The following herbicides were evaluated for relative phytotoxicity to cotton (Gossypium hirsutum L. ‘Acala SJ-1′), purple nutsedge (Cyperus rotundus L.), and yellow nutsedge (Cyperus esculentus L.) under greenhouse conditions: 2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide (alachlor); 2-chloro-2′,6′-diethyl-N-(butoxymethyl)acetanilide (CP-53619); 2-(3,4-dichlorophenyl)-4-methyl-1,2,4-oxadiazolidine-3,5-dione (VCS-438); 4-chloro-5-(dimethylamino)-2-α,α,α-trifluoro-m-tolyl)-3(2H)-pyridazinone (San-6706); 2-(α naphthoxy)-N,N-diethyl-propionamide (R-7465); and S-isopropyl 5-ethyl-2-methyl-piperidine-1-carbiothioate (R-12001). Herbicides were incorporated 6.35 cm deep, at rates of 1.12, 2.24, and 4.48 kg/ha, into a fine sandy loam prior to planting. All treatments except the low rate of alachlor and VCS-438 controlled yellow nutsedge for 8 weeks. R-7465 and R-12001 at 1.12 kg/ha and San-6706 at 2.24 kg/ha controlled purple nutsedge for 8 weeks. Alachlor and CP-53619 were somewhat less effective against purple nutsedge than yellow nutsedge, but their intermediate rates suppressed purple nutsedge for 4 weeks. Even the high rate of VCS-438 was ineffective against purple nutsedge. Cotton, in terms of fresh shoot weight, exhibited considerable tolerance to 1.12 and 2.24 kg/ha of VCS-438 and CP-53619 and 1.12 kg/ha of R-7465. Applications of 2.24 kg/ha of CP-53619 and 1.12 kg/ha of R-7465, however, suppressed the development of lateral roots of cotton. Other rates of these herbicides and all rates of alachlor, R-12001, and San-6706 moderately to severely injured cotton in most of the experiments.

The combined influence of plant competition and defoliation on development of Canada thistle was investigated in a 2-yr field study. Plant competition was induced by seeding tall fescue and crown vetch. Artificial defoliation was used to simulate various levels of leaf removal by insects. Both defoliation and induced competition reduced biomass of Canada thistle but their impact varied with environmental conditions. Defoliation had a greater detrimental influence than induced competition on thistle biomass in a dry year when growth of the plant competitors was suppressed. In a wet year, induced competition was more important in suppressing Canada thistle than defoliation, and moderate levels of defoliation (25%), applied once when the thistles were 12 to 15 cm in diam, stimulated root weight within the top 20 cm of soil. Reduction in thistle biomass increased with level of defoliation and was greatest when defoliation was applied repeatedly at 14-d intervals in the presence of induced competition. Crown vetch showed very little growth in one season and tall fescue was the primary source of competition for the thistles. The results confirm the hypothesis that combined stresses can substantially reduce biomass development of Canada thistle plants.

Degradation of SAN 6706 [4-chloro-5-(dimethylamino)-2-(α,α,α-trifluoro-m-tolyl)-3(2H)-pyridazinone] and SAN 9789 [4-chloro-5-(methylamino)-2-(α,α,α-trifluoro-m-tolyl)-3 (2H)-pyridazinone] was studied in a sandy loam soil at temperatures of 5, 20, and 35 C. After 210 days of incubation 10, 80, and 97% of SAN 6706 had been dissipated from soil at 5, 20, and 35 C., respectively. SAN 6706 was converted to SAN 9789 and a demethylated metabolite in the soil. SAN 9789 was converted to the demethylated metabolite. It was difficult to differentiate between first and second-order kinetics in the degradation of these compounds. SAN 6706, at 20 and 35 C, had a half life of 50 and 9 days respectively, and an Arrhenius activation energy of 15.8 Kcal/mole. SAN 9789, at 20 and 35 C, had a half life of 270 and 70 days, respectively, and an activation energy of 11.8 Kcal/mole. The degradation of SAN 6706 was inhibited by a chloroform treatment, and microbiological degradation is suggested.

A column leaching study was conducted in a greenhouse to determine the movement of atrazine and alachlor through soil as affected by alfalfa roots in three stages of decay. Alfalfa was grown in soil columns for 6 months and then killed with glyphosate. The 14C-atrazine and 14C-alachlor were applied to soil columns with bromide following 2, 13, or 26 weeks of root decomposition. Most of the herbicide remained in the top 9 cm of soil. However, atrazine and alachlor soil distribution profiles indicated greater preferential movement in columns with roots than in columns without roots. Higher levels of atrazine and alachlor were bound to soil at lower depths in the presence of roots than without roots. Adsorption of atrazine and alachlor on soil with and without alfalfa roots was not significantly different as determined by Freundlich adsorption isotherms. Degradation rates for atrazine and alachlor were not substantially different between soil with and without roots. Although only small amounts of the applied herbicide leached through the columns, preferential flow of herbicides through root-mediated soil macropores and cracks could be a mechanism of herbicide transport through soil under appropriate conditions.

The phytotoxicity of glyphosate [N-(phosphonomethyl) glycine] on seeds (protected or non-protected with soil) and seedlings of Italian ryegrass (Lolium multiflorum Lam.) and red clover (Trifolium pratense L.) was studied in the greenhouse. Percent germination of Italian ryegrass seeds covered with soil was significantly reduced with glyphosate applied at 4 kg/ha. The direct treatment over the seeds on the soil surface was more toxic, reducing the number of established seedlings at dosages of 3 and 4 kg/ha. In the two red clover seed treatments, germination was significantly reduced with 2 kg/ha of herbicide. Even though germination was reduced at intermediate rates of glyphosate, shoot growth and tillering were improved and yields of dry weight were equivalent to that of untreated plants. Postemergence applications of glyphosate were phytotoxic to both species with the effects becoming more pronounced at increasing dosages. However, at equal dosage, red clover was less susceptible than Italian ryegrass. The patterns of foliar absorption and translocation of 14C from 14C-glyphosate was similar in Italian ryegrass and red clover. 14C was translocated throughout the plants and exhibited apoplastic and symplastic movement.

Absorption of foliar-applied 14C-imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} 72 h after treatment was higher than 90% for soybean [Glycine max (L.) Merr. ‘Braxton’], peanut (Arachis hypogaea L. ‘Florunner’), common cocklebur (Xanthium strumarium L. # XANST), sicklepod (Cassia obtusifolia L. # CASOB), and Florida beggarweed [Desmodium tortuosum (SW.) # DEDTO]. Both symplasmic and apoplasmic translocation of the herbicide were evident. Imazaquin half-life was 4.4 days in soybean, 5.3 days in peanut, 9.6 days in Florida beggarweed, 12.7 days in sicklepod, and 39.8 days in cocklebur. Tolerance of these species to foliar-applied imazaquin as determined in greenhouse studies with 21-day-old seedlings was as follows: soybean = peanut > Florida beggarweed > sicklepod > cocklebur. Tolerance was directly correlated to imazaquin halflife within the tissue of the respective species.

In eastern Washington Dalmatian toadflax (Linaria dalmatica (L.) Mill) produces seeds for about 3 months, beginning in early July, with over 95% produced during the first 5 weeks. A large plant will produce a half million seeds under good conditions. Seeds are sharply angular, slightly winged, and 1 to 2 mm long. Approximately 7,000 weigh 1 g. Seeds are spread by wind and browsing animals. Some seeds will germinate under laboratory conditions the same season in which produced, but germination from dry storage increases for 2 to 3 years, with a periodic low in November and a high in April. Over 90% germination is obtained with 2 to 3-year-old seeds in the laboratory. Germination in the field occurs in both spring and fall, and seedlings can emerge from a soil depth of 2 to 2.5 cm.

Field experiments were conducted at five locations in Colorado, Kansas, and Wyoming in 1994–1995 and 1995–1996 to compare the effects of MON 37500 rate and application timings on downy brome control and winter wheat tolerance. MON 37500 at 18 to 35 g ha−1 applied preemergence or fall postemergence reduced downy brome density 71 to 92% in 1995. Spring-applied MON 37500 suppressed downy brome growth but did not reduce plant density. No application reduced downy brome density in 1996. At each location, downy brome was controlled best by MON 37500 applied preemergence or fall postemergence at 35 g ha−1. MON 37500 did not affect wheat height at Archer or Torrington, WY, and Burlington or Stratton, CO, but wheat treated preemergence or fall postemergence was taller than untreated wheat at Hays, KS, in 1995. Spring-postemergence-treated wheat at Hays in 1995 was shorter than untreated, preemergence-, or fall-postemergence-treated wheat. Wheat head density did not differ among treated and untreated plots at Torrington, but herbicide treatment increased wheat yields. Wheat head density increased with all MON 37500 treatments at Hays in 1995, as did yield. However, preemergence and fall-postemergence applications resulted in the highest wheat yields. No herbicide treatment affected head density or yield at Hays in 1996.

A liquid scintillation counting solution was developed which optimized the direct assay efficiencies of 14C-labeled 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), 3,6-dichloro-o-anisic acid (dicamba), and α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) in soil. In this counting technique, 3-g soil samples containing 14C-labeled herbicide were placed directly into a liquid scintillation vial containing a phosphorescent counting solution. Samples were shaken and allowed to settle before assaying using a liquid scintillation spectrometer. This direct radioassay was as efficient or more efficient than double or single extraction processes. Automatic external standardization in conjunction with the liquid scintillation counter was an effective tool for estimating the 14C present in soil samples.

Corn yield loss associated with common lambsquarters and giant foxtail in mixed-weed species communities was estimated from empirical equations based on early-season weed density, weed relative leaf area, or weed relative shoot volume in 1998 and 1999. The estimated maximum corn yield loss ranged up to 20% in 1998 but was 50% or more in 1999. Competition coefficients estimated from weed density (I values) or weed relative shoot volume (q V values) indicated that the weed species were equally competitive in 1998 but that common lambsquarters was more competitive than giant foxtail in 1999. In contrast, the relative leaf area–based competition coefficients (q L values) indicated that common lambsquarters and giant foxtail were equally competitive in both years. Weed species emerged at the same time as corn in 1998, whereas in 1999, common lambsquarters emerged 3 d earlier than corn and 1 d earlier than did giant foxtail. Earlier emergence of common lambsquarters was associated with greater cumulative intercepted photosynthetically active radiation (IPAR) per plant compared with that of giant foxtail. Competition coefficients estimated from weed relative leaf area were similar between years for common lambsquarters but differed for giant foxtail. Similarly, the relationship between cumulative estimated IPAR and early-season relative leaf area was stable between years for common lambsquarters but not for giant foxtail. Consequently, competition coefficients were more consistent for common lambsquarters than for giant foxtail in mixed communities. The results suggest that the competitive ability of common lambsquarters and giant foxtail may not differ greatly in corn, but variability in corn yield loss between years was not adequately explained by these empirical models.

Laboratory studies were conducted to characterize the adsorption-desorption and degradation of thiobencarb in soil. Thiobencarb adsorption on soil was correlated with soil organic carbon (r = 0.97) and values (ml g−1) for the following soils were: Pahokee muck 339, Everglades muck 169, and Immokalee sand 14. The adsorption values (ml g−1) per unit of organic carbon were Immokalee sand 1195, Pahokee muck 765, and Everglades muck 539. In desorption studies, thiobencarb Kf values increased 80 ml g−1 from the first to the third desorption step in the muck soils, but were similar in three desorptions from the sand. The half-lives of thiobencarb averaged 16, 18, and 24 days on surface samples of Pahokee muck, Everglades muck, and Immokalee sand, respectively, across the range of 25 to 35 C and 10 to 100 kPa soil water tension. Half-lives were generally longer with water contents equivalent to 100 kPa than 10 kPa soil water tension and incubated at 25 than 35 C. The relative percentages of extractable 14C, bound 14C, and 14CO2 from 14C-thiobencarb-treated soil were dependent on soil type, soil water content, incubation temperature, and time.

Weed population shifts occurred when simazine [2-chloro-4,6-bis(ethylamino)-s-triazine], diuron [3-(3,4-dichlorophenyl)-1,1-dimethyl-urea], terbacil (3-tert-butyl-5-chloro-6-methyluracil), and dichlobenil (2,6-dichlorobenzonitrile) were applied annually for 5 years to an apple orchard. The number of brambles (Rubus sp.) and Virginia clematis (Clematis virginiana L.) plants increased with terbacil. usage. Bramble populations increased with the high rate of dichlobenil and goldenrod (Solidago sp.) increased at the low rate. The major vegetation on plots receiving amitrole (3-amino-s-triazole) was not altered from that of a mowed check. However, amitrole reduced growth of shallow planted trees. Postemergence applications of paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) four times during each growing season resulted in more bare soil than was in the mowed checks at the end of the fifth growing season. Yields of apples were higher in the simazine, terbacil, and dichlobenil-treated plots than in the mowed checks. Simazine and terbacil residues did not accumulate in the upper 60 cm of soil during the study; a slight accumulation of dichlobenil appeared to occur in the surface 0 to 15-cm.

Waterhyacinth [Eichhornia crassipes (Mart.) Solms] grown in screen-enclosed, outdoor aquaria were treated with factorial combinations of 0, 0.75, 1.5, or 3 kg/ha of the experimental growth retardant EL-509 [α-(4-chlorophenyl)-α-(1-methylethyl)-5-pyrimidine-methanol] and zero, one, two, or three pairs of waterhyacinth weevils (Neochetina eichhorniae Warner)/plant, with three replicates of each weevil-number by retardant level combination. When plant growth was evaluated over a time course, significant effects of weevils and retardant were observed. After 127 days, however, waterhyacinth was reduced as weevil numbers increased, but the growth retardant was ineffective without weevils. Weevils appeared to be more effective when used in combination with the retardant, and the combined effects were additive.

Growers need affordable methods to sample weed populations to reduce herbicide use with site-specific weed management. Sampling programs and methods of developing sampling programs for integrated pest management are not sufficient for site-specific weed management because more and different information is needed to make treatment maps than simply estimate average pest density. Sampling plans for site-specific weed management must provide information to map the weeds in the field but should be developed for the objective of prescribing spatially variable management. Weed scientists will be most successful at designing plans for site-specific weed management if they focus on this objective throughout the process of designing a sampling plan. They must also learn more about the spatial distribution and dynamics of weed populations and use that knowledge to identify cost-effective plans, recommend methods to make maps as well as collect data, and find ways to evaluate maps that reflect management to be prescribed from the map. Foremost, sampling must be thought of as an ongoing process over time that uses many types of information rather than a single event of collecting one type of information. Specifically, scientists will need to identify common characteristics rather than just differences of the spatial distribution of weeds among fields and species, recognize that map accuracy may be a poor indicator of the value of a sampling plan, and develop methods to use growers' knowledge of the distribution of weeds and past spatially variable management within a field for both making a map and recommending a sampling plan. The value of proposed methods for sampling and mapping must also be demonstrated or adoption of site-specific weed management might be limited to growers who enjoy using sophisticated technology.

Six cropping/weed control systems were evaluated from 1978 to 1980 for the control of yellow nutsedge (Cyperus esculentus L.). Supplementing cultivation of cotton (Gossypium hirsutum L. ‘Acala SJ-2′) with either preplant applications of fluridone {1 - methyl - 3 - phenyl - 5 - [3 - (trifluoromethyl)phenyl] -4(1H-pyridinone} or two hoeings for 2 yr preceding cotton treated with DSMA (disodium methanearsonate) and MSMA (monosodium methanearsonate) reduced populations of viable yellow nutsedge tubers 98 to 99% within 3 yr. Dry- or wet - fallowing plus tillage after barley (Hordeum vulgare L. ‘Kombyne’), and double cropping potatoes (Solanum tuberosum L. ‘White Rose’) treated with EPTC (S-ethyl dipropylthiocarbamate) with soybeans [Glycine max (L.) Merr. ‘Williams’] treated with alachlor [2-chloro - 2′, 6’ - diethyl - N - (methoxymethyl)acetanilide] for 2 yr preceding cotton, reduced populations of tubers 98 to 99% within 3 yr. A similar reduction of tubers (97%) was obtained by double cropping potatoes with milo [Sorghum bicolor (L.) Moench. ‘NK- 265′] for 2 yr preceding cotton.

Absorption and metabolism of 14C-3-amino-2,5-dichlorobenzoic acid (amiben) by six plant species were investigated for correlation with species sensitivity. Plant sensitivity was determined from amiben concentrations required to give 50% reduction of radicle extension of germinating seedlings and shoot dry weight of 10-day-old plants (I50). The I50 values showed that soybean (Glycine max (L.) Merr., squash (Cucurbita pepo L.), and ivyleaf morningglory (Ipomoea hederacea (L.) Jacq.) were tolerant, cucumber (Cucumis sativus L.) was intermediate and giant foxtail (Setaria faberii Herrm.) and velvetleaf (Abutilon theophrasti Medic.) were susceptible. Neither the amount of amiben absorbed, the transport to shoots, the concentration of radioactivity soluble in methanol, nor the distribution of radioactivity among amiben, N-(3-carboxy-2,5-dichlorophenyl)-glucosylamine (hereinafter referred to as N-glucosyl amiben), and an unidentified amiben conjugate (amiben-X) was associated with species sensitivity in 11-day-old plants. In seedling tissues, neither the concentration of methanol-soluble nor methanol-insoluble radioactivity correlated with sensitivity. Highly significant correlations between the log percentage composition of amiben, amiben-X, and N-glucosyl amiben in the methanol-soluble fraction of seedlings and log I50 for radical elongation were obtained; the correlations were negative for amiben and amiben-X but positive for N-glucosyl amiben.

In growth chamber studies, we measured the movement of cotyledons and leaves or leaflets in seedlings of nine weed species to determine: the magnitude of movement, whether movements followed a rhythmic pattern, and if rhythmic movements were endogenously controlled. Seedlings were always entrained under a 15-h light:9-h dark regime. Measurements were made at 3-h intervals for 3 to 4 days under four light regimes: alternating 15-h light:9-h dark; continuous light; one 15-h light:9-h dark span, followed by continuous light; and a 15-h light:9-h dark regime, in which the dark span was advanced 9 h (phase shift) when measurements were begun. No clearly defined pattern of leaf movement was found in wild mustard [Brassica kaber (DC.) L. C. Wheeler var. pinnatifida (Stokes) L. C. Wheeler]. Daily rhythmic leaf movements that did not appear to be endogenously controlled were found in redroot pigweed (Amaranthus retroflexus L.) and black nightshade (Solanum nigrum L.). Circadian rhythmic leaf movements that appeared to be under endogenous control were found in jimson-weed (Datura stramonium L.), common lambsquarters (Chenopodium album L.), common cocklebur (Xanthium pensylvanicum Wallr.), prickly sida (Sida spinosa L.), sicklepod (Cassia obtusifolia L.), and coffee senna (Cassia occidentalis L.).

The degree and rate of volatilization, and the partial vapor pressures of the methyl, n butyl, and n octyl esters of 2,4-D [(2,4-dichlorophenoxy)acetic acid] were determined at ambient temperatures using a kinetic system approximating field-air conditions. The initial surface area/applied mass ratio (Q), as well as temperature and the type of compound, determined the volatility. Only when the Q values were kept constant did the volatility decrease with increasing chain length and decreasing saturated vapor pressure as expected. A mathematical model was constructed. Similar results were obtained for the amine salts which had saturated vapor pressures of ca 10−10 mm Hg at 38 C. Vapor drift can be essentially eliminated by use of the amine salts.

Apical dominance, as maintained by above-ground foliage or individual rhizome apexes, is very marked in johnsongrass. (Sorghum halepense [L.] Pers.). Axillary bud development in single-node segments excised from individual rhizome pieces was least at the proximal end with increasing activity toward the distal end (apex end). Within serially excised, multi-node sections, axillary bud development was least at the proximal end and greatest at the distal end, and there was an overall increase in bud activity from proximal to distal ends of the rhizome pieces. This was true irrespective of whether the multi-node sections were cultured vertically (with buds oriented above the nodes) or were inverted (with buds oriented below the nodes). Lateral rhizomes exerted a dominating influence on the development of axillary buds from their parent rhizomes, as did the apical meristems of the parent rhizomes.

Herbicides used in containerized plant production have a potential to move off-site in irrigation runoff water. A granular formulation of isoxaben plus trifluralin was applied to a commercial container plant nursery bed. Herbicides were monitored in irrigation runoff and collection pond water over a period of 60 d. Greatest quantities of both herbicides were lost during the first runoff event following application. Approximately 9.2 and 0.7% of the applied isoxaben and trifluralin, respectively, moved from the application site in runoff water within 5 d after treatment. Herbicide concentrations in runoff water were highest (0.75 μg ml−1 isoxaben and 0.08 (μg ml−1 trifluralin in 1992) during the first 0.25 h following herbicide application. Concentrations in the runoff collection pond climaxed following the first runoff event and decreased to less than 1 ng ml−1 within 60 d after treatment. Photodegradation of isoxaben within the surface 2.5 cm of pond water was greater in light as compared to dark. These studies indicate that isoxaben and trifluralin move from the site of application, but that neither accumulate in the runoff water collection ponds.