No abstract

Herbicide-resistant Echinochloa spp. pose a significant threat to U.S. rice production. Two surveys were conducted to characterize Echinochloa resistance to common rice herbicides and provide important demographic information on the populations in Arkansas: one was the Echinochloa Herbicide Resistance Confirmation Survey conducted annually since 2006; the other was the Echinochloa Herbicide Resistance Demographics Survey conducted since 2010. The Resistance Confirmation Survey showed that resistance to propanil (50%) was most prevalent, followed by quinclorac (23%), imazethapyr (13%), and cyhalofop (3%). Multiple resistance increased with time, with 27% of accessions being multiple-resistant, mostly to propanil+quinclorac (12%). The parallel Resistance Demographics Survey tested resistance by species. Of the 264 accessions collected, 73% were junglerice, 14% were rough barnyardgrass, and 11% were barnyardgrass. Overall, this survey also showed resistance to propanil (53%) and quinclorac (28%) being most prevalent, with low frequencies of resistance to cyhalofop (12%) and imazethapyr (6%). Resistance to herbicides was less frequent with barnyardgrass (54%) and rough barnyardgrass (28%) than with junglerice (73%). Multiple resistance was most frequent with junglerice (33%) and least frequent with rough barnyardgrass (8%). Across both surveys, the resistance cases were clustered in the northeast and Grand Prairie regions of the state. Herbicide resistance among Echinochloa populations in rice fields is continuing to increase in frequency and complexity. This is a consequence of sequential selection with different major herbicide sites of action, starting with propanil followed by quinclorac and others.

Changes in the environment, specifically rising temperature and increasing atmospheric carbon dioxide concentration [CO 2 ], can alter the growth and physiology of weedy plants. These changes could alter herbicide efficacy, crop-weed interaction, and weed management. The objectives of this research were to quantify the effects of increased atmospheric [CO 2 ] and temperature on absorption, translocation and efficacy of cyhalofop-butyl on multiple-resistant (MR) and susceptible (S) Echinochloa colona genotypes. E. colona , or junglerice, is a troublesome weed in rice and in agronomic and horticultural crops worldwide. Cyhalofop-butyl is a grass herbicide that selectively controls Echinochloa spp. in rice. Maximum 14 C-cyhalofop-butyl absorption occurred at 120 h after herbicide treatment (HAT) with >97% of cyhalofop-butyl retained in the treated leaf regardless of [CO 2 ], temperature, or genotype. Neither temperature nor [CO 2 ] affected herbicide absorption into the leaf. The translocation of herbicide was slightly reduced in the MR plants vs. S plants either under elevated [CO 2 ] or high temperature. Although plants grown under high [CO 2 ] or high temperature were taller than those in ambient conditions, neither high [CO 2 ] nor high temperature reduced the herbicide efficacy on susceptible plants. However, herbicide efficacy was reduced on MR plants grown under high [CO 2 ] or high temperature about 50% compared to MR plants at ambient conditions. High [CO 2 ] and high temperature increased the resistance level of MR E. colona to cyhalofop-butyl. To mitigate rapid resistance evolution under a changing climate, weed management practitioners must implement measures to reduce the herbicide selection pressure. These measures include reduction of weed population size through reduction of the soil seedbank, ensuring complete control of current infestations with multiple herbicide modes of action in mixture and in sequence, augmenting herbicides with mechanical control where possible, rotation with weed-competitive crops, use of weed-competitive cultivars, use of weed-suppressive cover crops, and other practices recommended for integrated weed management.

South African lovegrass (Eragrostis plana Nees) is the most important weed of native pastures in southern Brazil. Management options are limited under water stress conditions and glyphosate has been the main tool for control. This study compared four salts of glyphosate applied at three growth stages, and determined the glyphosate tolerance level. In addition, the performance of ammonium sulfate (AMS) under two soil moisture conditions (50% and 100% of water holding capacity), and the effect of AMS on absorption and translocation of radiolabeled 14C-glyphosate were evaluated. The potassium salt of glyphosate had the fastest activity across growth stages of E. plana, which is more vulnerable to glyphosate at panicle initiation stage. Isopropylamine salt was the slowest-acting glyphosate formulation. Younger plants were typically more easily controlled than older plants at full tillering stage. The addition of AMS increased the level of control of drought-stressed E. plana, compared to glyphosate alone, by increasing translocation out of the treated leaf and consequently increasing the concentration of glyphosate in the primary culm. This data can be used to plan an effective management program for E. plana considering the developmental stage of desired pasture grass species.

Herbicide-resistant Echinochloa species are among the most problematic weeds in agricultural crops globally. Recurring herbicide selection pressure in the absence of diverse management practices has resulted in greater than 20% of sampled Echinochloa populations from rice (Oryza sativa L.) fields demonstrating multiple resistance to herbicides in Arkansas, USA. We assessed the resistance profile and potential mechanisms of resistance in a multiple herbicide–resistant junglerice [Echinochloa colona (L.) Link] (ECO-R) population. Whole-plant and laboratory bioassays were conducted to identify the potential mechanisms of non–target site resistance in this population. ECO-R was highly resistant to propanil (>37,800 g ha−1) and quinclorac (>17,920 g ha−1) and had elevated tolerance to cyhalofop (R/S = 1.9) and glufosinate (R/S = 1.2) compared to the susceptible standard. The addition of glufosinate (590 g ha−1) to cyhalofop (314 g ha−1), propanil (4,500 g ha−1), or quinclorac (560 g ha−1) controlled ECO-R 100%. However, cyhalofop applied with propanil (48% control) or quinclorac (15% control) was antagonistic. The application of the known metabolic enzyme inhibitors malathion, carbaryl, and piperonyl butoxide increased control of ECO-R with propanil (>75%) but not with other herbicides. Neither absorption nor translocation of [14C]cyhalofop or propanil was different between ECO-R and ECO-S. [14C]Quinclorac absorption was also similar between ECO-R and ECO-S; however, translocation of quinclorac into tissues above the treated leaf of ECO-R was >20% higher than that in ECO-S. The abundance of metabolites was higher (∼10%) in the treated leaves of ECO-R than in ECO-S beginning 48 h after treatment. The activity of β-cyanoalanine synthase, which detoxifies hydrogen cyanide, was not different between ECO-R and ECO-S following quinclorac treatment. Resistance to propanil was due to herbicide detoxification by metabolic enzymes. Resistance to quinclorac was due to a detoxification mechanism yet to be understood. The reduction in sensitivity to cyhalofop and glufosinate might be a secondary effect of the mechanisms conferring high resistance to propanil and quinclorac.