A 2-yr field study was conducted to determine whether the addition of wetting agents to diclofop {2-[4-(2,4-dichlorophenoxy) phenoxy] propanoic acid} could enhance its postemergence control of grass weeds without altering its crop tolerance. In each year, 12 species were treated at two stages of growth with at least two concentrations of diclofop. Regardless of the treatment, selectivity was not altered on wheat (Triticum aestivum L.), soybeans [Glycine max (L.) Merr.], cucumber (Cucumis sativus L.), or sorghum [Sorghum bicolor (L.) Moench]. The addition of wetting agent to diclofop was of no value on highly susceptible grass weed species such as barnyardgrass [Echinochloa crus-galli (L.) Beauv.] or on the more resistant species such as large crabgrass [Digitaria sanguinalis (L.) Scop.]. Only on an intermediately susceptible species such as giant foxtail (Setaria faberi Herrm.) could any significant differences in control be attributed to a wetting agent. The stage of growth of the grass weed species at time of treatment and the diclofop concentration were more important than the presence of a nonionic wetting agent.

Greenhouse and field experiments were conducted with a roller applicator at Lincoln, Nebraska, during 1979 and 1980. Glyphosate [N-(phosphonomethyl)glycine] concentrations of 5, 10, and 20% and carpet saturations of 50 and 75% controlled shattercane [Sorghum bicolor (L.) Moench] when applied to the top 30 cm of the plant in greenhouse research. In the field, glyphosate concentrations of 5 to 20% with a carpet saturation of 50% controlled shattercane acceptably in soybeans [Glycine max (L.) Merr.], but a concentration of 2.5% with 25% carpet saturation did not. Weed control was comparable whether speed of application was 3.2, 6.4, or 9.6 km/h. Shattercane control in grain sorghum [Sorghum bicolor (L.) Moench.] was excellent at glyphosate concentrations of 5, 10, and 20% and at carpet saturations of 50 and 75%, and sorghum injury was minimal at 25 and 50% carpet saturations. The roller applicator was compared to a ropewick applicator for shattercane control in sorghum. Excellent weed control (90% or greater) with minimal crop injury was obtained with the roller applicator at glyphosate concentrations of 10 and 20% at application speeds of 3.2 and 6.4 km/h and with the ropewick applicator with glyphosate concentrations of 35 and 50% applied at 3.2, 6.4, and 9.6 km/h.

Foliage-only, soil-only, and foliage plus soil applications of picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid), clopyralid (3,6-dichloro-2-pyridinecarboxylic acid), chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzenesulfonamide}, and metsulfuron {2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]amino]sulfonyl]benzoic acid} were used to determine the contribution of soil spray deposit to the control of Canada thistle [Cirsium arvense (L.) Scop. ♯ CIRAR] under greenhouse conditions. Herbicide reaching the soil contributed significantly to the suppression of secondary shoot growth, particularly when pots were watered from above. In subirrigated pots, the herbicides showed activity if they were sprayed on moist soil, but not if they were sprayed on dry soil. These results suggest that it is important to shield the soil during postemergence applications of herbicides in greenhouse experiments in order to avoid overestimating foliar activity.

Field experiments were conducted to investigate methods of controlling red rice (Oryza sativa L. ♯ ORYSA) in drill-seeded rice (O. sativa). Treatments included the rice cultivar ‘Mars', coated with calcium peroxide (CaO2) at 40% (w/w) and a crop protectant, R-33865 (O,O-diethyl-O-phenyl phosphorothioate) at 0.5 and 1% (v/w). Molinate (S-ethyl hexahydro-1H-azepine-1-carbothioate) at 6.7 kg ai/ha was applied preplant incorporated (ppi). The land was flooded (2.5 to 5 cm deep) after seeding with rice (100 kg/ha, 2.5 cm deep), and the water was maintained throughout the growing season. CaO2, with or without molinate, increased rice grain yield 50% and increased rice culm density fivefold above untreated rice. Molinate applied ppi controlled 96% of the red rice. Rice seed coated with only CaO2 or with CaO2 plus R-33865 at 0.5%, each combined with ppi molinate, produced 5690 and 6030 kg/ha of grain, respectively. These high yields were associated with red rice control by molinate and good stands of rice provided by O2 supplied by CaO2. R-33865 applied to rice seed at 1% (v/w) injured rice by reducing rice culm densities 41%, compared with rice without protectant.

The range weed, Scotch thistle (Onopordum acanthium L.), is an increasing problem in the western United States. It competes with desirable forage plants and the flower stalks are a barrier to livestock movement. Prolific production of viable but dormant achenes makes this species a persistent pest. Applications of 4-amino-3,5,6-trichloropicolinic acid (picloram) in the spring or fall, at rates as low as 0.03 lb/A, gave control of Scotch thistle.

Horseweed has rapidly become a major weed in soybean and cotton production fields of the United States, and Indiana farmers ranked horseweed as one of the five worst weeds in their fields during a mail survey in 2003. Glyphosate resistance in horseweed is conferred by a single, incompletely dominant gene. Horseweed populations possess a high level of variability in their response to glyphosate. Horseweed has also evolved resistance to acetolactate synthase (ALS) inhibitors, and biotypes resistant to ALS-inhibiting herbicides and glyphosate are in many of the same areas. An experiment was designed to determine whether glyphosate resistance can be transferred by pollen. We found glyphosate-resistant plants in 1.1 to 3.8% of the progeny. Segregation ratios fit the expected 3 : 1 resistant : sensitive ratios confirming that glyphosate resistance in horseweed can transfer to closely located glyphosate-susceptible biotypes under open-pollinated conditions at low frequencies. The hypothesis of a follow-up experiment was that first-generation progeny of parent plants, selected on a continuum of low to high phenotypic response to glyphosate, will inherit respective low to high phenotypic responses to glyphosate. The variability in field-collected populations (low-level to high-level glyphosate resistance) ranged from 2 to 14 times the commonly recommended field use rate of glyphosate. However, low- and high-levels of glyphosate resistance were not observed in first-generation progeny. We conclude that differential glyphosate responses observed among parental populations was due to different frequencies of the resistance allele within the populations, rather than the presence of different resistance alleles.

Common lambsquarters tolerance to glyphosate is problematic because of the species' widespread distribution, competitive ability with many crop species, the widespread use of glyphosate in agriculture, and the weed's potential to develop decreased sensitivity to multiple herbicide sites of action. The mechanism that confers common lambsquarters tolerance to glyphosate is not known. Therefore, we conducted experiments to determine the mechanism of tolerance to glyphosate in an accession of common lambsquarters from Indiana relative to a sensitive accession from Wisconsin. The ED50 (the effective dose that reduced shoot mass 50% relative to nontreated plants) value for the tolerant accession (1.6 kg ae ha−1 ± 0.4 standard error of the mean [SEM]) was eightfold greater than the ED50 for the sensitive accession (0.2 kg ae ha−1 ± 0.2 SEM) 28 d after treatment. The glyphosate target-site (EPSPS) DNA sequence at proline 106, shikimate accumulation as an estimate of EPSPS sensitivity, and EPSPS protein abundance did not differ between accessions. Absorption of 14C-glyphosate was slightly greater in the tolerant accession than it was in the sensitive accession at 48 and 72 h after treatment (HAT). However, the tolerant accession translocated a smaller percentage of absorbed 14C-glyphosate to the tissue above the treated leaf, which included the shoot apical meristem, at 24, 48, and 72 HAT (P ≤ 0.05, 0.01, and 0.10, respectively). These results suggest an important role of reduced translocation in conferring tolerance of common lambsquarters to glyphosate.

Harvest weed seed control (HWSC) may control problematic weeds by decreasing contributions to the weed seedbank. However, HWSC practices will not be effective if plants have shed a great part of their seeds before harvest or if a low proportion of seed production is retained at a height that enables collection during harvest. The seed-shattering pattern of several weed species was evaluated over three growing seasons to determine their potential to be controlled with HWSC in the Pacific Northwest (PNW). The studied weed species were downy brome (Bromus tectorum L.), feral rye (Secale cereale L.), Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot], and rattail fescue [Vulpia myuros (L.) C.C. Gmel.]. Seed retention at harvest, seed production, and plant height differed among species, locations, and years. Environmental conditions influenced seed-shattering patterns, particularly the time plants started to shatter seeds and the rate of the shattering. Agronomic factors such as herbicide use, interrow space, or crop height/vigor also seemed to affect shattering patterns and seed production, but more specific studies must be conducted to determine their individual effects. Bromus tectorum, L. perenne ssp. multiflorum, and V. myuros had an average seed retention at harvest of less than 50%. In addition, the low seed retention height of V. myuros makes this species a poor candidate for HWSC. Secale cereale had average seed retention at harvest greater than 50%, and seed retention height was greater than 30 cm. The variability of seed retention in different species will make the efficacy of HWSC practices species and environment dependent in PNW winter wheat (Triticum aestivum L.) cropping systems. Harvesting the wheat crop as early as possible will be crucial to the success of HWSC.

Knowledge of crop–weed interference effects on weed biology along with yield penalties can be used for the development of integrated weed management (IWM) tactics. Nevertheless, little is known about the beneficial effects of soybean [Glycine max (L.) Merr.] density, an important aspect of IWM, on late Palmer amaranth (Amaranthus palmeri S. Watson) establishment time. Two field experiments were conducted in 2014 and 2015 to investigate how various soybean densities and A. palmeri establishment timings in weeks after crop emergence (WAE) affect height, biomass, and seed production of the weed but also crop yield in drill-seeded soybean. Soybean density had a significant impact on dry weight and seed production of A. palmeri that established within the first 2 wk of crop emergence, but not for establishment timings of the weed 4 wk and later in relation to crop emergence. Differential performance of A. palmeri gender was observed, regarding greater biomass production of female than male plants under crop presence, and merits further investigation. Grain yield reductions were recorded at earlier A. palmeri establishment timings (i.e., 0 and 1 WAE) compared with 8 WAE establishment timing in 2014 and 2015. High soybean densities resulted in greater soybean yields compared with low soybean density, but no grain yield benefits were observed between medium and high soybean densities. Crop budget analysis revealed the benefits of moderate seeding rate (i.e., 250, 000 seeds ha−1) increases in comparison to lower (i.e., 125,000 seeds ha−1) or high (i.e., 400,000 seeds ha−1) on crop revenue, net income returns, and breakeven price. Earlier A. palmeri establishment timings (i.e., 0, 1, and 2 WAE) resulted in lower crop revenue and net income returns compared with later establishment timings of the weed.

Herbicide-resistant (HR) crops are widely grown throughout the United States and Canada. These crop-trait technologies can enhance weed management and therefore can be an important component of integrated weed management (IWM) programs. Concomitantly, evolution of HR weed populations has become ubiquitous in agricultural areas where HR crops are grown. Nevertheless, crop cultivars with new or combined (stacked) HR traits continue to be developed and commercialized. This review, based on a symposium held at the Western Society of Weed Science annual meeting in 2021, examines the impact of HR crops on HR weed management in the U.S. Great Plains, U.S. Pacific Northwest, and the Canadian Prairies over the past 25 yr and their past and future contributions to IWM. We also provide an industry perspective on the future of HR crop development and the role of HR crops in resistance management. Expanded options for HR traits in both major and minor crops are expected. With proper stewardship, HR crops can reduce herbicide-use intensity and help reduce selection pressure on weed populations. However, their proper deployment in cropping systems must be carefully planned by considering a diverse crop rotation sequence with multiple HR and non-HR crops and maximizing crop competition to effectively manage HR weed populations. Based on past experiences in the cultivation of HR crops and associated herbicide use in the western United States and Canada, HR crops have been important determinants of both the selection and management of HR weeds.

Leaching of N,N-dimethyl-2,2-diphenylacetamide (diphenamid) was measured with bioassays in three soils; a fine sand, a silt loam, and a muck. The movement of diphenamid was reduced as the clay or the organic matter content of these soils increased. Efficiency of organic matter for adsorption of diphenamid decreased as the organic matter content of soil mixtures increased. Clay (bentonite) was less efficient than organic matter in adsorbing diphenamid. Desorption of diphenamid from muck soil was very slow.

When 2-(2,4,5-trichlorophenoxy)propionic acid-1-C14 (silvex) was applied to Opuntia polyacantha Haw. (prickly pear), pad absorption, root absorption, and translocation were low. The rate of translocation, as measured by acid fuchsin movement, was one cm per hour, about one tenth that found in soybean. Increasing the temperature from 21 to 32 C only slightly increased root absorption and upward translocation of silvex. Pre-treatment of a prickly pear pad, either with a 4% 2,2-dichloropropionic acid solution or mechanical removal of the cuticle, increased the absorption and translocation of subsequently applied silvex. The decarboxylation of silvex was 2 to 3 times greater in soybean than in prickly pear plants. Sixteen days after silvex was applied to prickly pear, four C14-metabolites were found in addition to unaltered silvex.

Comparative studies of herbicides and herbicide mixtures were conducted at Tifton, Georgia, in 1959 and 1960. Certain mixtures, which contained sodium 2,4–dichlorophenoxyethyl sulfate (sesone), tris(2,4–dichlorophenoxyethyl) phosphite (2,4–DEP), or the triethylamine salt of 3–amino–2,5–dichlorobenzoic acid (amiben) mixed with the alkanolamine salt of 4,6–dinitro–o–sec-butylphenol (DNBP), gave seasonal weed control on band-treated peanuts. Compared to herbicides applied singly, mixtures not only increased the spectrum of weeds controlled but also permitted the use of the lowest rate of each component on the weeds for which it was most effective.

Honey mesquite (Prosopis glandulosa Torr. # PRCJG) was treated with four herbicides at 1.1 kg ae/ha at 14 dates from April 30 through September 23 over a 2-yr period. Overall, clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) was the most effective herbicide, killing 80% or more of the honey mesquite at most dates. Picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid), triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid}, and 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid] were less effective than clopyralid. Honey mesquite control was correlated with Julian day of year sprayed, 15 plant, 11 environmental, and 8 herbicide concentration variables. Most effective control was negatively correlated with Julian day of year sprayed and new xylem ring thickness. Control was positively correlated with total daily apparent photosynthesis, upward methylene blue dye movement in the xylem, and herbicide concentration in the xylem 3 days after spraying. The only environmental variable of 11 measured that was significantly correlated with honey mesquite control was soil water and that was only with picloram. The concentration of herbicide in the stem was highest for clopyralid, followed by picloram, and then by 2,4,5-T and triclopyr, which were about equal. Variables most useful in regression equations for predicting control with herbicides were total daily apparent photosynthesis and rate of upward movement of methylene blue dye. Variables most incorporated into regression equations relating honey mesquite control with herbicide content were herbicide concentrations in the xylem 3 or 30 days after treatment.

Florida hydrilla populations have shown an alarming increase in resistance to fluridone, an herbicide used extensively for controlling invasive US hydrilla populations. A rapid PCR and sequencing method was developed to identify and screen hydrilla genomic DNA for three previously identified phytoene desaturase (pds) gene mutations that confer resistance to fluridone. Ninety hydrilla accessions were screened for fluridone resistant genotypes including 46 accessions from the US and 44 accessions from 15 other countries. In Florida, hydrilla from five of nine sites tested was heterozygous for wild-type and herbicide-resistant alleles. Additionally, a new resistant population was identified from Lake Seminole in Georgia, the first genetically confirmed strain of resistant hydrilla outside of Florida. All resistance-conferring mutations were located on the same homologous haplotype of US dioecious hydrilla. All other hydrilla samples tested possessed only wild type alleles, including monoecious strains that had been exposed to fluridone. Management implications are discussed.

Subsurface application of liquid or granular formulations of ethyl N,N-dipropylthiocarbamate (EPTC) at planting time effectively controlled annual grass weed species in new seedings of alfalfa (Medicago sativa L.) and birdsfoot trefoil (Lotus corniculatus L.). Control of annual broadleaf weeds by EPTC was incomplete. A postemergence application of 4-(2,4-dichlorophenoxy)butyric acid (2,4-DB) in addition to the EPTC treatment resulted in excellent overall weed control. Benefits of weed control were more evident in birdsfoot trefoil than alfalfa.

The photosynthetic responses of rice (C3) and three Echinochloa species (C4), barnyardgrass, early watergrass, and late watergrass, to changes in CO2 intercellular partial pressure, light intensity, and leaf temperature were investigated under laboratory conditions. The three Echinochloa species exhibited photosynthetic responses characteristic of C4 plants. The three weedy species showed higher efficiency for CO2 utilization at low CO2 intercellular partial pressure (CO2i) than rice. Compensation and saturation of CO2i for photosynthesis were lower in the weedy species than in rice. The maximum photosynthetic rates at high light intensity were 33.5, 32.7, 30.5, and 21.5 μmol CO2 m-2s-1 for barnyardgrass, early watergrass, late watergrass, and rice, respectively. Photosynthesis temperature optimum was 35 to 37 C for the three Echinochloa species and 33 C for rice. Overall, under simulated summer conditions, the four taxa showed a photosynthetic ability hierarchy with regard to gas exchange performance as follows: barnyardgrass ≥ early watergrass > late watergrass > rice.

We determined the effects of ethyl N,N-dipropylthiocarbamate (EPTC) in soil placed at different shoot zones of barley (Hordeum vulgare L.), wheat (Triticum aestivum L.), oats (Avena sativa L.), and sorghum (Sorghum vulgare Pers.) below the soil surface after emergence. A wax-barrier allowed separate exposure of the shoot zones to treated soil. Wheat, barley, and oats were severely injured when treated at the coleoptilar internode. Exposure of the remaining shoot did not affect growth. Sorghum was severely injured regardless of the shoot zone exposed. On this basis, sorghum was the most susceptible, while wheat, barley, and oats were equally tolerant. Uptake of 14C-EPTC from the soil by the sorghum shoots was double that of wheat, a fact which might explain the differential susceptibilities. These studies indicate that growth responses of species to EPTC applied to shoots is dependent on the stage of plant development at which treatment occurs.

Sublethal concentrations of isopropyl N-(3-chlorophenyl)- carbamate (CIPC) labeled with 14C in the ring or side chain were applied to all leaves present or to the roots of redroot pigweed (Amaranthus retroflexus L.), pale smartweed (Polygonum lapathiofolium L.), and parsnip (Pastinaca sativa L.). These species were selected because of their different susceptibilities to CIPC. The herbicide did not move out of the treated leaves in pigweed and smartweed and only slightly in parsnip in 21 days. In root treatment (3 days), the herbicide moved to all plant parts and the extent of movement was essentially the same in all species. Water soluble metabolites, which differed in Rf values were extracted from all three species. The metabolites apparently were not the result of cleavage of the CIPC molecule, but were more likely conjugates of CIPC with natural plant component(s). Very little 14CO2 was released by any of the species in 3 days. These data indicate that differences in movement and metabolism are not sufficient to account for the different susceptibilities of these three plant species.

We studied the uptake of ethyl N,N-dipropylthiocarbamate (EPTC) by barley (Hordeum vulgare L.), wheat (Triticum aestivum L.), oats (Avena sativa L.), sorghum (Sorghum vulgare Pers.), and giant foxtail (Setaria faberii Herrm.) using a double plastic pot technique that allowed separate exposure of the shoot and root zones to treated soil. Barley was more tolerant than wheat, whereas oats, sorghum, and giant foxtail were the most susceptible to EPTC at 1/4 to 1 1/2 lb/A-inch. The roots were the major site of uptake in barley, but injury to the other species from root exposure was equal to or slightly less than that from shoot exposure. The seed, or first 2 to 4 mm of shoot or both, were more sensitive in wheat than in barley. Such differential sensitivity was not evident in the shoot zone of the other species. These experiments, coupled with 14C-EPTC studies, indicated that differences in tolerance can be associated with the sites of uptake.