Non-target site (NTS) herbicide resistance by degradation enhancement is an increasing problem in several weeds around the world. In this study, the occurrence of degradation enhancement by cytochrome P450 monooxygenases (cytP450) was evaluated as the mechanism of resistance to imazethapyr in barnyardgrass. The cytP450 inhibitors malathion and piperonyl butoxide (PBO) and the inducer naphthalic anhydride (NA), applied in mixture or sequentially with imazethapyr, were evaluated on imidazolinone-susceptible and -resistant barnyardgrass byotipes. In addition, the degradation of imazethapyr was analyzed in plants treated with imazethapyr applied alone or two hours after malathion or NA. The spraying of malathion and PBO reduced the resistance factor (RF) from 15.92 to 3.44 and 4.99, respectively, in the resistant population PALMS01. Conversely, the cytP450 inducer NA increased the RF from 4.45 to 8.32. Malathion increased imazethapyr concentrations in resistant barnyardgrass in comparison with plants sprayed with the herbicide alone, indicating the inhibition of imazethapyr degradation. The simultaneous spraying of malathion and imazethapyr was less efficient than the previous application of this cytP450 inhibitor. These results indicate that degradation enhancement caused by cytP450 enzymes is involved in the resistance mechanism of barnyardgrass to imazethapyr, and appropriate measures should be taken to manage these populations.

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Environmental factors and the addition of adjuvants to the spray tank mix may interfere with glyphosate efficiency in hairy fleabane control. The objective of this study was to evaluate the effect of air temperature and the addition of ammonium sulfate (NH 4 ) 2 SO 4 to glyphosate in the control of glyphosate-resistant (GR) and -susceptible (GS) hairy fleabane. Treatments consisted of air temperatures of 12°C and 25°C, six doses of glyphosate from zero to 2,880 g • ha −1 , the presence or absence of (NH 4 ) 2 SO 4 in the spray solution, and one GS and another GR biotype. At the lowest tested dose (180 g • ha −1 ), control of the GR biotype was 91% and 20% when the plants were kept at 12°C and 25°C, respectively, reducing the resistance factor (RF) by 9.30 times and was associated to the reduction of temperature. The addition of (NH 4 ) 2 SO 4 increased the control by 10−20% at high glyphosate doses and at 25°C. The resistance of hairy fleabane to glyphosate was completely reversed when the plants were maintained at 12°C. At this temperature, resistant plants were controlled even at doses well below that recommended for the control of this species. At 25°C, a dose four times higher than that recommended was required for satisfactory control. At the field level, under situations of low temperatures, it was possible to improve the efficacy of glyphosate applications in hairy fleabane control, if there were no other mechanisms of resistance involved.

-Non-target-site resistance (NTSR) to herbicides mainly caused by enhanced degradation is highly problematic due to the occurrence of biotypes with multiple resistance and is a new challenge for weed management and herbicide use. Recently, enzymes associated with xenobiotic degradation especially cytochrome P450, GSTs and ABC transporters have been associated with the herbicide resistance in several weeds. This knowledge opens a new window to understand the evolution of NTSR. The aims of this review are to discuss the current knowledge of the gene regulation associated with the herbicide resistance caused by enhanced herbicide degradation and to analyze the main consequences of this problem for the adequate herbicide use and weed management. Multiple herbicide resistance caused by degradation enhancement occurs in Lolium rigidum, Alopecurus myosuroides, Echinochloa phyllopogon and in several other species. The level of herbicide resistance caused by degradation enhancement is affected by environmental factors in several cases, which difficult the resistance diagnostic and facilitate its distribution. The modern weed management should consider the characteristics of the herbicide degradation, since the simple rotation of herbicides mechanism of action might not be enough to prevent the herbicide resistance. The use of synergistic mixtures of herbicides and other enzyme inhibitors may contribute to prevent the evolution and spreading of NTSR weed herbicide resistance. Keywords: multiple resistance; cytochrome P450; detoxification; NTSR Resumo -A resistência a herbicidas causada por mecanismos não relacionados ao local de ação (NRLA), principalmente causada pelo incremento de metabolização, é altamente problemática devido à ocorrência de biótipos com resistência múltipla, e é um novo desafio para o manejo de plantas daninhas e utilização de herbicidas. Recentemente, enzimas associadas à degradação de xenobióticos, especialmente citocromo P450, GSTs e transportadores ABC, têm sido associadas à resistência a herbicidas em várias plantas daninhas. Os objetivos desta revisão são descrever o conhecimento atual da regulação gênica associada à resistência a herbicidas causada pelo incremento de metabolização e analisar as principais implicações deste problema para definir estratégias adequadas de manejo com o uso de herbicidas. A resistência múltipla a herbicidas causada pelo incremento da degradação ocorre em Lolium rigidum, Alopecurus myosuroides, Echinochloa phyllopogon e outras várias espécies. O nível de resistência a herbicidas causada pelo incremento de metabolização é afetado por fatores ambientais em vários casos, o que dificulta o diagnóstico da resistência e facilita sua distribuição. O manejo moderno de plantas daninhas deve considerar as características de degradação do herbicida, uma vez que a simples rotação de mecanismo de ação de herbicidas pode não ser suficiente para prevenir a ocorrência da resistência a estes produtos. A utilização de misturas sinergísticas de herbicidas e outros inibidores e...

Environmental conditions can influence herbicide metabolism and interfere with the evolution of weed resistance. This study aimed to evaluate the effect of thermal regimes on the control of barnyardgrass (Echinochloa crus-galli) susceptible and resistant to herbicides of the imidazolinones chemical group. Factor A (biotypes) consisted of one biotype that is susceptible and one that is resistant to imazethapyr; factor B (dose) comprised six imazethapyr doses between 0 and 800 g ha-1 for the resistant biotype and between 0 and 100 g ha-1 for the susceptible biotype; factor C (temperature) consisted of four thermal regimes (16/16 ºC, 25/25 ºC, 16/25 ºC and 25/16 ºC) to which the plants were submitted before and after the herbicide application. Concerning the resistant biotype, the highest control levels were observed in the thermal regimes with temperature fuctuation, especially at 25/16 ºC. At the recommended imazethapyr dose (100 g ha-1), the herbicide resulted in a control level over 90 % at 25/16 ºC and less than 50 % at 16/16 ºC, in the resistant biotype. The shoot dry mass was reduced by more than 80 % at 25/16 ºC and 16/25 ºC and less than 30 % at 16/16 ºC. The temperature conditions to which the plants are subjected in the pre- and post-application period interfere with the level of imazethapyr resistance in barnyardgrass.