At both 15 and 25 C, following a 24-h root absorption period, absorbed 14C-metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] was metabolized approximately 30% more rapidly to water-soluble and terminal fiber metabolites by winter wheat (Triticum aestivum L.) than by downy brome (Bromus tectorum L. # BROTE). Both species metabolized a greater proportion of metribuzin in leaf sheaths and roots than in the leaf blades. This was attributed to the increased incorporation of metribuzin into fiber. After an initial leaf extraction, metribuzin and the metabolites deaminated metribuzin (DA), deaminated diketo metribuzin (DADK), and diketo metribuzin (DK) partitioned into a chloroform fraction and five unidentified water-soluble metabolites into an aqueous fraction. At both 15 and 25 C, downy brome absorbed approximately three times more metribuzin per fresh weight than did winter wheat. The mechanism of differential tolerance of downy brome and winter wheat to metribuzin was attributed to the ability of winter wheat to metabolize metribuzin more rapidly and absorb less metribuzin than downy brome.

The effect of herbicides on the concentration of miserotoxin, the toxic compound in Wasatch milkvetch [Astragalus miser var. oblongifolius (Rydb.) Cronq. # ASAMO], was investigated. Eight herbicides or herbicide combinations were applied to Wasatch milkvetch in June 1985 and 1986. The concentration of miserotoxin was determined weekly for 4 weeks after treatments. All treatments in both years significantly reduced the miserotoxin concentration when compared to the controls, except for the 0.6 kg ae/ha rate of clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) in 1986. Wasatch milkvetch treated with triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid} at 1.7 and 2.2 kg ae/ha desiccated rapidly and had consistently low concentrations of miserotoxin 4 weeks after treatment.

Phytotoxicity, absorption, translocation, and metabolism of root-applied norflurazon [4-chloro-5-(methylamino)-2-(3-(trifluoromethyl)phenyl)-3(2H)-pyridazinone] in tolerant narrowleaf and susceptible broadleaf guineagrass (Panicum maximum Jacq. # PANMA) biotypes were examined under greenhouse and laboratory conditions. Dose response studies conducted in the greenhouse showed greater injury to the shoots of broadleaf than narrowleaf biotypes. 14C-norflurazon absorbed was 0.3 and 0.5% and 1.6 and 2.4% of that applied, 3 and 7 days after application for narrowleaf and broadleaf biotypes, respectively. Of the total radioactivity absorbed after 3 and 7 days, 70 and 76% and 57 and 61% were translocated to the shoots of susceptible and tolerant biotypes, respectively. Gas chromatographic analyses revealed that broadleaf shoots had 3.08 μg/g dry wt of the parent compound while its tolerant counterpart had 0.40 μg/g dry wt, 7 days after application. The toxic parent compound was metabolized more rapidly to its non-phytotoxic desmethyl analog by narrowleaf biotypes, thereby preventing its accumulation in the shoots. Differences in absorption, translocation, and metabolism were responsible for the differential response of the two biotypes of guineagrass.

Various surgical cuts were used to induce germination of dormant barnyardgrass [Echinochloa crus-galli (L.) Beauv. # ECHCG] caryopses. Greater percentages of germination were obtained when the cuts were made closer to the embryo. However, removal of only the caryopsis coat covering the coleoptile or midembryo axis induced up to 10% germination compared to 70% germination for caryopses with the coat covering the radicle removed. Dormancy of barnyardgrass was not caused by the inability of the embryo to imbibe, imposed mechanical resistance of the caryopsis coat, or presence of an inhibitor. A wounding response, other than ethylene production, by the cut caryopses was possibly responsible for stimulating germination. Similar responses to cutting observed in afterripened caryopses were consistent with the suggested hypothesis.

A quantitative assay using 14C-chloramben (3-amino-2,5-dichlorobenzoic acid) revealed that velvetleaf (Abutilon theophrasti Medic. # ABUTH) leaves absorbed a greater amount of chloramben formulated as an acid than as a sodium salt. A surfactant increased 14C-chloramben absorption 5 days, but not 1 day, after application. Translocation of 14C-label out of treated leaves was greater in velvetleaf than in soybean [Glycine max (L.) Merr.]. Differences in the amount of 14C-label as unmetabolized chloramben in ethyl acetate extracts of treated leaves were not detected between velvetleaf and soybean. The amount of unmetabolized chloramben was greater in the shoot above the treated leaf in velvetleaf (58% of total label recovered) than in soybean (35% of recovered label). The amount of recovered 14C-label in unextractable form was greater in treated leaves of soybean compared to velvetleaf.

A comparison of the structure of the α-methylbenzyl analogue (MBAT) of atrazine with the Photosystem II herbicides atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine] and diuron [N′-(3,4-dichlorophenyl)-N,N-dimethylurea] suggested that N-alkylbenzyltriazines may be a structural bridge between the triazines and the phenylureas. In the phenylureas, the addition of chlorines at the meta and/or para positions produces a marked increase in activity. Chloro-substituted derivatives of MBAT were synthesized to determine whether this structure-activity relationship also applies to the alkylaryltriazines. Addition of a chlorine to MBAT at the 4-position (CMBAT) caused a substantial increase in intrinsic activity, and a second chlorine at the 3-position (DCMBAT) caused a further increase. In direct comparisons, DCMBAT was more active in vitro than terbuthylazine [6-chloro-N-ethyl-N′-(1,1-dimethylethyl)-1,3,5-triazine-2,4-diamine], the most active chlorotriazine, and was also more active than diruon. The effects of DCMBAT were also measured on triazine-resistant pigweed (Amaranthus hybridus L. # AMACH) both in vivo and in vitro. The activity of this compound in triazine-resistant chloroplasts was intermediate between that of atrazine and diruon both in inhibition of photosynthetic electron transport and in competition for diuron binding sites, with half-maximal values falling in the micromolar range. Whole plant phytotoxicity of DCMBAT on triazine-resistant pigweed was also intermediate between that of diuron and atrazine. Since DCMBAT is a triazine with biological properties similar to that of a urea, we conclude that in a functional as well as structural sense, DCMBAT is a herbicide that is a hybrid between a triazine and a urea.

The mode of action of two experimental pyridinyloxyphenoxy propionate herbicides, haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy] phenoxy] propanoic acid} and CGA-82725 {2-propynyl-2-[4-[(3,5-dichloro-2-pyridinyl)oxy] phenoxy] propanoic acid} was studied. Oat (Avena sativa L.) roots showed a rate-dependent sensitivity to both herbicides. Consequently, a series of assays was performed to study cell division, cell cycle dynamics, and nucleic acid and protein synthesis in oat root tips in order to characterize growth inhibition. Both herbicides inhibited cell division, apparently by inhibiting protein synthesis in the G2 stage of interphase. Supporting evidence for this assumption comes from the following observations: a) There was almost complete absence of leucine incorporation within 1 h of exposure to 2.7 × 10−4 M of either herbicide; b) the number of 3H-labeled dividing cells was reduced to about 20% of the control within 4 h, and essentially to zero within 8 h, after exposure to 2.7 × 10−4 M of herbicide; c) the number of unlabeled cells was reduced to less than 10% of the control within 8 h of exposure to 2.7 × 10−4 M of herbicide; d) during this 8-h period, the number of 3H-labeled cells in interphase which had been exposed to herbicide greatly exceeded the control; e) thymidine incorporation into DNA was inhibited but was not proportional to reduction in cell division; f) uridine incorporation was enhanced during the first 8 h of exposure before a reduction was evident at 12 h; and g) an oat coleoptile elongation bioassay showed that cell enlargement was not inhibited at herbicide concentrations less than 2.7 × 10−4 M.

Immediately following postemergence application, similar quantities of metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] were retained on the foliage of downy brome (Bromus tectorum L. # BROTE) and winter wheat (Triticum aestivum L.). Three days following treatment, more metribuzin was present on the surface of the foliage of downy brome than of winter wheat and similar quantities of metribuzin had been absorbed into the interior of the foliage of both species. Fourteen days after treatment, metribuzin had disappeared from the surfaces of the foliage of both species but more metribuzin was present in the interior of the foliage of downy brome than of winter wheat. Results from petri dish studies in the field suggested that the amount of unaltered metribuzin available for absorption by leaves was influenced primarily by volatilization and secondarily by photodecomposition. In addition, small quantities of rainfall may have washed metribuzin from leaf surfaces. In 1984, yield of winter wheat was greater in metribuzin-treated plots than in nontreated check plots.

Duration and density experiments were conducted in the field to measure horsenettle (Solanum carolinense L. # SOLCA) interference with Spanish and runner-type peanuts (Arachis hypogaea L. ‘Pronto’ and ‘Florunner′). Spanish peanut yield generally was higher with 6 to 8 weeks of weed-free maintenance. Horsenettle interference for 6 to 8 weeks did not decrease the yield of Spanish peanuts, and interference for 6 weeks did not decrease yields of runner peanuts. Weed-free maintenance for 2 or more weeks allowed increased runner peanut yield when compared to weedy plots. Linear regression predicted a 69 kg/ha Spanish peanut yield increase for each week of weed-free maintenance. Linear regression predicted a Spanish peanut yield reduction of 40 kg/ha for each week of weed interference in 1983, the only year in which the slopes of the regressions were statistically significant Curvilinear equations with the runner-type cultivar predicted an 81 kg/ha yield increase or 96 kg/ha decrease for each week of weed-free maintenance or weed interference, respectively. In 1 of 2 yr, Spanish peanut yield was reduced by horsenettle at a density of 32 plants/10 m of row.

Field experiments were conducted during 1981 and 1982 in Ontario, Canada, on the effects of quackgrass [Agropyron repens (L.) Beauv. # AGRRE] interference in soybean [Glycine max (L.) Merr.] and the usefulness of infrared thermometry in predicting critical periods of weed interference. Soybean seed yield, dry weight, number of leaves, height, and number of pods were substantially reduced due to quackgrass interference. High levels of P and K fertility did not overcome the quackgrass interference. Part of the competitive effects of quackgrass was alleviated by irrigation. Infrared thermometry successfully detected the first occurrence of quackgrass-induced stress during the early soybean flowering stage, when the quackgrass was in the four-leaf gtowth stage. This coincided with the onset of the first significant soybean yield loss. No additional soybean yield loss occurred after quackgrass reached the five-leaf growth stage. There was an inverse relation between accumulated stress degree days and soybean yield reductions due to quackgrass interference. The use of the stress degree day concept may be a valuable tool in predicting soybean yield losses due to quackgrass interference.

Density of seed of three species of locoweed, salt milkvetch [Astragalus lentiginosus var. salinas (Howell) Barneby], Lambert locoweed (Oxytropis lambertii Pursh. # OXRLA), and white locoweed (O. sericea Nutt ex. T.&G. # OXRMA) in the soil was determined at four locations in New Mexico, Wyoming, and Utah. Seed density ranged from 50 to 2820/m2. Greater than 77% of the seed were viable as determined by tetrazolium tests. Longevity of white locoweed seed was determined by sewing seed into nylon mesh packets and returning them to the site of origin in northwestern Utah. Greater than 50% of seed buried 1 cm deep in the soil were still present after 6 yr, but only 33% of seed placed on the surface remained. Germinability of the buried seed was less than 5%, but an average of 14% of seed placed on the soil surface germinated each year. Viability of retrieved seed ranged from 89 to 99%.

Sorghum-almum (Sorghum almum Parod. # SORAL) is a weakly perennial, rhizomatous grass that in northern climates reproduces mainly by seeds. Environmental factors influencing seed germination and the longevity of buried seeds were studied in growth chamber and field experiments. Alternating temperatures of 20 C for 16 h and 30 C for 8 h were optimum for germination. Light was not critical to germination. Germination varied only slightly across a pH range from 5.0 to 8.8 but decreased sharply below pH 5. Osmotic potentials greater than −0.9 MPa reduced germination substantially, and no germination occurred at −1.5 MPa. Limited germination occurred at NaCl concentrations of 340 mM, and no germination occurred at 510 mM. Seeds that were either mechanically scarified or afterripened for 6 months had a higher percent germination and germination rate than untreated seeds. Sorghum-almum seedling emergence was 11% from seeding depths of 20 cm, but emergence was 67% or higher from seeding depths of 15 cm or less. In field studies, viability of seeds placed on the soil surface or buried 3 or 8 cm was 2% or less at 12 months and 0% at 18 months after burial. Viability of seeds buried 15 and 23 cm was 0.3 and 26%, respectively, at 24 months after burial. Thus, tillage that buried sorghum-almum seeds 20 cm or more could reduce emergence but would also prolong seed persistence. Shallow tillage should favor depletion of sorghum-almum seed reserves.

Effects of ethylene, gibberellins, kinetin, nitrate, light, and temperature, each alone or combined with other factors, were studied on the germination of dormant seed of field pennycress (Thlaspi arvense L. # THLAR). Treated seed were incubated at a range of constant temperatures, at temperatures alternating daily between 12 and 22 C (12 h each), or at 12 C followed by transfer to 22 C. No germination ocurrred in water under any conditions. When applied in the dark KNO3 had no effect on germination under any of the temperature regimes. A combination of KNO3, light, and a shift in temperature from 12 to 22 C induced 100% germination. Four days of incubation at 12 C induced maximal seed sensitivity towards KNO3 + light, and the sensitivity remained unchanged if the incubation at 12 C was extended to 15 days. Upon transfer to 22 C, the effect of low-temperature incubation was lost within 3 days. Of the two seed lots studied, one had a relatively lower chilling requirement, but both had similar requirements for light and KNO3. Gibberellic acids4+7 (GA4+7) induced 100% germination in the dark or light at all temperature regimes. Gibberellic acid3 (GA3) was promotive under isothermal conditions, but its effectiveness was enhanced by a shift in temperature from 12 to 22 C. Kinetin or ethephon, each applied alone, had little effect on germination. Ethephon and GA3 together induced 100% germination.

Full-season interference from unicorn-plant [Proboscidea louisianica (Mill.) Thell. # PROLO] at densities ranging from 0 to 32 weeds/10 m row was measured on weed and cotton (Gossypium hirsutum L. ‘Westburn M′) parameters in three Oklahoma environments. Linear regression models using log10 transformations were superior to linear and comparable to quadratic models in estimating the impact of inter- and intraspecific competition. Weed measurements on an individual plant basis were not reliable indicators of interference. Weed dry biomass, ground cover, and seed capsule production/plot generally increased with increasing weed densities. In the three environments, densities of two, four, and eight weeds/10 m row initially reduced cotton plant height. Maximum height reductions averaged 20, 28, and 43% in the three environments but did not detrimentally affect the mechanical harvest of cotton. As unicorn-plant density (expressed in log10 units) doubled (within a range of 1 to 32 plants/10 m of row), lint yield reductions ranged from 84 to 146 kg/ha. Maximum lint yield losses averaged 59, 65, and 74% in the three environments. Transforming lint yield/plot to a percentage of that from weed-free plots resulted in a single quadratic equation applicable over environments. Unicorn-plant at the highest weed density reduced fiber fineness, uniformity, and length, but not fiber strength.

Full-season interference of silverleaf nightshade (Solanum elaeagnifolium Cav. # SOLEL) with dryland and irrigated cotton (Gossypium hirsutum L. ‘Paymaster 145′) was evaluated in five field experiments during 1984 and 1985, Weed densities ranged from 0 to 32 plants/10 m of crop row. Dry weight of silverleaf nightshade increased from 0.08 to 0.39 kg/plot for each additional weed/10 m of row. Intraspecific competition among silverleaf nightshade plants was not evident. However, cotton height was reduced at weed densities of 4 plants/10 m of row or more. The densities at which initial lint yield reductions occurred ranged from 4 to 32 weed plants/10 m of row. Irrigated cotton more effectively competed with the weed than did dryland cotton, suggesting that soil water was a primary competition factor between the two species. Boll size was reduced at densities of 2 weeds/10 m of row and above. Silverleaf nightshade reduced mechanical harvest efficiency only at densities of 16 and 32 plants/10 m of row. Fiber properties were not affected. Linear regression predicted that lint yield would be reduced 1.54% for each silverleaf nightshade plant/10 m of cotton row. The distance silverleaf nightshade was established from the crop row did not affect the weed's interference with cotton within a range of 0 to 30 cm.

Wheat (Triticum aestivum L.) grain yields were reduced an average of 4.2% for every 10 Italian ryegrass [Lolium multiflorum (Lam.) # LOLMU] plants/m2 within the range of 0 to 100 Italian ryegrass plants/m2. Yield reductions caused by Italian ryegrass were attributed primarily to decreased crop tillering. Italian ryegrass densities as high as 80 plants/m2 had little effect on wheat head or kernel weights. In greenhouse experiments, the growth response of Italian ryegrass to increasing concentrations of NO3– and K+ was greater than that of wheat. Net uptake rates for NO3– by both species growing in nutrient solution were 1.5 times greater than net uptake rates for K+. Nitrate and potassium Imax values for Italian ryegrass were approximately twice the corresponding values for wheat. Although Italian ryegrass responded more to changes in nutrients and had greater ion uptake rates compared to wheat, Italian ryegrass accumulated more biomass when grown in monoculture than when grown in association with wheat. This difference was probably due to the initial size of the seedlings. Wheat seedlings were much larger than Italian ryegrass seedlings during the first 20 days following emergence.

Imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} was evaluated in the field for weed control in flue-cured tobacco (Nicotiana tabacum L.) during 1983 and 1984. The 75 DG formulation of imazaquin was used at 0.28 and 0.42 kg ai/ha. Methods of application were: preplant incorporated, over the top immediately after transplanting, postbed incorporated, and early postemergence. Imazaquin controlled 90%, or better, of common lambsquarters (Chenopodium album L. # CHEAL), common ragweed (Ambrosia artemisiifolia L. # AMBEL), redroot pigweed (Amaranthus retroflexus L. # AMARE), spiny amaranth (Amaranthus spinosus L. # AMASP), ivyleaf morningglory [Ipomoea hederacea (L.) Jacq. # IPOHE], sicklepod (Cassia obtusifolia L. # CASOB), and prickly sida (Sida spinosa L. # SIDSP) with all application methods except early postemergence. It controlled 80 to 89% of large crabgrass (Digitaria sanguinalis L. # DIGSA) by the soil-applied methods. Postemergence applications of imazaquin controlled common ragweed, pigweed spp., ivyleaf morningglory, prickly sida, and sicklepod. Imazaquin was less effective postemergence on lambsquarters and large crabgrass. Tobacco tolerated use of imazaquin by all methods of application except preplant incorporation.

A 3-yr (1982, 1983, and 1984) study was conducted to determine the relationship between tillage and six weed control treatments for soybean [Glycine max (L.) Merr. ‘Williams 79′] production on silt loam (Udollic and Mollic Ochraqualfs). Conventional tillage consisted of spring moldboard plowing and secondary tillage with a combination tool just before planting. No-tillage consisted of a foliar application of glyphosate [N-(phosphonomethyl)glycine] at the time of planting. Weed control treatments included combinations of no weed control with and without soybean plants, preemergence herbicide application only, postemergence herbicide application only, and combined preemergence and postemergence applications with and without additional handweeding. In 1982, a year of above-normal seasonal rainfall, conventional tillage had significantly greater soybean grain yields than no-tillage for all weed control treatments except the preemergence-only treatment. Yields within tillage systems and between weed control treatments in 1982 were not significantly different because adequate rainfall reduced the effect of weed competition for soil moisture. Soybean seed yields in 1983 and 1984 in no-tillage were equal to or significantly greater than those of conventional tillage. No-tillage treatments had greater soil moisture conservation and soil moisture availability resulting in less plant water stress during podfilling in periods of drought in 1983 and 1984. In all 3 yr, conventional tillage had significantly greater early weed growth than no-tillage in the treatments with and without soybean plants where no preemergence or postemergence herbicides were used. Comparing treatments with and without soybean plants indicated an average increase of 36 and 38% weed control for no-tillage and conventional tillage, respectively.

Picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid) and metsulfuron {2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] amino] sulfonyl] benzoic acid} were applied to broom snakeweed (Gutierrezia sarothrae [Pursh.] Britt. and Rusby # GUESA) on two spring dates and three fall dates from October 1983 to 1985. Applications of picloram and metsulfuron in the fall more effectively controlled broom snakeweed than applications in the spring. Lower herbicide rates were necessary for 90% or greater control on a sandy loam than on a silty clay loam. Grass yield increased significantly the first, second, and third growing seasons when a majority of the broom snakeweed were killed by either herbicide, but was not different from untreated rangeland when less than 50% of the plants were killed. Reduction in broom snakeweed densities of 90% or greater following applications of picloram or metsulfuron resulted in 3- to 5-fold increases in the estimated carrying capacity of rangeland in east-central New Mexico.

Five rice (Oryza sativa L.) cultivars were evaluated in the greenhouse for tolerance to fenoxaprop {(±)-2-[4-[(6-chloro-2-benzoxazolyl)oxy]phenoxy]propanoic acid}. Rice was more tolerant to fenoxaprop when flooded at 7 days after spraying, based on trends in reduction of weight of seedling topgrowth. ‘Lebonnet’ was the only cultivar that responded differently when five cultivars were compared. In the field, rice yield was not adversely affected by fenoxaprop applied at 168 g ai/ha and flooded 1 to 10 days after treatment, but yield increased as the time interval between application of 336 g ai/ha and flooding increased. Yield data suggested that flooding should be delayed at least 5 days when rice is treated with the high rate. The cultivar ‘Newbonnet’ was slightly less tolerant than 'Starbonnet’ in the field.