Mixed stands of big sagebrush (Artemisia tridentata Nutt.) and green rabbitbrush (Chrysothamnus viscidiflorus (Hook.) Nutt.) were sprayed with butyl ester of 2,4-dichlorophenoxyacetic acid (2,4-D) at 1, 2, 3, and 4 lb/A on six phenological dates in 1956, 1957, and 1958 at the Squaw Butte Experiment Station, Burns, Oregon. Data on soil moisture; soil temperature; growth development of herbaceous species; twig elongation of big sagebrush and green rabbitbrush; dry-matter, crude-protein, and total-carbohydrate contents of green rabbitbrush herbage; and crude-protein and total-carbohydrate contents of big sagebrush and green rabbitbrush roots were obtained at weekly to biweekly intervals in all 3 years to evaluate the ecological and physiological conditions that might indicate seasonal patterns of susceptibility to 2,4-D. Big sagebrush, a non-deciduous, non-sprouting plant, was easily killed with 2,4-D at 2 lb/A when soil temperatures and moisture contents were satisfactory for vigorous growth. The development of abundant leaf area on green rabbitbrush, a deciduous, sprouting plant, was important for spray interception and a photosynthetic rate sufficient to promote carbohydrate accumulation. Green rabbitbrush was controlled about 80% with a single application of 2,4-D butyl ester at 3 lb/A after the new twigs were 3 inches long and while soil moisture contents were sufficient for active growth. Differences in susceptibility between the two species are compared with morphological, phenological, and physiological differences, and seasonal patterns of susceptibility are further compared with environmental conditions.

Long-term compensation points in waterstargrass (Heteranthera dubia (Jacq.) Mac M), coontail (Ceratophyllum demersum L.), and duckweed (Lemna minor L.) were near 2–3% full sunlight during a 28-day period. Fresh weight of H. dubia increased as temperature increased from 15 to 30 C and as light intensity increased from 110 to 1000 foot-candles. Fresh weight of C. demersum increased as temperature increased above 20 C and as light intensity increased from 120 to 550 foot-candles. The L. minor strain used in this experiment was not temperature-dependent and increased in fresh weight and frond number at 25 and 30 C as light intensity increased from 77 to 550 foot-candles.

Patterns of absorption and distribution of dalapon (2,2–dichloropropionic acid) were determined by radioautography and counting. Dalapon was absorbed through the cuticle and through the large open pores of hypostomatous leaves. Stomata were clearly the most expeditious routes of entry, but results were erratic when no surfactant was included. An anionic surfactant, sodium dioctylsulfosuccinate, enhanced both (a) cuticular and (b) stomatal plus cuticular penetration 3- to 4-fold during a 2-hr. absorption period. Studies confirmed the view that dalapon is translocated to regions of high metabolic activity and/or storage in association with and dependent upon the movement of food materials. Dalapon did not move appreciably out of albino leaves or green leaves previously depleted of food reserves, but was transported readily out of normal leaves in the light when a surfactant was used to enhance cuticular penetration.

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.

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.

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.

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.

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.

We studied the effect of tillage systems (no-till, chisel, and moldboard plow) and the presence or absence of Zea mays L. (corn) on soil temperature, moisture, and, subsequently, the emergence phenology and density of Chenopodium album L. (common lambsquarters) at two sites (Elora and Woodstock) from 1993 to 1995. The tillage system affected the phenology of C. album seedling emergence only in 1995. In that year, more days were required to reach 80% cumulative seedling emergence in no-till than in the chisel or moldboard plow treatments. The delay in obtaining 80% cumulative emergence was attributed to a dry period from days 159 to 177 at Elora and from days 155 and 176 at Woodstock. The presence or absence of Z. mays did not affect soil temperatures, soil moisture, or C. album seedling emergence phenologies. Chenopodium album seedling density was influenced by tillage and environmental conditions. Large variations in seedling density were attributed to environmental conditions. The presence or absence of Z. mays did not affect C. album seedling density.

The residual effect of three cotton (Gossypium hirsutum L.) herbicide programs, including the use of no herbicides, a minimum program consisting of fluometuron {N,N-dimethyl-N′-[3-(trifluoromethyl)phenyl] urea} and MSMA (monosodium salt of methylarsonic acid), and an intensive program consisting of trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine], fluometuron, MSMA, and linuron [N′-(3,4-dichlorophenyl)-N-methoxy-N-methylurea], were evaluated from 1976 to 1982. Herbicide injury to wheat (Triticum aestivum L.) and hairy vetch (Vicia villosa Roth.) on three soils showed carryover effects in the following sequence: Sharkey silty clay > Dundee silt loam > Loring silt loam. The intensive program was the most injurious on the Sharkey silty clay. The effects of the two herbicide programs were nearly equal on the Dundee and Loring silt loams. Possible replacement crops for cotton, such as grain sorghum [Sorghum bicolor (L.) Moench.] and corn (Zea mays L.) suffered the least damage from carryover; rice (Oryza sativa L.), soybeans [Glycine max (L.) Merr.] and cucumber (Cucumis sativis L.) suffered severe damage. Greenhouse bioassays generally confirmed field results, and fluometuron appeared to be the major component of carryover.

Three musk thistle (Carduus thoermeri Weinm.) sites that were infested with a weevil (Rhinocyllus conicus Froel.) and a fourth non-infested site were evaluated for numbers of insects and seeds per seed head. At four successive weekly intervals, seed heads were sampled just after flowering. Initial sampling was in 1978 and was repeated in 1979. Weevil incidence in heads blooming during the first week was 6.7/head in 1978 and 28.0 in 1979. At infested sites, production of fully developed seeds averaged 28% less than at the non-infested site in 1978 and 78% less in 1979. Heads that were in bloom during the fourth sampling week had an infestation rate less than one weevil/head in both years. Fully developed seeds germinated well (96 to 99%) and were not affected by date of bloom. Because many heads bloomed during the third and fourth weeks, a large number of viable seeds were still produced at infested sites.

Cotton (Gossypium hirsutum L. ‘Stoneville 7A’) grown for 3 yr in competition with various weed densities and nitrogen levels showed annual weeds could reduce cotton heights as much as 75% and cotton yields as much as 88%. Cotton yields were increased by 18 to 30% by adding nitrogen to plots with no weeds. In 1973, cotton supplied with 224 kg/ha of nitrogen could tolerate up to 50% weeds without loss of yield. Weeds left to grow and reseed with no control resulted in increasingly greater competition with each succeeding year.

In field experiments at Prosser, WA, black nightshade (Solanum nigrum L. # SOLNI), eastern black nightshade (S. ptycanthum Dun. # SOLPT), American black nightshade (S. americanum Mill. # SOLAM), hairy nightshade (S. sarrachoides Sendtner # SOLSA), and sometimes accessions within a species responded differentially to herbicides applied preplant incorporated, preemergence, or postemergence. Also, the response of the nightshades to some of the herbicides varied significantly between years. Generally, black nightshade was more tolerant of herbicides than were the other three species. An accession of black nightshade from England and an accession of eastern black nightshade from Minnesota were the most tolerant of herbicides of eight accessions evaluated. Of the 25 herbicides evaluated, preplant-incorporated applications of ethalfluralin [N-ethyl-N-(2-methyl-2-propenyl)-2,6-dinitro-4-(trifluoromethyl) benzenamine] and preemergence applications of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], chloramben (3-amino-2,5-dichlorobenzoic acid), lactofen [1′-(carboethoxy)ethyl 5-[2-chloro-4-(trifluoromethyl) phenoxy]-2-nitrobenzoate], and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl] acetamide] controlled all species best. These herbicides reduced the oven-dry weights of both accessions of all four species of nightshades by 93% or more. Nightshades varied widely in their response to the thiocarbamates.

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.

Germination studies on camphorweed [Heterotheca subaxillaris (Lam.) Britt. & Rusby] revealed that freshly harvested disk achenes germinated best at 17.5 C (88%) while ray achenes were dormant. Camphorweed seed from disk achenes also germinated at temperatures as low as 3 C. Seedlings grown under long-day conditions at 23 C day and 8 C night temperatures for 144 days elongated at the rate of 0.18 cm/day. Plants grown under short-day conditions at the same temperature regime elongated at the rate of 0.06 cm/day. Total leaf surface area, fresh weight, and dry weight of shoots of plants grown under long days were at least 1.5 times greater than plants grown under short day conditions. Camphorweed, regardless of size and age, survived a 2-h exposure at −5 C. All plants in the rosette stage survived at −15 C in the freezer and an overnight temperature of −11.7 C in the field, whereas larger plants were killed at these temperatures. Stage of growth was an important factor in the herbicidal control of camphorweed. Plants in the rosette stage were generally more susceptible to herbicides than older plants. Simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] at 3.4 kg/ha, paraquat (1,1′-dimethyl-4,4′-bypyridinium ion) at 0.6 kg/ha, methazole [2-(3,4-dichlorophenyl)-4-methyl-1,2,4-oxadiazolidine-3,5-dione] at 5.0 kg/ha, and a formulated mix of diuron [3–3,4-dichlorophenyl)-1,1-dimethylurea] and terbacil (3-tert-butyl-5-chloro-6-methyluracil) at 4.5 kg/ha provided adequate control of camphorweed in the rosette stage. Asulam (methyl sulfanilylcarbamate) at 2.2 or 4.5 kg/ha applied alone did not control camphorweed in the rosette form but was more effective on older plants. Various combinations of these herbicides were generally effective at both stages of growth.

Sequential applications of preemergence (PRE) and postemergence (POST) herbicides were applied over a two-year period for large crabgrass and goosegrass control in common bermudagrass. Large crabgrass control was better when prodiamine at 0.3 kg ai ha−1 or oryzalin at 0.8 kg ai ha−1 were applied in late February, and followed by MSMA at 1.1 kg ai ha−1 in June than when prodiamine (0.8 kg ha−1) or oryzalin (2.4 kg ha−1) were applied at recommended rates. Goosegrass control was better when prodiamine was applied at 0.3 kg ha−1 followed by MSMA plus metribuzin at 2.2 + 0.1 kg ai ha−1, or when oryzalin was applied at 1.7 kg ha−1followed by MSMA plus metribuzin at 1.1 + 0.1 kg ha−1 than when prodiamine was applied at 0.8 kg ha−1 or oryzalin was applied at 3.3 kg ha−1. There was no increase in goosegrass control when diclofop was applied in sequence after prodiamine or oryzalin compared to diclofop alone. POST applications of diclofop, MSMA, and MSMA plus metribuzin did not injure common bermudagrass more severely when applied to plots previously treated with prodiamine and oryzalin than with POST herbicides alone.

Leafy spurge (Euphorbia spp.), an aggressive noxious perennial weed of North America, is a complex group that has been designated as several different species, including E. esula L. (# EPHES) and E. virgata Wald. & Kit. [E. waldsteinii (Sojak) Radcliffe-Smith]. Current classification keys are unsatisfactory in assigning plants in the field to specific taxa (or to a single taxon). These keys rely heavily upon morphological characteristics of leaves, but great variation in the leaves has been noted by us and previously reported by others. In this study we demonstrate qualitatively and quantitatively that the within-plant, within-clone, among-clone, and among-site variation in leaf morphology and triterpenoid content of the latex of leafy spurge is inherently high. Leaf characters were of little value in separating any of the accessions considered in our study. Latex triterpenoid profiles were useful in distinguishing E. lucida W. & K. × salicifolia Host. and E. salicifolia from European E. esula, E. waldsteinii, and E. sequieriana Neck. ssp. seguieriana, and all Montana accessions previously described from morphological studies. We concluded that Montana leafy spurge and the European E. esula, E. waldsteinii, and E. sequieriana belong to a single taxon: Euphorbia esula L.

Reports of unacceptable shattercane (Sorghum bicolor) control with acetolactate synthase (ALS)-inhibiting herbicides prompted the investigation of 29 fields in central and south-central Nebraska for ALS-resistant (ALSr) shattercane. These fields were located in three distinct geographical areas designated C, G, and P. Shattercane from 13 fields spanning all three areas was resistant to 80 g ai ha−1 (2 X field rate) primisulfuron. Accessions C and G were more resistant than accession P to primisulfuron and nicosulfuron. Accessions C and G were susceptible to imazethapyr, whereas accession P was resistant. The ALS resistance was associated with alterations in the ALS enzyme. Primisulfuron I50 values for ALS from ROX (forage sorghum), C, G, and P were 7, 8,510, 8,870, and 714 nM, respectively; nicosulfuron I50 values were 647, 4,110, 4,070, and 1,460 nM, respectively; and imazethapyr I50 values were 5,440, 13,100, 11,800, and 51,700 nM, respectively. Based on cross-resistance and enzyme sensitivities, at least two biotypes are represented in the three accessions of ALSr shattercane. Shattercane individuals from accessions C, G, and P were intercrossed to determine if the ALSr genes in each of the accessions were at independent loci. All the F2 populations were resistant to 80 g ai ha−1 primisulfuron, suggesting that the ALSr alleles in the three accessions are at the same locus or possibly linked loci. When the C, G, and P accessions were crossed with the wild type (WT), comparisons between the F1, susceptible, and resistant populations showed that primisulfuron resistance was expressed as a dominant, partially dominant, and additive trait for the C, G, and P accessions, respectively. The differences in ALSr allelic interactions indicate that primisulfuron resistance developed independently in each of the three accessions.

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.