Fall management of fleabane based on glyphosate + 2 , 4-D , MSMA and glufosinate applied isolated or in tank mixture with residual herbicides

Fleabane (Conyza spp.) has spread in no-tillage areas of Paraná State (PR), Brazil, and currently represents one of the main challenges related to weed control, particularly during off-season period. For this purpose, two experiments were carried out in Campina da Lagoa (PR) and Floresta (PR) with the aim of assessing the efficacy of herbicide treatments applied to fleabane areas during the offseason period between maize harvest (June to July) and soybean sowing (November). Treatments consisted of herbicide treatments (glyphosate + 2, 4-D; glufosinate, MSMA) targeting the control of fleabane plants in POST, tank-mixed with residual herbicides (metsulfuron, chlorimuron, diclosulam, imazethapyr, imazaquin, flumioxazin, metribuzin, amicarbazone and isoxaflutole) to control the emergence and growth of new flushes in PRE. Applications were performed when fleabane plants reached a height of 2 cm. Evaluation on both efficacy and residual weed control was accomplished from the first day of application day to 75 days after application (soybean crop sowing). Glyphosate + 2, 4-D mixture was efficient for burndown of Conyza spp. in all situations. During 75 day off season period, diclosulam and chlorimuron were the best options for controlling fleabane emergence when mixed with any other options of herbicide treatments, for the control in POST. Mixtures of glyphosate + 2, 4-D with metribuzin and glufosinate with flumioxazin, metribuzin and isoxaflutole maintained good fleabane control throughout the whole 75 days off season period. Fall management was an effective option for fleabane control.


INTRODUCTION
Fleabane (Conyza spp.) ranks among the top ten main weeds distributed around the world.Different species in this genus stand out in this ranking due to their ability to develop resistance to herbicides of different mechanisms *Corresponding author.E-mail: am.oliveiraneto@gmail.com.Tel: +55 (47) 3531 3733.
Author(s) agree that this article remains permanently open access under the terms of the Creative Commons Attribution License 4.0 International License of action, making its control even more difficult (Trainer et al., 2005).In the United States, the controlling failures were observed after only three years of continued use of glyphosate in post-emergence (POST).
In 2001, the first case of Conyza canadensis resistant to glyphosate was reported Vangessel (2001).In Brazil, glyphosate-resistant Conyza bonariensis and C. canadensis were first reported in 2006 both in citrus orchards located in São Paulo State (Moreira et al., 2007) and in grain-producing areas in Rio Grande do Sul (Vargas et al., 2007).Besides that, resistant biotypes of Conyza sumatrensis were also reported in Paraná State (Santos et al., 2014).
Depending on rain availability and fall/winter temperatures, two consecutive cropping cycles can be achieved within the same agricultural year in some regions of Brazil.The first and usually more important growing season starts in early spring (September to October), and ends in the mid-summer (February to March).This is the most favorable growing season, due to abundant rain and warm temperatures.The second growing season ("safrinha") starts as soon as the main growing season ends which usually finish in June to August, depending on crop cycle and climate.The second growing season is characterized by a higher risk of low temperatures (South and Southeast regions) or water deficit (Southeast and West Central regions).
The control of winter annual emerged with weed species which may not ensure an adequate control throughout the next summer crop sowing, once, new flushes of weeds may emerge after herbicide application, infesting the area again.Thus, the addition of residual herbicides such as acetolactate synthase inhibitors, photosystem II inhibitors or PROTOX inhibitors (Armel et al., 2009) may help to extend residual weed control until crop sowing (Owen et al., 2009).
The concept of fall burndown is related to the group of off-season weed control strategies, that is, those weed control methods adopted in the period of time between crop harvest and next crop sowing, when such period is relatively long.In Brazil, such situation will likely happen after the second growing season (June/August to September/October) (Constantin et al., 2012).
Fall burndown as a fleabane management strategy meets one of the basic assumptions on hard-to-control weed management, that is, to intervene with a control measure when the weed is most susceptible.In field conditions, fleabane emergence peak occurs from June through September.Shortly after emergence, plants are usually in a stage in which chemical control is easier due to increased susceptibility of fleabane plants, coinciding with the period when fall management can be performed.
The length of that period is directly related to when second maize is harvested, and may last from 45 to 90 days.In those areas, maize harvest can be divided into so called early harvest maize when harvest is performed not later than the first half of July (off-season longer than 60 days) and late harvest maize when harvest is performed after July 15 (off-season shorter than 60 days).This division is important because each harvest demands exclusive management practices in off-season time.For the first harvest season, fleabane emergence peak generally occurs after maize harvest, whereas for second harvest, it occurs also during the end of maize cycle.Thus, because of the length of off-season period, different approaches as related to herbicide residual control may be demanded.
The aim of this research was to evaluate the efficiency of three chemical options for burndown (glyphosate + 2, 4-D, glufosinate or MSMA) applied either isolated or in tank mixtures with residual herbicides for PRE and POST fleabane control.
Soils from experimental areas were identified as Red Oxisoil eutrophic (Embrapa, 2013), clay texture.In Campina da Lagoa, main soil properties included 60.0 % clay, 27.0 % sand, pH H20 = 6.0, and 28.58 g dm -3 of organic carbon (OC), while in Floresta, 61.0 % clay, 29.0 % sand, pH H20 = 6.0, and 20.89 g dm -3 OC.Regional climate is subtropical with rainy summers and dry winters (Cfa, according to Köppen climate classification).Monthly rainfalls observed during the period of time when experiments were in the field are presented in Figure 1.At each location, the experiment was initiated right after second maize harvest (July 08, 2009 in Campina da Lagoa andJuly 17, 2009 in Floresta).Those maize harvest dates provided an off-season period longer than 60 days, so both experiments were therefore considered as first-season maize experiments.
After maize harvest, there was a 15-days interval before the treatments application, due to the need to stabilize crop residues on soil surface, in order to enable both burndown efficacy and uniform distribution of residual herbicides on the soil.Treatments were identical for both experiments.They comprised three chemical treatments (glyphosate + 2,4-D 960 + 536 g ha -1 , MSMA 2370 g ha - 1 and ammonium-glufosinate 400 g ha -1 ) applied either isolated or in tank mixed with nine herbicide treatments with residual activity in soil (metsulfuron 3.6 g ha -1 , chlorimuron 20 g ha -1 , diclosulam 33.6 g ha -1 , imazethapyr 100 g ha -1 , imazaquin 180 g ha -1 , flumioxazin 125 g ha -1 , metribuzin 480 g ha -1 , amicarbazone 420 g ha -1 and isoxaflutole 56.25 g ha -1 ) with an additional treatment and no herbicide application.The combination of MSMA + imazaquin was replaced by amicarbazone (560 g ha -1 ) due to tank mixture incompatibility.Both experiments were arranged in a completely randomized block design, with four replicates.
Treatments with glyphosate + 2,4-D, glyphosate, MSMA and glufosinate were used to control emerged plants of Conyza spp. at the moment of application and were considered as without or limited residual treatments, as compared to the tank mixture, which were considered as soil residual treatments.Herbicide applications were done when fleabane plants were in a very early development stage (≤ 2 cm).That stage was chosen due to both the increased sensitivity to herbicides as compared to more developed plants, and to the lower sprouting capacity in early stages of fleabane  development (Vangessel et al., 2009;Moreira et al., 2010;Oliveira Neto et al., 2010).Herbicide applications were made with a backpack CO2 sprayer calibrated to deliver 200 L ha -1 using 207 kPa as CO2 pressure and five XR-110.02nozzles.Climatic conditions, weed densities and stages in each location are shown in Table 1.Visual weed control ratings were collected at 15 and 30 days after applications (DAA).The ratings were based on a 0 to 100% scale, where 0 means no control and 100 correspond to weeds completely dead or absence of weeds.
Density of emerged Conyza spp.plants was determined fortnightly starting at 30 DAA up to soybean planting (75 DAA).Fleabane densities were counted using four randomized sampling frames of 0.25 m 2 of inner area, each were counted per experimental unit.In these evaluations, fleabane height was also measured from root collar to its apical growing region.Data from each experiment were submitted to the F-test variance analysis and means compared by Scott-Knott test (p<0.05).

RESULTS AND DISCUSSION
In Campina da Lagoa (PR) treatments with glyphosate + 2, 4-D and glufosinate provided >90% control for fleabane, while MSMA presented the lowest efficacy (81%) among burndown options.However, for this particular option, tank mixing these herbicides with residual herbicides increased control to >87% (data not shown).Improved levels of weed control might be related to the POST effect promoted by residual herbicides, once metsulfuron, chlorimuron and diclosulam are considered efficient for both PRE and POST fleabane control (Vargas et al., 2007).Tank mixing of residual herbicides with glufosinate improved control of C. canadensis when compared to glufosinate alone (Steckel et al., 2006).
An efficient burndown is essential for fleabane fall burndown success, since eventual escaped plants begin to grow again freely during off-season time, when there is no soil cover until next summer crop sowing.When a summer crop is planted in areas where fleabane is already established, its competitive capacity is affected, once a very low fleabane density is enough to cause significant crop yield losses (Trezzi et al., 2015).
In Floresta (PR), except for MSMA alone, all the other herbicide treatments provided 82% of weed control after burndown.In other circumstances, this level of weed control could be satisfactory; however in fleabane areas this is not enough once non-controlled plants continue to grow during off-season and become even more serious problem afterwards.
Glyphosate + 2, 4-D-based treatments were the most efficient in weed control after burndown (data not shown).Although 2,4-D has proven to be an efficient postemergent alternative fleabane glyphosate-resistant biotypes (Oliveira Neto et al., 2010), this should not be seen as the only herbicide option, not only for its narrow weed spectrum control but also for the potential selection of resistant biotypes.Differential tolerance levels for 2, 4-D have been detected among C. canadensis populations in studies conducted by Kruger et al. (2008).
Residual herbicide treatments have proved to be more effective in decreasing densities of new emerged fleabane plants, especially from 45 to 75 DAA, in Campina da Lagoa (PR) (Table 2).Tank mixing with diclosulam provided the lowest Conyza spp.densities until summer crop pre-sowing assessment (75 DAA) (Table 2).However, due to the long residual period, diclosulam should be used in a rational way in order to avoid damage to sensitive crops.According to Dan et al. (2011), residual activity of this herbicide may cause negative effects in crops considered to be sensitive such as millet, maize, sunflower, sorghum and brassicas.
Tank mixing metribuzin and amicarbazone applied with glyphosate + 2, 4-D, MSMA and glufosinate were ranked as an intermediate category by Scott-Knott test at 75 DAA (Table 2).However, these treatments provided good performance till 60 DAA thus, that can be considered as efficient management alternatives in situations where offseason period does not exceed 60 days between the first application and summer crop sowing.Eubank et al. (2008) observed that glyphosate, glufosinate and paraquat tank mixed with metribuzin (420 g ha -1 ) reduced C. canadensis density for 7 weeks.
In Floresta (PR), regarding weed emergence during offseason, all herbicide treatments resulted in low fleabane densities, regardless of whether treatments contained residual herbicides or not.This reflects the fact that at this site, there were virtually no new flushes of fleabane emergence after the application of fall burndown (Table 3).The evaluation performed at summer crop pre-sowing (75 DAA), treatments with glufosinate, MSMA + imazethapyr, MSMA + amicarbazone and glufosinate + imazethapyr produced the highest fleabane densities; all the other herbicide mixtures were efficient on controlling fleabane and presented densities ≤5.3 plants m -2 .Long diclosulam residual activity in soil provided excellent performance on new emergence flushes of fleabane, since no emerged plants were found in treatments containing this herbicide.This result is similar to that found in Campina da Lagoa and demonstrates the efficacy of diclosulam on fleabane in fall burndown application.
Plant density is an important variable component to both efficiency and residual activity of herbicides.However, when analyzed isolated, it is not appropriate for the decision-making process regarding fleabane plants management.The best strategy to evaluate fleabane control should consider not only the number of emerged plants but also plant height right when next crop is about to be planted, since the effectiveness of another presowing burndown application will depend on that.Fleabane height is probably the best single predictor to settle the best strategy for fleabane control in no-tillage areas, since it can be used to decide when applications should be done and how many will be needed.Eubank et al. (2008) concluded that mixtures of nonselective herbicides with auxin-derived herbicides provided inconsistent C. canadensis control for plants with >15 cm.Also, 16 cm was the upper size limit to ensure an effective control of fleabane plants in POST applications (Blainski, 2011).Based on that assumption, all herbicide treatments were considered as effective tools to manage fleabane at 30 and 45 DAA, since plants were not taller than 16 cm in Campina da Lagoa (PR) (Table 4).
Therefore, for those situations when the period of time between fall burndown and summer crop sowing is close to 45 days, treatments with no-residual herbicides might be recommended, once the new flushes of fleabane will still be within a growing stage that provides adequate weed control by a second, pre-sowing application.
At 60 DAA, three treatments had plants >16 cm in Campina da Lagoa (PR): glufosinate, MSMA and MSMA + imazethapyr (23, 23 and 20 cm, respectively) (Table 4).It is important to mention that glyphosate + 2, 4-D mixture performed well up to 60 DAA.Although there has been a high level of fleabane that emerged after fall burndown, plants were kept within a suitable size (<16 cm) to be managed at pre-sowing burndown.Thus, taking into account the evaluations performed up to 60 DAA, there was a large number of herbicide mixtures which might be considered effective for fall management (Table 4).At 75 DAA (pre-sowing), there was an increment of plant heights when compared to the previous evaluation, but treatments based on diclosulam or chlorimuron maintained good performance.Glyphosate + 2, 4-D + metribuzin, glufosinate + flumioxazin, glufosinate + metribuzin and glufosinate + isoxaflutole mixtures also provided efficient fleabane growth suppression for such a long off-season period (Table 4).Although herbicide combinations which presented plants ranging from 17 to 20 cm at 75 DAA were considered as inadequate tools for fall management, they should not be disregarded within fleabane integrated management.Fleabane with 20 cm height were effectively controlled with summer burndown options tank mixing with glyphosate + 2, 4-D (Oliveira Neto et al., 2010).
Under Campina da Lagoa conditions, longer residual herbicides such as diclosulam, chlorimuron and flumioxazin performed best in terms of reduction of new fleabane flushes and, or growth suppression of emerged plants.This is presumably due to the intense fleabane emergence after fall burndown application, provided by climatic conditions (temperature around 20°C and adequate soil moisture conditions).ALS-inhibitor herbicides diclosulam and chlorimuron were efficient in controlling fleabane.However, herbicides with this mechanism of action should be rationally used, because their continuous use can lead to the selection of resistant biotypes.Studies carried out by Trainer et al. (2005) reported the existence of C. canadensis biotypes which is resistant to chlorimuron and cloransulam.Chlorimuron-resistant fleabane biotypes in Brazil were first found by Santos et al. (2014).In order to ensure long-term use of ALS-inhibitors, it is important to plan a rational mechanism of action rotation and/or herbicide mixtures with different mechanisms of action.
Long-term residual activity herbicides such as diclosulam and chlorimuron can potentially cause carryover effects on sensitive crops like maize, bean, sorghum, sunflower and cotton, so crop succession should be carefully planned before deciding the best herbicide options for off-season fall management.
Fleabane plants were kept under a stage of development suitable for chemical control up to 45 DAA, for all herbicide treatments in Floresta (PR) (Table 5).At Due to the limited emergence of new fleabane flushes at Floresta site, traditional burndown treatments with no residual activity such as glyphosate + 2,4-D and glufosinate were considered effective to control emerged fleabane, which was enough to keep plants within the adequate size necessary to achieve efficacy with the chemical control at pre-sowing (Table 5).

Conclusion
Glyphosate + 2, 4-D was effective for emerged fleabane burndown in all situations in this study.For a 75 day offseason period, diclosulam and chlorimuron were the best options for burndown plus fleabane residual control when combined with glyphosate + 2, 4-D, MSMA and glufosinate.Besides that, glyphosate + 2, 4-D + metribuzin, glufosinate with flumioxazin, metribuzin and

Figure 1 .
Figure 1.Monthly rainfall during the time of conducting experiments in two locations in Paraná.

Table 1 .
Climatic conditions, site description and characterization of Conyza spp.densities and stage of development at the dates of spraying of treatments in two locations in Paraná.