Multiple herbicide classes resistant (MHCR) kochia poses a serious concern for producers in the Central Great Plains, including western Kansas. Greenhouse and field experiments were conducted at Kansas State University Research and Extension Centers near Hays and Garden City, KS to evaluate pyridate-based postemergence (POST) herbicide mixtures for controlling MHCR kochia. One previously confirmed MHCR population (resistant to atrazine, glyphosate, dicamba, and fluroxypyr) and a susceptible (SUS) kochia population were tested in a greenhouse study. The kochia population at Hays field site was resistant to atrazine, dicamba, and glyphosate while kochia population at Garden City site was resistant to atrazine and glyphosate. Colby’s analysis revealed synergistic interactions when pyridate was mixed with atrazine, dicamba, dichlorprop-p, fluroxypyr, glyphosate, or halauxifen/fluroxypyr and resulted in ≥94% control and shoot dry biomass reduction of MHCR kochia in a greenhouse study. Similarly, synergistic interactions were observed for MHCR kochia control in fallow field studies at both sites when pyridate was mixed with glyphosate or atrazine. Kochia control was increased from 26% to 90% with the application of glyphosate + pyridate and from 28% to 95% with atrazine + pyridate at both sites as compared to separate applications of glyphosate or atrazine. This is the first report for such a strong synergistic effect for both glyphosate and atrazine mixtures with pyridate on a weed resistant to both. All other pyridate-based herbicide mixtures showed an additive interaction and resulted in better control of MHCR kochia (87 to 100%) as compared to their individual applications (23 to 92%) across both sites except 2,4-D. These results suggest that pyridate can play a crucial role in various POST herbicide mixtures for effective control of MHCR kochia.

Weeds are among the main limitations on chickpea production in Iran. The efficacy of herbicide treatments including linuron PPI, imazethapyr PPI, PRE, and POST, pendimethalin PPI and POST, bentazon POST, pyridate POST, and oxadiazon POST along with one or two hand weedings were evaluated for weed control and yield response in rain-fed chickpea in Aleshtar, Lorestan, Iran in 2015 and 2016. Wild safflower, threehorn bedstraw, wild mustard, and hoary cress were the predominant weed species in both experimental years. Total weed dry biomass in weedy check plots averaged 187 and 238 g m−2 in 2015 and 2016, respectively, and weed density and biomass were reduced in all treatments compared to the weedy check in both years. Treatments composed of pyridate followed by one hand weeding or imazethapyr POST followed by two hand weedings resulted in the lowest weed biomass. The presence of weeds reduced yield by 74% and 66% in the weedy check plots compared to the weed-free control plots in 2015 and 2016, respectively. Application of oxadiazon, bentazon, and imazethapyr PPI, PRE, and POST resulted in lower chickpea yields. All herbicides tested injured chickpea slightly, with pyridate causing the least injury.

Chickpea producers currently have no POST applied herbicides labeled for broadleaf weed control and rely heavily on PRE herbicides to manage weeds. Severe crop losses from broadleaf weed competition and harvest losses from weeds impeding harvest can occur when PRE herbicides perform poorly. Chickpea tolerance to POST applications of acifluorfen at 0.42 kg ai ha-1 and fomesafen at 0.28 kg ai ha-1 was tested at two sites in 2015. In 2016, both herbicides were tested on chickpeas when applied alone and in combination with pyridate at three sites. Acifluorfen and fomesafen injured chickpeas from 8 to 25% at 1 week after treatment (WAT) and 3 to 8% at 4 WAT in 2015 and from 16 to 40% at 1 WAT and 2 to 36% at 4 WAT in 2016. Pyridate applied POST at 1.00 kg ai ha-1 did not injure chickpeas or reduce yields. When pyridate was tank mixed with either acifluorfen or fomesafen, chickpea injury increased, but chickpeas recovered and yielded similar to nontreated checks or pyridate-treated plots. A low rate of metribuzin at 0.06 kg ai ha-1 tank mixed with pyridate had little impact on chickpea injury or weed control. In 2015, Russian thistle was controlled 100% by acifluorfen and fomesafen at Prosser at 28 DAT and both herbicides controlled the weed only 63% at Wilbur at 25 DAT. In 2016, all herbicide treatments reduced broadleaf weed densities equally ranging from 95 to 100% at Paterson, 50 to 100% at Prosser, and 78 to 98% at Wilbur. Chickpea yield was similar among POST herbicide treatments in all site-years. Acifluorfen, fomesafen, and pyridate have potential to improve control of susceptible broadleaf weeds that escape PRE herbicides chickpea production, but the potential for crop injury with acifluorfen and fomesafen warrant further evaluation.

Herbicidal control of triazine-resistant (TR) kochia was evaluated in no-till grain sorghum. Herbicides were applied early preplant (EPP) or POST. In EPP experiments, herbicides plus nonionic surfactant at 0.25% v/v were applied 7, 2, or 0 wk before planting (WBP). Fluorochloridone at 0.8 kg ai/ha applied 7 or 2 WBP, pyridate at 1.0 kg ai/ha applied 2 WBP, and paraquat at 0.4 kg ai/ha applied 0 WBP controlled 94 to 99% of TR kochia; a prepackaged mixture of glyphosate plus 2,4-D at 0.3 plus 0.6 kg ae/ha and paraquat at 0.4 kg/ha applied 7 or 2 WBP controlled 71 to 82% of TR kochia; and 2,4-D ester at 0.6 kg ae/ha applied 2 WBP controlled 75% of TR kochia. Linuron at 0.8 kg ai/ha and atrazine at 2.2 kg ai/ha were ineffective. In the POST herbicide experiments, where paraquat plus metolachlor at 0.6 plus 1.7 kg ai/ha were applied 17 d before planting, various combinations and rates of bentazon plus atrazine, bromoxynil, and dicamba with adjuvants provided good control of TR kochia that was less than 8 cm tall.

We conducted several experiments on linuron-resistant and -susceptible Portulaca oleracea and on atrazine-resistant and -susceptible Chenopodium album to determine their immediate and long-term responses to photosynthesis-inhibiting herbicides. Several photosynthesis-inhibiting herbicides were used, and O2 evolution was measured with a Clark-type O2 electrode. Resistance ratios (RRs) for P. oleracea, based on O2 evolution inhibition, were 8 and > 6 for linuron and diuron, respectively; > 800 for atrazine; and > 20 for terbacil. Linuron-resistant P. oleracea was negatively cross-resistant to bentazon and pyridate (RR = 0.5 and 0.75, respectively). Time-course measurements of fresh weight, photosynthetic CO2 assimilation, and photochemical efficiency indicated that linuron and atrazine inhibited electron transport in susceptible (S) P. oleracea and C. album, ultimately resulting in death. Measurements of photochemical efficiency and CO2 assimilation of linuron-resistant P. oleracea treated with linuron indicated a transient injury from which plants recovered within 14 d. Recovery of linuron-resistant P. oleracea from atrazine injury was more rapid than from linuron injury for all measured variables. Atrazine-resistant C. album had no cross-resistance to linuron. Sequence analysis of the D1 protein revealed that linuron-resistant P. oleracea had a serine-to-threonine substitution at position 264.

Seven postemergence (POST) herbicides were evaluated alone and in various mixtures for Palmer amaranth control in peanut from 1992 through 1994. AC 263,222 at rates as low as 0.04 kg/ha controlled ≥ 95% Palmer amaranth. Imazethapyr at rates of 0.05 to 0.07 kg/ha controlled ≥ 90% Palmer amaranth in 2 of 3 yr. Acifluorfen alone or in combination with bentazon or 2,4-DB and lactofen alone controlled > 90% Palmer amaranth in 2 of 3 yr. Bentazon or pyridate failed to provide adequate control (≤ 60%), while 2,4-DB provided good control (80%) in only 1 of 3 yr.

Field studies were conducted from 1986 through 1988 to evaluate various herbicides for yellow nutsedge control and peanut yields. Three applications of pyridate provided control comparable to two applications of bentazon with yellow nutsedge regrowth beginning 3 to 4 wk after application depending on moisture conditions. Crop oil concentrate did not improve the activity of pyridate. Flurtamone provided control comparable with that of metolachlor. Nutsedge control with fomesafen was erratic with peanut injury noted. Peanut yields did not reflect the competitive nature of nutsedge.

DC X2-5309 organosilicone adjuvant added at 1% (v/v) spray solution increased the efficacy of bentazon, pyridate, and terbacil on kochia in greenhouse trials. DC X2-5309 decreased the rain-free period needed after application of bentazon and bromoxynil to 6 and 1 h, respectively, and increased the uptake of 14C-bentazon and 14C-bromoxynil by kochia at 1, 3, 6, and 24 h after application compared with crop oil concentrate (COC). In field trials, bentazon and bromoxynil rates required to control equal amounts of kochia were reduced by half when adding DC X2-5309 compared with adding crop oil concentrate. DC X2-5309 added at 0.5% (v/v) spray solution with bentazon did not injure spearmint and controlled kochia equal to or better than bentazon with COC added at 1% (v/v) spray solution.

Eight POST herbicides were evaluated alone or in various mixtures for spiny amaranth control in peanut in 1991 and 1992. Acifluorfen alone or in combination with bentazon, lactofen alone, or 2,4-DB alone applied late POST controlled spiny amaranth more than 80% at an early season rating. Lactofen applied early POST and 2,4-DB applied late POST controlled more than 90% late season. Imazethapyr and AC 263,222 controlled 72 to 90% of spiny amaranth, whereas bentazon and pyridate failed to provide adequate control.

Field experiments were conducted from 1985 through 1989 to evaluate herbicide selectivity and impact on seeding-year yields of spring oat and underseeded alfalfa, and carryover weed control benefits from increased legume-fixed N for second-year dryland no-till corn. PRE metolachlor, pendimethalin, and prodiamine controlled green foxtail and POST bromoxynil or 2,4-DB controlled broadleaf weeds. These herbicides caused 0 to 20% alfalfa injury and 0 to 17% oat injury, and increased oat yield one of three years but did not increase the yield of underseeded alfalfa. POST pyridate, thifensulfuron, and tribenuron were too injurious to either oat, alfalfa, or both crops. Forage yields of annual ‘Nitro’ and perennial ‘Wrangler’ alfalfa seeded alone were greater than when they were underseeded in oat, with herbicides applied in both systems. As a result of drought in 1988 and 1989, yield of second-year corn planted after one-year alfalfa was not increased from potentially greater legume-fixed N. Dryland corn yield following monoculture oat or corn was 254% higher than corn following alfalfa.

Greenhouse and field experiments were conducted to evaluate the control of eastern black nightshade and velvetleaf in tomato with bentazon, metribuzin, pyridate, and acifluorfen. Metribuzin controlled velvetleaf but not eastern black nightshade, whereas pyridate controlled eastern black nightshade but not velvetleaf. A combination of metribuzin + pyridate controlled both weeds, but caused tomato injury 4 wk after application. Bentazon neither controlled the weeds nor injured the crop. Acifluorfen controlled both eastern black nightshade and velvetleaf. Diammonium phosphate and oxysorbic (5 POE) improved bentazon and acifluorfen activity against velvetleaf. Although initial crop injury from some treatments applied prior to fruit initiation was severe, the crop recovered and there were no reductions in fruit weight compared with the control. However applying acifluorfen with oxysorbic (5 POE) during fruit development reduced yield 50%.

Field experiments in 1988 and 1989 evaluated five POST herbicides alone or combined for horse purslane control in peanut. Acifluorfen, acifluorfen plus bentazon, pyridate plus 2,4-DB, and lactofen controlled more than 75% when rated up to 26 days after treatment (DAT). However at 67 DAT, acifluorfen alone, acifluorfen plus bentazon or 2,4-DB, or lactofen applied EPOST controlled horse purslane greater than 60% in 1988. In 1989, only 2,4-DB or pyridate plus 2,4-DB controlled horse purslane at least 70% at 116 DAT. Pyridate alone did not control horse purslane adequately.

Field experiments were conducted from 1995 to 1997 to test approaches to managing atrazine-resistant Datura stramonium. Eight weed management programs in a Zea mays and Glycine max rotation were evaluated for their effects on the dynamics of atrazine-resistant and -susceptible D. stramonium populations. Overall D. stramonium density and relative abundance of resistant (R) biotypes were greatly reduced in no-till Secale cereale L. (rye) cover crop management programs without triazine compared to conventional-tillage systems with the application of triazine herbicides. The negative effects of no-till on D. stramonium were greater under a G. max–Z. mays—G. max (SCS) rotation than under a Z. mays–G. max—Z. mays (CSC) rotation. A cropping system involving more G. max phases under no-till reduced both the resistant and susceptible D. stramonium populations. Results from this study support the use of soil management, crop rotation, and negative cross-resistant herbicides to manage atrazine-resistant biotypes.

Greenhouse and field experiments were conducted to identify alternative herbicide treatments for the control of a biotype of American black nightshade which has been shown to tolerate paraquat. In greenhouse experiments the combination of paraquat and the metal chelator diethyldithiocarbamate at either 1% w/v or 3% w/v, and diquat alone significantly lowered I50 values, based on dry weight, when compared with paraquat alone. None of these treatments provided acceptable control (>90%) in field experiments. Treatments that controlled American black nightshade under field conditions were: monocarbamide dihydrogensulfate at 9 kg ai ha-1, monocarbamide dihydrogensulfate at 9 kg ha-1 + paraquat at 0.6 kg ha-1, lactofen at 0.6 kg ha-1, oxyfluorfen at 0.6 kg ha-1, and acifluorfen at 0.6 kg ha-1. Addition of tridiphane at 1.7 kg ha-1 to paraquat at 0.6 kg ha-1 did not improve control of American black nightshade under field conditions.

Tank-mixed combinations of pyridate and 2,4-DB were synergistic with respect to sicklepod control and independent with respect to Florida beggarweed control and peanut tolerance in greenhouse studies. Leaf absorption of 14C-pyridate was generally enhanced by the addition of 2,4-DB. Translocation of 14C-pyridate out of the treated leaf did exceed 21% of the amount absorbed across all species, and 14C-pyridate was not affected by the addition of 2,4-DB. Sensitivity of peanut, Florida beggarweed, and sicklepod to pyridate was independent of both adsorption and translocation. Metabolic detoxification of pyridate into a benign metabolite was highest in the tolerant peanut, lowest in highly susceptible Florida beggarweed, and intermediate in slightly susceptible sicklepod. Absorption of 14C-2,4-DB by peanut was increased by approximately 10% with the addition of pyridate. No such increase was observed in either Florida beggarweed or sicklepod. Translocation of 14C-2,4-DB out of the treated leaf was not affected by the addition of pyridate in any of the three species evaluated.

Control of Texas panicum and southern crabgrass with sethoxydim was reduced under field conditions when bentazon, acifluorfen, bentazon + acifluorfen, pyridate, or naptalam plus 2,4-DB was added. The addition of lactofen to sethoxydim did not result in antagonism. Complete control of annual grasses did not appear to be a necessity in order to improve peanut yields. Peanut yields in plots with 70 to 80% control of annual grasses were comparable to those where annual grass control was 99%.

Field studies were conducted from 1995 through 1998 to evaluate citronmelon control with postemergence (POST) herbicides. Imazapic at any application timing and late postemergence (LPOST) applications of 2,4-DB were the only herbicides that provided >80% control of citronmelon late season. Other herbicides such as acifluorfen, imazethapyr, lactofen, and pyridate provided acceptable (>75%) early-season control of citronmelon, but control was inadequate at harvest.

Field studies were conducted near Archer, FL, and Vienna, GA, in 1995 and 1996 to investigate the effects of pyridate formulation and adjuvants on broadleaf weed control in peanut (Arachis hypogaea). Pyridate formulations SAN 319H 450EC 361LZ, SAN 319H 450EC 216LZ, and SAN 319H 600EC 418LZ were evaluated at two rates, 1.07 and 2.14 kg/ha. Pyridate at 1.07 kg/ha plus 2,4-DB at 0.23 kg/ha were evaluated alone and with five adjuvants. The adjuvants included a crop oil concentrate, a nonionic surfactant, a nonionic surfactant with organosilicone blend, urea ammonium nitrate plus a nonionic surfactant, and chlorothalonil (a fungicide) plus a nonionic surfactant. No pyridate treatment injured peanut. Pyridate formulation did not affect broadleaf weed control. Increasing pyridate rate increased weed control and yield. Mixing 2,4-DB with pyridate generally enhanced sicklepod (Senna obtusifolia) and common cocklebur (Xanthium strumarium) control. Florida beggarweed (Desmodium tortuosum), smallflower morningglory (Jacquemontia tamnifolia), hairy indigo (Indigofera hirsuta), sicklepod, and common cocklebur control with pyridate was not enhanced by adjuvants. Adding chlorothalonil to pyridate plus 2,4-DB did not affect weed control or peanut injury.

Lentil is vulnerable to weed competition because of its short stature, slow establishment, and limited vegetative growth. Although the vast majority of lentil production is under rain-fed conditions, there is a little published information on weed control with herbicides in rain-fed lentils. Field experiments were conducted to determine the efficacy of nine herbicide treatments including fomesafen, imazethapyr, linuron, pendimethalin, and pyridate alone or in combination compared with one or two hand weeding(s) on weed control and yield response in rain-fed lentil in Khorramabad, Iran in 2012 and 2013. Weed species included catchweed bedstraw, cowcockle, haresear mustard, hoary cress, wild mustard, and wild safflower. Total weed dry biomass in weedy check plots averaged 156 and 170 g m−2 in 2012 and 2013, respectively, and weed density and biomass were reduced in all treatments compared to the weedy check in both years. Plots that were hand weeded twice reduced weed biomass the greatest, whereas fomesafen, linuron, or one hand weeding did not control weeds satisfactorily. Noncontrolled weeds reduced lentil yield by 67% both years compared to the weed-free control. Lentil yield in 2013 (1,370 kg ha−1) was higher than in 2012 (1,150 kg ha−1). All herbicides tested injured lentil slightly, with pyridate (1,200 g ai ha−1) and pendimethalin (660 g ai ha−1 plus imazethapyr at 250 to 500 g ai ha−1) causing the least injury. Across all treatments, imazethapyr plus pendimethalin PRE, pyridate POST, and two hand-weeding treatments had the best performance for weed control and lentil yield.

Winter sowing of lentil results in higher yields than a spring-sown crop. However, when lentil is winter-sown, the critical period of weed control is extended and might require sequential herbicide management strategies to obtain maximum yields. The efficacy of POST alone or soil applied followed by (fb) POST herbicides or hand weeding were evaluated in 2002 and 2003. Trifluralin (PPI) or pendimethalin (PRE) were applied at planting, and pyridate or oxyfluorfen were applied POST at the three- to four-leaf stage of lentil. The lowest level of weed biomass was recorded with all treatments, except pyridate alone in both years, pendimethalin PRE fb pyridate in 2002, and oxyfluorfen in 2003. Application of pyridate or oxyfluorfen POST alone resulted in lower yields. In 2002, trifluralin PPI or pendimethalin PRE fb one hand weeding or pendimethalin fb pyridate resulted in yields similar to the weed-free treatment. In 2003, yields were similar to the weed-free treatment, except pyridate or oxyfluorfen alone and trifluralin PPI fb oxyfluorfen. Weed species observed in the present study required sequential management to achieve both effective control and maximum lentil yield.