Previous research revealed that resistance to cloransulam in at least one population of common ragweed was conferred by an altered herbicide target site, specifically, by a tryptophan-to-leucine amino acid substitution at position 574 (W574L) of acetolactate synthase (ALS). In this study, 22 common ragweed populations, several of which were suspected cloransulam resistant, were assayed to determine if the W574L ALS substitution was correlated with resistance to ALS inhibitors. From each population, 16 greenhouse-grown plants were treated with cloransulam, and another 16 were treated with imazamox. Plant dry weights were recorded 20 d after treatment and individual plants were considered resistant if their dry weight exceeded 50% of that of nonherbicide-treated controls. For each herbicide-treated plant, allele-specific primers were used in polymerase chain reactions to determine whether the ALS alleles contained leucine or tryptophan codons at position 574. Of the 352 plants treated with cloransulam, 70 were determined to be resistant, and all but two contained one or more Leu574 alleles. The frequency of imazamox resistance was much higher than that of cloransulam in the populations, with 149 of 352 plants identified as imazamox resistant. However, only about half (80) of the imazamox-resistant plants contained one or more Leu574 alleles. Correlation of imazamox resistance and Leu574 alleles was population dependent. ALS activity assays confirmed that imazamox resistance in plants from at least one population was due to an altered target site, even though plants from that population did not have a W574L substitution. These results lead to the conclusion that a Leu574 allele is the predominant basis for cloransulam resistance in common ragweed; however, other mechanisms of resistance to ALS inhibitors exist in some populations.

Field experiments were conducted to determine the effect of DPX-PE350 on the efficacy of fluazifop-P, sethoxydim, and quizalofop. DPX-PE350 severely antagonized the activity of fluazifop-P on large crabgrass. Antagonism of sethoxydim and quizalofop was dependent on rate of graminicide and large crabgrass stage of growth at application. DPX-PE350 did not reduce control of johnsongrass with quizalofop. Sethoxydim alone controlled 36 to 71% more johnsongrass than sethoxydim mixed with DPX-PE350 at 35 to 105 g ai ha-1. Antagonism of fluazifop-P activity was dependent upon the rate of DPX-PE350 and fluazifop-P applied. Applying DPX-PE350 3 d after fluazifop-P or sethoxydim reduced antagonism. Grass species, growth stage, rate, and sequential application timing all appear to play a role in the severity of antagonism.

Greenhouse and laboratory research was conducted to determine the antagonistic effects of various tank mixtures on BAS 625 efficacy. Bensulfuron at 60 g ai ha−1 and BAS 635 at 40 g ai ha−1 did not antagonize control of Echinochloa crus-galli or Brachiaria platyphylla by BAS 625 at 30 g ai ha−1 in greenhouse experiments. Tank mixtures of BAS 625 with 1,000 g ai ha−1 bentazon reduced BAS 625 control of E. crus-galli from 100 to 40%. Antagonism of BAS 625 activity by bentazon or chlorimuron at 10 g ai ha−1 was similar with B. platyphylla, reducing control from 100 to 28 and 32%, respectively. Addition of 5% (v/v) ethanol eliminated all antagonism with any of the herbicides used with either weed species. Uptake and translocation of 14C-BAS 625 1 and 12 h after treatment was not enhanced, either alone or in tank mixtures, with the addition of ethanol. Uptake of 14C-BAS 625 1 and 12 h after treatment was lower in both species when tank-mixed with bentazon. There was no effect of any of the antagonizing herbicides or ethanol on the metabolic degradation of the BAS 625 that was taken up by the plant. The herbicide concentration for 50% inhibition of activity (I50) for BAS 625 on Triticum aestivum acetyl coenzyme A carboxylase (ACCase) was 125 µM. Bentazon, BAS 635, and NC-311 at 1 mM each did not alter the inhibition on ACCase by BAS 625. BAS 635, NC-311, and bentazon at 1 mM inhibited the activity of ACCase 12, 16, and 29%, respectively. Our results indicate that antagonism of the weed control activity of BAS 625 by bentazon may be partly caused by reduced uptake. Other mechanisms may be involved to explain the antagonism of BAS 625 by bentazon and the sulfonylurea herbicides used in this study.

Isopropyl m-chlorocarbanilate (chlorpropham) at 6 lb/A, the standard treatment, usually controlled dodder (Cuscuta spp.) completely for 3 to 5 weeks. Of 27 other herbicides compared with chlorpropham, seven controlled dodder completely for longer periods (6 to 18 weeks), and nine for comparable or slightly shorter periods. Eleven controlled dodder only partially or completely for a period shorter than that from chlorpropham. The period of dodder control from chlorpropham was doubled when p-chlorophenyl N-methylcarbamate at 1.5 lb/A was included with it.

Sorghum genes responsive to purple witchweed parasitism were isolated, and their expression was analyzed. Using the suppression-subtractive hybridization strategy, 30 genes that were up-regulated in response to purple witchweed parasitism were isolated from the roots of a susceptible sorghum cultivar ‘Abu 70’. The changes in the expression of each gene were investigated in the roots and leaves of the sorghum cultivars ‘Wad Ahmed’ and ‘Tabat’ were parasitized by purple witchweed. Tabat is more susceptible and Wad Ahmed is less susceptible to purple witchweed than Abu 70. Further, the changes in the gene expression and host susceptibility to purple witchweed were studied in the roots of the three sorghum cultivars after treatment with salicylic acid (SA) or methyl jasmonate (MeJA). Purple witchweed parasitism induced jasmonic acid (JA)-responsive genes and suppressed SA-responsive genes in the roots of the cultivars Abu 70 and Tabat. In contrast, purple witchweed parasitism in the less-susceptible cultivar Wad Ahmed induced SA-responsive genes and induced JA-responsive genes to a small extent. SA, but not MeJA, decreased the susceptibility of all the sorghum cultivars to purple witchweed. Systemic expression of a few genes was observed in the leaves of the purple witchweed–parasitized sorghum cultivars.

Corn (Zea mays L. ‘Pioneer 3732′) showed little to no injury following the postemergence-directed application of sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio) propyl]-3-hydroxy-2-cyclohexen-1-one} plus crop oil concentrate (COC) at 56 g/ha plus 1.25% (v/v) at nine locations across Midwestern U.S. in 1984 and 1985. Little corn injury also occurred for the postemergence-directed application of sethoxydim plus COC at 110 g/ha plus 1.25% (v/v) at most locations in both years. Considerable variation in tolerance was seen across locations for over-the-top applications of sethoxydim at all rates tested and for the directed application at 220 g/ha. Although corn at most locations showed no yield reduction with the over-the-top application of sethoxydim plus COC at 56 g/ha plus 1.25% (v/v), a 70% yield reduction occurred in one location in one year. For an over-the-top application of sethoxydim plus COC at 110 g/ha plus 1.25% (v/v), yields ranged from 3 to 95% of the untreated check in 1984, and from 3 to 88% in 1985. Stand reductions from an over-the-top application of sethoxydim plus COC at 220 g/ha plus 1.25% (v/v) ranged from 0 to 99%. A significant negative correlation was found between yield of corn treated over the top with sethoxydim and precipitation on the day of application and in the week following application. Air temperature on the day of application was positively correlated with corn injury from over-the-top and directed sethoxydim applications, but no correlation existed between percent relative humidity and corn injury. Open pan evaporation and solar radiation before and after application were not correlated with corn injury from sethoxydim.

The importance of including crop density in studies of weed-wheat competition and in making estimates of crop losses due to weeds was demonstrated. Wheat density in adjacent quadrats of 1 m2 varied by an average of 25 plants in the farm fields and research plots examined. Differences between adjacent quadrats were in the same range for fields sown with different implements. Yield per plant, dry weight per plant, and tillering per plant decreased with increasing wheat density. For weed-crop competition studies in small-grain crops, the yields of weed-free and weedy quadrats are normally compared to estimate crop loss caused by competition. It was demonstrated that such estimates can be highly distorted if crop density is not taken into account. Current procedures used widely in agronomic competition studies are inadequate to properly define the relationship between crops and weeds. A more accurate method for performing such investigations is described in which both weed and crop abundance are determined. These variables are related to crop loss using multiple regression.

Following foliar application of 14C-bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-(4)3H-one 2,2-dioxide], a promising selective postemergence herbicide for the control of broadleaf weeds in soybean [Glycine max (L.) Merr.] and navy bean (Phaseolus vulgaris L.), the movement of the 14C-label was primarily acropetal in both species, with some basipetal movement. Metabolism was more rapid in the trifoliate navy bean leaf than in the unifoliate navy bean leaf and the trifoliate soybean leaf. Prior treatment with several other herbicides did not decrease metabolism of bentazon in navy bean. The metabolites in navy bean and soybean appeared similar. Four apparent 14C-conjugates were isolated from soybean plants harvested at full maturity.

A bioeconomic weed management model was tested as a decision aid for weed control in corn at Rosemount, MN, from 1991 to 1994. The model makes recommendations for preemergence control tactics based on the weed seed content of the soil and postemergence decisions based on weed seedling densities. Weed control, corn yield, herbicide active ingredient applied, and economic return with model-generated treatments were compared to standard herbicide and mechanical control treatments. Effects of these treatments on weed populations and soybean yield the following year were also determined. In most cases, the model-generated treatments controlled weeds as well as the standard herbicide treatment. The quantity of herbicide active ingredient applied decreased 27% with the seed bank model and 68% with the seedling model relative to the standard herbicide treatment. However, the frequency of herbicide application was not reduced. In 1 yr, seed bank model treatments did not control weeds as well as the standard herbicide or seedling model treatments. Corn yields reflected differences in weed control. Net economic return to weed control was not increased by using model-generated control recommendations. Weed control treatments the previous year affected weed density in the following soybean crop. In 2 of 3 yr, these differences did not after weed control or soybean yield. Although tactics differed, the bioeconomic model generally resulted in weed control and corn yield similar to the standard herbicide. The model was responsive to differing weed populations, but did not greatly after economic returns under the weed species and densities in this research.

Johnsongrass (Sorgum halepense (L.) Pers.) is a serious problem in many parts of the United States but little is known about the carbohydrate metabolism of this weed. Rapp studied the seasonal variation of various carbohydrates in Johnsongrass grown in pots. These plants were placed in the greenhouse for several weeks. The purpose of the present investigation was to study carbohydrates in field-grown plants and to obtain information on the effects of herbicidal treatments on the carbohydrate balance.

Following root application of a selective rate of 3-amino-2,5-dichlorobenzoic acid-C14 (amiben-C14), the translocation of radioactivity from roots to shoots was greater in pigweed (Amaranthus retroflexus L.), a susceptible plant, than in soybeans, a resistant one. Most of the radioactivity extracted from soybean and pigweed plants was present as conjugates of amiben-C14. The conjugate extracted from soybean was chromatographically identical with an N-glycoside prepared from glucose and amiben. In soybean plants, 3-amino-2,5-dichlorobenzoic acid also was converted to N-glycosylamiben.

The sensitivities of chlorophyll fluorescence and fresh weight as bioassay techniques for the determination of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5 (4H)-one], diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], and atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] concentrations in soil were compared. The ratio of the initial inflection point (I) to the variable fluorescence maximum (P) of intact oat (Avena sativa L. ‘Chilocco’) leaves was determined for plants seeded directly into herbicide-treated soil and for those transplanted into treated soil after 14 days of growth in nutrient solution. Using the chlorophyll fluorescence of transplanted oats bioassay, 0-, 0.13-, 0.25-, and 0.50-ppm concentrations could be distinguished from one another within 8 h for metribuzin, within 24 h for diuron, and within 48 h for atrazine. These distinctions between rates could not be made 17 days after seeding into treated soil when using fresh weight as the bioassay indicator. Chlorophyll fluorescence of oats seeded directly into treated soil was also a reliable technique, but required much more time to attain sufficient plant size for convenient chlorophyll fluorescence determinations.

The interaction of cultivation and photoperiod on the initiation of Canada thistle rosettes, and the effect of growth stage on control of Canada thistle with glyphosate were determined in a series of field experiments. Under the natural photoperiod occurring in the southern Canadian Prairies, rosettes of Canada thistle were initiated by cultivation to remove the shoot growth during the last week of July. Regrowth remained as rosettes without any stem elongation and formed dense clusters of leaves. Application at the “August rosette stage” improved the effectiveness of the glyphosate treatment and resulted in consistent control of Canada thistle with less than half as much herbicide as recommended for control when applied at the bud-stage. The number of shoots of Canada thistle on plots treated with glyphosate was less than on the intensively summer-fallowed check plot. Check plots received 5 cultivations during the summer-fallow season. One year after application of glyphosate at the rosette stage, the reduction in shoot density was 99% compared to the summer-fallowed check. By year three, without glyphosate applications in years two and three, the benefits of applying glyphosate at the rosette stage rather than the bud-stage were very evident. When applied at the bud-stage the shoot density on plots treated with glyphosate at 2.25 kg ha−1 was 24 m−2 compared to only 10 shoots m−2 when applied at the rosette stage, (35% vs 72% control). Glyphosate at 0.9 kg ha−1 or less than half of the rate recommended for application at the bud-stage, applied to Canada thistle in the rosette stage, resulted in consistently fewer shoots 2 and 3 yr after treatment. Physical removal of shoot top growth, simulating an in-crop herbicide treatment, improved the consistency of control. Two years after application of 0.9 kg ha−1 of glyphosate at the rosette stage, the control of Canada thistle was 98% compared to the summer-fallowed checks.

A laboratory study was conducted to evaluate the magnitude of imazethapyr resistance of field corn hybrids with altered acetolactate synthase (ALS) by comparing the responses of ALS extracted from homozygous and heterozygous resistant corn to nicosulfuron, primisulfuron, and imazethapyr. ALS isolated from Pioneer 3162 IR, Pioneer 3180 IR, Pioneer 3377 IR, and Ciba 4393 RSC hybrids was much more resistant to inhibition by these herbicides than ALS from the hybrids Pioneer 3162, Pioneer 3180, Pioneer 3377, and Ciba 4393, and the public inbred LH 74. Corn hybrids heterozygous for the Pioneer resistance gene were intermediate in response to the herbicides. ALS from ICI 8532 IT was resistant to imazethapyr but not to primisulfuron or nicosulfuron. The level of resistance of ALS from ICI 8532 IT was similar to that of ALS from Pioneer 3162 IR x Pioneer 3162 and Pioneer 3180 IR x Pioneer 3180 crosses that were also heterozygous for the resistance gene.

The number of vascular bundles in the rhizomes of different ecotypes of johnsongrass (Sorghum halepense (L.) Pers.) varied from 71 to 154. The size of vascular bundles varied from 60 to 150 μ in an ecotype from California to 100 to 230 μ in an ecotype from Georgia. The size of xylem cells in these vascular bundles also varied widely, but the size of phloem cells was more constant. The number of vascular bundles in culms ranged from 43 to 123. The average number of vascular bundles in individual leaves varied from 46 to 158 for different ecotypes. The number of stomata on leaf surfaces deviated from 63 to 148/sq mm for 10 different ecotypes. An average of 95 stomata/sq mm occurred on the upper surface of johnsongrass leaves while an average of 113 stomata occurred on lower surfaces. The arrangement and size of stomata varied in different johnsongrass ecotypes.

A population of oriental mustard from Port Broughton in South Australia was reported as not being controlled by 2,4-D. Dose response experiments determined this population was resistant to both 2,4-D and MCPA, requiring greater than 20 times more herbicide for equivalent control compared to a known susceptible population (from Roseworthy, South Australia) and a population resistant only to the acetohydroxyacid synthase (AHAS)-inhibiting herbicides (from Tumby Bay, South Australia). The Port Broughton population was also found to be resistant to three chemical groups that inhibit AHAS; however, the level of resistance was lower than the known acetolactate synthase–resistant population from Tumby Bay. Herbicides from other modes of action were able to control the Port Broughton population. Assays of isolated AHAS from the Port Broughton population showed high levels of resistance to the sulfonylurea and sulfonamide herbicide groups, but not to the imidazolinone herbicides. A single nucleotide change in the AHAS gene that predicted a Pro to Ser substitution at position 197 in the protein was identified in the Port Broughton population. This population of oriental mustard has evolved multiple resistance to AHAS-inhibiting herbicides (AHAS inhibitors) and auxinic herbicides, through a mutation in AHAS and a second nontarget-site mechanism. Whether the same mechanism provides resistance to both AHAS inhibitors and auxinic herbicides remains to be determined. Multiple resistance to auxinic herbicides and AHAS inhibitors in the Port Broughton population will make control of this population more difficult.

Solanum viarum, commonly known as tropical soda apple (TSA), is native to Brazil and Argentina but has become a harmful weed in many countries with tropical climates. This study was conducted to reassess the seed biology of TSA found in South Africa. Cold stratification (14 d), acid scarification (20% H2SO4 for 5 min), and sandpaper scarification (30 s) significantly improved percentage germination when compared to the control. The highest germination (99.5%) was achieved when seeds were germinated in 50% Hoagland's nutrient solution (HS). The lowest germination (66%) was recorded in the absence of phosphorus (P) under alternating light conditions. HS without nitrogen (N) completely inhibited seed germination of TSA under constant light conditions. These findings are useful in controlling TSA by amending the levels of N and P in soils. Seed germination of TSA was significantly enhanced by different concentrations of smoke-water and butenolide solution. Smoke-water dilution of 1:500 v/v and butenolide concentration of 10−8M showed the highest seedling vigor indices (6,688 and 6,666, respectively) in comparison to the control (1,251) and gibberellic acid (GA3) concentrations (< 5,327). These findings suggest that germination of seeds or seedbanks of TSA might be successfully stimulated using smoke solutions. Subsequently, patches of seedlings emerging after treatment can be mechanically uprooted to reduce the infestation of TSA. However, justifying this with field trials is essential.

Control of seedling and rhizome johnsongrass [Sorghum halepense (L.) Pers. # SORHA] with five herbicides was evaluated after postemergence application in various diluents and spray volumes using one to three types of applicators from 1983 to 1985. The rate of each herbicide required to control seedling johnsongrass using paraffinic oil as the diluent in applications with an external- or internal-mixing air-assist sprayer at a spray volume of 9.4 L/ha was one-half the rate required when water plus 1.25% (v/v) paraffinic oil concentrate (POC) or water plus 1.25% (v/v) soybean oil concentrate (SOC) were the diluents in applications with a conventional sprayer equipped with fan jet spray nozzles at a spray volume of 187 L/ha. Rates were also reduced when once-refined soybean oil was the diluent at a spray volume of 9.4 L/ha, except for the ethyl ester of quizalofop {(±)-2-[4-[(6-chloro-2-quinoxalinyl)oxy] phenoxy] propanoic acid}. Sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} was equally effective on seedling johnsongrass growing in soybeans [Glycine max (L.) Merr. ‘DPL 105’] when applied at 224 g ai/ha with an internal-mixing air-assist sprayer or a controlled-droplet applicator (CDA) at a spray volume of 9.4 L/ha and a conventional sprayer at a spray volume of 187 L/ha, regardless of diluent. Conventional application required the use of 1.25% (v/v) SOC or POC as part of the diluent to be effective. For three of the five herbicides evaluated on rhizome johnsongrass, paraffinic oil at a spray volume of 9.4 L/ha was the only diluent that resulted in season-long control. Rhizome johnsongrass control with this diluent also resulted in significantly higher soybean yields than all other diluent-spray volume treatments.

Seeds of five yellow foxtail [Setaria lutescens (Weigel) Hubb.] biotypes, collected from Connecticut, Iowa, Massachusetts, Pennsylvania and California, were tested for dormancy and germination characteristics. Seeds of the California biotypes, while dormant at time of dispersal, germinated readily following a 4-month after-ripening period in dry storage. California biotype seeds did not require stratification. The eastern biotypes also showed high initial dormancy. Prolonged dry storage (16 months) or stratification overcame this dormancy to varying degrees. All five biotypes germinated over the range of 15 to 35 C, but the California biotype exhibited much higher germination at 35 C than any eastern biotype. The California biotype germinated more rapidly, by 1 to 3 days, at any given temperature than the eastern biotypes. These differences in dormancy and germination requirements between the five biotypes are considered as evidence to support a hypothesis of genetically controlled, physiological differences between geographically separated biotypes.

The germination behavior of giant foxtail and shattercane seed was investigated to determine whether environment in which maternal plants grow and progeny seed develop affects seed dormancy. Giant foxtail and shattercane seed was collected in 1985, 1991, and 1992. Plants from this seed were grown to maturity in the field and the greenhouse in 1993 and 1994. Experiments were conducted using mature progeny seed collected from these plants to determine the influence of environment of maternal growth on seed germination. Greenhouse-raised giant foxtail seed was more dormant than field-raised seed. Progeny of the 1991 giant foxtail collection had the most dormant seed, whereas progeny of the 1992 collection had the least dormant seed. Field-raised shattercane seed was more dormant than greenhouse-raised seed, but this dormancy was easily overcome by placing them in fluctuating temperatures. Moist stratification of giant foxtail seed increased its overall germination and reduced the differences in germination between individual collections and between field- and greenhouse-raised seed lots. Moist stratification of shattercane seed resulted in increased variability in germination within and among seed lots. Subjecting shattercane seed to freezing and thawing was most detrimental and caused an 89% reduction in germination due to seed death. These data indicate that before overwintering, giant foxtail seed differed in germination percentage, but overwintering reduced these differences. Shattercane seed produced in different environments had similar germination percentage. This suggests that successful overwintering depends on its ability to survive freezing and thawing conditions.