Greenhouse and laboratory experiments were conducted to determine the physiological basis for differential tolerance of four Zea mays L. hybrids to RPA 201772. Differences in Zea mays tolerance were quantified by determining the herbicide rate required to injure and reduce Z. mays height 50% (GR50). GR50 values indicated that the Z. mays hybrids ‘Pioneer 3751’ and ‘Pioneer 3737’ were less tolerant to RPA 201772 than the hybrids ‘Pioneer 3394’ and ‘Pioneer 3963.’ Experiments using 14C-RPA 201772 were conducted to determine if hybrid sensitivity was due to differential uptake, translocation, or metabolism of the herbicide. Differences in hybrid tolerance were primarily due to differential herbicide metabolism rates. The time required for 50% inactivation (T½) of RPA 201772 was 42 and 52 h for the more tolerant hybrids and 66 and 93 h for the more sensitive hybrids. Increased uptake of RPA 201772 was also a contributing factor to the sensitivity of one of the hybrids.

Postemergence applications of BAS 9052 OH {2-[1-(ethoxyimino) butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexene-1-one} and Ro-13-8895 {acetone-O-[d-2-[p-[(α,α,α-trifluoro-p-tolyl-oxy] phenoxy] propionyl]oxime} at 0.3 kg/ha selectively controlled four- to five-leaf-stage quackgrass [Agropyron repens (L.) Beauv. ♯3 AGRRE] 75 and 80%, respectively, 105 days after treatment (DAT). KK-80 {4-[4-[4-(trifluoromethyl)phenoxy) phenoxy]-2-pentenoic} at the same rate and stage of growth controlled quackgrass only 45%. None of the herbicides had preemergence activity on quackgrass. All herbicides applied at 0.8 kg/ha controlled two- to three- or four- to five-leaf-stage quackgrass an average of 88%, whereas applications at the six- to seven-leaf stage controlled quackgrass only 27%, 105 DAT. Cultivation alone 1 DAT provided quackgrass control that gave soybean [Glycine max (L.) Merr.] yields equal to any herbicide or herbicide plus cultivation treatment In a greenhouse experiment, BAS 9052 OH killed 50-bud rhizome systems in 1 day, whereas KK-80 and Ro-13-8895 required 14 and 7 days of translocation, respectively, to provide similar control.

Repeated use of the preemergence herbicide triallate has selected for wild oat populations that are resistant (R) to field use rates. Field collections and an inbred R line were shown in greenhouse and petri dish dose response experiments to be 6- to 20-fold more tolerant to triallate than susceptible (S) lines. R populations and the inbred line were also resistant (8-fold) to the related thiocarbamate herbicide diallate, as well as to the chemically unrelated postemergence herbicide difenzoquat (60-fold). 14C-triallate uptake and translocation patterns were similar between R and S lines for the first 24 h after application. However, translocation of radioactivity was more rapid in S lines than R lines from 24 through 60 h after application. 14C-difenzoquat uptake was the same in R and S lines 12 h after application, but was 10 to 20% higher in R lines than S lines by 24 through 96 h after application. Similarly, translocation of radioactivity after 14C-difenzoquat application was 7 to 14% greater in R than S lines after 12 h, although translocated radioactivity amounts were not significantly different between R and S lines. The relatively minor differences in triallate and difenzoquat uptake and translocation patterns between R and S lines are most likely not of sufficient magnitude to explain the observed resistance levels.

Turbinella oak was sprayed with picloram-containing herbicides on 90 dates throughout 81 months. A mixture of triisopropanolamine salts of picloram and 2,4-D was compared to the potassium salt of picloram on 44 dates, and to a mixture of isooctyl ester of picloram and propylene glycol butyl ether esters of 2,4-D on 42 dates. Low rates of herbicide treatments were used to increase the sensitivity to environmental factors and plant growth stages. Turbinella oak mortality was greater with the mixture of picloram and 2,4-D amines than with picloram alone or the mixture of picloram and 2,4-D esters. The mixture of picloram and 2,4-D esters did not kill any plants. The combination of available soil moisture at the 60-cm depth, no senescent or falling leaves, and full-sized leaves was the best indicator of when to apply picloram or the mixture of picloram and 2,4-D amines. No turbinella oak were killed when soil at the 60-cm depth was dry or leaves were falling at the time of treatment.

A series of greenhouse studies examined the effectiveness of PRE- and POST-applied sulfentrazone in controlling purple and yellow nutsedge as influenced by selective tissue exposure. In addition, 14C-sulfentrazone was utilized to contrast absorption and translocation resulting from these exposures. Consistent control with preemergence applications to germinating tubers was obtained with a combined root and shoot zone exposure. Yellow nutsedge was more susceptible than purple nutsedge. Performance of the separate root and shoot zone exposure was soil pH- and nutsedge-species dependent. POST-foliar applications to established nutsedge were more effective when sulfentrazone was allowed to contact the soil surface. 14C-sulfentrazone was readily absorbed by the roots and translocated to the foliage of both species in hydroponic culture.

The life cycle of Notaris bimaculatus Fab. and the influence of this weevil on the control of quackgrass [Agropyron repens (L.) Beauv.] with glyphosate [N-(phosphonomethyl) glycine] were studied. Observations on this insect in 1978 and 1979 showed that its life cycle occurs in close association with quackgrass. The adult weevils feed on quackgrass culms and caryopses and use the inside of the culms for ovipositing. Adult populations, measured in quackgrass infestations during the summer months, ranged from 3 to 44/25 sweeps of an insect net. Larvae emerge from the eggs after 2 weeks, feed down the inside of the culms, chew an exit hole, and move into the soil where they attack the rhizomes. Larval numbers ranged from two to six/28 dm3 of soil during the summer months. Larvae feed on the rhizome surface or enter the rhizomes where they devour the vascular and cortical tissue. In dense quackgrass sods treated with glyphosate at 1.7 kg/ha, feeding damage on quackgrass rhizomes caused by the larvae reduced the control of quackgrass by disrupting the translocation of glyphosate in the rhizomes. Although soil-borne larvae of several insects were found in quackgrassinfested soil, larvae of N. bimaculatus were always present. Controlling these soil-borne insect larvae with a soil-applied insecticide for 2 months before applying glyphosate resulted in significantly increased quackgrass control. Shoot regrowth several months after the application of glyphosate at 1.4 kg/ha to quackgrass grown in cages infested with 400 adult weevils was 298 and 611 shoots/m2 in the 1979 and 1980 experiments, respectively. Similar glyphosate applications to weevil-free quackgrass resulted in only 26 and 15 shoots/m2 in the 1979 and 1980 experiments.

Previous reports have suggested that bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] tolerance among soybean genotypes is the result of differential translocation or metabolism. The basis for tolerance was reexamined using susceptible and tolerant genotypes. Tolerant genotypes (‘Hill’ and ‘Clark 63’) were found to tolerate 100- to 300-fold more bentazon than susceptible genotypes (‘L78–3263’, ‘Hurrelbrink’, and ‘PI 229.342’). Minor differences in absorption and translocation occurred among the genotypes but they did not correlate with tolerance. Tolerant genotypes metabolized 80 to 90% of absorbed bentazon within 24 h, while susceptible genotypes metabolized only 10 to 15%. Two major metabolites, the glycosyl conjugates of 6- and 8-hydroxybentazon, were formed in tolerant genotypes. Susceptible genotypes did not form the hydroxybentazon conjugates but instead produced relatively low levels of two unidentified metabolites. It is concluded that differential bentazon tolerance among soybean genotypes is linked to the ability to form both the 6- and 8-hydroxybentazon conjugates.

A series of experiments was carried out on oat to test the efficacies of fluazifop and glyphosate against water-stressed plants, plants grown in low and high nitrogen (N), and plants treated with gibberellic acid (GA). In the laboratory, plants maintained at wilting point for 5 days before and 9 days after spraying with fluazifop (0.5 kg ae/ha) appeared healthy 32 days after herbicide application, while plants supplied with water throughout the experiment were completely chlorotic/necrotic and had main stem detachment from within the leaf sheaths. In the field, plants maintained unirrigated until 14 days after spraying with fluazifop (0.25 kg/ha) or glyphosate (0.18 kg ae/ha) showed greater tolerance of the herbicides than plants irrigated regularly. Under well-watered conditions in the laboratory and field, fluazifop (0.25 kg/ha) and glyphosate (0.18 kg/ha) were less toxic at low N than high N. Increased fluazifop activity at high N was associated with increased transport of herbicide to apical meristems. Addition of 200 μg GA into the leaf sheaths 2 days prior to spraying with fluazifop or glyphosate increased the efficacy of both herbicides at low N.

Picloram uptake, translocation, and efficacy were studied using greenhouse-grown Russian knapweed plants. Uptake of 14C-picloram, applied as discrete droplets to adaxial leaf surfaces of well-watered plants, averaged less than 10% of that applied. Most uptake occurred within 30 min of application. Uptake increased linearly with external picloram concentrations from 6.2 to 74.5 mM and was proportional to picloram concentration. Only about 10% of absorbed picloram was translocated out of the treated leaf of well-watered plants within 96 h, with approximately equal acropetal and basipetal translocation. Water stress before, at the time of, and after picloram application did not affect picloram uptake, but reduced total translocation and increased the relative amount translocated basipetally. Water stress also reduced picloram efficacy. Although Silwett L-77 increased picloram uptake into Russian knapweed leaves, it did not increase efficacy under water stress conditions.

Resistance to imazethapyr was identified in a population of common sunflower that had been treated with imazethapyr for seven consecutive years. The imazethapyr-resistant biotype of common sunflower was approximately 170 times more resistant to imazethapyr than the susceptible biotype based on the rate required for 25% control. Resistance was due to altered acetolactate synthase (ALS) that is less sensitive to imazethapyr. The imazethapyr concentration required to inhibit in vitro ALS activity by 25% was 210–fold higher in the resistant biotype than in the susceptible biotype. Differences in absorption, translocation, and metabolism of imazethapyr in common sunflower biotypes were not sufficient to explain the resistance to imazethapyr.

The effect of photoperiod and growth stage on translocation of 14C-glyphosate was compared in Canada thistle plants at the bud and rosette stage of growth. Canada thistle plants grown under controlled environment conditions with a 10 h photoperiod remained as low growing rosettes and developed a mature root system. When the photoperiod for half of these plants was increased to 16 h, stem elongation occurred and flowering was initiated. Growth stage at the time of application affected the distribution of 14C-glyphosate within the elongated shoot and between the shoot and root. The shoot tissue of the bud stage plants contained 25.9% of the 14C-glyphosate recovered, while the rosette plants contained only 3.6%; a seven-fold difference. 14C-glyphosate was applied to leaves 19 and 20, which corresponded to the mid-point of the elongated stem of the bud-stage plants. In the bud-stage plants, 14C-glyphosate moved preferentially into the apical portion of the stem, with three to four times as much in the apical portion of the elongated stem as in the basal portion. In the roots, the effect of growth stage on distribution of 14C-glyphosate was reversed, application at the rosette stage resulted in a four-fold increase in the amount of 14C-glyphosate in the root. When applied in the rosette stage, 19.1% of the 14C-glyphosate detected was in the root compared to only 4.9% when applied at the bud stage. Although the root of the rosette plants was larger than in bud-stage plants, the concentration of 14C-glyphosate in the root tissue of the rosette plants was three times greater. Photoperiod indirectly affected the distribution of 14C-glyphosate in Canada thistle by its effect on growth.

Foliar-applied clopyralid was translocated much more readily than chlorsulfuron in the phloem of Tartary buckwheat plants. This result was not due to greater penetration of clopyralid into the treated leaf or to greater retention of chlorsulfuron in the cuticle. Experiments with excised leaf disks indicated that chlorsulfuron was taken up more readily by the leaf tissue and accumulated in the tissue to a higher concentration than clopyralid. Both herbicides effluxed readily from the tissue after transfer to herbicide-free medium, indicating that the accumulation was not due to irreversible binding within the tissue. Chlorsulfuron (2.8 nmol) applied with 14C-sucrose reduced 14C export from the treated leaf. Chlorsulfuron also reduced export of 14C following exposure of the treated leaf to 14CO2 at 6, 12, or 24 h after herbicide application. This effect of chlorsulfuron could be partially reversed by pretreating the plants with a combination of 1 mM valine, leucine, and isoleucine. In similar experiments clopyralid had no effect on assimilate transport. It is concluded that phloem translocation of chlorsulfuron in sensitive species is limited by a rapid, indirect effect on phloem transport that reduces both its own translocation and that of assimilate.

Field and laboratory experiments were conducted to evaluate the effect of glyphosate concentration, surfactant concentration, and ammonium sulfate on the control of giant burreed when applied through a ropewick applicator. Under non-flooded conditions, a 30% (v/v) solution of glyphosate applied with a ropewick applicator gave the highest giant burreed injury in 1987 and 1988, and reduced corm and rhizome viability compared to the nontreated control in 1987. Warm, dry conditions may have been responsible for poor long-term control in 1988 by reducing translocation of glyphosate to rhizomes. Addition of ammonium sulfate or surfactant did not influence glyphosate toxicity to giant burreed when applied with a ropewick applicator. In the laboratory, leaf necrosis at the point of application was observed with surfactant at 0.6% or 1.2% regardless of glyphosate concentration. Absorption of 14C-glyphosate increased two-fold with the addition of surfactant, and translocation to roots and rhizomes decreased almost two-fold as surfactant concentration was increased from 0 to 1.2%. Electrolyte leakage increased with 30% glyphosate when surfactant concentration was increased from 0 to 1.2%. Localized leaf necrosis and disruption of tissues inside the leaf may be responsible for reduced giant burreed control with glyphosate and surfactant applied in high concentrations.

Space-planted johnsongrass [Sorghum halepense (L.) Pers. ♯ SORHA] was clipped back to a height of 26 cm after flowering and, when 45 to 61 cm in height, was treated with various rates of the sodium salt of dalapon (2,2-dichloropropionic acid), MSMA (monosodium methanearsonate), or the sodium salt of asulam (methylsulfanilylcarbamate). Treatments were reapplied as needed during the growing season whenever a majority of surviving plants in a plot had initiated new foliar growth. Most plants were not killed by a single application of any herbicide, although rhizome development, as measured by the number of rhizome buds, was inhibited by the initial herbicide treatments. MSMA at 4.5 kg ai/ha was more phytotoxic to johnsongrass than other herbicide treatments or MSMA at 2.3 kg ai/ha; a single application at the higher rate reduced the original number of rhizome buds on plants by 50 to 60%, and two applications killed 86 to 93% of the plants. Asulam at 4.5 and 6.7 kg ae/ha and dalapon at 6.7 and 9.0 kg ae/ha primarily controlled plants by inhibiting growth for an extended period. After two or three applications of asulam or dalapon at 6.7 kg ae/ha, only about 40% of the plants had been killed, but the original number of rhizome buds on surviving plants had been reduced by about 80%.

Antagonism of quizalofop-P efficacy on annual grasses by bromoxynil has been noted in both the field and greenhouse. Laboratory experiments were conducted on yellow foxtail (Setaria glauca) to determine the effect of mixing bromoxynil with the ethyl ester of quizalofop-P on absorption, translocation, and metabolism of 14C-quizalofop-P Applying bromoxynil in mixture with quizalofop-P reduced 14C-label absorption from 63 to 51%, 73 to 52%, 77 to 68%, and 90 to 80% at 4, 8, 24, and 96 h after treatment, respectively. Translocation of 14C-label from the treated leaf into the shoot above or shoot below was unaffected by bromoxynil. However, translocation into the roots was reduced from 0.9 to 0.4% and 1.0 to 0.5% at 4 and 8 h after treatment, respectively, but differences were not noted at 0.5, 1, 24, and 96 h after treatment. Bromoxynil increased deesterification of quizalofop-P-ethyl into quizalofop-P acid from 45 to 60% in the shoot above the treated leaf. However, bromoxynil did not affect metabolism of quizalofop-P in the treated leaf or shoot below the treated leaf. These results suggest that antagonism of quizalofop-P activity by bromoxynil is primarily due to decreased absorption of quizalofop-P, whereas effects on translocation and metabolism were minor.

Kochia is a troublesome weed throughout the western United States. Although glyphosate effectively controls kochia, poor control was observed in several no-till fields in Kansas. The objectives of this research were to evaluate kochia populations response to glyphosate and examine the mechanism that causes differential response to glyphosate. Glyphosate was applied at 0, 54, 109, 218, 435, 870, 1305, 1740, 3480, and 5220 g ae ha−1 on 10 kochia populations. In general, kochia populations differed in their response to glyphosate. At 21 d after treatment, injury from glyphosate applied at 870 g ha−1 range from 4 to 91%. In addition, glyphosate rate required to cause 50% visible injury (GR50) ranged from 470 to 2149 g ha−1. Differences in glyphosate absorption and translocation and kochia mineral content were not sufficient to explain differential kochia response to glyphosate.

Studies were conducted to evaluate 14C-aminocyclopyrachlor absorption and translocation in alligatorweed, waterhyacinth, and waterlettuce. Alligatorweed plants were treated at the seven-node stage, waterhyacinth was treated at the five-leaf stage, and waterlettuce was treated at the eight-leaf stage. All plants were pretreated with nonlabeled aminocyclopyrachlor at 0.14 kg ai ha−1 with 1% (v/v) methylated seed oil (MSO). 14C-aminocyclopyrachlor was then applied to a protected leaf, and plants were harvested at 1, 2, 4, 12, 24, and 96 h after treatment (HAT). Radioactivity was determined in the treated leaf, shoots above treated leaf, shoots below treated leaf, roots, and growing solution. Absorption was rapid in all species and reached a maximum of 73, 72, and 73% of applied radioactivity for alligatorweed, waterhyacinth, and waterlettuce, respectively. In alligatorweed at 96 HAT, 43% of absorbed carbon-14 (14C) was translocated to shoots above the treated leaf and 17% was translocated to lower shoot tissue. In waterhyacinth at 96 HAT, 56% of absorbed 14C remained in the treated leaf, whereas 14 and 13% were found in parts above and below the treated leaf, respectively. In waterlettuce at 96 HAT, 50 and 33% of absorbed radioactivity was located above the treated leaf and in the growing solution, respectively. The low recovery of aminocyclopyrachlor in alligatorweed roots and growing solution might explain regrowth potential after herbicide treatment. These results also indicate that the lack of waterlettuce control with aminocyclopyrachlor is not due to reduced absorption or translocation.

Selective placement studies were conducted under greenhouse conditions to determine the relative importance of root vs. foliar absorption of postemergence-applied CGA-362622 by torpedograss. All application methods were equally effective in reducing torpedograss foliage as measured 4 wk after treatment. However, foliar + soil and soil-only were more effective than foliar-only in suppressing regrowth at 10 wk after treatment. Foliar absorption by torpedograss and subsequent translocation was determined with radiotracer techniques. After 72 h, 29% of the applied CGA-362622 had been absorbed, and 2 and 7% of the amount applied had accumulated in developing rhizomes and roots, respectively. CGA-362622 was more readily absorbed and translocated by the root. Hydroponically grown plants were transferred to a hydroponic solution spiked with CGA-362622 at 200 ppb. After 6 h, whole plant concentration was 113.1 ng/plant. Only 56% of amount absorbed remained in the roots, the remainder having been translocated to other tissues. The youngest leaf and the immature rhizomes accumulated 2 and 15%, respectively. CGA-362622 soil adsorption was slightly influenced by CGA-362622 concentration and greatly influenced by soil pH. Average percent recovered in the soil solution (i.e., not absorbed) was 15.3 and 27.4% at pH 5.7 and 6.7, respectively. Soil mobility was also pH dependent. Soil solution and soil mobility data support the observation that soil application followed by root entry is more effective in delivering phytotoxic concentrations to the regenerative tissues of torpedograss than foliar application.

Growth room studies were conducted to determine the physiological basis of reduced glyphosate efficacy under low soil nitrogen using velvetleaf, common ragweed, and common lambsquarters as model species. Glyphosate dose–response experiments of weeds grown under low (1.5 mM) and high (15 mM) soil N were conducted. Velvetleaf and common lambsquarters grown under low N required 169 g ae ha−1 glyphosate for a significant reduction in biomass, but only 84 g ae ha−1 were required when grown under high N. However, when common ragweed was grown under low or high soil N there was no significant difference in response to glyphosate at all doses tested. The reduced glyphosate efficacy on velvetleaf and common lambsquarters under low N was primarily due to decreased herbicide translocation to the meristem. It appears that low N may decrease the net assimilation of carbon in plants, resulting in a decrease in the net export of sugars and hence glyphosate from mature leaves. Understanding the relationship between soil N and herbicide efficacy may help explain observed weed control failures with glyphosate and may contribute to our knowledge of the occurrence of weed patchiness in fields. This is the first report illustrating a physiological basis for decreased glyphosate efficacy under low soil N in selected weed species.

Experiments were conducted to determine the efficacy of glyphosate on four common lambsquarters populations collected from Kansas, Nebraska, North Dakota, and Ohio. Glyphosate dose-response studies for common lambsquarters treated at 2.5-, 7.5-, and 15-cm heights showed that glyphosate at 1.1 kg ae ha−1 caused more than 80% injury to 2.5-cm plants but less than 55% injury to 7.5- and 15-cm plants. All populations were susceptible to glyphosate at the 2.5-cm height. The glyphosate rate required to cause 50% injury (GR50) was 430, 500, 500, and 560 g ha−1 for the Kansas, North Dakota, Ohio, and Nebraska populations, respectively. Differential response of common lambsquarters populations was evident with 15-cm plants where the GR50 was glyphosate at 1,010, 1,230, 1,650, and 2,770 g ha−1 for the Kansas, North Dakota, Nebraska, and Ohio populations, respectively. Reduced injury on 15-cm common lambsquarters plants by glyphosate may be partly attributed to reduced glyphosate accumulation per unit of plant tissues and enhanced calcium content in more-developed plants. All four common lambsquarters populations at the early seedling stage were susceptible to glyphosate, but tolerance increased as the plant developed and the extent of tolerance differed among populations.