Reid J. Smeda

Screening for Herbicide Resistance in Weeds1

Abstract: Diagnosing herbicide-resistant weeds as a first step in resistance management and monitoring their nature, distribution, and abundance demands efficient and effective screening tests. This review summarizes and recommends appropriate seed sampling techniques, protocols for screening weeds for resistance to herbicides of different sites of action, interpretation of results, and information given to the grower. Elements common to all screening procedures are reviewed. Choosing appropriate discriminating doses to distinguish between resistant and susceptible weed biotypes is the most important factor in achieving accurate and consistent results. Interpretation of results is also critical because resistant weeds may comprise a small portion of the population in suspected accessions or biotypes. Additional index words: Bioassay, discriminating dose, seed sampling, site of action, surveys. Abbreviations: ACCase, acetyl-CoA carboxylase (EC 6.4.1.2); ALS, acetolactate synthase (EC 4.1.3.18); AOPP, aryloxyphenoxy propionate; CHD, cyclohexanedione; DAT, days after treatment; EPSP synthase, 5-enolpyruvylshikimate-3-phosphate synthase (EC 2.5.1.19); KARI, ketol-acid reductoisomerase (EC 1.1.1.86); POST, postemergence; PRE, preemergence; R, resistant; S, susceptible.

Response of Sorghum (Sorghum bicolor) to Atrazine, Ammonium Sulfate, and Glyphosate1

Abstract: Experiments were conducted to determine whether antagonism between atrazine and glyphosate on shattercane observed in field studies could be duplicated under greenhouse conditions on ‘Rox Orange’ forage sorghum and whether it could be overcome by the addition of ammonium sulfate, other adjuvants, or additional glyphosate. Atrazine or surfactant added to glyphosate did not significantly affect sorghum dry weights compared to glyphosate alone. The Colby equation for synergism indicated that atrazine did not antagonize sorghum control with glyphosate in the greenhouse. Glyphosate at 0.43 kg ae/ha plus ammonium sulfate provided greater control of sorghum than glyphosate at 0.43 kg/ha without ammonium sulfate; however, glyphosate at 0.84 kg/ha plus ammonium sulfate did not provide greater control of sorghum than glyphosate at 0.84 kg/ha without ammonium sulfate. Reduced activity of glyphosate at 0.43 kg/ha in the absence of ammonium sulfate was likely due to an abundance of calcium cations in the carrier water that associated with glyphosate molecules and subsequently reduced herbicide uptake by plants. Thus, antagonism observed under cool conditions in field studies was not evident in controlled-temperature greenhouse studies. Nomenclature: Atrazine, 6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine; glyphosate, N-(phosphonomethyl)glycine; ‘Rox Orange’ sorghum and shattercane, Sorghum bicolor (L.) Moench #3 SORVU. Additional index words: Antagonism, surfactant.

Differential Susceptibility to Glyphosate among the Conyza Weed Species in Spain

Greenhouse and laboratory experiments were conducted to investigate differences in glyphosate susceptibility among three species of the genus Conyza introduced as weeds in Spain: tall fleabane (Conyza sumatrensis), hairy fleabane (Conyza bonariensis), and horseweed (Conyza canadensis). Plant material was obtained from seeds collected in weed populations growing in olive groves and citrus orchards in southern Spain, with no previous history of glyphosate application. Dose-response curves displayed ED(50) values of 2.9, 15.7, and 34.9 g ai ha(-1), respectively, for C. sumatrensis, C. bonariensis, and C. canadensis plants at the rosette stage (6-8 leaves). Significant differences were found among the three species in the glyphosate retention on leaves as well as the leaf contact angle. The species order according to glyphosate retention was C. sumatrensis > C. bonariensis > C. canadensis, while the mean contact angles of glyphosate droplets were 59.2, 65.5, and 72.9 degrees , respectively. There were no significant differences among species in the absorption of [(14)C]glyphosate (ranged from 37.4% for C. canadensis to 52.4% for C. sumatrensis), but the order among species was the same as glyphosate retention. The amount of radioactivity translocated from treated leaves was lower in C. canadensis as compared to the other two species (C. sumatrensis > C. bonariensis > C. canadensis). Combined, all of the studied parameters identified differential susceptibility to glyphosate among the Conyza species. Each species accumulated shikimate in leaf tissues following application of glyphosate at 200 g ai ha(-1). However, C. canadensis exhibited lower shikimate levels than the other two species at 168 h after herbicide application. For hairy fleabane, a greenhouse study explored its susceptibility to glyphosate at three developmental stages: rosette, bolting (stem height, 10-15 cm), and flowering. The ED(50) was lower at the rosette stage (15.7 g ai ha(-1)) as compared to bolting (86.6 g ai ha(-1)), with the highest ED(50) values occurring at flowering (117.5 g ai ha(-1)); plants at the earlier developmental stage retained more glyphosate. These results agree with field observations that plants at early developmental stages are more sensitive to glyphosate.

Weed Management Programs in Glufosinate-Resistant Soybean (Glycine max)1

Field trials were conducted at two sites in both 1997 and 1998 to evaluate soybean response and weed control with glufosinate alone or combined with quizalofop, lactofen, imazethapyr, flumiclorac, or bentazon plus acifluorfen in narrow-row, glufosinate-resistant soybean. Soybean injury ranged from 0 to 21% at 2 wk after treatment (WAT) and from 0 to 5% by 4 WAT. Glufosinate alone at 0.29 and 0.4 kg ai/ha controlled velvetleaf, common waterhemp, common ragweed, morningglory species, and giant foxtail greater than 85% in all studies. Mixtures containing glufosinate and other herbicides controlled these species greater than 81% but did not improve control over glufosinate alone. Estimates of weed biomass closely reflected visual control evaluations. However, giant foxtail biomass was higher for mixtures of glufosinate plus lactofen, flumiclorac, or bentazon and acifluorfen, indicating possible antagonism of glufosinate activity. At both locations, soybean yields were similar among most treatments, but that of the glufosinate plus lactofen treatment was lower when compared with other treatments. Additional trials evaluated soybean response and weed control with a preemergence herbicide followed by glufosinate postemergence (POST), glufosinate applied once or twice POST, and mixtures of glufosinate plus imazethapyr or flumiclorac POST in wide-row soybean. Glufosinate applied twice controlled common waterhemp, morningglory species, prickly sida, common cocklebur, and giant foxtail up to 39% greater than did glufosinate applied once. The addition of imazethapyr, but not flumiclorac, to glufosinate improved weed control when compared with glufosinate alone. Nomenclature: Acifluorfen; bentazon; flumiclorac; glufosinate; imazethapyr; lactofen; quizalofop; common cocklebur, Xanthium strumarium L. #3 XANST; common ragweed, Ambrosia artemisiifolia L. # AMBEL; common waterhemp, Amaranthus rudis Sauer # AMATA; giant foxtail, Setaria faberi Herrm. # SETFA; morningglory species, Ipomoea spp. # IPOSS; prickly sida, Sida spinosa L. # SIDSP; velvetleaf, Abutilon theophrasti Medik. # ABUTH; soybean, Glycine max (L.) Merr. ‘Asgrow 2704 LL’. Additional index words: Herbicide mixtures, herbicide-resistant crop, soybean injury. Abbreviations: COC, crop oil concentrate; EPOST, early postemergence; LPOST, late postemergence; MPOST, midpostemergence; POST, postemergence; PRE, preemergence; WAT, weeks after treatment.

Late-Emerging Common Waterhemp (Amaranthus rudis) Interference in Conventional Tillage Corn'

Waterhemp has emerged as one of the most problematic weeds in agronomic crops in the Midwest because of an extended germination period and widespread occurrence of biotypes resistant to atrazine and sulfonylurea herbicides. However, the competitive effects of late-emerging cohorts on corn yield are not known. Field studies were conducted in 2001 and 2002 at Columbia, Novelty, and Albany, MO, to determine the effects of late-emerging waterhemp interference on corn growth, nitrogen (N) accumulation, and yield. Waterhemp emerged approximately 20 d after planting (DAP) and was treated at heights of 8, 15, 23, 31, 38, or 46 cm with directed applications of dicamba + diflufenzopyr followed by hand hoeing. Soil water status, corn leaf chlorophyll content, and corn and common waterhemp height were recorded at the time of waterhemp removal. N stress was detected with a chlorophyll meter at four of six removal timings at high waterhemp densities (362 or more plants/m2) but only at one of six removal timings at lower densities (82 or less plants/m2). Water stress was observed at five of the six removal timings at high densities but at none of the removal timings at low densities. High waterhemp densities reduced corn yield when allowed to reach 15 cm before removal, and yields were reduced 36% when not controlled. At low densities, yield losses did not occur unless waterhemp was allowed to remain with corn season long. Our research suggests that waterhemp is less competitive with corn than redroot pigweed, smooth pigweed, and Palmer amaranth. In addition, low densities of late-emerging waterhemp would not warrant removal to protect corn yield. Nomenclature: Atrazine; dicamba; diflufenzopyr; sulfonylurea; common waterhemp, Amaranthus rudis Sauer. #3 AMATA; Palmer amaranth, Amaranthus palmeri (S.) Wats.; redroot pigweed, Amaranthus retroflexus L.; smooth pigweed, Amaranthus hybridus L.; corn, Zea mays L. ‘Pioneer 34B28’. Additional index words: Chlorophyll content, nitrogen accumulation, soil moisture deficit. Abbreviations: CEC, cation exchange capacity; DAP, days after planting; K, potassium; N, nitrogen; OM, organic matter; P, phosphorus; SPAD, single photon avalanche diode; TDR, time domain reflectrometry; VWC, volumetric water content.

Weed diversity and soybean yield with glyphosate management along a north–south transect in the United States

Abstract There are many concerns about the effects of repeated use of glyphosate in glyphosate-resistant (GR) crops, including two that are seemingly contradictory. These are (1) weed escapes and (2) loss of weed diversity. Weeds that escape glyphosate treatment represent species that likely will become troublesome and difficult to control in the future, and identifying these future problems may allow more effective management. In contrast, complete weed control directly reduces the weed component of agroecosystem biodiversity and may lower other components indirectly (e.g., weed-dependent granivores). During 2001 and 2002 effects of glyphosate and conventional weed control treatments on weed community composition and GR soybean yields were studied. Field studies were conducted along a north–south transect of sites spanning a distance of 1600 km from Minnesota to Louisiana. Low-intensity use (single application yr−1) of glyphosate allowed more escapes and maintained higher weed diversity than high-intensity use (two applications yr−1) of glyphosate, and it was equivalent to or even higher than diversity in non-GR systems. Although the same weeds escaped from low- and high-intensity glyphosate treatments, frequency of escapes was higher with less intensive use. These results suggest that limited use of glyphosate would not have profound effects on weed diversity. In addition, crop yield did not differ between GR and non-GR treatments at high latitudes, but below 40° N latitude, with a longer cropping season, yields with low-intensity glyphosate use decreased by about 2% per degree latitude because of competition from escaped weeds. Nomenclature: Soybean, Glycine max (L.) Merr.

Diurnal fluctuations and leaf angle reduce glufosinate efficacy

Velvetleaf plants have diurnal leaf movements, which may result in decreased interception of herbicides when applications are made near sunset. However, it is not known if leaf angle alone accounts for diurnal fluctuations in efficacy. Greenhouse experiments were conducted to determine the effect of time of day (TOD) of application and velvetleaf leaf angle on glufosinate efficacy and spray interception. Glufosinate at 90, 180, and 360 g ai/ha was applied to 10-cm-tall plants at 4:00, 6:00, 7:00, 7:30, and 8:00 p.m., respectively. Leaf angles were either manipulated physically to −90° or the plants natural 2:00 p.m. leaf angle (approximately −10°) or were allowed to exhibit their natural leaf movements. Plant dry weight 3 wk after treatment revealed that TOD effects were observed for all leaf angle treatments after glufosinate application at 90 g/ha. At 180 g/ha glufosinate, there was no TOD effect for plants with 2 p.m. leaf angles, whereas there was a TOD effect for plants with −90° and natural leaf angles. At 360 g/ha glufosinate, biomass for the −90° leaf angle plants was similar to that for the natural and the 2:00 p.m. leaf angle plants when glufosinate was applied at 4:00 p.m. but was significantly different at or after 6:00 p.m.. This suggests that at least 4 h of light is needed to provide optimum herbicide activity when spray interception is reduced as a result of leaf movements. Leaf angle decreased by as much as 70% from 4:00 to 8:00 p.m., which resulted in approximately 50% less spray interception at 8:00 p.m. than at 4:00 p.m. These data provide evidence that leaf angle plays a pivotal role in reducing glufosinate efficacy when applications are made near sundown. However, leaf angle is not the sole reason for reduced efficacy because TOD effects were observed at different leaf angles with 4 h of light, after an application of 360 g/ha glufosinate. Nomenclature: Glufosinate; velvetleaf, Abutilon theophrasti Medicus #3 ABUTH. Additional index words: Application timing, glufosinate, herbicide interception, time of day. Abbreviations: GS, glutamine synthetase; POST, postemergence; TOD, time of day; WAT, weeks after treatment.

Physiological basis for resistance to diphenyl ether herbicides in common waterhemp (Amaranthus rudis)

Abstract Common waterhemp seeds were collected from two Missouri soybean fields where biotypes were not controlled by acifluorfen. Plants grown from these seeds were tested for resistance to the diphenyl ether herbicides acifluorfen and lactofen. Resistance to susceptibility (R/S) ratios, calculated as the ratio of the dose required to inhibit dry weight accumulation by 50% (GR50) in resistant plants to that for susceptible plants, were 9.5 and 11 for the Meadville biotype and 28 and 44 for the Bethel biotype exposed to acifluorfen and lactofen, respectively. Electrolyte leakage assays determined that light-induced lipid peroxidation by acifluorfen was greatest on a control population (Bradford), intermediate for the Meadville biotype, and lowest for the Bethel biotype. Levels of the photodynamic pigment protoporphyrin IX (Proto) accumulating in leaf disks exposed to acifluorfen were much lower in the resistant biotypes than in the susceptible wild type, and the level of Proto accumulation was significantly correlated to the degree of membrane disruption. Although the binding of acifluorfen to protoporphyrinogen oxidase in chloroplasts may have been altered in the resistant biotypes, the molecular and biochemical factors involved in the mechanism of resistance remain to be fully characterized. However, this study establishes that the physiological basis for the evolved resistance to diphenyl ethers in common waterhemp rests on the reduction of Proto accumulation. Nomenclature: Acifluorfen; lactofen; common waterhemp, Amaranthus rudis Sauer AMATA; soybean, Glycine max (L.) Merr.

Use of preplant sulfentrazone in no-till, narrow-row, glyphosate-resistant Glycine max

Abstract Field studies were conducted in 1998 and 1999 to evaluate crop response, weed control, Glycine max yield, and economic returns with sulfentrazone alone and tank-mixed with glyphosate, cloransulam, or chlorimuron at two preplant application timings in no-till, narrow-row, glyphosate-resistant G. max. No significant crop injury was observed. Setaria faberi and Polygonum pensylvanicum control 5 wk after planting (WAP) was generally greater with sulfentrazone applied early preplant (EPP) than with sulfentrazone applied at planting (AP). When applied AP, glyphosate plus sulfentrazone provided greater S. faberi control than sulfentrazone alone. Control of Amaranthus rudis, Ambrosia artemisiifolia, and Ipomoea hederacea was greater in 1998 than in 1999 because of more timely early-season precipitation. Sulfentrazone-based programs provided 80 to 100% control of A. rudis in 1998, but control in 1999 ranged from 72 to 95% at Columbia and 46 to 83% at Novelty. Cloransulam alone, at either application timing, was the only treatment that provided greater than 80% control of A. artemisiifolia at each site in each year. All sulfentrazone-based treatments provided greater than 80% control of I. hederacea in 1998, but control was less in 1999 and ranged from 54 to 91%. Xanthium strumarium control ranged from 5 to 94% with sulfentrazone alone; however, the addition of cloransulam or chlorimuron provided 75 to 99% control regardless of application timing. A blanket application of glyphosate was made 6 WAP over all preplant herbicide treatments, and weed control 5 wk after this treatment was greater than 79% with all sulfentrazone-based treatments. Sulfentrazone plus cloransulam or chlorimuron plus glyphosate EPP or AP followed by (fb) glyphosate postemergence (POST) generally provided the greatest weed control. Overall weed control was generally greater with the use of residual herbicides vs. glyphosate alone, although yield and net returns were not always greater. A greenhouse study was conducted to determine if altering the preplant application timing reduced sulfentrazone injury to G. max. Treatment variables included herbicide rate, temperature during a preplant incubation period, and application timing. Glycine max, Zea mays, and Sorghum bicolor were used as indicator species. Sulfentrazone caused less injury to G. max, Z. mays, and S. bicolor in soils incubated at 30 C when applied 20 d before planting compared to 0 d before planting. Equivalent amounts of crop injury were noted with sulfentrazone applied 20 or 0 d before planting in soils incubated at 5 C with all indicator species. Nomenclature: Chlorimuron, cloransulam, glyphosate, sulfentrazone; Xanthium strumarium L. XANST, common cocklebur; Ambrosia artemisiifolia L. AMBEL, common ragweed; Amaranthus rudis Sauer AMATA, common waterhemp; Setaria faberi Herrm. SETFA, giant foxtail; Ipomoea hederacea (L.) Jacq. IPOHE, ivyleaf morningglory; Polygonum pensylvanicum L. POLPY, Pennsylvania smartweed; Zea mays L. ‘Pioneer 3394’, corn; Glycine max (L.) Merr. ‘Asgrow 3601’, ‘Pioneer 9362, soybean; Sorghum bicolor (L.) Moench’. ‘Pioneer 8500’, grain sorghum.

Influence of formulation and glyphosate salt on absorption and translocation in three annual weeds

Abstract Absorption and translocation of three commercial formulations of glyphosate, the isopropylamine salt formulated as Roundup Ultra™ (IPA1) and Roundup UltraMax™ (IPA2) and the diammonium salt formulated as Touchdown™ IQ (DA), were compared in three- to five-leaf velvetleaf, common waterhemp, and pitted morningglory. Absorption of 14C-glyphosate in velvetleaf was not significantly different among the three formulations up to 50 h after treatment (HAT). More absorption of 14C-glyphosate occurred in the IPA1 (26.0%) vs. the IPA2 (17.7%) formulation over 74 h. Of the total 14C-glyphosate absorbed, 20 to 35% was translocated from the treated leaf to the rest of the plant. Initial absorption of 14C-glyphosate was rapid in common waterhemp with the IPA1 (42.7%) and IPA2 (30.7%) formulations; both were higher compared with absorption of the DA formulation (11.5%) by 2 HAT. These differences continued up to 26 HAT, but no differences were evident by 74 HAT. Up to 65% of the 14C-glyphosate absorbed was translocated out of the treated leaf by 74 HAT, with roots the primary sink. Initial absorption of 14C-glyphosate was slow in pitted morningglory compared with the other species. More foliar absorption occurred in plants treated with the DA (13.6%) vs. the IPA2 formulation (4.9%) by 6 HAT. Absorption beyond 26 HAT was not different among the three glyphosate formulations. Translocation of 14C-glyphosate to roots was 27% greater as the DA salt than IPA1 and IPA2 by 74 HAT. The distribution pattern of glyphosate was similar in all species; phosphorimages demonstrated movement both acropetal and basipetal, with accumulation in roots greater than in any other plant parts. An efficacy study parallel to the 14C study showed no difference among the three glyphosate formulations on the species investigated at both 74 HAT and 2 wk after treatment. Nomenclature: Glyphosate; common waterhemp, Amaranthus rudis Sauer AMATA; pitted morningglory, Ipomoea lacunosa L. IPOLA; velvetleaf, Abutilon theophrasti Medicus ABUTH.