A 4-yr field test was initiated in 1975 to determine susceptibility of a natural infestation of six morningglory species to herbicides which can be applied to the soil and foliage in soybeans [Glycine max (L.) Merr.]. The species studied were pitted morningglory (Ipomoea lacunosa L.), entireleaf morningglory [Ipomoea hederacea (L.) Jacq. var. integriuscula], ivyleaf morningglory [Ipomoea hederacea (L.) Jacq.], purple moonflower [Ipomoea muricata (L.) Jacq.], palmleaf morningglory [Ipomoea wrightii (Gray)], and small flower morningglory [Jacquemontia tamnifolia (L.) Griseb.]. Control with herbicides applied preplant incorporated and preemergence was dependent on morningglory species and on rainfall to allow plant uptake of the herbicide. Herbicides applied to the soil were not as effective as those applied postemergence. Oxadiazon [2-tert-butyl-4-(2,4-dichloro-5-isopropoxyphenyl)-δ2-1,3,4-oxadiazolin-5-one] gave the best and longest lasting preemergence control, averaging 79% control for all species. Preemergence control with metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] was dependent on species, with 84% control of small flower morningglory and only 26% for ivyleaf and 18% for entireleaf morningglory. At V2 (one trifoliolate) and V5 stages of soybean growth, acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} at 0.56 kg/ha applied over-the-top and oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene] at 0.28 kg/ha applied post-directed gave 90 and 92% control of all species, respectively. Most effective were repeated post-directed applications at V5 and V7 stages of soybean growth. Metribuzin + 2,4-DB [4-(2,4-dichlorophenoxy)butyric acid] (99%), linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] + 2,4-DB (98%), 2,4-DB (95%), and paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) + 2,4-DB (93%) provided excellent control regardless of the morningglory species.

Greenhouse experiments were conducted to determine the persistence of imazapyr and the tolerance of a wide variety of bioassay plants to imazapyr residues in central Texas soils. Eleven bioassay crops were planted in pots 1 d before and 2, 6, and 12 mo after imazapyr application at rates of 0, 0.02, 0.04, 0.07, 0.14, 0.28, 0.56, and 1.12 kg ai ha−1. Imazapyr was phytotoxic 12 mo after treatment to all crop plants in at least one rate. Forage grasses and herbs were highly susceptible to imazapyr residual carryover damage, as was corn and cabbage. Bean and squash were tolerant of imazapyr residues. No statistical differences occurred in imazapyr phytotoxicity to corn grown in leached vs. nonleached pots.

Root absorption of 14C-5 (ring)-metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] from hydroponic culture and its subsequent translocation in tolerant (‘Russet Burbank’) and susceptible (‘Chipbelle’) potato (Solanum tuberosum L.) cultivars were measured. Differences in root absorption did not appear to be as important to differential tolerance as did translocation differences. Total available metribuzin absorbed was 6 and 7%, 18 and 29%, and 31 and 45% after 1, 4, and 8 days for Russet Burbank and Chipbelle, respectively. Radioactivity was concentrated in stems, petioles, and leaf veins in the tolerant cultivar, whereas interveinal leaf tissue was the major accumulation site for 14C in the susceptible cultivar. Of the total radioactivity in the plants after 8 days, 82 and 87% were in the foliage for the tolerant and the susceptible cultivar, respectively. Translocation was apoplastic. Metribuzin tolerance by Russet Burbank is due in part to restricted translocation to the leaf blades. A model for differential transport between cultivars is proposed.

Root and foliar absorption and translocation of 14C-metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] were determined in tolerant (‘Steptoe’) and susceptible (‘Morex’) barley (Hordeum vulgare L.) cultivars grown in nutrient solution culture under greenhouse conditions. Root-applied metribuzin toxicity to the two barley cultivars was also examined. A 50% reduction in growth occurred at 0.22 and 0.72 μM metribuzin for Morex and Steptoe, respectively. Root absorption was similar for both cultivars at 1 day, but Steptoe absorbed about two times more metribuzin 4 and 8 days after application than did Morex. Steptoe absorbed 19% of the total applied metribuzin by 8 days. Root absorption was positively correlated with water uptake (r≥ 0.87). Leaf absorption was three times greater for Morex than for Steptoe after 4 days. Transport to leaves from root application was rapid, and, by 1 and 8 days, leaves of Steptoe contained 71 and 82% and leaves of Morex 78 and 84% of the total absorbed 14C, respectively. Translocation was apoplastic following both root and leaf absorption and was similar for both cultivars. Differential tolerance could partially be accounted for by differences in foliar absorption but not by differences in root absorption.

A wild population of a plant species, especially a cross-pollinated species, can display considerable genetic variation. Genetic variability is evident in differential susceptibility to an herbicide because the population can show continuous phenotypic variation. Recent, recurrent selection studies have revealed that phenotypic variation in response to low herbicide rates is heritable and can result in rapid evolution of herbicide resistance in genetically variable cross-pollinated rigid ryegrass. In this study, the heritable genetic variation in an herbicide-susceptible rigid ryegrass population was exploited to shift the population toward greater herbicide susceptibility by recurrent selection. To enhance herbicide susceptibility, herbicide-susceptible rigid ryegrass plants were divided into two identical clones, and one series of cloned plants was treated with a low rate of herbicide (diclofop). The nontreated clones of individuals that did not survive the herbicide treatment were selected and bulk-crossed to obtain the susceptible progeny. After two cycles of selection, the overall susceptibility to diclofop was doubled. The results indicate that minor genes for resistance are present in an herbicide-susceptible rigid ryegrass population, and their exclusion can increase susceptibility to diclofop.

Field studies were conducted in Fayetteville, AR, to determine the response of 38 pitted morningglory accessions to fomesafen, chlorimuron, and glyphosate when applied postemergence over-the-top (POT) at 9 g ai/ha, 420 g ai/ha, and 840 g ae/ha, respectively, to four-leaf, 15-cm-tall pitted morningglory. Visual control following chlorimuron application ranged from 82 to 95% 3 wk after treatment (WAT). Visual control of accessions with fomesafen ranged 34 to 84% 3 WAT. Variability in visual control following fomesafen application was documented among accessions collected from similar geographic locations in west-central and southwest Arkansas, central and southeast Louisiana, and west-central Mississippi. Glyphosate controlled pitted morningglory accessions 81 to 89% 3 WAT. Data indicate that weed management programs should be field specific where fomesafen use is intended.

Field studies were conducted to compare the response of one inbred (‘CL161’) and two hybrid (‘CLXL729’ and ‘CLXL745’) Clearfield (CL) rice cultivars to imazamox. Imazamox was applied at 44 and 88 g ai ha−1 to rice in the panicle initiation (PI) and PI plus 14 d (PI + 14) growth stages and at 44 g ha−1 to rice in the midboot growth stage. Maturity of hybrid CL cultivars was delayed following imazamox at 44 g ha−1 applied at PI + 14 and midboot. Furthermore, imazamox at 44 g ha−1, applied at midboot, delayed maturity of CLXL745 more than CLXL729. Expressed as a percentage of the weed-free control plots, rough rice yields for CLXL729 were 91% following imazamox at 44 g ha−1 applied at PI + 14, 78% following imazamox at 44 g ha−1 applied at midboot, and 77% for imazamox at 88 g ha−1 applied at PI + 14. Rough rice yield for CLXL745 was 77 to 92% of the control following all imazamox treatments. All imazamox treatments reduced CLXL745 rough rice yield compared with CL161. Rough rice yield, pooled across CL cultivar, varied with imazamox treatment between years, and these differences may have been a consequence of lower temperatures and solar radiation in the first year. Hybrid CL cultivars CLXL729 and CLXL745 were less tolerant than was CL161 when imazamox was applied at nonlabeled rates (88 g ha−1) and/or timings (PI + 14 or midboot). Because of variability in rice growth stages and irregularities in imazamox application in commercial fields, inbred CL cultivars should be planted where an imazamox application will likely be required.

Field studies were conducted to evaluate the tolerance of peanut cultivars ‘Florunner’, ‘Georgia Green’, ‘Sunoleic 95R’, ‘AgriTech GK7’, ‘NC-7’, ‘ViruGard’, and ‘Spanco’ to sulfentraone. Herbicide treatments included sulfentrazone applied as a single treatment preemergence (PRE) at 0.14, 0.21, 0.28, 0.35, or 0.42 kg ai/ha or as a PRE followed by (fb) an at cracking (AC) application (0.14 kg ai/ha PRE fb 0.14 kg ai/ha AC, 0.21 kg ai/ha PRE fb 0.14 kg ai/ha AC, 0.21 kg ai/ha PRE fb 0.21 kg ai/ha AC, 0.28 kg ai/ha PRE fb 0.07 kg ai/ha AC, or 0.28 kg ai/ha PRE fb 0.14 kg ai/ha AC). Imazapic and paraquat applied early postemergence (EPOT) were included along with a weed-free control. NC-7 exhibited higher early-season injury (ranging from 1 to 29%) than other cultivars across all sulfentrazone applications. However, this injury did not affect yield when compared with the untreated weed-free check. Overall, peanut tolerance to sulfentrazone was high across all varieties.

Rice cultivar, growth stage at application, or both may influence rice tolerance to quinclorac. Field studies were conducted to compare the response of five rice cultivars ‘Bowman’, ‘Cheniere’, ‘CL161’, ‘Cocodrie’, and ‘XL723’ to postflood quinclorac applications. Quinclorac at 0.56 kg ai ha−1 was applied 2 and 4 wk after flood (WAF). Pooled across quinclorac application timings, no differences in maturity were detected among the cultivars in 2008, but maturity of Cheniere and XL723 were delayed compared with CL161 and Cocodrie in 2007. Maturity of Cheniere and XL723 was delayed in 2007 compared with 2008. Pooled over cultivar, maturity was similar for 2 and 4 WAF applications in 2007 but was delayed for 2 WAF treatments in 2008. Regardless of year, postflood quinclorac applications reduced rough rice yield of all cultivars except Bowman. Cheniere and XL723 had lower rough rice yields compared with other cultivars in 2007; however, in 2008, rough rice yields of Cheniere, CL161, Cocodrie, and XL723 were similar, but still lower, than that of Bowman. Pooled over cultivar, postflood quinclorac reduced rough rice yields more when applied 4 WAF than at 2 WAF during both years. Our results demonstrate that Cheniere and XL723 are less tolerant than Bowman is to postflood quinclorac applications and that all evaluated cultivars are more susceptible to quinclorac applied at later developmental stages. Consequently, if circumstances necessitate a postflood quinclorac application, the herbicide should be applied no later than panicle initiation and should not be applied to Cheniere or XL723.