A triazine-resistant (TR) biotype of Powell amaranth was discovered in 1989 in a potato field treated with metribuzin. A survey of all agricultural counties in Idaho showed that the TR Powell amaranth infestation was localized in the southeastern corner of Gooding county in southern Idaho. To determine the mechanism of triazine resistance, I50 values for inhibition of photosystem II were determined for atrazine, metribuzin, and diuron using thylakoids isolated from TR and triazine-susceptible (TS) biotypes. TR/TS ratios based on I50 values were 134 for atrazine, 62 for metribuzin, and 1.9 for diuron. Results of binding studies with atrazine and metribuzin were consistent with the I50 studies, indicating that resistance was due to reduced binding of triazines to the thylakoid membrane D1 protein. Sequencing the chloroplast psbA gene from TR and TS biotypes revealed a serine 264 to glycine change in the TR biotype. The mutation presumably resulted in reduced hydrogen bonding between triazine herbicides and the Dl protein.

The uptake, translocation, and metabolism of the herbicide fluridone 1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone was studied in cotton (Gossypium hirsutum L. ‘Stoneville 213’) (fluridone tolerant) and corn (Zea mays L. ‘Migro 5040’), soybean [Glycine max (L.) Merr. ‘Calland’], and rice (Oryza sativa L. ‘Nato’) (fluridone susceptible). Uptake studies with 14C fluridone demonstrated that all species absorbed radioactivity with rice having the highest concentration. Equivalent concentrations of radioactivity were observed in corn, cotton, and soybean. Absorbed fluridone was retained primarily in the roots and the basal region of the stem of cotton, while in the herbicide susceptible species fluridone was readily translocated into the shoots. No appreciable metabolism of fluridone was observed in the roots or shoots of cotton and soybean. Tolerance of cotton to fluridone was primarily attributed to the limited translocation of the compound.

Experiments were conducted in growth chambers to evaluate the effect of soil temperature and soil moisture on the distribution of 14C-photoassimilates and 14C-glyphosate [N-(phosphonomethyl) glycine] in quackgrass [Agropyron repens (L.) Beauv. ♯3 AGRRE]. When 14C-glyphosate was applied to leaves, the radioactivity was less in the rhizome buds of plants exposed to 7-C soil temperature than in plants exposed to 12- and 18-C soil temperatures after 2 days. In plants with leaves exposed to 14CO2, the radioactivity from 14C-photoassimilates was greatest in rhizomes and rhizome buds of plants at the 12-C soil temperature. As soil moisture levels were decreased, uptake of C-glyphosate into leaves declined, and transport to the daughter shoots, rhizomes, and rhizome buds was reduced. The concentration of 14C-photoassimilates in the rhizome system of water-stressed quackgrass plants was similar to that in nonstressed plants. This study shows that the patterns of glyphosate distribution differ from those of photoassimilate distribution in quackgrass plants exposed to water stress.

Field experiments were conducted in 1988 and 1989 to evaluate imazethapyr for weed control in peanuts. Imazethapyr was applied PRE or POST at 3, 5, or 7 weeks after crop emergence (WAE) at 0.071 kg ai ha–1. Imazethapyr applied PRE controlled common lambsquarters 85%, prickly sida 92%, and a mixture of entireleaf, ivyleaf, pitted, and tall morningglory species 77%. Morningglory control was at least 91% with imazethapyr plus metolachlor PRE followed by imazethapyr plus 2,4–DB or imazethapyr plus acifluorfen at 3, 5, or 7 WAE. Yields from systems that included metolachlor plus imazethapyr PRE followed by imazethapyr plus acifluorfen, imazethapyr plus 2,4–DB, or acifluorfen plus 2,4–DB at 3 WAE were greater than yields from the handweeded check. All systems with imazethapyr plus metolachlor PRE followed by any POST treatment except imazethapyr plus acifluorfen 7 WAE provided net returns equivalent to the herbicide standard of metolachlor PRE and acifluorfen and bentazon plus 2,4–DB 3 WAE. All systems except imazethapyr PRE provided greater net returns than the handweeded weed–free check.

Sicklepod (Cassia obtusifolia L. ♯6 CASOB), hemp sesbania [Sesbania exaltata (Raf.) Cory ♯ SEBEX], and showy crotalaria (Crotalaria spectabilis Roth. ♯ CVTSP) were grown in controlled-environment greenhouses with day/night temperatures of 23/17, 26/20, 29/23, and 32/26 C. Plant height, leaf area, and total dry weight at 53 days after planting increased significantly with temperatures between 23/17 and 29/23 C in sicklepod and showy crotalaria, and between 23/17 and 32/26 C in hemp sesbania. In all species, leaves were produced more rapidly as temperature increased. Axillary branch leaf production was greatest at 29/23 C in sicklepod, and at 29/23 and 26/20 C in showy crotalaria; hemp sesbania showed little axillary branch development. Total dry-matter production in all species was closely correlated with leaf area duration, but not with net assimilation rate. Differences in total main stem leaf area in all species were primarily due to differences in leaf number rather than to differences in leaf size at maturity.

The presence of a common cocklebur (Xanthium strumarium L. # XANST) population resistant to organic arsenical herbicides was confirmed in three adjoining counties of South Carolina's ten-county cotton-growing region, but little evidence of resistance was found in the remaining seven counties. This population was resistant to both disodium salt of methylarsonic acid (DSMA) and monosodium salt of methylarsonic acid (MSMA) but not to herbicides from nine other chemical groups. A much greater level of resistance was apparent in the field than under greenhouse conditions and larger plants were more resistant than small plants.

Soybean (Glycine max (L.) Merr., var. Chippewa 64) plants were grown in culture solutions containing 14C ring-labeled 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine). Culture solution temperatures were varied but ambient temperature about the shoots was constant. The amount of atrazine absorbed increased with increases in herbicide concentration, absorption time, and root temperature. Root temperature had more influence on atrazine accumulation in stems and leaves than in roots. Transpiration rates of atrazine-treated plants were reduced at least 50%.

Witchweed [Striga asiatica (L.) Kuntze] seeds conditioned for 3 weeks in either water or 0.01% (w/v) sodium hypochlorite (NaOCl) had 83 to 95% germination after being treated with dl-strigol, kinetin, zeatin, or natural stimulants from corn (Zea mays L.) root exudates. None of these compounds, however, induced germination of seeds conditioned in 0.025 or 0.050% (w/v) NaOCl. Seeds conditioned in 0.075, 0.1, or 0.15% (w/v) NaOCl had 8, 56, and 76% germination, respectively, during the first week of conditioning and 10, 100, and 100% germination after 2 weeks of conditioning. Germination in these concentrations of NaOCl was termed “atypical”.

The substituted urea herbicides were rated according to their effectiveness as aquatic herbicides in this order: diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], monuron [3-(p-chlorophenyl)-1,1-dimethylurea], neburon [1-butyl-3-(3,4-dichlorophenyl)-1-methlyurea] and TCA(trichloroacetic acid) mixtures with them. They showed greatest potential in controlling certain aquatic plants in pre-emergence and early postemergence applications. However, relatively high concentrations were required to control filamentous algae (Cladophora, Pithophora and Spirogyra), chara (Chara), coontail (Ceratophyllum), naiad (Najas), and pondweeds (Potamogeton) for periods of time exceeding three months and up to three years. Granular formulations achieved better distribution of herbicides for control of rooted aquatic plants along the margins of lakes and ponds. Wettable powder and liquid emulsifiable concentrates were superior to granular formulations for the control of algae and emergent or floating aquatic plants.

Monuron and fenuron were less toxic to fish than were diuron and neburon. The TCA mixtures were somewhat more toxic than the simple ureas. Some species of fish were more sensitive than others and fingerlings more sensitive than adults of the same species. Fish-food organisms were reduced appreciably in plastic enclosures at herbicidal concentrations.

The temperature at which seeds of witchweed (Striga asiatica (L.) Kuntze), an obligate parasite, were stored markedly influenced the length of time that the seeds remained dormant. Dormancy was broken completely by storing seeds collected in August, 1960, at 88 F for 6 weeks, at 75 for 24 weeks, at 40 for 32 weeks and at 0 F for 40 weeks. Seed viability was lost rapidly when the storage of seeds was continued at 88 or 100 F plus 100% relative humidity after seed dormancy had been broken. All of the seeds stored under those conditions were non-viable within 20 weeks after storage began. The viability of seeds stored at 75, 40 or 0 F had not decreased after 26 months for 1960 seeds. Seeds collected in 1961 and stored at those three temperatures or at 88 F plus 20% relative humidity for 14 months did not decrease in per cent viability. A hypothesis on the physiology of the dormancy, viability, and germination of witchweed seeds is presented.

Experiments were carried out to investigate weed seed production in widely spaced spring wheat crops that received aggressive mechanical weed control (hoeing and harrowing) compared with that in narrowly spaced crops receiving less aggressive mechanical control (harrowing only). Three species (wild buckwheat [Polygonum convolvulus], ladysthumb [Polygonum persicaria], and common chickweed [Stellaria media]) were studied in three row-spacing treatments (10, 20, and 30 cm) and two sowing densities (140 and 180 kg ha−1). Average seed production per surviving plant was up to three times higher in the 30-cm treatments compared with the 10-cm treatments. Taking into account the 40–50% weed mortality resulting from control in the 30-cm treatments, seed production per seedling was still higher in the 30-cm treatments than in the 10-cm treatments. Differences in wheat yield were not found among treatments. From the perspective of long-term weed population management, using a narrow row spacing would be more effective in spring wheat on the basis of experiments with the weed species considered here.

A mathematical model was developed to predict common lambsquarters seedling emergence in southwestern Quebec. The model was based on the thermal-time concept, using air temperatures in the double-sine calculation method. The model was built using data from five experiment-years for corn naturally infested with weed populations. Once developed, the model was calibrated using different crop seedbed preparation times. The base temperature was then adjusted for each time of seedbed preparation. A power regression function was used to relate adjusted base temperatures and the accumulated thermal units at seedbed preparation time. A modified Weibull function was then fitted to the field emergence data, expressed as the cumulative proportion of the total seedling emergence over the growing season as a function of cumulative thermal units. The simplicity and accuracy of this model would make it an excellent tool to predict common lambsquarters seedling emergence in field situations, facilitating the determination of the timing of scouting in integrated weed management systems.

The bacterially produced phytotoxin 2-amino-4-(2-amino-3-hydroxypropoxy)-trans-3-butenoic acid (rhizobitoxine) was tested for herbicidal properties in comparison with 3-amino-s-triazole (amitrole) and 4-chloro-5-(dimethylamino)-2-(a,a,a,-trifluro-m-tolyl)-3(2H)-pyridazinone (metflurazone). In postemergence tests with various plant seedlings, rhizobitoxine and amitrole were approximately equal in phytotoxicity on a weight basis, and both were generally much more phytotoxic than metflurazone. Phytotoxicity of rhizobitoxine varied markedly among grass species. Sorghum (Sorghum bicolor (L.) Moench ‘Hegari’) was very sensitive to rhizobitoxine, large crabgrass (Digitaria sanguinalis (L.) Scop.) was moderately sensitive, wheat (Triticum aestivum L. ‘Thorne’) was tolerant, and Kentucky bluegrass (Poa pratensis L.) was very tolerant. In contrast, amitrole was almost as phytotoxic to bluegrass as to large crabgrass, and metflurazone had little effect on either. In a preemergence test with mustard (Brassica japonica (L.) Coss. ‘Southern Giant Curled’) rhizobitoxine was as effective as amitrole, when compared on a molar basis, in inhibiting chlorophyll synthesis and root growth and was superior in retarding overall growth and development.

Crops transformed to provide resistance to herbicides with two different mechanisms of action provide new opportunities for control of herbicide-resistant weeds. However, unexpected interactions may develop, especially for herbicides not generally used in tank-mixtures. The objectives of this study were to evaluate weed control and determine herbicide interactions and fluorescence responses with combinations of glyphosate and glufosinate on selected weeds prevalent in Michigan cropping systems. Field studies to determine herbicide interactions resulted in synergism only at 0.84 kg ae ha−1 of glyphosate and 0.47 kg ai ha−1 glufosinate in 2008. Early synergism (7 d after treatment [DAT]) was observed in the field at several combined rates for common lambsquarters and velvetleaf in 2009, and in the greenhouse for giant foxtail. Differences between years were perhaps due to the effect of environmental conditions on herbicide absorption and translocation. Antagonism was observed in the field in 2009 for velvetleaf, common lambsquarters, and giant foxtail especially at 840 g ae ha−1 glyphosate and 118 g ai ha−1 glufosinate, 28 DAT. Antagonism was also observed in the greenhouse for giant foxtail and Canada thistle, 28 DAT. Fluorescence measurements on Canada thistle in the greenhouse showed that glufosinate and glufosinate plus glyphosate acted rapidly to quench electron transport of photosystem II (PS II) system of photosynthesis, and the fluorescence characteristics of the glyphosate and glufosinate combinations were indistinguishable from glufosinate alone.

GESTINF is a decision tool for postemergence weed control based on the equivalent density approach. Using observed weed densities just before treatment, the program estimates the economic return from the treatment, thus indicating whether to treat or not and, if a treatment is needed, the most economical weed control solution. Each treatment is also characterized by an environmental pollution index. GESTINF has been tested in wheat and soybeans on a farm in northeastern Italy with a total cropping area of 60 ha of wheat and 40 ha of soybean. For both crops, weed control followed the suggestions of GESTINF, whereas the remaining cropped areas were treated according to standard farm weed control practices. To compare the two weed control systems, weed control efficacy, average crop yield, and the extra time required for scouting and treatments were measured. In both crops, the treatments suggested by GESTINF showed good efficacy, and yields proved to be no different from those obtained in the fields treated with standard farm weed control practices. In most cases, GESTINF selected treatments with a lower environmental effect. The most critical point was the time required to scout the weed population that, in low-value crops or when very cheap treatments were available, reduced the weed control economic return. In wheat, GESTINF indicated that fewer fields needed to be treated than did the conventional system. However, extra costs due to both scouting and more expensive treatments balanced the savings obtained from nontreated areas. For soybean, the treatments adopted by the farm were based on a combination of pre- and postemergence practices. In this case, GESTINF identified cheaper but still efficacious treatments, significantly reducing the total cost of weed control.

The effect of temperature and relative humidity on root and foliar uptake of C14-labeled atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) in quackgrass (Agropyron repens (L.) Beauv.) was studied in a controlled environment growth chamber. Uptake and translocation of root-fed atrazine increased as temperature increased. Greater uptake and translocation of root-fed atrazine occurred under low relative humidity than under high relative humidity. Slightly greater uptake and translocation of foliar-applied atrazine occurred as temperature was increased from 60 to 80 F.

A bioeconomic model was tested as a decision aid for weed control in soybean at Rosemount, MN, from 1991 to 1994. The model makes recommendations for preplant incorporated and preemergence control tactics based on the weed seed content of the soil and postemergence decisions based on weed seedling densities. Weed control, soybean yield, herbicide use, and economic return with model-generated treatments were compared to standard herbicide and mechanical control systems. Effects of these treatments on weed populations and corn yield the following year were also determined. In most cases, the model-generated treatments controlled weeds as well as a standard herbicide treatment. Averaged over the 3 yr, the quantity of herbicide active ingredient applied was decreased by 47% with the seedbank model and 93% with the seedling model compared with a standard soil-applied herbicide treatment. However, the frequency of herbicide application was not reduced. Soybean yields reflected differences in weed control and crop injury. Net economic return to weed control was increased 50% of the time using model-recommended treatments compared with a standard herbicide treatment. Weed control treatments the previous year affected weed density in the following corn crop but had little effect on weed control or corn yield. The bioeconomic model was responsive to differing weed populations, maintained weed control and soybean yield and often increased economic returns under the weed species and densities in this research.

We investigated germination enhancement of downy brome (Bromus tectorum L.) seeds (caryopses) recovered from herbicide fallows in degraded big sagebrush (Artemisia tridentata Nutt.) communities. Numbers of germinable downy brome seeds were increased most by adding 0.1 mmole potassium nitrate (KNO3) or combinations of 0.1 mmole KNO3 and 0.14 mmole gibberellic acid (GA3). Germinability was enhanced in the fall and spring but not in midwinter when dormancy was greatest. Improved downy brome control and better establishment of seeded forage species with a fallow technique in sagebrush-dominated communities look promising if these growth regulators can be effectively applied in the field. Without germination enhancement, many germinable seeds of downy brome are present in litter under shrub canopies at the end of the fallow period. With near-optimal microenvironment under shrub canopies for plant growth, severe competition between downy brome and the seeded forage species can result.

Field experiments conducted over a 2-yr period demonstrated that common bermudagrass [Cynodon dactylon (L.) Pers. # CYNDA] inhibited growth of newly planted peach (Prunus persica L. ‘Norman’) trees. Common bermudagrass densities of 100, 75, 50, and 25% ground cover reduced tree fresh weight by 86, 64, 43, and 19%, respectively, the first year (1978) and 87, 62, 44, and 28%, respectively, the second year (1979) after planting. Tree trunk diameter relative growth rate (RGR) was reduced by 75 and 100% common bermudagrass ground cover densities at all measurement dates only in 1978. Tree leaf N and K were reduced in both years by common bermudagrass; however, only at the 100% common bermudagrass density in 1978 was N at a deficient level. Leaf chlorophyll was reduced in trees grown in all densities of common bermudagrass only in 1978. Reduced tree growth cannot be explained entirely by competition for essential nutrients; thus an allelopathic effect of the bermudagrass on young peach roots is suspected.

Adsorption and desorption of metribuzin and metolachlor were studied for 0- to 15- and 30- to 45-cm soil depths and at 5 and 28 C temperatures for two subarctic Alaskan agricultural soils. Surface soils had five to eight times the organic carbon content of deeper soils and had lower Freundlich isotherm slopes (1/n) for both herbicides. Surface soil Freundlich coefficients (Kf) were affected by both soil type and equilibration temperature, with soil type accounting for greater than 80% of the variation in Kf. Surface soil mean Kf values ranged from 1.5 to 2.4 for metribuzin and 4.4 to 9.2 for metolachlor. For soils from the 30- to 45-cm depth, neither soil type nor temperature affected Kf. Isotherm slopes for desorption were less than adsorption, indicating hysteresis. Regressions between desorption Kf and maximum herbicide adsorbed prior to desorption were highly significant with coefficients of determination (r2) between 0.50 and 0.99.