Study On The Control Effect Of Tiafenacil On Orchard Weeds And Its Residue And Hydrolysis Behavior In Soil

Tiafenacil is an active ingredient of a new type of protoporphyrinogen oxidase（PPO）inhibiting herbicide Terrad’or（trade name）.It was successfully developed and registered for application by Farm Hannong Co,Ltd.（South Korea）in 2018.The active ingredient has the characteristics of low toxicity,quick effect and non-selective properties,which can effectively control broadleaf weeds and grass weeds,such as barnyard grass,beef tendon and abutilon,etc.At the same time,it also has good control effect against glyphosate weeds.Its strong anti-rain erosion and specificity to plants can prevent animals and humans from harm,and it is very friendly to the environment.The herbicide has been registered and listed in the United States,Australia and other countries in recent years,and is widely used to control weeds in apple orchards,non-arable land and before crops planting.The research of this paper is carried out on the basis of summarizing the previous research results and the actual situation in China.Taking the citrus,the largest type of fruit in China as the test object,through the control effect of orchard weeds and the residual digestion test of soil and relevant detection and analysis,this paper has further understand the characteristics of tiafenacil and its application value in China.It provides a scientific basis for rational use in the future.The content of this test is as follows:（1）In order to explore the possibility of tiafenacil in controlling weeds in citrus orchards outdoors,5%tiafenacil suspension agent was selected as the application formulation,and the experiment was conducted in Jiangxi Agricultural University in July2019.The experiment was carried out in the citrus garden of the experimental station,and the control effect of 5%tiafenacil suspension agent on weeds in citrus orchards under the treatment of 40.13～160.50 g a.i./ha was tested by the stem-leaf directional spraying method.The results showed that 5%tiafenacil suspension agent could effectively control grass weeds in citrus orchards,with strong quick-acting effect and a lasting period of up to30 days,and had no adverse effect on the growth of citrus trees.The control effects of100.31～160.50 g a.i./ha on grass weeds were 88.71%～96.15%and 82.33%～91.29%on 15and 30 days after treatment,respectively,and its fresh weight efficacies were89.27～95.81%at 30 days after spray.The control effect on broad-leaved weeds was relatively low,and the control effect of all treatments was lower than that of the control agent 70%saflufenacil water dispersible granules at 65.63 g a.i./ha at 15 and 30 days after treatment.Therefore,it is considered that 5%tiafenacil suspension agent can be used to control grass weeds in citrus orchards,and its recommended dosage is 100.31～160.50 g a.i./ha.（2）In order to rapidly and accurately detect the active ingredients of tiafenacil,a quantitative analysis method of high performance liquid chromatography-tandem mass spectrometry（HPLC-MS）was established.Zorbax Eclipse XDB-C18 chromatographic column was used under the conditions of flow rate 1 m L/min,column temperature 40℃,acetonitrile-0.1%formic acid water（volume ratio 50:50）was used as the mobile phase,and the content of tiafenacil was determined by tandem mass spectrometry in ESI positive ion mode.Under the detection conditions,tiafenacil was in the range of 0.01～1 mg/L,with good linearity and specificity,high sensitivity,standard deviation of 0.059,coefficient of variation of 1.16%,and recovery rate of 96.93%～101.92%,the average recovery rate was99.01%,all of which met the requirements of trace detection,and it was an ideal detection and analysis method for determining the content of tiafenacil in preparations and matrices.The composition of the soil matrix is complex,and it is easy to cause interference when detecting low-concentration pesticides in the soil.In order to eliminate the influence of soil impurities and improve the detection and analysis ability,loessial soil（Gansu）,brown soil（Shandong）,purple soil（Sichuan）,red soil（Jiangxi）and latosol（Hainan）were used as the research substrates,and the more mature quick,easy,cheap,effective,rugged and safe pretreatment technology（QuEChERS）was used to separate tiafenacil from soil,and finally determined to use acetonitrile as the extraction solvent,and C18（octadecyl-bonded silica gel）was used as the purified adsorbent.The average recovery rate was86.31%～101.66%.At the same time,the absolute value of matrix effect was less than 10%when using this method to detect samples,which had little impact on the detection signal.In conclusion,it is considered that the optimized residue analysis method in this study can meet the detection requirements of tiafenacil in soil matrix.（3）In order to understand the dissipation of tiafenacil in the soil,the tiafenacil dissipation experiment was conducted at citrus orchard sites in five provinces of China（Gansu,Shandong,Sichuan,Jiangxi,and Hainan）in 2019 and 2020.The dose selected for the test is the highest recommended dose（160.50 g a.i./ha）of 5%tiafenacil suspension agent（active ingredient）and 1.5 times the highest recommended dose（240.75 g a.i./ha）.The results show that the dissipation of tiafenacil in two years and five experimental sites conformed to the first-order kinetic equation C_t=C₀e^-kt,R²is 0.8130～0.9967,and the half-life t_0.5is 0.26～4.19 d.The dissipation rate of tiafenacil was positively correlated with soil organic matter content and negatively correlated with soil pH.In conclusion,tiafenacil could readily dissipate in soil,and it belonged to easily degradable pesticides and was safe for the agricultural environment.（4）The hydrochemical degradation of pesticides is an important indicator for evaluating whether they are safe to the environment.In order to determine the hydrolysis kinetics of tiafenacil and its influencing factors,the experiment was simulated by an indoor incubator,the effects of microorganisms,water quality,pH,temperature and initial concentration on the hydrolysis rate were briefly discussed,and the kinetic and thermodynamic functions were used to analyze.The results showed that the indoor hydrolysis process of tiafenacil met the first-order kinetic formula,R²was 0.7796～0.9997,and the hydrolysis half-life was 0.22 d～88.85 d,which belongs to the moderate degradation level.Among them,the effects of water quality,pH and temperature was more dominant,and the effects of microorganisms and initial concentration were secondary.When analyzing the temperature effect coefficient（Q）,activation energy（E_a）,activation enthalpy（ΔH）and activation entropy（ΔS）in the thermodynamic function,it is found that the activation energy and activation enthalpy of the hydrolysis reaction have no obvious correlation with temperature,but show a good correlation with activation entropy.The average value of activation entropy is-68.26 k J/（mol k）,which decreases with the increase of temperature.In this experiment,the hydrolysis process of tiafenacil in ultrapure water,tap water and lake water was compared and analyzed,and its t_0.5was 47.14 d,6.72 d and10.41 d,respectively,indicating that in the actual complex environment,tiafenacil are more easily hydrolyzed.