Sweet Sorghum Establishment after Application of Residual Herbicides

Imazethapyr, sulfentrazone, clomazone, diclosulam, trifloxysulfuron-sodium and trifluralin are residual herbicides commonly used for weed control in soybean or sugarcane crops. The sorghum crop implanted succeeding sugarcane, can be affected by the carryover effect of these herbicides. In this context, we aim with this work to evaluate the minimum period between application of herbicides with residual effect (imazethapyr, sulfentrazone, clomazone, diclosulam, trifluralin and trifloxysulfuron-sodium) and the planting of sorghum so that there is no impairment in growth and establishment of this crop due to the herbicide carryover effect. The experiment was installed in randomized blocks design with four replications, under field conditions. The herbicides were applied to the previously tillaged soil, with sorghum being planted 0, 14, 28, 42, 56 and 70 days after herbicide application (DAA). The percentage of germination was evaluated daily from planting, and 7, 14, 21 and 28 days after emergence (DAE) of each planting, the phytotoxicity was evaluated. Thirty five DAE of each planting season, ten plants were collected per plot for measurement of leaf area, fresh and dry mass of plants, leaves and stems. The minimum time interval for planting sorghum after application of these herbicides varies, but imazethapyr is highlighted by causing high and durable toxicity to sorghum even when planting sorghum after 70 days of its application.


INTRODUCTION
The different types of sorghum (grain, forage and saccharine) are cult ivated in different regions of the world and have wide adaptability to environmental conditions, especially under water deficiency, establishing themselves in more varied environments than other commercial species (Francisco, 2016). In addition, research on sorghum in Brazil has been boosted in recent years, mainly due to its applicability to ethanol production in situations or regions of the country where sugarcane may either not present high yields, or is not available fo r processing, since the entire sugarcane-based alcohol and sugar industry structure is suitable also for sorghum processing (Almodares & Hadi, 2009). Thus, sorghum has increasingly become an option for cult ivation in Bra zil, mainly in succession to soybeans (Dan et al., 2010). Sugarcane makes an average of five to six successive crops, demanding a p lantation reform after this period (Durães, 2011), fo r a new cropping cycle. Sorghum, with a short cycle -90 -130 days fro m emergence to harvest, is ideal for co mp lementing ethanol production during the sugarcane off-season, or when sugarcane is still with lo w sugar concentration, allowing to extend the period of use of the ethanol production plants in up to three months (Almodares & Hadi, 2009). It should be noted that sorghum requires less fertilizer amounts, and stores sugars in its stems at different times, compared to sugarcane (Lourenço et al., 2007). In addition, it may also be suitable in an integrated system o f rural property exploitation, aiming at self-sufficiency in energy, together with other activit ies focused on agricultural production . Weed control is essential in cash crops due to competition for environmental resources such as water, light, nutrients and physical space (Silva et al., 2007). In contemporary agriculture, herbicides stand out as one of the main tools for weed control, being its use economically viable (Inoue et al., 2011). However, herbicides that have a long residual effect in soils may not be degraded during the main crop cycle, leaving residues that harm the germination and development of succeeding crops (Werle et al., 2017). Several authors report effects of residual herbicides to succeeding crops, as for rice ( (Silva et al., 1999;Dan et al., 2010) and millet (Dan et al., 2011). The impact of herb icide residues (carryover effect) on crops grown succession depends on several factors, among them the natural susceptibility of the planted species, the herbicide half-life and the environmental conditions that affect the herbicide degradation rate in soil (Silva et al., 2007). Imazethapyr, sulfentrazone, clo mazone, diclosulam, trifluralin and triflo xysulfuronsodium are herb icides co mmonly used in soybean or sugarcane cultivation (Monquero, 2014), where sorghum can be planted in succession; all these compounds are considered at least moderately soil persistent (IUPAC, 2018). With the possibility of gro wing sorghum in succession to these crops, it is a priority to study the residual effect of these mo lecules and their potential to cause damage to the establishment of sorghum planted in succession.

II. OBJECTIVE
In this context, we aimed with this work to evaluate the minimu m period between the application of the residual herbicides imazethapyr, sulfentrazone, clo mazone, diclosulam, triflura lin and triflo xysulfuron-sodium, and the planting of sorghum so that there is no damage to the growth and establishment of this crop.

III.
MATERIAL AND METHODS The experiment was installed in field conditions on a Red Dystroferric Latosol with 60% clay, in the experimental area of Emb rapa Agropecuária Oeste, Dourados -MS, Brazil, in the 2013/2014 cropping season. We used the strip-plot experimental design, comprising a factorial scheme 7 x 6, with four replications.
Factor A (horizontal bands) was represented by the treatments: Test (T-01); Clo mazone 1.25 kga.i. ha -1 (T-02); Triflo xysulfuron-sodium 0.0075 kga.i. ha -1 (T-03); Trifluralin 2.4 kga.i. ha -1 (T-04); Diclosulam 0.042 kga.i. ha -1 (T-05); Imazethapyr 0.15 kga.i. ha -1 (T-06); and Sulfentrazone 0.6 kga.i. ha -1 (T-07). Factor B (vertical bands) was composed by sorghum planting, variety BRS 511, at intervals of 0, 14, 28, 42, 56 and 70 days after application (DAA) of the herbicides. These intervals were chosen in order to identify the minimu m period required between the application of these herbicides and the implementation of the sorghum crop in a way that does not hinder its growth and development. The physico-chemical characteristics of the screened herbicides are listed in Table 1. Planting was accomp lished manually, where 3 cm deep furrows were opened in rows spaced at 0.45 m, and 7 seeds m -1 were uniformly deposited, resulting in an approximate final density of 150,000 plants ha -1 (15 plants m -2 ). The area was tillaged with plo wing and harrowing, prev iously fertilized according to soil analysis and technical reco mmendations for the crop (May et al., 2012). The area had no history of application of residual herbicides for five years prior to the installation of the experiment. Soil characteristics are listed in Table 2.   Herbicide application and the first planting season were acco mp lished on Oct. 18,2013. For this, we used a CO2-pressurized backpack sprayer, connected to a bar equipped with nozzles 110.02 working at the recommended pressure, delivering 120 L ha -1 of herbicide solution. The application was done at early mo rning, right after the planting of the first season. The soil was about 80% of field capacity by the time of the application. Basic  Phytotoxicity evaluations were performed 7, 14, 21 and 28 days after emergence (DA E), through visual symptoms measured on a scale varying from 0 to 100, where zero represents no symptoms and 100% the death of the plants. The emergence was evaluated by daily counting in a previously marked section of 3 m o f planting row in each replication, daily fro m 0 to 14 days after planting (DAP), being considered as "emerged" seedlings with height equal or superior to 1 cm. Th irty DAE, in each planting season and for each herbicide treatment, the fresh and dry mass of of shoot, leaves and stems of sorghum plants were evaluated. At 103 DA E, plant height, fresh and dry mass and density were assessed.
The data set was submitted to analysis of variance in the statistical software R (R Core Team, 2012), being explored by 3D response surfaces, and linear o r non-linear regressions, according to the significances. Fo r percentage of emergence and phytotoxicity, the Gaussian equation was used to obtain the response surfaces, as follows: (1)

IV.
RESULTS AND DISCUSSION The average daily air temperature during the conduction of the experiment ranged fro m 15 to 25 °C, and at least 16 rainfall events with considerable volu me were observed (Figure 1), demonstrating good conditions for conducting the experiment.
The regression parameters for all t reatments are summarized at Table 3. The number of emerged plants (Z axis) was modeled according to the sorghum planting interval after herb icide application (X axis) and the period in days after each planting (Y axis), by using Gaussian response surfaces (Figure 2). For all herbicide treat ments the percentage of emergence increased until the eighth day after planting, reaching the apex between the eighth and the tenth day; due to unfavorable environmental conditions and pest attacks there was a decrease in the number of p lants after the tenth day. It can also be observed that all herbicides affected the number of emerged plants, and the lower the interval between herbicide application and planting, the lower the sorghum germination. When sorghum was planted at the day of the application, for examp le, there were 15 seeds germinated at the control plot, while for the herbicide treat ments, only about 10 seedlings were present. Diclosulam was the least impacting herbicide on sorghum in concomitant planting/application, with appro ximately 13 seeds in a 3m row (Figure 3).   /dx.doi.org/10.22161/ijaers.5.9.35  ISSN: 2349-6495(P) | 2456-1908(O) germination ( Figure 3) decreased considerably and all treatments were similar to the control plot ( Figure 2).The treatment T-02 (clo mazone), T-04 (t riflu ralin) and T-07 (sulfentrazone) were the ones that most affected the emergence of sorghum seedlings. Maladão et al. (2013) reported that only 1 /4 of the co mmercial dose of sulfentrazone was sufficient to significantly reduce the emergence of sorghum. According to Stougaard et al. (1990) and Brighenti et al. (2002), diclosulam (T-05) and sulfentrazone (T-06) present long residual effect and they may, depending on climat ic and soil conditions, cause damage to crops planted in succession. Vencill (2002) also observed that trifluralin has physical and chemical characteristics that allow it to persist in soil for a certain period of time, as observed in this work. Machado et al. (2016) verified 46% and 50% to xicity and stand reduction in sorghum planted soon after application of t riflu ralin and clomazone, respectively.
Although there was seed germination in treat ments where the herbicides were applied, many of these plants showed toxicity symptoms, wh ich was higher when the planting was carried out closer to the herbicide application date (Figure 3). Each herbicide presented a different percentage of to xicity in sorghum that is native to its molecule; that is, the natural differential level o f tolerance to a specific treatment. Thus, 35 DA E the first planting season, all herb icides -except trifluralin (T-04), scored toxicity levels above 70%. By considering the response surfaces altogether, it can be seen that in each planting season the degree of phytotoxicity increases throughout the evaluation period.
Trifluralin also presented a shorter period of influence on sorghum development, co mpared to the other herbicides( Figure 3). Most of the herb icides tend not to cause significant phytotoxicity to sorghum when it is planted after 70 DAA of the herbicides (40 DAA for trifluralin ). Ho wever, Imazethapyr at the end of the evaluations still presented an average 8% of phytotoxicity on sorghum plants, suggesting that the safety interval is above the range evaluated and that more studies are needed for this herbicide.
ALS-inhib iting herbicides (triflo xysulfuronsodium, diclosulam and imazethapyr) had similar behavior, with persistent symptoms and toxicity above 80% in the first planting season, with the greatest symptoms reported 14 DA E of each planting season (Figure 4). The main symptoms were intense chlorosis, striae, followed by necrosis, reduction of growth rate and even plant death. PROTOX-inhib iting (sulfentrazone) and carotenoid biosynthesis inhibiting (clo mazone) herbicides, were highly harmful to sorghum; seedlings that were able to emerge already presented more than 40% phytotoxicity 7 DAE (Figure 4), in agreement with data reported by Machado et al. (2016). Maladão et al. (2013) also observed high impact of sulfentrazone in Sorghum bicolor.
Fresh and dry mass of the plants, leaves and stems that were able to emerge, were smaller in plantings closer to the application of the herbicides (Figures 5; 6), corroborating with the data of phytotoxicity (Figure 4). These variables are closely linked to the dissipation of herbicides fro m soil, which strongly affects soil persistence. Persistence corresponds to the time when a herbicide remains active in soil, which is of fundamental importance in weed management (Karam, 2005). However, mo re persistent herbicides, if they are not selective to the crop, can cause losses as reduced fresh and dry mass, leaf area and productivity.
It was observed ( Figures 5; 6) that all herbicides caused damage to the sorghum. Fo r the first planting season (same day of herbicides application), the dry mass measured 35 DA E, corresponded to approximately 2 -5 g plant -1 in all treat ments; for the planting performed 60 DAA, dry mass was superior to 15 g p lant -1 , also 35 DAE. Treatments with clo mazone, triflo xysulfuron-sodium, trifluralin, diclosulam and sulfentrazone were statistically equal, so for better co mprehension, they were grouped for the variables fresh and dry shoot mass, leaf and stem dry mass, and leaf area. There was reduction in all these variables, for imazethapyr at the 70 DAA planting compared to the control treat ment; this corroborates the phytotoxicity data, although this herbicide did not differ statistically fro m the other treatments. This is also in agreement with results by Dan et al. (2012), who reported reductions in maize shoot growth when using 0.1 kg ha -1 imazethapyr, even when planting it 97 DAA.
For the same variables, no differences were observed between the control and the herbicide treat ments at the 70 DAA planting. However, Dan et al. (2010) found negative effects of diclosulam on sorghum plants grown in succession to soybean in the Brazilian Cerrado (savanna-like biome) region.

International Journal of Advanced Engineering Research and Science (IJAERS)
[ application of such herbicides and sorghum planting. Thus, sorghum can be considered an alternative in areas previously managed with clo mazone (2.5 L ha -1 ), triflo xysulfuron-sodium (30 g ha -1 ), trifluralin (4.0 L ha -1 ) and sulfentrazone (1.2 L ha -1 ) since they are applied at the beginning of the cycle of the preceding crop, confering at least 70 days between its application and sorghum planting. On the other hand, attention should be given to areas applied with imazethapyr and diclosulam, where the carryover effect is potentially damaging even after 70 days after its application.

V. CONCLUSIONS
The best planting time for sorghum, after application of residual herbicides, varies for each co mpound, being the toxicity as smaller as longer the time between the application of such herbicides and sorghum planting. Thus, sorghum can be considered an alternative in areas previously managed with clo mazone (2.5 L ha -1 ), triflo xysulfuron-sodium (30 g ha -1 ), trifluralin (4.0 L ha -1 ) and sulfentrazone (1.2 L ha -1 ) since they are applied at the beginning of the cycle of the preceding crop, confering at least 70 days between its application and sorghum planting. On the other hand, attention should be given to areas applied with imazethapyr and diclosulam, where the carryover effect is potentially damaging even after 70 days after its application.