Evaluation of Sensitivity of Some Plants to Simulated Trifluralin Residues in Soil

Trifluralin is one of the important soil applied selective, pre-sowing or pre-emergence herbicide used to control many annual grasses and broadleaf weeds in a large variety of arable and horticultural crops. It is a generally regarded as a moderately persistence herbicide with a half-life of 1.5 to 6.5 months. Several factors determine the length of time herbicides persist and their carryover. These factors fall into three categories: soil factors, climatic conditions, and herbicide properties. Soil related factors including soil temperature, soil texture, and soil moisture and soil organic matter are the most important factors in herbicide persistence. However physicochemical of herbicide such as absorption properties are also important factor. So different herbicides may have different persistence in soil and have different effects on agroecosytems and also on subsequent rotational crops. Normally residual herbicides extend the period of weed control, and increase the efficiency of weed management efforts. However, they may persist longer than desired and injure or kill subsequent rotational crops. Most herbicide labels include crop rotation guidelines, but rotational restrictions are often not listed for many crops. Several experiments have demonstrated that trifluralin herbicide can per sist in the soil and carryover to affect succeeding crops. Some crops tolerate a particular herbicide residue and can be replanted soon after that herbicide is applied, while other crops remain sensitive to the herbicide for a longer time after application. This experiment was conducted to investigation of some plants sensitivity to soil residue of trifluralin in greenhouse conditions. To evaluate the sensitivity of some plants to simulated trifluralin soil residues, an experiment was conducted based on completely randomized design as factorial layout and four replications in Research Greenhouse of Faculty of Agriculture, Ferdowsi University of Mashhad. Treatments included seven plants (sorghum, millet, barley, wheat, oat, alfalfa and cucumber) and eight levels of concentrations of simulated trifluralin residue in soil (0, 0.002, 0.004, 0.021, 0.043, 0.046, 0.086 and 0.129 mg a.i. kg-1 soil). Trifluralin aquatic solution incorporated to soil and transported to the plastic pots with a 15-cm diameter. Plants emergence percentage was recorded and their shoot and root biomass were measured 35 days after emergence. The data were statistically analyzed using analysis of variance, and differences among mean values of treatments were compared by Duncan's test using SAS software. For determination the required dose of herbicides to reduce 50% of plants response (ED50), the dose response curves were fitted simultaneously using the following three-parameter logistic dose-response model with the lower limit equal to zero.
Where f is the response (above ground dry weight and root dry weight), d is the upper limit, b is the curve slope, e denotes the dose required to give a response halfway between the upper and lower limits (ED50); and x is the herbicide concentration in soil. The validity of the above model and the comparisons between the parameters were made using F-test for lack-of-fit with a 5% level of significance. Results showed that trifluralin residue decreased plants mergence and shoot and root biomass. By increasing the trifluralin soil residue, reduced all above mentioned parameters significantly (p≤0.01) in all plants. Plants showed different response (p≤0.01) to trifluralin soil residue. The highest and the lowest percent shoot and root dry weight to control treatment were observed in alfafa and sorghum, respectively. Based on ED50 parameter, alfafa (0.164 mg a.i. trifluralin kg-1 soil) and sorghum (0.048 mg a.i. trifluralin kg -1 soil) appeared the most tolerant and the most sensitive plants to trifluraline soil residue, respectively. The other crops sensitivity to trifluralin in soil residue followed the following order: sorghum> millet >barley> wild oat > wheat > cucumber >alfafa. According to the results, trifluralin residue in soil can affect the growth of rotational crops specially sorghum and millet. Therefore, regarding to the distance planting of sensitive crops after trifluralin application is necessary. Based on the results, plant response to trifluralin residues in soil were different significantly. However some sensitive plants such as sorghum and millet may be used for soil residue of trifluraline in bioassay experiments.