SELECTIVITY OF HERBICIDES APPLIED ALONE AND IN TANK MIXTURES TO SWEET SORGHUM IN POST-EMERGENCE

The small number of herbicides registered for sorghum (Sorghum bicolor (L.) Moench) restricts its cultivation. The aim of this study was to evaluate the selectivity of herbicides applied alone or in tank mixtures to sweet sorghum in post-emergence. Two experiments were conducted in greenhouse, the first with herbicides applied alone and the second with herbicide mixtures. Based on the results of greenhouse experiments, treatments were selected to evaluate selectivity in the field. In the field experiment, the herbicides applied alone and herbicide mixtures did not differentiate from the control without herbicide application regarding to phytotoxicity, fresh mass of the aerial part, percentage of dry mass of the aerial part and Brix of sweet sorghum at 28 days after application. The treatments considered selective were: atrazine (1000 to 2000), bentazon (360 to 720), S-metolachlor (576 to 864), mesotrione (48 to 150), carfentrazone (4 to 8), 2,4-D amine (335 to 670), besides mixtures with [atrazine + S-metolachlor] [601 + 471.2] and [901.5+ 706.8], atrazine + 2,4-D amine (1000 to 2000 + 100.5 to 268), atrazine + tembotrione (1000 + 42 to 63) and atrazine + mesotrione (1000 + 48 to 72) (doses in g a.i. ha-1).

With the increase in the world population, there is greater demand for the supply of energy from a wide variety of sources. However, the energy matrices of most countries are composed of sources derived from the burning of fossil fuels, which are polluting and non-renewable.
For the production of ethanol, sugarcane currently stands out as the main alternative of plant origin, however, it is essential to develop new agricultural crops as options for renewable sources that have bioenergetic potential .

Sweet sorghum (Sorghum bicolor (L.)
Moench) is cultivated all over the world, and in Brazil, its main use is as a food source in livestock (Menezes et al., 2014). The crop has a short developmental cycle, tolerance to drought periods, high biomass production and high sugar content in the stalks (Teixeira et al., 1999). In addition, it represents an excellent alternative for bioenergy production as it presents desirable agronomic characteristics for the sugar and alcohol industries, as it allows its use in the sugarcane off-season and its implantation and harvesting is fully mechanized (Albuquerque et al., 2012;Solano et al., 2017).
One of the obstacles to the production of sweet sorghum is weeds. The lack of control over them during the crop development cycle negatively affects the height and diameter of the plants, making them more susceptible to lodging, making harvesting difficult and resulting in productivity losses that can reach 50% . A common practice in Brazil for weed control is the adoption of tank mixtures, in order to reduce costs and expand the spectrum of control (Gazziero, 2015). Generally, tank mixtures are carried out in post-emergence of the cultures, however, there are few options of herbicides registered in this modality of application for the control of weeds in the sorghum crop, among them, atrazine and 2,4-D (Correia & Gomes, 2015;Cunha et al., 2016;Rodrigues & Almeida, 2018).
Although the crop is an interesting renewable source alternative for biomass and ethanol production, the scarcity of publications and recommendations on the use of herbicides in post-emergence can lead to incorrect use in agricultural areas, and thus compromise the productivity and quality of the sweet sorghum.
Registered herbicides only for the corn crop are frequently applied to the sorghum crop, which can result in phytotoxicity due to the plant's greater sensitivity to certain molecules. Therefore, it is necessary to identify herbicides that are selective for the crop. Based on the above, the objective of this work was to evaluate the selectivity of herbicides applied alone and in tank mixtures in post-emergence of sweet sorghum crop.
Treatments were defined based on recommendations for maize cultivation (Rodrigues & Almeida, 2018) and based on other studies in the literature (Campos et al., 2016;Galon et al., 2016;Machado et al., 2016).
The experimental units consisted of pots with a capacity of 3.5 L of soil, with two sweet sorghum plants.

Application of Treatments
Herbicide applications were carried out with a precision backpack sprayer pressurized with CO 2 was used, equipped with a bar containing three flat jet nozzles, model XR 11002,

Assessments
In both experiments, the phytotoxicity of sorghum plants was evaluated at 28 days after In each evaluation, scores were assigned in a percentage scale, where 0 represents the absence of phytotoxicity and 100% the death of the plants (Velini et al., 1995). At 28 DAA, the plants were collected, placed in paper bags and taken to a greenhouse with forced air circulation at 58°C until they reached a constant mass. Then, the samples were weighed to determine the aerial part dry mass (APDM).

Origin and Production of Plant Material
The period for conducting the two field experiments (one for herbicides applied alone and the other for tank mixtures) on the Experimental Meteorological data related to the period of conducting experiment were collected at the agrometeorological station of the National

Institute of Meteorology (Instituto Nacional de
Meteorologia, 2020), and are presented in Figure   1.

Application of Treatments
The herbicide applications were carried out with the a precision backpack sprayer pressurized with CO 2 was used, equipped with a bar containing to an oven with forced air circulation at 58°C until they reached a constant mass. Then, the samples were weighed to determine the APDM.

Statistical Analyses
Data from the greenhouse and field experiments were analyzed for error normality and homogeneity of variances using Shapiro-Wilk and Levene tests, respectively (p ≤ 0.05).
After accepting the assumptions, the analysis of variance was performed using the F test. The means were compared using the Scott-Knott cluster test (p ≤ 0.05).  (Abit et al., 2009), resulting in less damage to sweet sorghum plants.

Experiment -Greenhouse -Herbicide mixtures
The results obtained with the applications of herbicide mixtures are shown in Table 2.
Phytotoxicity levels from 0 to 3.75% were it is possible that the herbicide is metabolized into non-toxic compounds due to the high presence of benzoxazinones (Shimabukuro, 1968;Jachetta & Radosevich, 1981;Rodrigues & Almeida, 2018 showed results similar or closer to the control in terms of aerial part dry mass.

Experiment -Field -Herbicides applied alone
The treatments considered more selective in the greenhouse experiments were evaluated in the field, aiming to determine the effect of herbicides on characteristics related to the production of sorghum biomass for energy generation.
Only for the treatments composed by the herbicides atrazine (2000 g a.i ha -1 ), S-metolachlor (864 g a.i. ha -1 ) and 2,4-D amine (502.5 g a.i. ha -1 ) there was no decrease in the levels of phytotoxicity from the first (14 DAA) to the second assessment (28 DAA). In the evaluation at 14 DAA, the levels of phytotoxicity of herbicide treatments ranged from 1.33 to 20%. As for the evaluation at 28 DAA, the levels ranged from 0 to 10.83% (Table 3) g a.i. ha -1 ), mesotrione (150; 96 and 72 g a.i. ha -1 ), carfentrazone (8, 6 and 4 g a.i. ha -1 ) and 2,4-D amine (670 g a.i. ha -1 ) showed higher levels of phytotoxicity at 14 DAA, however, at 28 DAA, no differences were observed between the herbicide treatments and the control. The application of the mixture of atrazine + S-metolachlor (1500 + 384 g a.i. ha -1 ) in post-emergence caused phytotoxicity below 5% in hybrids ESX5200 and EJX7C5110 at 35 days after emergence (Maciel et al., 2017), similar to the results found for the hybrid N31G2091 used in this study. Although the results are similar, differential tolerance to herbicides may occur due to genetic variations in the hybrids, affecting herbicide absorption, translocation and metabolism (Bunting et al., 2004;Galon et al., 2016).
In the first two weeks after the application of the herbicides, some symptoms of phytotoxicity were observed, however, the sweet sorghum plants showed the capacity to recover from the symptoms after 28 DAA. Similar results were obtained by Archangelo et al. (2002), who observed absence

Experiment -Field -Herbicide mixtures
In the evaluation at 14 DAA, the lowest phytotoxicity values were observed in treatments with the mixture of atrazine + 2,4-D amine, and the lowest dose did not differ from the control without herbicide (Table   4). The atrazine + mesotrione and atrazine + tembotrione mixtures showed phytotoxicity levels ranging from 28 to 43%. The plants showed symptoms of leaf whitening, which is related to the mechanism of action of the herbicides mesotrione and tembotrione, which blocks the enzyme p-hydroxyphenylpyruvate dioxygenase (HPPD), responsible for the carotenoid biosynthesis pathway (Senseman, 2007;Rodrigues & Almeida, 2018). In the case of atrazine + 2,4-D amine mixtures, the maximum level of phytotoxicity was approximately 20%.  should take into account the spectrum and density of weed species present and the possible differential tolerance of other cultivars.

Conclusions
There is initial phytotoxicity in sorghum