The relative competitiveness of triazine-resistant (TR) and -susceptible (TS) common waterhemp, as a component of fitness, was assessed using TR and TS biotypes collected from the same field in Fillmore County, Nebraska. Competition between biotypes was evaluated under greenhouse conditions using a set of replacement series experiments with five ratios of TR:TS (1:0, 3:1, 1:1, 1:3, and 0:4) and four plant densities (50, 100, 150, and 300 plants m−2). No difference in final individual plant weight between TR and TS plants was observed in the absence of interbiotypic competition at all plant densities examined. Interbiotypic competition was indicated by ANOVA of plant weight and modified relative crowding coefficients (RCC) for plant weight, height, and volume. Replacement series diagrams showed TS plants were more competitive than TR plants, as measured by plant weight at densities of 50, 100, and 150 plants m−2. TS and TR plants were equally competitive at 300 plants m−2. Individual TS plant weight for 50, 100, and 150 plants m−2 was greatest at the 3:1 ratio (TR:TS) and decreased as the proportion of susceptible plants increased. Individual TR plant weight for 50, 100, and 150 plants m−2 was greatest at the 1:0 ratio and decreased as the TR proportion decreased. Lack of interbiotypic competition at 300 plants m−2 and decreasing TS plant weight with increasing ratios suggest intrabiotypic competition. RCC values for volume and height were calculated weekly during the experiment and indicated the competitive advantage of TS plants was constant from transplanting through harvest.

Reduced effectiveness of carbamothioate (thiocarbamate) herbicides in certain soils has been attributed to rapid herbicide degradation by soil microorganisms. Studies were conducted to determine if greater populations of EPTC (S-ethyl dipropyl carbamothioate)-degrading microorganisms were responsible for increased rates of degradation observed following repeated applications of EPTC to a Grenada silt loam soil. EPTC-degrading microorganism populations, measured with a 14C-MPN (most-probable-number) technique, were not larger in soils with accelerated rates of EPTC degradation, and degrader populations did not increase after application of 6 mg EPTC/kg of soil. Degrader populations increased after application of 60 mg EPTC/kg of soil only in soil previously treated for 6 yr with EPTC. Increased rates of metabolism of EPTC were apparently responsible for the increased rates of degradation, rather than increased populations of degraders.

Freshly harvested Florida pusley (Richardia scabra L.) seeds do not germinate in continuous dark, but do germinate if exposed to more than 2 hr of light each day. Increasing periods of illumination up to 16 hr per day increases the germination percentages. Scarification increases the rate of germination in the presence of light. Scarification also causes germination of seeds stored for 8 months or longer in total darkness. The seeds fail to germinate at constant temperatures of 15 C or less and at 40 C. Almost complete germination occurs at a constant temperature of 30 C, or at alternating temperatures of 20 and 30 C. The seeds germinate equally well in the pH range of 3 to 8. Increasing depths of planting reduces percent emergence, and none of the seedlings emerge from a depth of 1.5 cm or more. Air-dried seeds can be stored either at 5 or at 25 C without losing viability, at least up to a period of 1 year after harvest. Moist storage of seeds at 5 C reduces germinability.

Enzymatically isolated leaf cells from cotton (Gossypium hirsutum L. ‘Stoneville’) and soybean (Glycine max (L) Merr. ‘Kino’) were used to study the effect of SAN 6706 [4-chloro-5-(dimethylamino)-2-(α,α,α-trifluoro-m-tolyl)-3(2H)-pyridazinone] on photosynthesis, protein, RNA, and lipid synthesis during the first 19 hr of herbicidal treatment. Cotton plants are tolerant to SAN 6706, whereas soybean plants are susceptible. SAN 6706 inhibited the incorporation of 14C-bi-carbonate, uridine, and acetate into cotton leaf cells but stimulated incorporation of leucine. Incorporation of all of these precursors into soybean cells was inhibited. Cotton and soybean cells each took up equivalent amounts of labeled SAN 6706. The incorporation activity of the cotton and soybean cells appeared to regulate the entrance of the precursors into the cells by depleting the intracellular precursor pools. It is suggested that the inhibition of RNA and lipid synthesis in cotton and soybean cells is an indirect result of inhibited photosynthesis. Isolated cotton cells were not tolerant of the herbicide.

Field experiments established that tetraploid soybean [Glycine max (L.) Merr.] plants were relatively tolerant while diploid plants were highly susceptible to metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] applied both preemergence and postemergence. Sensitivity of diploids and tolerance of tetraploids was also found in growth chamber experiments. Autoradiographs prepared 4 days after 14C-metribuzin application showed a high level of 14C-translocation to all parts of diploid plants including meristems. In contrast, only low levels of 14C were translocated in tetraploid plants, with no 14C-movement into meristems. Tetraploid plants rapidly transformed absorbed metribuzin to nontoxic products. Diploids were incapable of inactivating absorbed metribuzin at a rate sufficient to prevent injury. Differences in absorption, translocation, and metabolism of metribuzin appeared to be the main factors in the diploid and tetraploid differential response in field and growth chamber experiments. Differences in the rate of metribuzin metabolism appeared to be the factor responsible for the differential response in diploid and tetraploid cell suspension cultures. The primary polar metabolites were conjugates of metribuzin and deaminated metribuzin [6-tert-butyl-3-(methylthio)-1,2,4-triazin-5(4H)-one] with a ratio of 8:1, respectively.

Quackgrass (Agropyron repens (L.) Beauv.) was defoliated for 25 to 30 days with applications of 1.0 lb/A of l,l′-dimethyl-4–4′-bipyridinium salt (paraquat). Applications were more effective made prior to darkness than at midday. Phytotoxicity was increased by adding surfactants in the field, and experiments utilizing 14C-paraquat under controlled environmental conditions indicated that the surfactant X-77 increased foliar absorption by 20%. Quackgrass plants grown in nutrient solutions exhibited chronic paraquat toxicity which increased as the nitrogen levels increased. Paraquat also inhibited the growth of lateral buds on quackgrass rhizome segments cultured in the dark. Greater movement of 14C-paraquat in quackgrass leaves occurred under 6 hr of light as compared to 6 hr of darkness following application. However, a 6-hr dark period after application prior to the light period further enhanced the movement from the treated leaf.

Lithium salt of (2,4-dichlorophenoxy)acetic acid (2,4-D) (1000 ppm) was sprayed on plants of curly dock (Rumex crispus L.) 12 days before anthesis, at an thesis, 7 days after anthesis, and 34 days after anthesis. Treatment 12 days before anthesis completely prevented the formation of viable seeds. Two percent of the seeds had minute embryos after treatment at anthesis, but even these seeds did not germinate. Embryos were present in 91% of the seeds from plants treated 7 days after anthesis. These embryos ranged in size from 0.5 to 2.5 mm. The total weight of seeds obtained was significantly less than the control and only 5 to 15% of these seeds were capable of germination. Spraying the inflorescences 34 days after anthesis did not affect the number of seeds produced or seed viability. Almost all plants remained alive after the harvest.

The effect of herbicides applied to V3 soybeans on race 3 soybean cyst nematode (SCN) reproduction and glyceollin production in roots was measured. Soybeans were treated postemergence with 1 × and 2 × rates of herbicides plus adjuvants or with adjuvants alone, and SCN development was measured. Acifluorfen, bentazon, lactofen, crop oil concentrate (COC), and nonionic surfactant (NIS) applications reduced SCN egg population densities 50 to 60% compared with the untreated control 4 and 8 wk after application. The SCN reproduction on plants treated with fluazifop-P, sethoxydim, and imazethapyr was similar to the untreated control. Crop oil concentrate or NIS applications alone were as effective as acifluorfen, bentazon, or lactofen applications for reducing SCN reproduction. However, no additive effect of adjuvant-herbicide combinations was observed, nor did herbicide rate affect SCN reproduction. Treatments reduced SCN reproduction only when applied to soybeans and had no effect on SCN reproduction when applied directly to the soil. No treatment stimulated SCN reproduction relative to the untreated control. Soybeans treated with COC, NIS, acifluorfen, and bentazon also had more glyceollin detected than the untreated control. Herbicide-induced glyceollin production may have increased the resistance of soybean to SCN.

Four experiments were conducted in central Iowa during 2001 and 2002 to determine the effects of weed emergence time and corn row spacing on common waterhemp growth and fecundity. Four common waterhemp emergence cohorts were established in each experiment and corresponded to the VE, V3, V5, and V8 stages of corn grown in rows spaced 38 and 76 cm apart. Common waterhemp mortality averaged 20, 56, 97, and 99% for the first, second, third, and fourth cohorts, respectively. Mean mature common waterhemp height for the first cohort was 140 cm, whereas plants emerging at the V8 corn stage were only 5 cm tall. Biomass of the first cohort was 20% less in 38-cm rows than in 76-cm rows, but later cohorts were not affected by row spacing. Biomass and seed production of waterhemp emerging at the V3, V5, and V8 corn stages decreased 80, 97, and 99%, respectively in comparison with the first cohort. Overall results indicate that common waterhemp biomass, survival, and fecundity decline sharply with delayed emergence relative to corn, but weeds emerging at or before the V5 corn stage may still contribute significantly to the seed bank.

Field, greenhouse, and laboratory experiments were conducted to investigate resistance to glyphosate in tropical sprangletop biotypes (Lv8 and Lv9) collected in Persian lime from Veracruz, Mexico. Assays to determine the dose required to reduce seedling fresh weight by 50% indicated a resistance factor (RF) of 4.9 and 3.2 for biotypes Lv8 and Lv9, respectively; whereas the LD50 showed a RF of 4.4 and 3.3 for biotypes Lv8 and Lv9, respectively. On the other hand, the RFs using whole plant dose–response assays were lower (RF of 3 for Lv8 and 2.3 for Lv9). The susceptible biotype (LvS) accumulated 5.5 and 11.8 times more shikimate than biotypes Lv8 and Lv9, respectively, at 96 h after treatment (HAT). In field experiments, alternatives to glyphosate-resistant tropical sprangletop management were identified. Indaziflam + glufosinate and paraquat + diuron provided over 80% control of in-field populations of tropical sprangletop at 60 d after treatment (DAT). These results confirmed the first reported case of glyphosate-resistant tropical sprangletop.

In spring 2005, a downy brome population with possible resistance to fluazifop-P, an acetyl-CoA carboxylase (ACCase) inhibitor (group 1) herbicide was found in a commercial creeping red fescue seed production field, near La Grande, OR, where fluazifop-P had been used to control downy brome repeatedly over 7 yr. Greenhouse experiments were conducted to confirm resistance to a number of group 1 herbicides. The suspected resistant downy brome accession was tested for resistance to fluazifop-P and tested for cross-resistance to other aryloxyphenoxy propionate (APP) and cyclohexanedione (CHD) herbicides, including quizalofop-P, sethoxydim, and clethodim. Data recorded included plant-mortality counts and biomass. Tests revealed that the La Grande downy brome accession was highly resistant to fluazifop-P and sethoxydim at all tested rates. The La Grande accession suffered no mortality from fluazifop-P or sethoxydim treatments up to the maximum tested rate of eight times (8×) the labeled recommendation. The La Grande accession was resistant to quizalofop-P and clethodim at the labeled rate or less but was susceptible to application rates higher than the labeled rate. The control downy brome accession was completely susceptible to fluazifop-P, quizalofop-P, and clethodim at all rates and exhibited increasing susceptibility with increasing sethoxydim rate. This pattern of cross-resistance differs from that of a previously reported case of ACCase resistance in downy brome.

Several investigations were undertaken to study the metabolism of alpha,alpha,alpha-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) by intact peanut (Arachis hypogaea L.) and sweet potato (Ipomoea batatas L.) plants and crude extracts of these plants. Attempts also were made to determine the biological activity of three isolated degradative intermediates. Accumulated data indicated that trifluralin was degraded by both intact and crude extracts of these plants; the degradation pathway differing in each instance. In crude extracts of peanuts, the initial degradation product was an incomplete dealkylated derivative of the parent compound, alpha, alpha, alpha-trifluro-2,6-dinitro-N-propyl-p-toluidine. The initial degradation product formed in sweet potato crude extracts was a reduced derivative of trifluralin, N2,N2-di-n-propyl-3-nitro-5-trifluromethyl-o-phenylenediamine. All isolated degradation intermediates used in bioassay studies demonstrated root inhibition in cucumber (Cucumis sativus L.) at many of the concentration levels utilized. The greatest amount of root inhibition was observed in cucumber seeds germinated in alpha,alpha,alpha-trifluro-2,6-dinitro-N-(n-propyl)-p-toluidine.

Experiments were conducted to determine the influence of soybean row width and glyphosate application timing on survival, biomass production, and fecundity of three sicklepod cohorts along with soybean seed yield. The first cohort comprised sicklepod plants that emerged from soybean planting through the V3 stage of soybean (two fully developed trifoliate leaves plus the unifoliate leaves; cohort 1). The second cohort comprised plants that emerged between the V3 to V6 stages of soybean (five fully developed trifoliate leaves plus the unifoliate leaves; cohort 2), and the third cohort emerged after the V6 stage through the R2 stage of soybean (full bloom; cohort 3). Glyphosate was applied at V3; V6; V3 and V6; and V3, V6, and R2 in rows 19 and 97 cm wide, and a nontreated control was included for comparison in each row width. Survival of cohort 1 in 2004 in glyphosate-treated plots occurred only after a single glyphosate application at V3 or V6 in wide rows. Narrowing the soybean row width reduced sicklepod survival throughout the growing season, even without glyphosate. Total biomass production from all cohorts averaged over years was 1,602 g m−2 in wide rows compared with 648 g m−2 in narrow rows. Cohort 1 accounted for 70 and 77% of the total sicklepod biomass in wide and narrow rows, respectively. Cohort 2 contributed 29% of the total sicklepod biomass in wide rows and 22% in narrow rows. Cohort 3 produced minimal biomass, contributing no more than 1% of the total sicklepod biomass. Sicklepod emerging after V6 failed to produce seed in 2004, and no sicklepod seed were produced in 2005 by plants emerging after V3. Averaged over years, sicklepod from cohort 1 in nontreated controls produced 3,695 seed m−2 in narrow rows compared with 6,685 seed m−2 in wide rows. Nontreated sicklepod from cohort 2 in 2004 produced 510 seed m−2 in narrow rows compared with 1,640 seed m−2 in wide rows. Soybean yields were similar among all glyphosate applications averaged over years and row widths, ranging from 3,340 to 3,700 kg ha−1 compared with 1,290 kg ha−1 without glyphosate (61 to 65% yield loss).

Laboratory studies were conducted in 2006 and 2007 to evaluate the effects of temperature and propanil on alligatorweed control with penoxsulam. Biomass reduction of alligatorweed at 42 d after treatment (DAT) compared to nontreated was greatest at 21/11 C (day/night) compared to 26/18 C or 30/25 C for all treatments. Propanil plus penoxsulam reduced biomass less than penoxsulam applied alone, independent of temperature. At 21 and 27 C, delaying propanil application 3 d after penoxsulam provided similar biomass reduction to penoxsulam applied alone. At 27 C and 30 C, delaying propanil application 10 d after treatment was required to achieve biomass reduction greater or equal to penoxsulam applied alone. Absorption and translocation of 14C penoxsulam indicated that propanil reduced absorption of penoxsulam into the treated leaf of alligatorweed 48 h after treatment. This research demonstrates the potential for propanil to antagonize penoxsulam when applied to alligatorweed. Under the most severe antagonistic conditions (30 C) propanil applications following penoxsulam needed to be delayed 10 d to avoid antagonism.

There was no antagonism between dalapon (2,2-dichloropropionic acid) or TCA (trichloroacetic acid) and BAS 9052 {2 - [1 -(ethoxyimino)butyl] - 5 - [2 -(ethylthio) -propyl] -3-hydroxy-2-cyclohexen-1-one} at 0.3 kg/ha for control of volunteer barley (Hordeum vulgare L. ‘Bonanza’) and wheat (Triticum aestivum L. ‘Neepawa’) in flax (Linum usitatissimum L. ‘Dufferin’). However, MCPA {[(4-chloro-o - tolyl)oxy] acetic acid} amine at 0.2 to 0.6 kg/ha had an antagonistic effect on BAS 9052, especially at 0.1 kg/ha, for control of wild oat (Avena fatua L.). BAS 9052 at 0.3 kg/ha overcame this antagonistic effect of MCPA. Field results confirmed that wild oat control with BAS 9052 at 0.3 kg/ha was not reduced by the mixtures with each of four broadleaf weed herbicides at 0.6 kg/ha. The mixtures gave excellent control of grass and broadleaf weeds. Flax seed yields were five times that of the unsprayed check and were equal to the yield of the handweeded treatment.

Field experiments were conducted at four locations to evaluate the efficacy of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one], simazine [2-chloro-4,6-bis(ethylamino)-s-triazine], buthidazole {3[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-4-hydroxy-1-methyl-2-imidazolidinone} and 2,4-DB [4-(2,4-dichlorophenoxy) butryic acid] treatments for common dandelion (Taraxacum officinale Weber) control in established alfalfa (Medicago sativa L.). Most herbicides temporarily reduced common dandelion populations at all locations, but by the end of the first year only buthidazole reduced them on fine-textured soils, but buthidazole, metribuzin and fall-applied simazine reduced populations on coarse-textured soils. Only buthidazole controlled common dandelion in the year following application at all locations. Control of common dandelion did not consistently increase total forage yield or quality or increase alfalfa yield and stands compared to the untreated check. Buthidazole reduced alfalfa and total forage yield on coarse-textured soils.

Four herbicides were applied in the spring and fall over a 3-yr period to Kentucky bluegrass [Poa pratensis (L.) ‘Common′] in the Mountain Region and bermudagrass [Cynodon dactylon (L.) Pers. ‘Common′] in the Piedmont Region of Georgia at different frequencies of treatments for summer and winter weed control. Large crabgrass [Digitaria sanguinalis (L.) Scop.] was controlled throughout the 3-yr period from spring treatments of bensulide [O,O-diisopropyl phosphorodithioate S-ester with N-(2-mercaptoethyl)benzenesulfonamide] at 11.2 kg/ha and oxadiazon [2-tert-butyl-4-(2,4-dichloro-5-isopropoxyphenyl)-δ2-1,3,4-oxadiazolin-5-one] at 4.5 kg/ha in the first year followed by 5.6 kg/ha for bensulide and 2.3 kg/ha for oxadiazon the following 2 yr. To control large crabgrass with benefin (N- butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine) it was necessary to apply 3.4 kg/ha for 2 consecutive yr before rates could be reduced to 1.7 kg/ha. Oxadiazon was the only herbicide that controlled goosegrass [Eleusine indica (L.) Gaertn.] completely. Control was excellent throughout the 3-yr period when oxadiazon was applied at full rate in the spring of the first year with no additional treatments during the following 2 yr. Optimum control of winter weeds was obtained throughout the 3-yr period when herbicides were applied at full rates for the spring and fall treatments the first year followed by one-half rates at similar dates the following 2 yr. Bensulide treatments increased cover of corn speedwell (Veronica arvensis L.) and hop clover (Trifolium agrarium L.); DCPA (dimethyl tetrachloroterephthalate) and benefin increased spur weed (Soliva spp.) and wild parsnip (Pastinaca sativa L.); and oxadiazon increased wild parsnip and thymeleaf sandwort (Arenaria serpyllifolia L.).

Exotic annual weeds on western U.S. rangelands owe at least part of their invasion success to prolific seed production. Reproductive output is determined largely by timing of flowering in relation to soil moisture, light, and competing vegetation. This study investigated the influence of temperature (including vernalization), photoperiod, and intraspecific competition on reproductive phenology in three populations (two varieties, typica and bracbypappa) of common crupina. Monthly plantings at four densities were grown in field plots through two consecutive cycles, and the date each plant reached bud and flowering stage was recorded. Development of greenhouse-grown plants from the same populations was compared with those grown outside for 4, 6, 8, and 10 wk of cold induction. Reproductive development in common crupina was influenced by all factors investigated: temperature, photoperiod, density, and vernalization. Conditions fostering the shortest thermal time from emergence to floral induction were a period of cold temperatures/short photoperiod as seedlings followed by warming temperatures and lengthening photoperiod. Both varieties of common crupina responded quantitatively to vernalization, but lack of vernalization caused a greater delay of flowering in var. brachypappa than in var. typica. Vernalization contributed to convergence of spring flowering among the August to December plantings, but earlier emergence and lower plant density greatly increased final plant size and fecundity. Lack of vernalization, high temperatures, and shortening day length delayed flowering of spring/summer emergents, increasing the probability of succumbing to drought before producing seed.

Weeds are often portrayed as growing in resource-rich environments. However, weeds growing within crops often deal with variable nitrogen (N) availability and reduced levels of light quantity and quality as a result of the crop canopy. In order to explore how weeds adapt to such stressful growing conditions, growth-cabinet studies were conducted using common lambsquarters as a model weed to determine how light, defined in terms of photosynthetic photon flux density (PPFD) and quality (red to far-red light ratio [R/FR]), and N stress influence the expression of adaptive traits that contribute to survival. Development rate of common lambsquarters was not influenced by low N; however, low N in addition to low R/FR delayed the rate of leaf appearance. Main-stem leaf number was reduced by low PPFD but was insensitive to N and R/FR. Neither doses of N had any influence on the shoot-to-root ratio. Plants also responded to the interaction of light and N. Under low PPFD and high N, plants adapted by growing taller, increasing biomass allocation to leaves, and producing more leaf area per mol of accumulated incident PPFD. Plants adapted to the most stressful treatment combination of low PPFD and low N by producing thinner leaves and increasing inflorescences per mol of accumulated incident PPFD. Seed production was reduced under low PPFD, but 1,000-seed weight and carbon concentration was unaffected. Although reduced in number, the total N concentration of the seed increased under low PPFD treatments, especially under low N. The adaptive traits identified in this study provide a greater understanding of the survival and persistence of common lambsquarters.

Geostatistical techniques were used to describe and map the spatial distribution of crenate broomrape populations parasitizing broad bean over 6 yr (from 1985 to 1990). In the first year, the spatial distribution was random, but from 1986 to 1989, crenate broomrape populations were clearly aggregated. The crenate broomrape infection severity (IS: number of emerged broomrape m−2) increased every year, from an average of 0.45 in 1985 to 29.4 in 1989, with a slight decrease the following year (IS = 27.4). Spherical functions provided the best fit because the cross-validation criteria were accomplished in all study cases. Kriged estimates were used to draw contour maps of the populations. About 34.3, 43.3, and 74.3% of the field plot surface exhibited an IS ≥ 1 (economic threshold) in 1985, 1986, and 1987, respectively, and nearly 100% of the area exceeded the economic threshold from 1988 to 1990; 1985 and 1986 were key years for control of the parasitic weed population. The percentage of infested area at different IS intervals in each year's map obtained by kriging was used to estimate the percentage of yield losses in each infested area (YA) with the equation: YA = A ∗ Ymax ∗ (1 − IS ∗ 0.124), where A is the infested area at a given IS interval and Ymax is the expected broomrape-free broad bean yield. Yield losses under different IS intervals were compared with yield loss attributable to a uniform distribution of crenate broomrape. Results showed that yield loss assuming a uniform distribution of crenate broomrape was clearly overestimated, which is important to avoid overuse of herbicides.