Field experiments were established in 1998 and 1999 to evaluate the effect of giant smutgrass competition and hexazinone application on bahiagrass forage yield. The experimental design was a split-plot, with low (< 20% groundcover), medium (20 to 70% groundcover), and high (> 70% groundcover) giant smutgrass density as the main plot factors and hexazinone application or no hexazinone application as the subplot factors. In 1998, without hexazinone, bahiagrass biomass accumulation was 1,164 kg ha−1 mo−1 under low giant smutgrass infestation but 590 and 154 kg ha−1 mo−1 under medium and high giant smutgrass densities, respectively. From harvests occurring 1 yr after hexazinone application, bahiagrass yield in the weed-free area was similar to that growing under low giant smutgrass density. However, as giant smutgrass density increased to moderate or high levels, bahiagrass yield was reduced relative to the weed free. Giant smutgrass biomass accumulation was also measured over time. Giant smutgrass biomass, in both years, increased dramatically in the late summer months at the medium and high densities but not at the low density. It was concluded that bahiagrass was competing with the giant smutgrass at low density and depressed late season growth but was not capable of doing so at higher infestation levels. A rapid increase in late-season giant smutgrass growth was partially explained by the fact that bahiagrass is a short day plant that begins to flower in mid- to late summer, and aboveground biomass production decreases in late summer. This shift in carbon allocation in bahiagrass would exert less competition on giant smutgrass and thus partially be responsible for the late season increase in giant smutgrass growth. Economic analysis performed on these data illustrated that a net loss of $15.20 per stocking unit (cow–calf pairs) would be realized if hexazinone were used to control low densities of giant smutgrass. However, a net gain of $29.28 and $55.75 per stocking unit was observed if hexazinone was used to control giant smutgrass that had reached medium or high levels of infestation, respectively. It was concluded from these data that giant smutgrass should not be controlled until densities reach approximately 35% infestation.

The conversion of 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone (pyrazon) to N-chloro-4-phenyl-3(2H)-pyridazinone glucosamine (hereinafter referred to as N-glucosyl pyrazon) in intact seedlings and in leaf discs of red beets (Beta vulgaris L., var. Detroit Dark Red) was significantly greater in the light than in the dark. Pyrazon conjugation with glucose was dependent on the carbohydrate status of the leaf tissue, since the formation of the N-glucoside in the dark was further reduced with dark pretreatment but could be restored to the level observed in the light by infiltrating the leaf discs with supplemental sucrose.

Clods of three sizes (1.5-, 3.3-, and 5.3-cm diameter) and two hardnesses [soft (0.3 MPa) and hard (2.2 MPa)] were made by incorporating seed from one of the five following weed species: corn poppy (Papaver rhoeas L. # PAPRH), shepherdspurse [Capsella bursa-pastoris (L.) Medic. # CAPBP], smooth crabgrass Digitaria ischaemum (Schreb.) Muehlenb. # DIGIS], common lambsquarters (Chenopodium album L. # CHEAL), and Canada thistle [Cirsium arvense (L.) Scop. # CIRAR], listed by increasing seed size. Under conditions of this experiment, higher germination occurred with larger seed. The higher germination was connected with fewer dormant seed remaining in the clods. Germination significantly decreased (13%) in the larger soft and hard clods with the smaller seed sizes (less germination associated with more dormant seed remaining in the clod). Germination of the two larger seed decreased (23%) only in the larger soft clods but was less significant with both small seed. Non-emerged seedlings increased with larger clods and was most significant with the three large seed in the hard clods and absent with the three large seed in the soft ones. On the average, 70% of the germinated seed emerged. Differences in germination as well as in nonemerged seedlings caused decreased emergence on larger clods. Clod influence in germination and emergence was explained by the volumes of the clod layers involved, and in some comparable aspects corresponded with volumes (depths) of soil layers.

Tomatoes (Lycopersicon esculentum ‘Roma VF’) and weeds were seeded at two locations in 1973 and 1974. In separate plots, jimsonweed (Datura stramonium L.), tall morningglory [Ipomoea purpurea (L.) Roth.], and common cocklebur (Xanthium pensylvanicum Wallr.) seedlings were thinned to densities ranging from 2.7 to 86 plants/m2. Densities of large crabgrass [Digitaria sanguinalis (L.) Scop.] ranged from 11 to 430 plants/m2. There were a few instances of difference in nutrient concentrations in tomato and weed leaf tissue, but no clear relationship was evident between concentration of N, P, K, Ca, Mg, and S and weed density. Considerably more fruit weight was produced per kilogram of total assimilated N, P, and K in the weed-free than in the weed-infested tomato plots.

Field research was conducted in Tennessee at Jackson in 1991 and at Milan in 1992 to compare the effect of MSMA and pyrithiobac on cotton development, yield, and quality. In separate treatments to different plots, pyrithiobac at 0.14 kg ai ha–1 did not affect development, yield, and quality of cotton. MSMA at 2.24 kg ai ha–1 decreased cotton plant internode length 10 to 15% and height by 15% 5 wk after late treatment both years. MSMA applied to 50 cm cotton reduced subsequent plant height in 1992, but other plant characteristics were not affected. In 1991, MSMA increased squares and decreased blooms and bolls for monopodia and sympodia position one and two, which suggested a delay in plant development. In 1991, mechanical harvest lint yields were decreased by MSMA at first harvest while increasing second harvest lint yields. However, in 1991 only MSMA applied late decreased total harvest lint yield 20%. Plant mapping data determined that the yield decrease was a result of decreased yields at sympodia positions one and two. Cotton seed arsenic analysis indicated that MSMA-late increased arsenic levels by ≈ 1 ppmw in 1991 for sympodia at position one and two compared to the untreated check, while position two contained the highest level of 1.8 ppmw.

Experiments were conducted to examine the effects of low soil moisture on yellow nutsedge emergence in relation to thermal and chronological time and test the ability of thermal models generated previously to predict emergence under water stress. Unsprouted tubers from California and Arizona were planted in pots, buried at field sites in California and Arizona, and subjected to wet or dry treatments. Pots were monitored weekly to determine date of emergence and number of emerged shoots. The California genotype emerged 47 to 61 days after planting (DAP) in the dry treatment and 33 to 49 DAP in the wet treatment, depending on planting site. The range for the Arizona genotype was 51 to 76 DAP in the dry treatment and 43 to 61 DAP in the wet treatment. Days and degree-day intervals to first emergence differed between irrigation treatments and planting sites but interactions were not significant. All models were accurate in predicting emergence dates for genotypes in the wet treatment at the California site, while emergence in Arizona was underestimated by 9 d. Tubers subjected to the dry treatment needed a higher number of accumulated degree-day units before emergence occurred and had fewer emerged shoots as compared to the wet treatment. Degree-day models generated for yellow nutsedge under optimal conditions lack sufficient robustness to be predictive under water stress conditions.

Laboratory studies were conducted to determine the effect of rainfall timing and antecedent moisture on atrazine leaching through intact soil cores taken from no-till and conventional-till corn fields. Simulated rainfall was applied to no-till and conventional-till cores 1 to 14 d after atrazine application and, in a second study, one d after atrazine was applied to no-till and conventional-till cores at 1 to 800 kPa soil moisture. Increasing the lag time between atrazine application and rainfall from one to 14 d reduced the amount of atrazine leached by about 50% for both no-till and conventional-till soil cores. During the same time period, the amount of atrazine adsorbed to soil increased by about 30% for both tillages. Soil dryness (antecedent moisture) at the time of atrazine application had no effect on the amount of atrazine leached through conventional-till cores. However, leaching decreased in no-till cores as antecedent moisture decreased from 1 to 33 kPa; drying to 800 kPa had no further effect. The leaching rate of atrazine was much higher for the initial 0.5 pore volume than for the next 1.5 pore volume at all rainfall timing and antecedent moisture levels. This behavior is indicative of preferential transport.

In a 3-year study propanil (3′,4′-dichloropropionanilide) at 6.7 or 9.0 kg/ha was applied postemergence as single or split applications to rice (Oryza sativa L. ‘Starbonnet’ and ‘Nova 66′) 15 to 75 days after crop emergence. The effect on crop injury, maturity, grain yield, and seed and milling quality was measured on rice grown in weed-free plots. Residues of DCA (3,4-dichloroaniline) in grain and straw from treated rice plants and of propanil and TCAB (3,3′,4,4′-tetrachloroazobenzene) in treated soil were determined. Propanil applied at all times and rates in 1968 and 1970 or applied at all rates at 15 to 55 days after crop emergence in 1969 did not reduce grain yields, but 6.7 kg/ha applied 65 and 75 days after crop emergence in 1969 injured rice plants vegetatively and reduced grain yields. Plant maturity, seed viability and milling quality of grain were not affected. Residues of DCA in grain and straw were highest from treatments applied 65 and 75 days after crop emergence; rice straw contained more DCA than did grain.

In a 3-year field study in corn (Zea mays L.), several herbicides and combinations were studied for the control of yellow nutsedge (Cyperus esculentus L.), giant green foxtail [Setaria viridis var. major (Gaud.) Posp.], and fall panicum (Panicum dichotomiflorum Michx.). Best yellow nutsedge control (87 to 88%) resulted from applications of 4.48 kg/ha of either alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] preplant incorporated or atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] preplant incorporated plus atrazine postemergence with a phytobland oil. Between 72 to 83% control of yellow nutsedge resulted from applications of 2.24 kg/ha of alachlor, 4.48 kg/ha of butylate (S-ethyl diisobutylthiocarbamate), and combinations of atrazine plus butylate. Greater than 90% control of giant green foxtail and fall panicum resulted from butylate plus atrazine or alachlor; postemergence applications of atrazine resulted in significantly less control of fall panicum or giant green foxtail.

Several weed-free and bermudagrass competition periods were established and maintained in cotton to evaluate their influence on cotton yields and reproduction of bermudagrass. Cotton sprayed with fluazifop for bermudagrass control 4 or 8 weeks after emergence yielded 93 and 90% as much as weed-free cotton. This compared to losses of 16 and 26% when bermudagrass was permitted to compete with cotton for 12 and 25 weeks, respectively. Cotton handweeded for 8 to 12 weeks yielded the same as weed-free cotton and 9% more than cotton weed free for only 4 weeks. Bermudagrass seed production was prevented and rhizome production was negligible when weed competition did not exceed 8 weeks and weed-free periods exceeded 4 weeks. Bermudagrass produced significant amounts of seeds (45 to 88% of weedy checks) and rhizomes (9 to 31% of weedy checks) in plots weed free for 4 weeks and in plots where control was delayed for 12 weeks. Cotton grades were reduced because the cotton lint was contaminated with grass in plots weed free for only 4 weeks, and in plots where bermudagrass competed for 25 weeks.

Field experiments were conducted in 1997 and in 1998 to determine the effects of density and time of emergence on Eriochloa villosa growth and seed production in Zea mays. E. villosa was transplanted at four densities (3, 9, 27, and 81 plants m−2) to simulate emergence at four Z. mays growth stages (VE, V2, V5, and V10). Compared to E. villosa plants that emerged with Z. mays plants, total above-ground E. villosa biomass at maturity of plants grown at 3 plants m−2 was reduced by 54, 97, and 99% when emergence was delayed until the V2, V5, and V10 stages of Z. mays, respectively, in 1997. In 1998, total aboveground E. villosa biomass at maturity was reduced by 70, 87, and 99% when emergence was delayed until the V2, V5, and V10 stages of Z. mays, respectively. E. villosa aboveground vegetative biomass per plant at maturity was linearly related to seed production per plant in each year. E. villosa seed production m−2 decreased nonlinearly as density decreased and time of emergence was delayed. Based on estimated model parameters, maximum seed production was 57,100 and 12,700 seeds m−2 in 1997 and 1998, respectively. Within time of emergence, E. villosa density did not affect seed mass per seed, however, seed mass of late-emerging cohorts was less than that of early-emerging cohorts. Time of weed emergence relative to the crop was a very important factor in determining biomass and seed production. Results suggest that late-emerging plants may not be very important to long-term management of E. villosa.

Bioassay techniques were evaluated for the determination of chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} residues in soil and water. A linear response of corn (Zea mays L.) primary root length to chlorsulfuron concentrations (InX+1) of 0.0 to 16.0 μg/L was observed in the water bioassay. Several soil extraction bioassay methods were attempted and found to be highly variable. A Ca(OH)2 soil extraction bioassay technique produced a linear response of primary corn root length to chlorsulfuron concentrations ranging from 0.0 to 64.0 μg/kg. The efficiency of the Ca(OH)2 extraction bioassay was determined by the use of 14C-chlorsulfuron. Recovery efficiency at 1.0 μg/kg was 74% and averaged 62% at all higher chlorsulfuron concentrations.

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.

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.

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.

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.