The effect of glyphosate [N-(phosphonomethyl) glycine] on barley (Hordeum vulgare L.) and corn (Zea mays L.) shoot δ-aminolevulinic acid (ALA) production was examined by monitoring ALA content in the tissue and measuring incorporation of 14C precursors into ALA and chlorophyll a. Barley shoot ALA content was significantly decreased by 1 mM glyphosate after 9, 11, and 15 h of illumination. ALA production by treated barley shoots was 30 nmoles•g fresh weight-1•h-1 at each interval tested, compared with 75 to 120 nmoles•g fresh weight-1•h-1 for the control. In corn shoots, ALA content was reduced 32, 45, and 58% by 0.1, 1.0, and 10.0 mM glyphosate, respectively, after 12 h illumination. Incorporation studies with 14C-glutamate, 14C-α-ketoglutarate, and 14C-glycine into ALA showed a 77, 92, and 91% inhibition, respectively, in barley shoots treated with 1 mM glyphosate. Incorporation of 14C-ALA into chlorophyll a was not affected by 1 mM glyphosate. Thus, the site of action of glyphosate may involve two enzyme pathways:one controlling the conversion of α-ketoglutarate to ALA, and the other controlling the condensation of glycine with succinyl CoA to form ALA and carbon dioxide. Inhibition of ALA synthesis blocks synthesis of chlorophyll, as well as all other porphyrin ring compounds found in higher plants. Thus, inhibition of ALA synthesis may be an integral component of the herbicidal mode of action of glyphosate.

Mixtures of annual weeds emerging 1 week after and competing all season with potatoes (Solanum tuberosum L.) reduced tuber yields by an average of 54% compared to 16% in comparable treatments in which weeds emerged 3 weeks after potatoes. Weeds reduced tuber yields 19% 8 weeks after potatoes emerged and 25 to 45% 10 weeks after emergence. After full-season competition, for each additional 10% of the total dry biomass represented by weeds, there was a decrease of 12% in fresh tuber yield. This relationship was not valid if potatoes were harvested early, which reduced the deleterious effect of weeds on potato yields. Weed competition reduced both size and number of tubers, but had little effect on specific gravity of tubers. In some instances, the reduction in tuber size by weeds reduced the percentage of tubers that were marketable.

The effects of various densities and durations of wild oat (Avena fatua L.) competition in soybean [Glycine max (L.) Merr. ‘Evans’] were determined in the field during a 2-yr period. Season-long competition by densities of 1, 3,9, and 30 wild oat plants/m of row reduced soybean seed yield an average of 6, 17, 32, and 51%, respectively. An infestation of 30 wild oat plants/m of row did not reduce soybean yield if the period of competition was limited to 4 weeks after crop emergence; however, yields were reduced 29, 50, 63, 58, and 63% when wild oat competed for 5, 6, 7, and 8 weeks, or season long, respectively. Wild oat competition reduced soybean pods per plant and seeds per plant more than seeds per pod or seed weight.

Levels of MSMA (monosodium methanearsonate) that simulated drift (0.2, 0.39, and 0.78 kg/ha) were applied over-the-top to rice (Oryza sativa L. ‘Vista’, ‘Starbonnet’, or ‘Labelle’) after flooding at four developmental stages between early tillering and late jointing. The MSMA rates used were equivalent to, or lower than, the 0.78-kg/ha rate that would be applied as an over-the-top application to cotton (Gossypium hirsutum L.) for early season weed control. Over-the-top applications of MSMA did not affect heading and maturity dates of filled grains regardless of the rate or application timing. Reductions occurred, however, in plant height, number of panicles, and yield. In addition, the percentage of erect panicles showing typical “straighthead” symptoms increased. Rice sensitivity to MSMA depended on MSMA rate and stage of growth with injury being most severe as rice approached the reproductive stages of development. Because aerial applications of MSMA to cotton are normally made before rice reaches the reproductive stage, injury resulting from MSMA drift should be minimal.

Greenhouse and field experiments (at Rosemount and Roseau, Minnesota) were conducted to characterize the growth and development of 10 quackgrass [Agropyron repens (L.) Beauv.] biotypes selected near Roseau, Minnesota. The biotypes were highly variable for all traits studied. Eight of the biotypes had flag leaves approximately 1 cm wide, but two biotypes had flag leaves approximately 1.5 cm wide. Four biotypes were glaucous and blue-green in color, and the other biotypes varied from light to dark green and had varying leaf pubescence. In the field, plant height varied from 59 to 79 cm, and rhizome length varied from 105 to 135 cm. Spike production varied from 4 to 62 spikes per plant, and mid-season rhizome bud production varied from 246 to 1,211 buds, suggesting that the biotypes varied in their reproductive potential. Shoot dry weight varied from 48 to 362 g per plant, and daughter shoot production varied from 7 to 235 shoots per plant. The quackgrass near Roseau, Minnesota, appears to be made up of several distinct biotypes.

Weed and cucumber (Cucumis sativus L. ‘Poinsett’) growth was studied where herbicides were soil-applied or applied to emerged weeds in the established crop. Cucumber yields were decreased at least 36% in 2 of 5 yr when weeds interfered for 4 weeks after emergence. Sequential applications of bensulide [O,O-diisopropyl phosphorodithioate S-ester with N-(2-mercaptoethyl)benzenesulfonamide] applied preplant and incorporated (PPI) + trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) applied postemergence and incorporated (POI) were outstanding and controlled Japanese millet (Echinochloa crus-galli Link. var. frumentacea), Texas panicum (Panicum texanum Buckl.), Palmer amaranth (Amaranthus palmeri S. Wats.), and common purslane (Portulaca oleracea L.) without affecting the growth or yield of cucumber. Compared with bensulide, the selectivity of soil-incorporated butralin [4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamine] with cucumber was limited. Laboratory bioassays showed that under warm field soil conditions, PPI applications of 1.7 kg/ha of butralin persisted more than 2 months, but were dissipated after 4 months. Ethalfluralin [N-ethyl-N-(2-methyl-2-propenyl)-2,6-dinitro-4-(trifluoromethyl)benzenamine] reduced cucumber yields only when it was soil-incorporated to the crop-seed depth at planting. Applications to soil of bensulide + DCPA (dimethyl tetrachloroterephthalate) stunted cucumber plants, reduced leaf area, increased leaf thickness, and caused higher near-infrared (0.75 to 1.34 μm) leaf reflectance.

An experiment was initiated in 1970 and continued through 1979 by exhuming and germinating seed of 12 economic weed species buried beneath 23 cm of soil in eastern and western Nebraska. Loss in germination of exhumed seeds over years is mathematically characterized by the formula for the rectangular hyperbola, which represents many shapes of curves that have zero as their lower limit. Of the 12 weed species, only fall panicum (Panicum dichotomiflorum Michx.) and redroot pigweed (Amaranthus retroflexus L.) seed germination did not drop significantly over the 10-yr burial period. Germination of redroot pigweed seed was higher when buried in eastern Nebraska, but was higher for smooth groundcherry (Physalis subglabrata Mack&Bush.) and velvetleaf (Abutilon theophrasti Medic.) when buried in western Nebraska. Germination of the other nine species were not affected by burial location. The 12 weed species can be ranked as those showing most to least rapid loss of germination during burial for 10 yr as follows: honeyvine milkweed [Ampelamus albidus (Nutt.) Britt.], hemp dogbane (Apocynum cannabinum L.), kochia [Kochia scoparia (L.) Schrad.], sunflower (Helianthus annum L.), large crabgrass [Digitaria sanguinalis (L.) Scop.], common milkweed (Asclepias syriaca L.), musk thistle (Carduus nutans L.), velvetleaf, fall panicum, redroot pigweed, green foxtail [Setaria viridis (L.) Beauv.], and smooth groundcherry.

The response of several spring wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), and oat (Avena sativa L.) cultivars to preemergence and postemergence applications of chlorsulfuron (tested under code number DPX-4189) {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)aminocarbonyl] benzenesulfonamide} was evaluated in the field and greenhouse in 1979. In greenhouse experiments, ‘Butte’ spring wheat was less susceptible than six other spring wheat cultivars to chlorsulfuron at 1.0 kg/ha applied preemergence or postemergence. ‘Morex’, ‘Conquest’, and ‘Bonanza’ barley cultivars were less susceptible than six other barley cultivars to preemergence chlorsulfuron, but barley cultivars were similar in response to postemergence chlorsulfuron. The eight oat cultivars responded similarly to preemergence and postemergence chlorsulfuron at 1.0 kg/ha. Data from two field experiments, which included evaluation of durum wheat [Triticum durum (Desf.)] cultivars, indicated that the four small grains had adequate tolerance to postemergence chlorsulfuron at 0.125 and 0.25 kg/ha, except that durum wheat grain yield was reduced by 0.25 kg/ha at the Rosemount location. No differential cultivar responses to chlorsulfuron occurred among the small grains in the field.

Laboratory studies were conducted to investigate the possibility that herbicide runoff from treated fields might be adversely affecting submersed aquatic plants in the Chesapeake Bay. In laboratory studies, I1 and I50 values (the concentrations inhibiting growth 1 and 50%, respectively) were calculated for three herbicides and several aquatic plants. Exposure periods varied from 3 to 6 weeks, with the 3-week exposure being most common. For atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], I1 values were usually a few parts per billion (ppb), whereas I50 values varied from ca. 80 ppb for elodea (Elodea canadensis Michx) to ca. 1040 ppb for Eurasian watermilfoil (Myriophyllum spicatum L.). The toxicity of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] was similar to that of atrazine. Glyphosate [N-(phosphonomethyl)glycine] was essentially non-toxic. Interactions between salinity and atrazine were studied using the brackish water species, vallisneria (Vallisneria americana Michx.). Increasing salinity did not affect atrazine toxicity. When atrazine was present in both the hydrosoil and water, the concentration in the water determined the toxicity. At concentrations below 1000 ppb, atrazine in the hydrosoil did not adversely affect the plants. Based on the results of these studies, and the known concentrations of herbicides in runoff water, these herbicides do not appear to pose any threat to the species tested.

The effects of caffeic acid, chlorogenic acid, t-cinnamic acid, p-coumaric acid, ferulic acid, gallic acid, p-hydroxybenzaldehyde, 5-sulfosalicylic acid, vanillic acid, and vanillin on growth, photosynthesis, water relations, and chlorophyll content of 3-week-old soybeans [Glycine max (L.) Merr. ‘Tracy’] grown in aerated nutrient solution were determined. At concentrations of 10−3 M, caffeic, t-cinnamic, p-coumaric, ferulic, gallic, and vanillic acids significantly reduced dry matter production, leaf expansion, height, leaf production, net assimilation rate (rate of dry matter production per unit leaf area), and leaf area duration (total leaf area present during treatment interval). Chlorogenic acid, p-hydroxybenzaldehyde, 5-sulfosalicylic acid, and vanillin at 10−3 M did not inhibit growth. None of the 10 compounds at 10−4 M inhibited growth. At concentrations of 10−3 M, caffeic, t-cinnamic, p-coumaric, ferulic, gallic, and vanillic acids severely reduced net photosynthetic rate and stomatal conductance of single, fully expanded leaves. These same compounds also caused marked reductions in leaf chlorophyll content, with net losses of chlorophyll occurring over an 86-h period after treatment.

Toxicity of nonradiolabeled and translocation of 14C-labeled metriflufen3 {2-[4-(4-trifluoromethylphenoxy)phenoxy] propanoic acid} in cotton (Gossypium hirsutum L. ‘Stoneville 213’) were evaluated under different environmental conditions as affected by surfactant and by the maturity of stem or leaf tissue at the place of herbicide application. Cotton was very tolerant to metriflufen, with the greatest tolerance at high temperatures. The herbicide entered the plant quite rapidly and autoradiographs indicated that radioactivity from the herbicide moved freely throughout the plant. Relative humidity (RH) between 40 and 100% had little effect on metriflufen toxicity. Two weeks after treatment with metriflufen at 0.5 kg/ha, cotton was essentially free of injury symptoms at 35 C, but injury was as great as 41 and 13% at 18 and 25 C, respectively. All plants recovered fully after 6 additional weeks in the greenhouse. When surfactant was added to the spray solutions, cotton injury was frequently increased during the first 2 weeks but not after 8 weeks. Accumulation of 14C at the site of 14C-metriflufen application was 31 to 65% after 48 h. Between 16 and 51% of the applied 14C moved from the treated area by translocation into other parts of the plant, or by volatilization into the atmosphere. Movement was greatest from stem tissue at 35 C and 100% RH. There was no correlation between 14C movement and herbicide toxicity within the plant. The selectivity of metriflufen to cotton probably results from deactivation by, or tolerance within, the plant rather than from differential absorption and translocation of the herbicide.

Responses of soybeans [Glycine max (L.) Merr. ‘Bragg’] and red rice (Oryza sativa L.) to postemergence applications of mefluidide {N-[2,4-dimethyl-5-[[(trifluoromethyl)sulfonyl] amino] phenyl] acetamide} and bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] alone and in combinations were evaluated in greenhouse studies. Soybeans were tolerant to bentazon up to the highest tested rate of 2.0 kg/ha, but their height was reduced by mefluidide at 1.0 kg/ha. Soybeans were not injured by any tested combination of the two herbicides. Bentazon applied alone at rates of 5.0 kg/ha and above had a significant effect on red rice injury. Mefluidide applied at rates above 0.25 kg/ha severely reduced height and fresh weight of red rice. When these two herbicides were combined, a synergistic reaction occurred, and the combination was much more effective in controlling red rice than was either herbicide applied alone. The addition of a surfactant significantly increased the activity of the combination on red rice up to 0.25 kg/ha of mefluidide and at all rates of bentazon.

The effects of buthidazole {3-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-4-hydroxy-1-methyl-2-imidazolidinone} on the photochemical reactions of isolated pea (Pisum sativum L.) chloroplast thylakoids were analyzed. Buthidazole was found to inhibit electron transport at two distinct sites in the photosynthetic electron transport chain. The major site of inhibition was on the reducing side of photosystem II at the site of diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] and atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] inhibition. Buthidazole also had a secondary site of electron transport inhibition on the oxidizing side of photosystem II. No evidence was found for buthidazole to act as an uncoupler or as an energy transfer inhibitor.

The soil activity of foliar-applied HOE 29152 {2-[4-trifluoromethyl-phenoxy)phenoxy] propanoic acid} and diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid} on barnyardgrass [Echinochloa crus-galli (L.) Beauv.] and yellow foxtail [Setaria lutescens (Weigel) Hubb.] was evaluated in the greenhouse. HOE 29152 was applied at 0.1, 1.2, 2.4, and 3.5 kg/ha; and diclofop was applied at 0.5, 2.7, 4.9, and 7.2 kg/ha. Both compounds applied to roots or shoots before emergence reduced growth. Neither compound had much, if any, effect on the germination of barnyardgrass or yellow foxtail; however, the survival of both weed species that germinated in solutions of these two compounds was reduced. The number of seedlings surviving for both weed species was reduced by both compounds. Seedling survival from germination imbibition by diclofop and HOE 29152 was influenced by the presence of light at the time of seed germination.

The effectiveness of dicamba (3,6-dichloro-o-anisic acid), picloram (4-amino-3,5,6-trichloropicolinic acid), and triclopyr [(3,5-6-trichloro-2-pyridinyl)oxy]acetic acid and mixtures of these herbicides with 2,4-D [(2,4-dichlorophenoxy)acetic acid] for the control of horsenettle (Solanum carolinense L.) in Kentucky bluegrass (Poa pratensis L.) pastures was evaluated at three different field sites in Southwest Virginia over a period of 3 yr. Picloram applied at 0.6 and 1.1 kg/ha significantly reduced horsenettle roots, as indicated by a 98% reduction in shoots 1 yr after the first annual application. No shoots emerged within 18 months after three annual picloram applications. There was no significant difference between picloram applied alone and in a mixture with 1.1 kg/ha of 2,4-D. Three annual applications of triclopyr at 3.4 kg/ha reduced horsenettle shoots from 82% to 92%. The 3.4-kg/ha rate of triclopyr was slightly more effective in controlling horsenettle roots than was triclopyr at 1.7 kg/ha alone and in mixture with 1.1 kg/ha of 2,4-D. Dicamba at 0.6 and 1.1 kg/ha reduced the horsenettle shoots by 74% and 81%, respectively. The reduction was similar from mixtures of dicamba (0.1 and 0.3 kg/ha) with 2,4-D (0.4 and 0.8 kg/ha). There were no effects on the bluegrass forage that could be attributed to the herbicides.

Aspects of the life history and population dynamics of the Platte thistle (Cirsium canescens Nutt.), a rangeland weed, were studied in the Nebraska Sandhills from 1977 through 1979. This thistle is a monocarpic perennial, reproducing only by seed production, which typically occurs in small, scattered groups of plants. Seedling mortality at four sites was 75% during the first year, mostly because of dry moisture conditions. In plots with good vegetative cover, thistle rosette plants had a lower mortality rate, but growth rate was slower than that of rosette plants in plots with sparse vegetative cover. Seed heads developed during May and June and gained more than one-half of their dry weight shortly before maturing. Plants averaged 72, 197, and 303 seeds at three sites. Within sites, plants produced seeds in proportion to the number of seed heads per plant. Although naturally occurring biological-control agents have little effect on the vegetative growth phase of the life cycle, they greatly reduce the production of seeds.

Extensive variability in total lipid composition was observed in seeds of different plant species as well as cultivars within the same species. Susceptibility of plants to soil-incorporated trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) and oryzalin (3,5-dinitro-N4,N4-dipropyisulfanilamide) under greenhouse conditions decreased as the percentage of total lipid in dry seeds increased. Species with high seed lipid levels such as jimsonweed (Datura stramonium L.), soybean [Glycine max (L.) Merr.], and velvetleaf (Abutilon theophrasti Medic.) were less susceptible to these herbicides than were oat [Avena sativa (L.), giant foxtail (Setaria faberi Herrm.), and sorghum [Sorghum bicolor (L.) Moench.] with low seed lipid levels. Velvetleaf was the most tolerant to the herbicides. Corn (Zea mays L.) cultivars with a wide range of seed lipid content (4.45 to 17.0% of dry weight) were germinated in herbicide-free soil, but their roots were allowed to grow through trifluralin- or oryzalin-treated soil. Roots of seedlings from seeds with high lipid content had higher lipid contents than those from low-lipid seeds. There was a significant correlation between root lipid content and sensitivity to trifluralin or oryzalin. Roots high in lipids were less susceptible than those with low amounts of lipids. Similar results were obtained for oat cultivars. The fatty acid composition of plant roots differed among plant species and also among cultivars of the same species. The individual fatty acids did not significantly correlate with susceptibility of roots to trifluralin or oryzalin. Root lipid and fatty acid compositions were not significantly altered by trifluralin or oryzalin treatment.

Competition of green foxtail [Setaria viridis (L.) Beauv.] was studied in a semi-dwarf wheat (Triticum aestivum L. ‘Norquay’) and in two normal-height wheats (‘Napayo’ and ‘Sinton’) from 1975 through 1978. Green foxtail suppressed wheat growth as well as grain yield. Tiller number, leaf area, and dry weight of wheat were reduced. Green foxtail was more competitive in the semi-dwarf variety than in either normal height variety. The intensity of green foxtail competition could not be determined by density alone. In 1975, as few as 100 green foxtail plants/m2 significantly reduced yield of Napayo and Norquay wheat by 21 and 44%, respectively. In 1977, however, 1600 green foxtail plants/m2 did not reduce the yield of Sinton wheat significantly. The intensity of green foxtail competition was highly variable from one date of seeding to the next, but there was no correlation between the level of green foxtail competition and the date of seeding. Soil temperature and moisture at the time of seeding and early growth are thought to affect green foxtail competition in wheat critically.

Methyl ester of diclofop {2-[4-(2,4-dichlorophenoxy) phenoxy] propanoic acid} (diclofop as used herein refers to the methyl ester) added to nutrient solution inhibited adventitious root development and reduced the mitotic index in adventitious root tips of wheat (Triticum aestivum L. ‘Neepawa’) and wild oat (Avena fatua L.). Elongation of wild oat roots was significantly inhibited by 0.15 μM diclofop 24 h after treatment; whereas, wheat roots were unaffected at concentrations less than 1.5 μM even at 12 days, indicating at least a 10-fold difference in sensitivity. Initiation of new adventitious roots was reduced in both species by 0.30 μM diclofop. Mitotic index was more severely reduced at lower concentrations and after a shorter exposure in wild oat than in wheat. Histological studies showed that 24 h after wild oat roots were treated with 0.30 μM diclofop for 24 h, severe damage occurred to tissues within the developing central cylinder, close to the root apex. Disruption of cortical and epidermal cells generally was not as severe until 4 to 7 days after treatment. By 12 days the entire root tip was almost completely obliterated, with only the epidermal cell walls remaining intact.

A procedure is described for the rapid analysis of leaf samples to determine triazine resistance. The technique uses a commercially available fluorometer and is based upon the fact that photosynthesis-inhibiting herbicides increase chlorophyll fluorescence (because of dissipation of absorbed radiant energy in the absence of useful photochemistry). Resistant and susceptible biotypes of six weed species were assayed. Fluorescence of susceptible leaf sections increased dramatically over a 1- to 3-h exposure to atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], but resistant leaf sections showed no fluorescence increase. Substantial fluorescence increases of both resistant and susceptible leaf sections were induced by diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea]. In the absence of herbicides, fluorescence was higher in resistant than in susceptible leaf sections, suggesting that the resistant biotypes are less efficient photosynthetically. Fluorescence analysis was used to characterize atrazine inhibition of photosynthesis in leaf sections of three crop species. Differences in atrazine-induced fluorescence between crop lines were relatively small and were not well correlated with atrazine tolerance. The fluorometer is a convenient device for monitoring photosynthesis inhibition, however, and could be useful in detecting plants having resistance to photosynthesis-inhibiting herbicides.