Palmer amaranth is a common weed on levees in rice fields but has become increasingly problematic with adoption of furrow-irrigated rice and the lack of an established flood. Florpyrauxifen-benzyl has previously been found effective for controlling Palmer amaranth in rice, but the efficacy of low rates of florpyrauxifen-benzyl and the effect of Palmer amaranth size on control is unknown. The objective of this research was to find the level of Palmer amaranth control expected with single and sequential applications of florpyrauxifen-benzyl at varying weed heights. The first study was conducted at the Lon Mann Cotton Research Station near Marianna, AR, in 2019 and 2020 to determine the effect of florpyrauxifen-benzyl rate on control of <10 cm (labeled size) and 28- to 32-cm tall (larger-than-labeled size) Palmer amaranth. The second experiment was conducted in 2020 at Pine Tree Research Station and Lon Mann Cotton Research Station to compare single applications of florpyrauxifen-benzyl at low rates to sequential applications at the same rates with a 14-day interval on 20- and 40-cm tall Palmer amaranth. Results revealed that florpyrauxifen-benzyl at 15 g ae ha−1 was as effective as 30 g ae ha−1 in controlling <10-cm tall Palmer amaranth (92% and 95% mortality in 2019). Sequential applications of florpyrauxifen-benzyl at 8 g ae ha−1 was as effective as single or sequential applications at 30 g ae ha−1. However, no rate of florpyrauxifen-benzyl applied to 20- or 40-cm tall Palmer amaranth was sufficient to provide season-long control of the weed, with the escaping female plants producing as many as 6,120 seed per plant following a single application.

Environmental conditions surrounding herbicide applications are known to affect weed control and crop response. Variable levels of rice injury caused by florpyrauxifen-benzyl have been observed across cropping systems and environmental conditions, warranting research in which single environmental and management strategies are isolated to understand the effect of each factor on rice injury and subsequent reductions in rice growth. A field study was conducted to determine the effects of planting date, rice cultivar, and florpyrauxifen-benzyl rate on rice injury, maturity, and yield. Two greenhouse studies were conducted to determine the effect of soil moisture and time of flooding after florpyrauxifen-benzyl application on rice injury caused by the herbicide. Growth chamber experiments were conducted to isolate the effects of temperature and light intensity on rice injury caused by florpyrauxifen-benzyl. In the field study, levels of injury varied across planting dates in both years, indicating the influence of environment on the crop response to florpyrauxifen-benzyl applications. Under dry (40% soil moisture) and saturated (100%) soil conditions, rice injury increased to 36 and 35%, respectively, compared to 27 and 25% at 60 and 80% soil moisture, respectively. Flooding rice 0 to 6 days after florpyrauxifen-benzyl application reduced visible injury; however, a reduction in rice tiller production occurred when the rice was flooded the same day as application. Visible rice injury increased when florpyrauxifen-benzyl was applied under low light intensity (700 µmol m−2 s−1) and high temperatures (35/24 C day/night). Based on these findings, applications of florpyrauxifen-benzyl are least likely to cause unacceptable rice injury when applied to soils having 60 and 80% saturation in high light, low temperature environments, and the crop is flooded three to six days following application.

Varying levels of sensitivity to florpyrauxifen‐benzyl and benzobicyclon applications among rice (Oryza sativa L.) cultivars have been observed. The safe use of florpyrauxifen‐benzyl and benzobicyclon in the same herbicide program could provide broad‐spectrum weed control and delay resistance in weed species. Two experiments were conducted in 2019 and 2020 near Stuttgart, AR, to test cultivars’ sensitivity to multiple rates of florpyrauxifen‐benzyl and applications of florpyrauxifen‐benzyl followed by benzobicyclon. The first experiment was designed to determine the sensitivity of the rice cultivars ‘XP753’, ‘CL XL745’, ‘Gemini 214CL’, ‘Titan’, and ‘Diamond’ to applications of florpyrauxifen‐benzyl at 0.03 and 0.06 lb acid equivalent (a.e.) acre–1 and sequential applications of 0.03 lb a.e. acre–1. The second experiment determined the effect of benzobicyclon applied post‐flood on rice injury following an application of florpyrauxifen‐benzyl on the same cultivars used in the previous experiment. A rough rice yield loss of 17% was observed for sequential applications of florpyrauxifen‐benzyl at 0.03 lb a.e. acre–1 to XP753. Single applications of florpyrauxifen‐benzyl at 0.06 lb a.e. acre–1 caused a 45% groundcover reduction in XP753, whereas sequential applications of 0.03 lb a.e. acre–1 reduced groundcover on all hybrid cultivars tested. Applications of benzobicyclon alone never injured rice by more than 8%, and applying benzobicyclon after florpyrauxifen‐benzyl did not increase the observed injury over florpyrauxifen‐benzyl alone. Single applications of florpyrauxifen‐benzyl followed by benzobicyclon can be used in an herbicide program to control weeds without the risk of greater injury than is typically caused by florpyrauxifen‐benzyl alone.