Due to the ongoing evolution of herbicide-resistant weeds, new technologies are needed to maintain effective levels of control. A new rice variety that will be resistant to quizalofop, an acetyl coenzyme A carboxylase-(ACCase-) inhibiting herbicide, is currently under development. With the anticipated launch of this technology in 2018, multiple experiments were conducted to determine effectiveness of the quizalofop-resistant rice system for common grass weed species found in Arkansas rice production. One hundred and twenty-six barnyardgrass populations were collected across Arkansas and treated with quizalofop at 80 g ai hato determine a baseline of response. All populations evaluated were effectively controlled (≥92%) by quizalofop, with only 13 populations resulting in lower than 98% control. A greenhouse and field trial were conducted to compare efficacy of quizalofop to currently labeled rice graminicides for control of common rice grass weeds. Results from the greenhouse experiment showed that quizalofop treatments resulted in greater efficacy of common grass weeds compared to cyhalofop or fenoxaprop. This was especially apparent at the larger grass growth stages. A field experiment conducted compared season-long weed control programs of quizalofop to fenoxaprop and cyhalofop. The quizalofop-containing treatments were no better than fenoxaprop and cyhalofop for barnyardgrass and broadleaf signalgrass control. Barnyardgrass and broadleaf signalgrass control were greater than 96% for all herbicide treatments. An additional field experiment was conducted to determine the best rate structure for sequential applications of quizalofop in rice. Sequential applications of quizalofop at 120 g ha −1 followed by 120 g ha −1 two weeks later resulted in the highest barnyardgrass and broadleaf signalgrass control. Likewise, applying the full seasonal use rate of 240 g ha −1 of quizalofop resulted in greater control compared to 200 and 160 g ha −1 . Results from this research indicate a strong benefit from quizalofop use in rice.

Field experiments were conducted in 2014 and 2015 in Fayetteville, Arkansas, to evaluate the residual activity of acetyl-CoA carboxylase (ACCase)–inhibiting herbicides for monocot crop injury and weed control. Conventional rice, quizalofop-resistant rice, grain sorghum, and corn crops were evaluated for tolerance to soil applications of six herbicides (quizalofop at 80 and 160 g ai ha–1, clethodim at 68 and 136 g ai ha–1, fenoxaprop at 122 g ai ha–1, cyhalofop at 313 g ai ha–1, fluazifop at 210 and 420 g ai ha–1, and sethoxydim at 140 and 280 g ai ha–1). Overhead sprinkler irrigation of 1.3 cm was applied immediately after treatment to half of the plots, and the crops planted into the treated plots at 0, 7, and 14 d after herbicide treatment. In 2014, injury from herbicide treatments increased with activation for all crops evaluated, except for quizalofop-resistant rice. At 14 d after treatment (DAT) in 2014, corn and grain sorghum were injured 19% and 20%, respectively, from the higher rate of sethoxydim with irrigation activation averaged over plant-back dates. Conventional rice was injured 13% by the higher rate of fluazifop in 2014. Quizalofop-resistant rice was injured no more than 4% by any of the graminicides evaluated in either year. In 2015, a rainfall event occurred within 24 h of initiating the experiment; thus, there were no differences between activation via irrigation or by rainfall. However, as in 2014, grain sorghum and corn were injured 16% and 13%, respectively, by the higher rate of sethoxydim, averaged over plant-back dates. All herbicides provided little residual control of grass weeds, mainly broadleaf signalgrass and barnyardgrass. These findings indicate the need to continue allowing a plant-back interval to rice following a graminicide application, unless quizalofop-resistant rice is to be planted. The plant-back interval will vary by graminicide and the amount of moisture received following the application.

With the widespread occurrence of herbicide-resistant weeds in midsouthern U.S. rice, new technologies are needed to achieve adequate weed control. A new non–genetically modified rice trait has been commercialized that is resistant to quizalofop, an acetyl coenzyme A carboxylase (ACCase)-inhibiting herbicide. The addition of quizalofop-resistant rice to production systems will increase the use of quizalofop, possibly increasing the risk for injury to other grass crops. Experiments were conducted in 2014 and 2015 to determine the sensitivity of corn, grain sorghum, and conventional rice to low rates of quizalofop (1/10× to 1/200× of 160 g ai ha–1). Conventional rice was not affected by quizalofop rate or application timing. Corn displayed the greatest response to the 1/10× quizalofop rate at the two- to three-leaf stage, with 50% to 65% injury and 35% to 37% relative yield compared to the nontreated check. Grain sorghum was injured 31% to 34% by the 1/10× quizalofop rate applied at the two- to three-leaf stage, and there was 20% to 26% injury at the panicle exertion growth stage. The highest rate of quizalofop at the panicle exertion stage reduced yields 28% to 46%. Overall, risk for injury to any of the three evaluated crops from quizalofop appears low, with greatest injury observed at the highest quizalofop drift rate, with minimal injury at lower rates.

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To combat herbicide‐resistant weeds, new options are needed to effectively rotate herbicide modes of action and slow the development of additional herbicide resistance.  Thiencarbazone‐methyl (TCM), an acetolactate synthase (ALS)‐inhibiting herbicide, is currently being evaluated for preemergence (PRE) and postemergence (POST) activity on many troublesome summer annual and perennial weeds.  Multiple experiments were conducted to assess the utility of this product in soybean (Glycine max) production systems.  Field experiments were conducted across Arkansas in 2016 and 2017 to determine the tolerance of ALS‐resistant soybean varieties to PRE and POST applications of TCM at either 0.03 or 0.06 lb acre–1.  For all locations, no significant interactions or main effects were observed for any parameters evaluated.  Overall, soybean injury remained consistently low, with <3% injury at 21 days after treatment (DAT) on all ALS‐resistant varieties.  However, the ALS‐susceptible variety showed high levels of injury for both PRE (75%) and POST (98%) applications at 21 DAT.  Two additional experiments were conducted in 2016, 2017, and 2018 to evaluate the efficacy of PRE and POST applications of TCM in ALS/glufosinate‐resistant soybean.  For PRE applications, residual activity of TCM, imazethapyr, chlorimuron, nicosulfuron, halosulfuron, and flumetsulam at labeled rates for soybean or corn were evaluated.  TCM at 0.06 lb acre–1 was effective in controlling ivyleaf morningglory (91 %) and broadleaf signalgrass (97%) at 21 DAT, but was not effective in controlling ALS‐resistant Palmer amaranth (58%).  In the POST experiment, TCM at 0.03 or 0.06 lb acre–1 was compared to imazethapyr, chlorimuron, nicosulfuron, and halosulfuron with and without glufosinate (0.5 lb acre–1).  Alone, TCM produced higher control of ivyleaf morningglory (99 to 100%) and broadleaf signalgrass (94 to 99%) at 21 DAT compared to all other ALS‐inhibiting herbicides.  However, TCM alone was not effective in controlling ALS‐resistant Palmer amaranth (55 to 61%) at 21 DAT, but when combined with glufosinate control increased to 95 to 99%.  The results from this research suggest that TCM could have utility in soybean weed control systems due to the high level of resistance exhibited by ALS‐resistant soybean varieties and the efficacy of TCM on common soybean weed species at both the PRE and POST application timings.