Field experiments were initiated near Colt, AR, in the fall of 2016 and continued through the summer of 2018 to evaluate rice tolerance and weedy (or red) rice control after fall-applied very-long-chain fatty acid (VLCFA)-inhibiting herbicides. A split-plot design was used for the experiment, with the whole-plot factor being winter condition (flooded or non-flooded) and the split-plot factors being herbicide and rate. Herbicide treatments included acetochlor, dimethenamid-P, pethoxamid, pyroxasulfone, and S-metolachlor applied at 1,050, 525, 420, 205, and 1,070 g ai ha−1 and at 2,100, 1,050, 840, 410, and 2,140 g ha−1 for low rates and high rates, respectively. Herbicides were applied in the fall, then ‘CL172’ rice was drill seeded in the spring of the following calendar year. Weedy rice control differed between years, but acetochlor and pyroxasulfone consistently provided the greatest levels of control across rates and flood conditions. Consequently, herbicides that best controlled weedy rice also caused the greatest injury to cultivated rice. Rice injury did not exceed 13% regardless of herbicide treatment at 3 wk after planting (WAP). However, the high rate of pyroxasulfone caused 20% rice injury at 5 WAP in 2018. Although it was expected that winter condition may affect residual activity of the VLCFA-inhibiting herbicides, herbicide selection and application rate both had much greater effects on rice injury and on weedy rice control. Based on these results, rice injury would be minimal or nonexistent after fall applications of the tested VLCFA inhibitors, and intermediate levels of weedy rice control may be achieved. The implementation of VLCFA-inhibiting herbicides in rice production systems would offer a novel herbicide site of action and offer a degree of selective control of weedy rice.

The evolution of herbicide resistance is making it extremely difficult for US rice producers to use chemical control on weed species such as barnyardgrass and red rice. To combat herbicide resistance, it is imperative that alternative herbicide sites of action (SOAs) be incorporated into rice whenever possible. There are currently no very-long-chain fatty acid–inhibiting herbicides (WSSA Group 15) labeled for use in US rice; however, pethoxamid is one such herbicide currently under development. If appropriate rice tolerance and weed control can be established, pethoxamid would represent a unique herbicide SOA for use in US rice. We conducted field trials near Stuttgart, AR, in 2015 and near Colt and Lonoke, AR, in 2016 to assess selectivity of pethoxamid and weed control alone and in combination with other herbicides as a delayed preemergence (DPRE) application in drill-seeded rice. Pethoxamid was applied at 0, 420, or 560 g ai ha–1 alone and in combination with clomazone, imazethapyr, pendimethalin, and quinclorac. Minimal rice injury occurred with any treatment assessed. A reduction in rice shoot density and plant height compared to the nontreated control followed the use of pethoxamid; however, no decrease in yield resulted. The highest levels of barnyardgrass control followed the use of imazethapyr at 91% and quinclorac at 89% regardless of the presence of pethoxamid near Lonoke; however, pethoxamid applied at both rates in combination with clomazone and quinclorac increased barnyardgrass control compared to clomazone and quinclorac applied alone. Near Colt, barnyardgrass control of 92% and 96% resulted from pethoxamid alone, averaged over the high and low rates. Based on these data, rice can tolerate pethoxamid when applied DPRE, and adequate levels of barnyardgrass control can be achieved at the rates evaluated within a program; hence, pethoxamid appears to be a viable option for use in rice to allow for increased rotation of herbicide SOAs to combat herbicide-resistant and difficult-to-control weeds.

Herbicide resistance to several of the most common weed species in US rice production, such as barnyardgrass and red rice, has made weed control extremely difficult with available herbicide options. No very-long-chain fatty acid–inhibiting herbicides are labeled for use in US rice; however, pethoxamid is one such herbicide under development for soil-applied use to control grasses and small-seeded broadleaves in rice and various row crops. Field trials were conducted in 2015 and 2016 near Stuttgart, AR, for rice tolerance and in 2016 near Colt, AR, and Lonoke, AR, for weed control with the use of pethoxamid-containing rice herbicide programs. Pethoxamid was applied alone and in a program at 420 and 560 g ai ha–1 with other herbicides labeled in rice including clomazone, quinclorac, propanil, imazethapyr, and carfentrazone POST. Injury less than 10% was seen for all treatments 2 wk after application in 2015 and 2016, except for pethoxamid at 420 g ha–1 to clomazone to one-leaf rice. Rice injury dissipated to less than 5% following all treatments by 4 wk after flood establishment. Barnyardgrass was controlled 95% or more near Colt and 93% or more near Lonoke for herbicide programs including clomazone PRE followed by pethoxamid plus quinclorac or imazethapyr at three- to four-leaf stage rice. Considering the minimal injury and high levels of barnyardgrass control associated with pethoxamid-containing weed control programs, pethoxamid provides a unique and effective site of action for use in US rice production.

Rigid ryegrass is the most-troublesome, herbicide-resistant weed in cropping systems of southern Australia. Field experiments were undertaken at Roseworthy, South Australia, in 2013 and 2014, to identify effective herbicide options for the control of clethodim-resistant rigid ryegrass in Clearfield canola. PPI trifluralin + triallate followed by (fb) POST imazamox + imazapyr + clethodim + butroxydim had the lowest plant density of rigid ryegrass in 2014 and provided superior control compared with the standard grower practice of PPI trifluralin + triallate fb POST imazamox + imazapyr + clethodim in 1 of 2 yr. Propyzamide either alone or as a split application (PPI fb POST) or in combination with clethodim provided similar rigid ryegrass control to that of the standard grower practice (38 to 553 plants m−2). Rigid ryegrass treated with PPI dimethenamid-P, pethoxamid, pethoxamid + triallate, and PPI trifluralin fb carbetamide POST produced significantly more seeds than the standard grower practice, which would lead to reinfestation of subsequent crops. Canola yield responded positively to effective herbicide treatments, especially in 2014, when rigid ryegrass density was greater. PPI dimethenamid-P and pethoxamid alone or in combination with triallate and propyzamide were ineffective in reducing rigid ryegrass density and seed production to levels acceptable for continuous cropping systems.