Synthesizing Nanoencapsulated Sulfentrazone Herbicide and Optimizing Time and Dose for Season Long Weed Management in Irrigated Blackgram (Vigna mungo L.)

Pulses are nutritious edible seeds of leguminous plants, have become an essential part of the human diet. Among the pulses, blackgram (Vigna mungo L.) is an important legume crop cultivated in tropical and subtropical regions of the world. Even though there are so many factors responsible for the lower yield of blackgram weeds play a major role. Weeds are silent slayers of the crop because initially they grow along with crop, at certain stage they dominate crop and reduce the yield in a drastic way. 

Sulfentrazone is a herbicidal molecule belongs to the family of phenyl triazolinone which controls the weeds by the process of protoporphyrinogen oxidase (PPO) inhibition. It can be applied as pre plant, pre-emergence or post-emergence for broad spectrum weed control (Dayan et al., 1996). Sulfentrazone has high persistence and mobility with mean partition coefficient K_oc = 43 and sorption coefficient K_d < 1 and also has high horizontal and vertical leaching potential (Martinez et al., 2008). It has high Groundwater Ubiquity Score (GUS) of 6.75 which is far more than broad spectrum herbicides like pendimethalin and glyphosate which are having GUS of 0.66 and 0.42 respectively (Gustafson, 1989).

Even though generally used herbicides in blackgram like pendimethalin, imazethapyr and quizalofop-ethyl are helpful in managing weeds, they have to apply multiple times or have to integrate with other methods of weed management which is expensive. So as to reduce the usage of multiple herbicides, to avoid manual weeding and to achieve season long weed control without affecting the environment the nano-encapsulated sulfentrazone is the better alternative and it gives better solution for the above constraints besides increasing the productivity.

Preparation of encapsulated sulfentrazone
 
One g of sulfentrazone (a.i) [i.e. 2.525 ml of commercial formulation] was mixed with 10 ml of ultrapure water and stirred it using magnetic stirrer for 5 min. Then separately 2 ml of polyethylene glycol as polymer and 8 ml of dichloromethane were taken, mixed and stirred for 5 min using magnetic stirrer. Both the solutions were mixed and stirred for another 5 min thus organic phase was formed. After that 4 % starch solution was taken and stirred with magnetic stirrer for 1 hour thus formed the aqueous phase. Finally organic phase containing polymer with herbicide was added drop by drop to aqueous phase and stirred again with magnetic stirrer for 12 hours. Thus produced nanoparticles were collected as such (liquid formulation) in a vial or centrifuged it for 15 min at 5000 rpm then dried the solid particles in vacuum desiccator to get dried powder.

All the field experiments were conducted at wetland farms, Tamil Nadu Agricultural University, Coimbatore during the year 2019-20 in randomized block design with three replications. The screening trial comprised of seventeen different treatments viz., sulfentrazone with and without encapsulation of two different concentrations @ 0.30 and 0.40 kg a.i. ha^-1 and absolute control. The main field trial I and II comprised of nine treatments of which four best performed treatments were selected from the screening trial. Other treatments were pendimethalin @ 1.0 kg a.i. ha^-1 at 2 DAS fb quizalofop-ethyl @ 50 g a.i. ha^-1 and imazethapyr @ 50 g a.i. ha^-1 at 20 DAS, pendimethalin @ 1.0 kg a.i. ha^-1 at 2 DAS fb 1 HW at 20 DAS, HW twice at 15 and 30 DAS, Weed free check and absolute control.

Lab experiment
 
The encapsulated sulfentrazone particles were characterized in SEM (Fig 1) and also analysed with EDAX (Energy dispersive X-ray analysis) which is used for elemental analysis or chemical characterization of a sample. SEM images (Fig 1) were clearly showing that spherical or round particles which were encapsulated by the starch molecules. In the organic phase because of mixing and continuous stirring the polyethylene glycol polymerized and mixed with herbicide molecules in the presence of solvent dichloromethane. Similar results were observed by Mohanraj and Chen (2006). In the same way covalent bonding ability, mixing during preparation or surface adsorption ability of PEG was explained by Hans and Lowman (2002). Reis et al., (2006) explained in similar way about solvents used in organic phase as encapsulant and observed nano encapsulated particles were obtained by dispersion (Fig 1).



The SEM-EDAX image (Fig 2) is showing that presence of carbon and oxygen peak and also presence of little amounts of fluorine, Sulphur and chlorine confirmed the presence of active ingredient of sulfentrazone in the encapsulated herbicide. Dayan et al., (1998) given structure of sulfentrazone in the similar way which supports the above elemental composition. The spherical nanoparticles with smooth and shining surface is showing that herbicide was encapsulated with starch molecules (Fig 1).



In the particle size analyzer it was tested to know the size of encapsulated particles and zeta potential of the particles. The average particle size of encapsulated sulfentrazone and normal sulfentrazone were 186.9 nm (Fig 3) and 626.9 nm (Fig 4) respectively which were clearly showing that encapsulated herbicidal particle size was far less than normal herbicide because of using solvent evaporation method by the processes of polymerization, dissociation and dispersion by the presence of polyethylene glycol, dichloromethane and starch. The zeta potential of encapsulated sulfentrazone was -38.1 mV (Fig 5). Zeta potential is a measure of surface charges present on the nano particles. Zeta potential shows the stability of the colloids. Nano particles with zeta potential above (+/-) 30 mV have been considered to be stable in suspension, as the charge on the surface of particles prevents aggregation. As the zeta potential of encapsulated sulfentrazone was -38.1 mV which is more than (+/-) 30 mV, thus the encapsulated sulfentrazone herbicide is stable. This stability might be useful for slow release of the particle and also for reducing vertical and horizontal leachability.






 
Field experiment
 
Weed flora
 
Weed species vegetation in the screening trial consists of Dinebra retroflexa, Echinochloa colonum, Amaranthus viridis, Calotropis gigantea, Corchorus trilocularis, Euphorbia thymifolia, Malachra capitata, Parthenium hysterophorus, Portulaca oleracea, Trianthema portulacastrum. All these weeds were occurred in main field trial I and II also except Calotropis gigantea and Parthenium hysterophorus. Among these most dominant weed species were grasses. Similar findings were observed by Dayan et al., (1996).
 
Screening trial
 
Effect of weed management treatments on weeds
 
The higher weed density was observed in unweeded control respectively at 20, 40 and 60 DAS. Lower weed density was noticed in the treatments T₇ (Sulfentrazone @ 0.40 kg a.i. ha^-1 e⁺ at 2 DAS) and T8 (Sulfentrazone @ 0.40 kg a.i. ha^-1 e⁺ at 3 DAS) at 20 and 40 DAS respectively. But at 60 DAS lower weed density was noticed in T₁ (Sulfentrazone @ 0.30 kg a.i.ha^-1 e⁺ at 1 DBS) and T₅ (Sulfentrazone @ 0.40 kg a.i. ha^-1 e⁺ at 1 DBS) (Table 1). The results were in conformity with the findings of Srivastava (2003). He proved that in hand weeded plot weed density was 43.53 m^-2 whereas in the sulfentrazone (0.8 L ha^-1) treated plot the weed density was only 0.90 m^-2.



Effect of weed management treatments on blackgram crop
 
Plant height (cm) was lesser in all the sulfentrazone applied treatments compared to absolute control (15.87 and 49.73) at 20 and 40 DAS. But at 60 DAS the plant height was on par in T₁,T₃,T₄, T₅, T₉,and T₁₁ with the control (55.07) (Table 2). In case of plant dry weight (g plant^-1) at 20 DAS there was no significant difference among the treatments. At 40 and 60 DAS more dry weight was noticed in T₁,T₃, T₅, T₉ and T₁₁ compared to unweeded control (Table 2).



Seed yield, hull yield and haulm yield (kg ha^-1) of blackgram were higher in sulfentrazone @ 0.30 (T₁) and 0.40 (T₅) kg a.i. ha^-1 at 1 DBS which are 1349, 587 and 3851 kg ha^-1 in T₁ and 1355, 589 and 3842 kg ha^-1, respectively (Table 1). Similar results were recorded by Krausz et al., (1998) in soybean when sulfentrazone applied @ 0.42 kg a.i. ha^-1.
 
Main trial I and II

Effect of weed management treatments on weeds
 
Zero weed density was noticed with the application of encapsulated sulfentrazone at 1 DBS (T₁), hand weeding at 15 and 30 DAS (T₇) and weed free plot (T₈) in both the trials at 20 DAS. Very low weed density was observed in T₁ and T₂ (sulfentrazone without encapsulation applied at 1 DBS) and T₇ in both the trials at 40 and 60 DAS. All the other treatments also noticed lower weed population compared to absolute control in both main trials (Table 3). The results were in conformity with the findings of Srivastava (2003). In case of pendimethalin similar weed density was observed by Gupta et al., (2013).



Effect of weed management treatments on blackgram crop
 
In plant dry matter at 20 DAS there was no significant difference among the treatments. At 40 and 60 DAS more plant dry weight was noticed in all the treatments compared to control (Table 4). Seed yield, hull yield and haulm yield of blackgram were higher in T₁, T₂, T₅, T₇ and T₈ in both trials (Table 5). There was no significant difference in harvest index of all the treatments (Table 5). Higher gross returns (92,233 Rs ha^-1), net returns (64,548 Rs ha^-1) and B:C ratio (3.33) were acquired in T₁ followed by T₂ and least Gross returns (27,808 Rs  ha^-1) and net returns (3,033 Rs  ha^-1) and B:C ratio (1.12) were acquired in absolute control (Table 6). The results were supported by the findings of Shruthi and Salakinkop (2015).

In the laboratory experiment nano-encapsulated sulfentrazone herbicide was synthesized and characterized by using SEM and PSA which proved that the sulfentrazone was encapsulated perfectly and the size also reduced much compared to non-encapsulated (normal) sulfentrazone. In addition the zeta potential of encapsulated one is in correct range which made that powerful and stable. Then the synthesized encapsulated sulfentrazone was tested in the field experiment for its selectivity, efficacy and to optimize time and dosage of application. On the basis of both laboratory and field experiment (screening trial) it might be concluded that sulfentrazone @ 0.30 kg a.i. ha^-1 with encapsulation and without encapsulation applied at 1 DBS and 2 DAS were produced lesser weed density and higher plant dry weight, seed yield. Even though both T₁ and T₅ are giving better results, T₁-sulfentrazone @ 0.30 kg a.i. ha^-1 with encapsulation applied at 1 DBS is economically feasible to the farmer for getting higher gross returns, net returns and B:C ratio. These better performing treatments in screening trial were compared with other herbicides and weeding methods in main field trial I and II. These trials revealed that sulfentrazone @ 0.30 kg a.i. ha^-1 with encapsulation applied at 1 DBS was given lower weed density and higher seed yield, gross returns (92233 Rs  ha^-1), net returns (64,548 Rs  ha^-1) and B:C ratio (3.33). This concluded that sulfentrazone @ 0.30 kg a.i. ha^-1 with encapsulation applied at 1 DBS  is better alternative for the season long weed management in blackgram without affecting the soil and ground water, as well as increasing the productivity.

Authors are thankful to the Department of Agronomy and Department of Nano Science and Technology, TNAU, Coimbatore for rendering facilities whenever and wherever needed.