A giant foxtail (PCW1) population putatively resistant to fluazifop-P and sethoxydim was identified in a carrot, onion, and corn cropping system in Wisconsin during 1991. Previous, extensive use of fluazifop and fluazifop-P over several years imposed a high level of selection intensity on grass weeds. In a field experiment, fluazifop-P, sethoxydim, and quizalofop at recommended dosages resulted in 58, 53, and 45% plant survival, respectively, within the PCW1 giant foxtail population; no plants survived treatment with clethodim or nicosulfuron, and few plants survived treatment with imazethapyr at recommended dosages. Based on shoot dry biomass reduction in greenhouse experiments, a PCW1 giant foxtail biotype had 16-, > 9-, 4.9-, and 4.2-fold resistance to fluazifop-P, diclofop, quizalofop, and fenoxaprop, respectively, relative to a giant foxtail (AC1) accession that was susceptible to aryloxyphenoxypropionate (APP) and cyclohexanedione (CHD) herbicides. The PCW1 biotype had 134-fold resistance to sethoxydim and slight and inconsistent resistance to clethodim. The PCW1 biotype and AC1 accession were equally susceptible to imazethapyr, linuron, and oxyfluorfen. Based on plant survival, a PCW1 giant foxtail accession had 25- and > 143-fold resistance to fluazifop-P and sethoxydim, respectively, relative to the AC1 accession. The selection intensity associated with repeated use of fluazifop and fluazifop-P over 5 yr contributed to the cross-resistance of PCW1 giant foxtail to APP and CHD herbicides.

Synergistic interaction between the insecticide terbufos and the herbicide nicosulfuron may result in severe injury to corn. Greenhouse and laboratory experiments were conducted to determine if using the imidazolinone-resistant corn ‘Pioneer-3343 IR’ (P-3343 IR) or coating corn seeds with naphthalic anhydride (NA) would reduce herbicidal injury imposed by the nicosulfuron-terbufos interaction. Greenhouse experiments showed nicosulfuron-terbufos interactions resulting in herbicidal injury in both P-3343 IR and ‘DeKalb 689’ (D-689) corn varieties, but the D-689 was more sensitive than the P-3343 IR corn. The greenhouse experiments also demonstrated protection against the nicosulfuron-terbufos interaction by NA seed treatments. Studies with radiolabeled nicosulfuron showed that terbufos inhibited the metabolism of nicosulfuron, but pretreatment of D-689 and P-3343 IR corn seed with NA decreased the inhibition in excised corn leaves. The differences in sensitivity to nicosulfuron in the two corn varieties resulted in part from the differential metabolism and primarily from the differential sensitivity of the target enzyme, acetolactate synthase, to the herbicide.

The antagonistic effect of Na-bentazon on sethoxydim absorption and herbicidal activity has been documented. The addition of ammonium sulfate (AMS) with a surfactant overcomes the observed loss of sethoxydim activity. Nuclear Magnetic Resonance (NMR) was used to study the chemical effects of commercially formulated Na-bentazon, NaCl, NaHCO3, Na-bentazon plus AMS, and NaHCO3 plus AMS on commercially formulated sethoxydim. Technical grade Li-sethoxydim and sethoxydim were analyzed and 1H-NMR spectra were used as comparative standards. Data indicate an association of Na+ from Na-bentazon, NaHCO3, and NaCl with the sethoxydim molecule. NH4+ from AMS appears to associate spatially with sethoxydim but does not exert the same electronic effect on the sethoxydim ring protons as observed with Na+ or Li+. The addition of AMS to sethoxydim plus Na-bentazon or NaHCO3 treatments prevents the complexation of Na+ with the sethoxydim molecule. The data support the hypothesis that the observed Na-bentazon antagonism and ammonium sulfate reversal of antagonism are chemically based.

Puccinia coronata infection increased absorption of 14C-glyphosate in quackgrass 3 days after inoculation (DAI) when compared to 0,6,9, or 12 DAI. Days after inoculation had no effect on glyphosate translocation out of treated leaves until 9 to 12 DAI when 89% of the absorbed 14C-glyphosate was retained within the infected leaf. In contrast, only 43% was retained in leaves treated at 0 DAI. In other studies, 14C-glyphosate absorption increased linearly from 21 to 35% 12 to 96h after treatment (HAT). Differences between control and rust-infected plants were not significant. Sixty-three percent of absorbed 14C-glyphosate translocated out of control leaves by 96 HAT, while only 10% translocated out of rust-infected leaves. Additionally, 28 and 4% of the 14C absorbed was recovered from the rhizomes of control and rust-infected plants, respectively.

The goal of this research was to determine if differing levels of tolerance to AC 263,222 exist in soybean. 14C-AC 263,222 was foliar-applied to ‘Coker 6955,’ ‘Hartz 6686,’ ‘Hutcheson,’ ‘9581 Pioneer,’ ‘9681 Pioneer,’ and ‘RA 606’ soybean cultivars. Differential absorption of 14C-AC 263,222 was evident among cultivars 48 and 96 h after application. Movement out of the treated leaf was both acropetal and basipetal, indicating xylem and phloem mobility. Although absorption and translocation differences occurred among cultivars, these differences do not explain the differential response of soybean cultivars to AC 263,222. Metabolism of 14C-AC 263,222 differed greatly among cultivars and increased with time. Ninety-six h after application, 9581 Pioneer metabolized 66% of the absorbed 14C-AC 263,222, compared to RA 606, which metabolized only 41%. The large differences in metabolism that occurred 48 and 96 h after application suggest that metabolism is responsible for the differential response of soybean cultivars to AC 263,222.

Field studies have shown primisulfuron to be more injurious to sugarbeet than nicosulfuron 1 and 2 yr after herbicide application. Experiments were initiated to determine if primisulfuron is more injurious to sugarbeet grown in a nutrient culture and if the difference in sugarbeet response is a result of greater uptake, translocation, or acetolactate synthase (ALS) site sensitivity with primisulfuron. Concentrations of primisulfuron and nicosulfuron that reduced sugarbeet growth by 50% were 1.9 and 8.9 μg ai L−1, respectively, at pH 6.5. The pH of the nutrient solution did not influence sugarbeet response to either herbicide. Uptake of primisulfuron was greater (3%) than that of nicosulfuron (1%). Translocation (expressed as a percent of uptake) of nicosulfuron was more rapid than primisulfuron. Fifty-seven percent of the absorbed nicosulfuron translocated out of the root during the 12-h pulse period, while an equal concentration of primisulfuron was not translocated out of the root until 48 h after pulsing. The total nicosulfuron translocated after 144 h was half that of primisulfuron. The nutrient solution taken up by sugarbeet in the 12-h pulse period was reduced by 41% in the presence of either herbicide compared to the untreated control. The ALS enzyme was a minimum of 15 times more sensitive to primisulfuron compared to nicosulfuron which may account for greater sugarbeet response to primisulfuron.

More than 70% of all 14C-bentazon absorption occurred within 4 h after herbicide application regardless of adjuvant Moisture stress reduced 14C-bentazon absorption by common cocklebur and velvetleaf. Mature (second true leaf) and moisture-stressed leaves of velvetleaf had 50 and 17 μg cm−1 more epicuticular wax (ECW) than did juvenile and unstressed leaves, respectively. Common cocklebur had less 14C in the ECW and lower total 14C in treated mature leaves compared to juvenile leaves. The use of 28% urea ammonium nitrate (UAN) or crop oil concentrate (COC) increased 14C in ECW samples of both plant species, regardless of leaf age or moisture stress. More 14C in the ECW did not always correlate with more 14C in the leaf tissue. Adjuvants increased 14C-bentazon absorption into leaves of plants that had been stressed.

The time-dependent uptake, translocation, and metabolism of [14C]MG-191 safener and the influence of EPTC on these processes were studied using 5-d-old corn seedlings grown in nutrient solution. Plants absorbed relatively low levels of root-applied radiolabel at both 10 μM and 50 μM [14C]MG-191 rates during 6 d of exposure. Amounts of absorbed radioactivity increased with time. Addition of 50 μM EPTC, in general, did not alter amounts of [14C]MG-191 taken up. Initial translocation of radiolabel from roots to shoots was more rapid at 10 μM [14C]MG-191 treatment compared to 50 μM. EPTC had no effect on radiolabel movement at either safener rate. MG-191 was metabolized rapidly to water-soluble metabolites. Slower formation of water-soluble products was detected only during the initial 3 h in the presence of EPTC. Also, thin-layer chromatographic analyses of water- and hexane-soluble metabolite fractions confirmed rapid transformation of the parent molecule. Because addition of EPTC retarded parent MG-191 metabolism 3 h after treatment, we assume that initial availability of the MG-191 molecule may be prerequisite to its protective effect.

Hard-water cations, such as Ca+2 and Mg+2, present in the spray solution can greatly reduce the efficacy of glyphosate. These cations potentially compete with the isopropylamine in the formulation for association with the glyphosate anion. 14C-Glyphosate absorption by sunflower was reduced in the presence of Ca+2. The addition of ammonium sulfate overcame the observed decrease in 14C-glyphosate absorption. Nuclear Magnetic Resonance (NMR) was used to study the chemical effects of calcium and calcium plus ammonium sulfate (AMS) on the glyphosate molecule. Data indicate an association of calcium with both the carboxyl and phosphonate groups on the glyphosate molecule. Initially, a random association of the compounds occurred; however, the reaction progressed to yield a more structured, chelate type complex over time. NH4+ from AMS effectively competed with calcium for complexation sites on the glyphosate molecule. Data suggest that the observed calcium antagonism of glyphosate and AMS reversal of the antagonism are chemically based.

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.

The DNA sequence of an 83-base pair region of the acetolactate synthase (ALS) gene was compared for 10 chlorsulfuron-resistant (R) and three chlorsulfuron-susceptible (S) kochia biotypes. Point mutation in the codon for the proline residue at position 173 in Domain A of the ALS protein was observed in seven of 10 R biotypes. Among these seven R biotypes, mutation to threonine, serine, arginine, leucine, glutamine, and alanine was identified. The mechanism of resistance was determined for the R biotypes that did not have mutation in Domain A; all were resistant due to modified ALS, which indicated that at least one non-Domain A mutation site for resistance exists in kochia. Sequence results indicate that multiple mutations for resistance have occurred, and that geographically widespread ALS-inhibitor resistance in kochia is not the result of a single resistance allele.

The locoweeds, woolly loco and silky crazyweed, contribute to livestock poisoning in the western United States. Differences in response of these two locoweeds to foliar-applied picloram and metsulfuron was investigated by evaluating differences in herbicide uptake, translocation, and metabolism. Silky crazyweed compared to woolly loco was more than 10 times as sensitive to increasing rates of either herbicide. The two species absorbed 8 to 15%, and 11 to 17% of applied picloram and metsulfuron, respectively. Translocation of picloram and metsulfuron out of treated leaflets of either species was less than 3% of that absorbed after 96 h. Approximately 70 and 100% of the absorbed herbicides remained as picloram and metsulfuron, respectively, in both species. Therefore, differences in picloram or metsulfuron uptake, translocation, and metabolism appear inadequate to account for the differential response to each herbicide by the two locoweed species. Selectivity differences between these locoweed genera to picloram and metsulfuron most likely are due to sensitivity differences at sites of action.

Disappearance of rye cover crop residue and allelochemicals from rye residue were evaluated in a field study at Clayton, NC, in 1992 and 1993. The aerial portion of rye biomass declined linearly over time with 50% of the residue disappearing by 105 d after clipping. Extrapolations indicated all residue would disappear approximately 200 d after rye clipping. Total content of DIBOA and two related compounds, DIBOA-glucoside and BOA, in the residue also was measured over time. Fifty percent of the 0-d content of the compounds disappeared from residue 10 and 12 d after kill for 1992 and 1993, respectively. Extrapolations indicated content reached 0121 and 168 d after kill for 1992 and 1993, respectively. Reported duration of weed suppression by rye cover crops more closely follows disappearance of allelochemicals from rye residue than disappearance of the residue.

The effect of soil moisture, temperature, and light intensity on the spray deposition of fenoxaprop and imazamethabenz applied to wild oat plants was examined by using fluorescent tracer dye. Based on either biomass or total leaf area, the apparent deposition of the two herbicides diminished in the following order: shading > low temperature ≥ drought ≥ “optimum” > high temperature. The enhanced phytotoxicity of both herbicides under shading could be associated with increased spray deposition; and reduced fenoxaprop phytotoxicity under high temperature stress could be related to reduced deposition. Changes in spray deposition were attributed mainly to differences in herbicide interception due to altered plant morphology. Reduced retention for both herbicides was exhibited only in the plants grown at high temperature. Under “optimum” conditions, fenoxaprop phytotoxicity was directly associated with leaf orientation and thus with the proportion of projected leaf area at the time of herbicide spraying. Given similar application conditions, spray deposition of fenoxaprop and imazamethabenz on wild oat could be estimated by determining the ratio of the projected leaf area, as measured by an image analyzer, to the total leaf area.

Field and greenhouse research was conducted to quantify the level of resistance to atrazine in Wisconsin (WRB1) and Maryland (MRB) velvetleaf biotypes and to determine cross-resistance of the WRB1 and MRB biotypes to other selected herbicides as compared to a Wisconsin atrazine-susceptible velvetleaf accession (WSA1). In field studies, the WRB1 and MRB biotypes survived atrazine applied POST at dosages as high as 4.5 kg ha−1. In contrast, the WSA1 accession had 50% survival following a 1.1 kg ha−1 POST atrazine application. The WRB1 biotype demonstrated neither cross-resistance nor negative cross-resistance to alachlor, bentazon, bromoxynil, cyanazine, dicamba, linuron, metribuzin, or thifensulfuron. The MRB biotype demonstrated neither cross-resistance nor negative cross-resistance to alachlor, bentazon, dicamba, metribuzin, or thifensulfuron; slight negative cross-resistance was demonstrated to bromoxynil, cyanazine, and linuron. In greenhouse studies, the WRB1 and MRB biotypes were approximately 100-fold more resistant to atrazine than the WSA1 accession; the WRB1 and MRB biotypes demonstrated neither cross-resistance nor negative cross-resistance to alachlor, ametryn, bentazon, bromoxynil, cyanazine, dicamba, linuron, metribuzin, pendimethalin, terbacil, or thifensulfuron. Absence of cross-resistance to cyanazine, ametryn, metribuzin, and terbacil in the WRB1 and MRB biotypes of velvetleaf is in contrast to most other atrazine-resistant weed biotypes.

The uptake, translocation, and metabolism of 14C-MSMA in organic arsenical-resistant (R) and -susceptible (S) Mississippi biotypes of common cocklebur were investigated. The two biotypes did not differ significantly with respect to either uptake, total translocation, or translocation pattern of 14C-MSMA plus its radiolabeled metabolites regardless of whether plants were pre-treated with 1.12 kg ai ha−1 of unlabeled MSMA or treated only with radiolabeled material. Absorption of 14C-MSMA was greater in unlabeled MSMA-pretreated plants than untreated plants, whereas the percent of total absorbed 14C-MSMA that was translocated out of the 14C-MSMA-treated leaf was almost the same in MSMA-pretreated and untreated plants. Qualitative detection of 14C-MSMA distribution by autoradiographs confirmed quantitative results obtained with radioassays. Herbicide metabolism in the 14C-treated leaf showed that MSMA was not readily broken down in both biotypes and the extractable 14C was the same. Uptake, translocation, and metabolic degradation were not involved in the mechanism of resistance of the resistant-Mississippi biotype common cocklebur to MSMA.

The joint action of metribuzin and tridiphane was investigated in metribuzin-tolerant (T) and metribuzin-susceptible (S) soybean and tomato cultivars within species, respectively, under growth room studies. Maple Arrow (T) and Maple Amber (S) exhibited similar tolerance to tridiphane applied at soybean emergence. Vision (T) tomato was more sensitive to tridiphane than was Springset (S) tomato, the reverse of the relative tolerance to metribuzin. A phytotoxic interaction was demonstrated following application of tridiphane and metribuzin at the respective rates (kg ai ha−1) of 0.1 and 1.1 in Maple Arrow (T) soybeans, 0.05 and 0.25 in Maple Amber (S) soybeans, and 0.25 and 0.2 in Springset (S) tomato. Tridiphane applied 1 or 4 h before metribuzin caused the greatest phytotoxicity in Maple Amber (S) soybeans. Soybean field results generally supported those of growth-room studies. Foliar spray pretreatment with tridiphane increased total radioactivity in Springset (S), decreased the total root radioactivity and increased total shoot radioactivity in both Vision (T) and Springset (S) and decreased metabolism of metribuzin to water soluble conjugates in Springset (S) roots over 24 h following 14C-metribuzin application to roots of intact tomato seedlings. The increased uptake, translocation to the shoots, and decreased root metabolism of metribuzin in Springset when pretreated with tridiphane could explain the phytotoxic interaction (which was unexpected, based only on the glucose detoxification pathway of metribuzin in tomato).

Laboratory experiments were conducted to determine the physiological mechanism of tall morningglory resistance to the experimental cotton herbicide DPX-PE350. Tall morningglory, a resistant species, was compared with entireleaf morningglory, a sensitive species, to evaluate inhibition at the site of action, the acetolactate synthase (ALS) enzyme (E.C.4.1.3.18), by DPX-PE350 as well as uptake, translocation, and metabolism of DPX-PE350. No differences were found between species in the concentration required to inhibit the ALS enzyme by 50% (I50), or in uptake and translocation of the herbicide. Tall morningglory metabolized the herbicide more rapidly than did entireleaf morningglory. Tall morningglory contained 3.6 and 1.4 times more metabolites of DPX-PE350 than did entireleaf morningglory 6 and 24 h after treatment, respectively. Tall morningglory produced an O-desmethyl metabolite from the 3,5-dimethoxypyrimidine moiety of DPX PE350 that was not found in entireleaf morningglory. These data suggest that the ability of tall morningglory to more rapidly metabolize DPX-PE350, possibly through the production of the pyrimidinyldesmethyl metabolite, may be the mechanism of resistance to DPX-PE350.

Greenhouse and laboratory experiments were conducted to determine the effect of DPX-PE350 on the herbicidal activity of fluazifop-P on large crabgrass. Greenhouse experiments indicated that fluazifop-P activity was antagonized by DPX-PE350. This antagonism was reduced by increasing fluazifop-P rates or by applying DPX-PE350 after fluazifop-P. Applying DPX-PE350 24 h prior to fluazifop-P reduced fluazifop-P activity more than applying it 10 min prior. The effect of DPX-PE350 application to large crabgrass on extractable acetyl-CoA carboxylase activity, and the effect of DPX-PE350 on acetyl-CoA carboxylase activity in vitro also was studied. Applying DPX-PE350 to large crabgrass prior to protein extraction did not reduce fluazifop-P inhibition of acetyl CoA carboxylase activity in vitro. The presence of DPX-PE350 in the enzyme assay mix did not affect fluazifop-P inhibition of acetyl-CoA carboxylase activity. The effect of DPX-PE350 on uptake, translocation, and metabolism of fluazifop-P was considered. DPX-PE350 did not affect uptake of fluazifop-P by large crabgrass. Applying DPX-PE350 24 h prior to fluazifop-P decreased translocation of fluazifop-P out of the treated leaf 44 to 53% whereas mixing DPX-PE350 with fluazifop-P decreased fluazifop-P translocation only 15 to 17%. Metabolism of fluazifop-P was not affected by applications of DPX-PE350. These studies suggest that the antagonism of fluazifop-P activity by DPX-PE350 is due to decreased translocation of fluazifop-P out of the treated leaf.

Experiments were conducted in controlled environment chambers to evaluate the effects of temperature and soil water content on the time of dissipation of commercial (CF) and starch encapsulated (SE) atrazine formulations to one-half the original concentration (T50). SE samples were also analyzed for the amount of atrazine remaining within the starch particles (percent encapsulation). The dissipation of CF atrazine was affected by changes in temperature and soil water content. SE atrazine dissipation was most influenced by changes in soil water content rather than temperature. Independent of soil water, there was no atrazine dissipation from any formulation at 15 C. The T50 for CF atrazine at 20% soil water content was 53.4 and 29.9 d for 25 and 35 C, respectively. At 20% soil water content, all SE treatments gave a T50 greater than 60 d. The percent starch encapsulation at 20% soil water content was greater than or equal to 55.8 and 30.4% for SE large and SE small, respectively. This high level of encapsulated atrazine accounts for the reduced SE dissipation observed at 20% soil water content. At 40% soil water content, the dissipation of CF and SE small atrazine were not different for either 25 or 35 C. Compared to the CF, the SE large formulation extended the T50 by 7.4 and 6.7 d at 25 and 35 C, respectively. At 40% soil water content, there was no encapsulated atrazine present in SE formulations 60 DAT.