. rhoeas as well as computational capacity during the inference process. Of all the models, YOLOv5s performed best in the testing phase (75.3%, 76.2%, and 77% for F1-score, mean average precision, and accuracy, respectively). These results indicated that under real field conditions, DL-based object-detection strategies can identify P. rhoeas at an early stage, providing accurate information for developing SSWM.

Amiben (3–amino–2,5–dichlorobenzoic acid) was applied to the surface of various soil types in cylinders which were then leached with water. The leached soil was tested biologically for herbicide activity. Loss was most rapid from sand, followed by clay, with muck remaining phytotoxic much longer. There was only a slight increase in the toxicity of loam soil given repeated treatments with amiben. Incubation of treated soil for various periods of time indicated that microorganism breakdown is important in soils relatively high in organic matter.

Field studies were conducted at three locations in 1993 and 1994 to evaluate weed control and crop response to metolachlor plus combinations of 0.5 × and 1 × label rates of imazaquin applied preplant and imazethapyr applied early postemergence or postemergence in no-till narrow-row soybean production. Giant foxtail, common ragweed, common cocklebur, and large crabgrass population reductions were greater with sequential preplant metolachlor plus imazaquin followed by early postemergence or postemergence imazethapyr than with preplant metolachlor plus imazaquin or early postemergence/postemergence imazethapyr alone. Ivyleaf morningglory was not effectively controlled by any herbicide program. Pennsylvania smartweed populations were reduced with all herbicide treatments. Soybean yields with treatments utilizing 0.5 × rates were usually equal to 1 × rates if imazethapyr was applied early postemergence or postemergence. Net income with reduced herbicide rates was equal to full-label rates and provided no greater risk to net income.

Glyphosate [N-(phosphonomethyl)glycine] was applied in the spring and fall to leafy spurge (Euphorbia esula L. # EPHES) in the field at rates of 0.14 to 4.48 kg ai/ha. Fall applications of glyphosate at rates of 0.56 to 4.48 kg/ha stimulated axillary branching and caused an increase in the number of stems/m2 by the end of the following summer in a dense population. This was a result of shoot growth from buds on the crown region of the root system. The absorption and translocation of 14C-glyphosate applied to leaves of mature leafy spurge plants were evaluated at prebloom, full-bloom, and senescence phenological stages. Approximately 81% of the 14C-glyphosate applied to the leaves of senescing plants was absorbed. There was a decrease in the proportion of 14C translocated out of the treated leaf when applications were made after full bloom. Translocation of 14C to the treated stem, nontreated stems, root crown, and roots did not differ with phenological stage in 1983. Translocation and concentration of 14C in most plant parts in 1985 differed with phenological stage. Translocation to the crown buds as a percentage of the 14C absorbed was highest at the senescence stage. At senescence and before soil freezing, leafy spurge crown buds demonstrate transient but active elongation toward the soil surface. This active development may account for the enhanced translocation of 14C into the crown buds.

Bulb onions are poor competitors and volunteer potato, commonly observed in western USA onion fields, is difficult to manage. To improve the understanding of onion and weed interactions, relationships were quantified among volunteer potato density, onion yield, and volunteer potato tuber production using hyperbolic or linear models. Onion yield losses because of volunteer potato interference occur at densities commonly observed in the field. A volunteer potato density as low as 0.067 plants m−2 resulted in a 10% reduction in crop yield. Asymptotic yield loss (A parameter) was 100% and achieved with 4 volunteer potato plants m−2. Volunteer potato competition limits onion bulb size, resulting in a lower quality and thus a less-valuable crop. Volunteer potato tuber density and biomass increased linearly with initial weed density as high as 8 volunteer potato m−2. Onion yield loss from volunteer potato competition occurs to a greater extent and at a lower weed density than demonstrated in previous research on small-seeded annual weed species.

Herbicide-resistant weeds pose a considerable threat to agriculture, but their resistance mechanisms are poorly understood. Differential gene expression analysis of a weed subjected to herbicide treatment is a key step toward more mechanistic studies. Such an analysis, often involving quantitative real-time PCR (qPCR), requires suitable reference genes as internal controls. In this study, we identified optimal reference genes in the noxious weed, Japanese foxtail. This weed has evolved resistance to acetyl-coenzyme A carboxylase (ACCase) inhibitors. We analyzed the stability of eight commonly used candidate reference genes (glyceraldehyde-3-phosphate dehydrogenase [GAPDH]; ubiquitin [UBQ]; capsine phosphatase [CAP]; beta-tubulin [TUB]; eukaryotic initiation factor 4a [EIF4A]; elongation factor-1 alpha [EF1]; 18S ribosomal RNA [18S]; 25S ribosomal RNA [25S]) from root, stem, and leaf tissue of plants that were either resistant or sensitive to ACCase inhibitors, with or without herbicide stress, using qPCR. The results were further ranked and analyzed using geNorm, NormFinder, and BestKeeper software. These analyses identified EF1 and UBQ in roots, EF1, TUB, CAP, and 18S in stems, and EF1, GAPDH, and 18S in leaves as suitable references for qPCR normalization. We have identified a set of reference genes that can be used to study herbicide resistance mechanisms in Japanese foxtail.