Screening of solvent and adjuvant
The results of solvent screening are shown in Table 1. The original pesticide could not be completely dissolved using a single solvent. However, 5% N-methyl-2-pyrrolidone + 10% cyclohexanone could completely dissolve the original pesticide. There was no solid precipitation at room temperature, so the formulation could be used for the subsequent experiment. According to Table 2, a mixture of sulfonate adjuvants (70b) and fatty alcohol polyoxyethylene ether adjuvants (AEO-4, -5, -7, -9, 992) could stabilize the system in a single, transparent, homogeneous phase. Therefore, sulfonate adjuvant (70b) was selected and mixed with five adjuvants of the AEO series to prepare thidiazuron·diuron ultra-low-volume sprays, numbered 1–5 (as shown in Table 3).
Surface tension measurement
The critical surface tension of cotton leaves is 63.30–71.81 mN/m. Figure 1 shows that the surface tension of each sample was 31.67–33.37 mN/m, which was much lower than the critical surface tension of the leaf, indicating the agent was able to completely wet the leaf and be fully distributed on the leaf surface. The maximum surface tension of the reference product was 38.90 mN/m. Under the same dosage of adjuvant, sample 5 with adjuvant 992 had the smallest surface tension of 31.67 mN/m.
Surface tensions of different samples. Different letters (a–d) indicate significant differences between means. Means followed by the same letter are not significant at the 5% significance level by the LSD test (LSD = 0.05). Vertical bars indicate a standard deviation of the mean. The detailed data of the histogram is shown in Supplementary Table S1.
Contact angle measurement
According to Young’s equation, the smaller the surface tension, the smaller the contact angle40,41. Figure 2 shows the contact angle of different samples on cotton leaves and the change in contact angle over time. The contact angles of oil agents containing the adjuvant 992, AEO-7 and AEO-9 were smaller than that of the reference product, and the spreading effect was superior to that of the reference product. In the surface tension test, sample 5 had the smallest surface tension of 31.67 mN/m; this sample showed the minimum initial contact angle (39°) and a static contact angle (22°). The surface tension of the reference product was 38.90 mN/m., with the maximum initial contact angle (65.5°). Therefore, the relationship between surface tension and contact angle conformed to Young’s equation.
Contact angles of different samples on cotton leaves in 0–10 s. The detailed data of drawing the contact Angle curve is shown in Supplementary Table S2.
Volatilization rate measurement
As shown in Fig. 3, the volatilization rate of the oil agent was much lower than that of the reference product. The volatilization rate of the five treatments was 5.80–8.74%, while the volatilization rate of the reference product was 22.97%. The volatilization rate of the oil agent met the quality requirements of an ultra-low-volume spray (≤ 30%). A low volatilization rate helps with spraying defoliants in hot and dry areas such as Xinjiang, effectively preventing evaporation of the droplets and increasing deposition.
Volatilization of different samples on filter paper. Different letters (a–e) indicate significant differences between means. Means followed by the same letter are not significant at the 5% significance level by the LSD test (LSD = 0.05). Vertical bars indicate a standard deviation of the mean. The detailed data of the histogram is shown in Supplementary Table S3.
Viscosity measurement
Viscosity is an important factor affecting the atomization performance of a formulation42. Figure 4 shows that the viscosity of the five oil agents ranged from 12.9 to 18.3 mPa s, meeting the quality requirements of an ultra-low-volume spray (< 2 Pa s). The addition of chlorinated paraffin effectively increased the viscosity, facilitating analysis of the relationship between deposition and the viscosity of droplets in the subsequent experiment. This part of the work is described below under atomization performance.
Viscosity of each sample. Different letters (a–f) indicate significant differences between means. Means followed by the same letter are not significant at the 5% significance level by the LSD test (LSD = 0.05). Vertical bars indicate a standard deviation of the mean. The detailed data of the histogram is shown in Supplementary Table S4.
Screening for the best working conditions for the centrifugal spray atomizer
As shown in Fig. 5, the linear equation fitted to voltage and rotation speed was y = 79.2418x + 400.2857, and the correlation coefficient was 0.9998. This indicated a good linear relationship and that the working state of the rotary atomizer was stable, and met the requirements of the next experiment.
Relationship between voltage and rotation speed of the centrifugal spray atomizer. The detailed data of drawing the curve is shown in Supplementary Table S5.
The relationship between droplet size and the rotation speed of the rotary atomizer are shown in Fig. 6. The rotation speed of the rotary atomizer and the cumulative volume diameter had a binomial distribution. The equation fitted to D10 was y = 1E−05x2 − 0.2496x + 1440.4, with a correlation coefficient of 0.9947; the equation fitted to D50 was y = 8E−06x2 – 0.1672x + 934.73, with a correlation coefficient of 0.9791; the equation fitted to D90 was y = 5E−06x2 – 0.0983x + 539.55, with a correlation coefficient of 0.9005. The correlation between the cumulative volume diameter and the rotation speed was very high, and the cumulative volume diameter became smaller as the rotation speed increased. When the rotation speed exceeded 9600 rpm (voltage 20 V), the cumulative volume diameter hardly changed. As shown in Fig. 7, when the rotation speed was 6400–7600 rpm (voltage 10–15 V), the spectral width of droplets increased with increasing rotation speed, and the distribution of droplets became more uneven. When the rotation speed was 7600–9600 rpm (voltage 15–20 V), the spectral width of droplets decreased with increasing rotation speed, and the distribution of droplets became more uniform. The minimum droplet size distribution appeared at 9600 rpm (voltage 15–20 V). When the rotation speed surpassed 9600 rpm (voltage > 20 V), the droplet size distribution tended to be stable. This coincided with data shown in Fig. 6, where the inflection point appeared when rotation speed was 9600 rpm (voltage = 20 V).
Relationship between the rotation speed of the centrifugal spray atomizer and droplet size. D10: 10% cumulative volume diameter, D50: 50% cumulative volume diameter, D90: 90% cumulative volume diameter. The detailed data of drawing the curve is shown in Supplementary Table S6.
Relationship between the rotation speed of the centrifugal spray atomizer and the fog droplet spectrum. The detailed data of drawing the curve is shown in Supplementary Table S6.
Therefore, we determined that the optimal working conditions for the rotary atomizer were achieved by setting the DC voltage stabilized power supply current to 1.00 A and voltage to 20 V, which were used for subsequent experiments.
Atomization performance
The relationship between viscosity and droplet spectrum are shown in Table 4 and Fig. 8. The cumulative volume diameter for the five treatments was less than 150 μm meeting the requirements of the ULV spray32. The cumulative volume diameter for the five treatments was larger than that for the reference product, the width of the droplet spectrum was narrower, and the droplet distribution was more uniform. Droplet size affects the drift of droplets43. The D10 of the reference product was 25.62 μm under these working conditions. This droplet size was highly susceptible to drift and deposition on non-target organisms. Water suspension was not suitable for this application at low dosage.
Relationship between formulation viscosity and droplet spectrum. The detailed data of drawing the figure is shown in Supplementary Table S7.
As presented in Table 4, droplet size increased with increasing viscosity, which influenced the droplet spectrum. The results in Fig. 8 show that the span of droplet size decreased with the increase of viscosity, indicating that droplets with more uniform distribution could be obtained by increasing the viscosity of the formulation41.
Droplet deposition effect
We tested the efficacy of the ULV spray formulation by spraying cotton plants using an UAV. The test results in Table 5 indicate that increasing the dosage of application would increase droplet size, coverage, and deposition density. At the same application dosage, the droplet size of the ultra-low-volume spray was slightly larger than that of the reference product, and the coverage and deposition density were greater than those of the reference product. The droplet spectral width (Rs) of the five treatments was less than 1, and the coefficient of variation was less than 7%, indicating that the droplet distribution was relatively uniform. Among treatments, T2 had the narrowest Rs and coefficient of variation (CV), where the droplet size distribution was the most uniform. For the ultra-low-volume spray, at the application dosage of 4.5–9.0 L/ha, the droplet coverage gradually increased from 0.85 to 4.15%; the droplet deposition densities were 15.63, 17.24, 28.45, and 42.57 pcs/cm2, which were larger than requirements suggested in the literature. The droplet coverage of the reference product (T5) was 0.73%, and the deposition density was only 11.32 pcs/cm2.
Efficacy trials
The efficacy of cotton defoliant is reflected in the defoliation rate and boll opening rate of cotton after application. Therefore, we surveyed the defoliation rate and boll opening rate of cotton in the test area 3–15 days after application. The results are shown in Figs. 9 and 10.
Defoliation rate 3–15 days after treatment. The detailed data of drawing the curve is shown in Supplementary Table S8.
Boll opening rate 3–15 days after treatment. The detailed data of drawing the curve is shown in Supplementary Table S9.
Figure 9 indicates that the defoliation rates of the five treatments 15 days after the pesticide treatment were 59.82%, 63.96%, 71.40%, 77.84%, and 54.58%, respectively. The defoliation rates of T1, T2, and T5 were less than 70%.
Application of the ultra-low-volume spray at 4.50 L/ha or 6.00 L/ha and the reference product at 6.00 L/ha had a poor defoliation effect. T4 (9.00 L/ha) was superior to the others, and the defoliation rate reached 77.84% 15 days after application. As shown in Fig. 10, the boll opening rates of the five treatments were 58.54%, 67.74%, 95.35%, 100%, and 44.68% 15 days after application. Similarly, the boll opening rates of T1, T2, and T5 were poor, with the boll opening rate of the control T5 only 44.68%. We analyzed significant differences between the defoliation rates and boll opening rates of the five treatments. The results showed that the defoliation rate and boll opening rate associated with the thidiazuron·diuron ultra-low-volume spray on cotton plants were significantly different from those of the reference product.
Overall, the defoliation rate and boll opening rate produced by the ultra-low-volume spray were superior to those produced by the reference product. This result was consistent with data shown in Table 5. The higher the droplet coverage rate, the higher the droplet deposition density and the higher the defoliation rate and boll opening rate. T1, T2 and T5 had poor deposition effect on cotton plants, and the effective pesticide utilization rate was low, resulting in dissatisfactory defoliation rates and boll opening rates. Both the droplet coverage rate and the droplet deposition density of T3 and T4 were large. Therefore, droplets of pesticide solution could deposit more easily and uniformly on cotton leaves, allowing the plants to defoliate and open their bolls easily.
Source: Ecology - nature.com
