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Glufosinate serves as both a primary herbicide option and a complement to glyphosate and other postemergence herbicides for managing herbicide-resistant weed species. Enhancing broadleaf weed control with glufosinate through effective mixtures may mitigate further herbicide resistance evolution in soybean and other glufosinate-resistant cropping systems. Two field experiments were conducted in 2020 and 2021 at four locations in Wisconsin (Arlington, Brooklyn, Janesville, and Lancaster) and one in Illinois (Macomb) to evaluate the effects of postemergence-applied glufosinate mixed with inhibitors of protoporphyrinogen oxidase (PPO) (flumiclorac-pentyl, fluthiacet-methyl, fomesafen, and lactofen; Group 14 herbicides), bentazon (a Group 6 herbicide), and 2,4-D (a Group 4 herbicide) on waterhemp control, soybean phytotoxicity, and yield. The experiments were established in a randomized, complete block design with four replications. The first experiment focused on soybean phytotoxicity 14 d after treatment (DAT) and yield in the absence of weed competition. All treatments received a preemergence herbicide, with postemergence herbicide applications occurring between the V3 and V6 soybean growth stages, depending on the site-year. The second experiment evaluated the effect of herbicide treatments on waterhemp control 14 DAT and on soybean yield. Lactofen, applied alone or with glufosinate, produced the greatest phytotoxicity to soybean at 14 DAT, but this injury did not translate into yield loss. Mixing glufosinate with 2,4-D, bentazon, and PPO-inhibitor herbicides did not increase waterhemp control, nor did it affect soybean yield compared to when glufosinate was applied alone, but it may be an effective practice to reduce selection pressure for glufosinate-resistant waterhemp. Nomenclature: Bentazon; glufosinate; flumiclorac-pentyl; fluthiacet-methyl; fomesafen; waterhemp; Amaranthus tuberculatus (Moq.) Sauer; soybean, Glycine max (L.) Merr.

Greenhouse and laboratory studies were conducted to confirm and quantify glyphosate resistance, quantify pyrithiobac resistance, and investigate interaction between flumiclorac and glyphosate mixtures on control of Palmer amaranth from Mississippi. The GR50 (herbicide dose required to cause a 50% reduction in plant growth) values for two glyphosate-resistant biotypes, C1B1 and T4B1, and a glyphosate-susceptible (GS) biotype were 1.52, 1.3, and 0.09 kg ae ha−1 glyphosate, respectively. This indicated that the C1B1 and T4B1 biotypes were 17- and 14-fold resistant to glyphosate, respectively, compared with the GS biotype. The C1B1 and T4B1 biotypes were also resistant to pyrithiobac, an acetolactate synthase (ALS) inhibitor, with GR50 values of 0.06 and 0.07 kg ai ha−1, respectively. This indicated that the C1B1 and T4B1 biotypes were 7- and 8-fold, respectively, more resistant to pyrithiobac compared with the GS biotype, which had a GR50 value of 0.009 kg ha−1. Flumiclorac was antagonistic to glyphosate by reducing glyphosate translocation. The C1B1 and T4B1 absorbed less glyphosate 48 h after treatment (HAT) compared with the GS biotype. The majority of the translocated glyphosate accumulated in the shoot above the treated leaf (that contains the apical meristem) in the GS biotype and in the shoot below the treated leaf in the resistant biotypes, C1B1 and T4B1, by 48 HAT. The C1B1 biotype accumulated negligible shikimate levels, whereas the T4B1 and GS biotypes recorded elevated levels of shikimate. Metabolism of glyphosate to aminomethylphosphonic acid was not detected in either of the resistant biotypes or the susceptible GS biotype. The above results confirm multiple resistance to glyphosate and pyrithiobac in Palmer amaranth biotypes from Mississippi and indicate that resistance to glyphosate is partly due to reduced absorption and translocation of glyphosate. Nomenclature: Flumiclorac; glyphosate; pyrithiobac; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA.

In a repeated multi-year study, mother potato plants were exposed to herbicides at rates that simulated off-target application, such as through tank contamination. Following exposure of mother plants to herbicides, seed from mother plants was planted in the following growing season and crop growth, yield and tuber quality were quantified. Visual injury from herbicides was observed both in the mother plant and daughter tuber growing seasons and occasional impacts on tuber yield were noted. However, an inconsistent relationship was observed for herbicide related injury and tuber yield reductions of mother potato plants with daughter tuber growth and yield. The lack of consistency in the relationship between visual potato injury in the mother plant production and adverse daughter tuber growth and yield in the following year challenges traditional crop scouting as a tool to predict off-target herbicide risk near seed potato production. Nomenclature: 2,4-D; aminopyralid; cloransulam-methyl; dicamba; flumiclorac; fluthiacet; glyphosate; mesotrione; metribuzin; metsulfuron-methyl; S-metolachlor; tembotrione; thifensulfuron-methyl; topramezone; tribenuron-methyl; potato; Solanum tuberosum L. En un estudio repetido varios años, plantas madres de papa fueron expuestas a herbicidas a dosis que simularon aplicaciones accidentales, tales como las que se dan por contaminación en el tanque. Después de la exposición de las plantas madre a los herbicidas, semilla de estas plantas madre fue plantada en la siguiente temporada de crecimiento y se cuantificó el crecimiento del cultivo, el rendimiento y la calidad de los tubérculos. El daño visual causado por los herbicidas fue observado tanto en las temporadas de crecimiento de la planta madre como el de las plantas hijas y ocasionalmente se notó un impacto en el rendimiento de tubérculos. Sin embargo, se observó una relación inconsistente entre el daño y reducciones en el rendimiento de tubérculos en las plantas de papa madre causados por el herbicida y el crecimiento y rendimiento de tubérculo de plantas hijas. La ausencia de consistencia en la relación entre daño visual en la papa durante la producción de la planta madre y efectos adversos en el crecimiento y rendimiento de plantas hijas el siguiente año desafían la utilidad del muestreo tradicional del cultivo como herramienta para predecir el riesgo de daño accidental por herbicidas cerca de áreas para la producción de semilla de papa.

Injury to weeds from sublethal doses of POST herbicides may reduce the effect of weed interference on crop yield. Information on how herbicide dose influences weed mortality, growth, and seed production is needed to assess the potential benefit of applying reduced herbicide doses. Field experiments were conducted at Mead, NE, in 2001 and 2002 to quantify velvetleaf mortality, growth, and corn–velvetleaf interference in response to varying doses of three POST herbicides. Untreated velvetleaf at six densities (0, 1, 3, 6, 12, and 20 plants m−1 corn row) was grown in mixture with corn to establish a baseline corn–velvetleaf interference relationship. Treated velvetleaf at a density of 20 plants m−1 row received one of five doses of dicamba, halosulfuron, or flumiclorac. Untreated velvetleaf height, biomass, and seed capsule production were greater in 2002 than 2001 and declined with increasing velvetleaf density in both years. Corn yield was not affected by untreated velvetleaf in 2001, but yield loss increased with increasing velvetleaf density in 2002. Mortality of herbicide-treated velvetleaf was 56% greater in 2001 than 2002 and increased with increasing herbicide dose. Maximum height of treated velvetleaf was similar for all treatments in 2001 but declined with increasing herbicide dose in 2002. Biomass and seed production of treated velvetleaf varied among herbicides in 2002 and decreased with increasing dose. Corn yield was not influenced by velvetleaf in 2001, but yield loss in response to herbicide-treated velvetleaf declined with increasing herbicide dose in 2002. Results show that the assumption that weeds surviving herbicide application are as competitive as untreated weeds is incorrect. Reduction in growth and resource consumption by herbicide-damaged weeds reduced the negative effects of weeds on corn. Nomenclature: Dicamba; halosulfuron; flumiclorac; velvetleaf; Abutilon theophrasti Medic. ABUTH; corn; Zea mays L.

Common dandelion has developed into a troublesome agronomic weed for no-tillage corn producers. A postemergence herbicide application is often required to reduce common dandelion competition. Field experiments were conducted in 2002 and 2003 to evaluate 22 postemergence herbicide treatments for efficacy on established populations of common dandelion in no-tillage corn. All herbicides were applied to five- to six-collar corn at registered rates with typical adjuvants. At 28 d after treatment (DAT) the most effective treatments included glufosinate and mesotrione providing at least 76% control of common dandelion. All other herbicide treatments provided less than 40% common dandelion control 28 DAT. Common dandelion control was evaluated 56 DAT when regrowth of treated plants was observed for some herbicide treatments. AT 56 DAT, dicamba + diflufenzopyr was the most effective treatment, providing 83% control of common dandelion. In 2002, all herbicide treatments, with the exception of flumiclorac, resulted in corn yields greater than the nontreated. Nomenclature: Dicamba, diflufenzopyr, flumiclorac, glufosinate, mesotrione, common dandelion, Taraxacum officinale Weber TAROF, corn, Zea mays L

The effect temperature, light intensity, time to initial light exposure, relative humidity, and the presence of dew have on CGA-248757 and flumiclorac efficacy was evaluated in laboratory trials. Increasing temperature from 10 to 40 C increased CGA-248757 and flumiclorac activity on common lambsquarters by 79 and 87%, respectively. Similarly, increasing temperature from 10 to 40 C increased CGA-248757 and flumiclorac activity on redroot pigweed by 68 and 60%, respectively. Increasing light intensity from 0 to 1,000 μmol m−2 s−1 increased CGA-248757 activity on common lambsquarters and redroot pigweed by 92 and 93%, while flumiclorac activity increased 91 and 99%. Time to initial light exposure and relative humidity did not affect CGA-248757 or flumiclorac activity on common lambsquarters and redroot pigweed. The presence of dew reduced herbicidal activity of both compounds on common lambsquarters by 5% and redroot pigweed control with CGA-248757 and flumiclorac by 21 and 20%, respectively. Field applications of CGA-248757 or flumiclorac at 6:00 A.M., 2:00 P.M., and 10:00 P.M. indicate environmental conditions at application strongly influence soybean tolerance and weed control with CGA-248757 and flumiclorac. The greatest soybean injury occurred from CGA-248757 or flumiclorac applications at 6:00 A.M. compared with applications at 2:00 P.M. or 10:00 P.M. Common lambsquarters control was greatest when CGA-248757 or flumiclorac was applied at 6:00 A.M. or 2:00 P.M. compared with 10:00 P.M. However, redroot pigweed control was greatest when CGA-248757 or flumiclorac was applied at 2:00 P.M. Application time of day did not affect velvetleaf control with either herbicide.

Greenhouse and field experiments were conducted to evaluate the potential for antagonistic or synergistic interactions from tank mixtures of glyphosate plus a selective herbicide applied postemergence. In the greenhouse, glyphosate at 420 g ae ha−1 plus 28% liquid urea-ammonium nitrate (28% UAN) provided at least 89% control of common lambsquarters and common ragweed. Glyphosate at 1,680 g ha−1 plus 28% UAN provided less than 81% control of velvetleaf and less than 75% control of ivyleaf morningglory. Tank mixing bentazon at 1,120 g ai ha−1 with glyphosate at 420 g ha−1 synergistically increased control of velvetleaf. Tank mixtures of glyphosate plus a selective herbicide were predominately additive in control of common lambsquarters, common ragweed, and velvetleaf. Several tank mix combinations of chlorimuron or imazethapyr plus glyphosate plus 28% UAN were antagonistic in control of ivyleaf morningglory. In the field, glyphosate at 840 g ha−1 plus 28% UAN provided at least 88% control of common lambsquarters and velvetleaf in 1994. However, glyphosate at 840 g ha−1 plus 28% UAN provided only 60% control of velvetleaf in 1995. Tank-mixing bentazon or CGA-248757 with glyphosate at 420 g ha−1 increased velvetleaf and common lambsquarters control in 1995. In general, adding chlorimuron, imazethapyr, or thifensulfuron to glyphosate plus 28% UAN did not increase control of common lambsquarters or velvetleaf. Tank mixing imazethapyr with glyphosate plus 28% UAN antagonized velvetleaf control in 1994 and in 1995. The tank mixture of thifensulfuron at 2 g ha−1 plus glyphosate at 420 g ha−1 plus 28% UAN increased soybean injury in the field in 1994. However, tank mixing chlorimuron, imazethapyr, or thifensulfuron with glyphosate plus 28% UAN did not increase soybean injury in the greenhouse or in the field in 1995.

Knowledge of how reduction in the rate of herbicide application or rotation of their mode of action influences weed growth will provide insight into how successful these practices will be in an integrated weed management program. Field experiments were conducted in 1996 and 1997 to quantify velvetleaf growth response to three postemergence herbicides, each with a different mode of action. A monoculture of velvetleaf was treated with halosulfuron, dicamba, and flumiclorac at 0, 0.10, 0.25, 0.50, 0.75, and 1.0 × the labeled rate for weed control in corn. Percent plant mortality increased with rate of application; the greatest mortality occurred in flumiclorac treatments in 1996 and in halosulfuron and flumiclorac treatments in 1997. Growth rate temporarily decreased as application rate increased. Maximum height decreased as rate of application increased, with the dicamba treatment resulting in the greatest (27%) reduction. Early-season leaf area index decreased with increasing rate of application, the greatest reduction occurring with halosulfuron (1997) and flumiclorac (1996 and 1997) treatments. The number of leaves produced per plant was temporarily reduced by all treatments, but treatment with dicamba later resulted in larger numbers of small leaves. The number of velvetleaf seed capsules produced per surviving plant was not reduced by any treatment, but the number of capsules per square meter was reduced by the 0.5 × rate of flumiclorac (1996) and the 0.5- and 1.0 × rates of halosulfuron (1997). Research is needed to evaluate whether the temporary suspension of velvetleaf growth after herbicide treatment is sufficient to prohibit crop yield reduction and velvetleaf capsule production.

The effect temperature, light intensity, time to initial light exposure, relative humidity, and the presence of dew have on CGA-248757 and flumiclorac efficacy was evaluated in laboratory trials. Increasing temperature from 10 to 40 C increased CGA-248757 and flumiclorac activity on common lambsquarters by 79 and 87%, respectively. Similarly, increasing temperature from 10 to 40 C increased CGA-248757 and flumiclorac activity on redroot pigweed by 68 and 60%, respectively. Increasing light intensity from 0 to 1,000 μmol m−2 s−1 increased CGA-248757 activity on common lambsquarters and redroot pigweed by 92 and 93%, while flumiclorac activity increased 91 and 99%. Time to initial light exposure and relative humidity did not affect CGA-248757 or flumiclorac activity on common lambsquarters and redroot pigweed. The presence of dew reduced herbicidal activity of both compounds on common lambsquarters by 5% and redroot pigweed control with CGA-248757 and flumiclorac by 21 and 20%, respectively. Field applications of CGA-248757 or flumiclorac at 6:00 a.m., 2:00 p.m., and 10:00 p.m. indicate environmental conditions at application strongly influence soybean tolerance and weed control with CGA-248757 and flumiclorac. The greatest soybean injury occurred from CGA-248757 or flumiclorac applications at 6:00 a.m. compared with applications at 2:00 p.m. or 10:00 p.m. Common lambsquarters control was greatest when CGA-248757 or flumiclorac was applied at 6:00 a.m. or 2:00 p.m. compared with 10:00 p.m. However, redroot pigweed control was greatest when CGA-248757 or flumiclorac was applied at 2:00 p.m. Application time of day did not affect velvetleaf control with either herbicide. Nomenclature: CGA-248757, [[2-chloro-4-fluoro-5-[(tetrahydro-3-oxo-1H,3H-[1,3,4]thiadiazolo[3,4-a]pyridazin-1-ylidene)amino]phenyl]thio]aceate; flumiclorac; common lambsquarters, Chenopodium album L. CHEAL; redroot pigweed, Amaranthus retroflexus L. AMARE; soybean, Glycine max (L.) Merr. ‘Conrad’ GLYMA; velvetleaf, Abutilon theophrasti Medik. ABUTH.

Greenhouse and laboratory studies were conducted to determine the physiological basis for CGA-248757 and flumiclorac selectivity in five plant species. CGA-248757 and flumiclorac selectively control weeds postemergence (POST) by inhibiting protoporphyrinogen oxidase (Protox). Injury symptoms from CGA-248757 and flumiclorac include rapid desiccation and necrosis similar to injury from diphenyl ether and bipyridinium herbicides. Species sensitivity to CGA-248757 and flumiclorac was evaluated by comparing the dry weight reduction from POST applications. Abutilon theophrasti was sensitive to both herbicides, Amaranthus retroflexus was more sensitive to flumiclorac than CGA-248757, Brassica kaber was sensitive to CGA-248757 but tolerant of flumiclorac, and Zea mays and Glycine max were tolerant of both herbicides. Studies evaluated CGA-248757 and flumiclorac retention, absorption, translocation, and metabolism. Enhanced herbicide metabolism contributed to the tolerance of A. retroflexus to CGA-248757 and B. kaber to flumiclorac. Decreased herbicide retention, absorption, and translocation coupled with increased metabolism contributed to Z. mays tolerance of CGA-248757 and flumiclorac. Decreased herbicide retention and increased herbicide metabolism provided G. max tolerance of both herbicides.