Better Droplet Deposition and Internode Shortening Effects of Plant Growth Regulator EDAH on Maize Applied by Small Unmanned Aerial Vehicle Than Electric Knapsack Sprayer

Abstract

Maize (Zea mays L.) lodging is an important factor limiting its yield increase worldwide. EDAH (containing 27% ethephon and 3% DA-6) is commonly used to decrease lodging. There is an urgent need to select efficient application methods of agrochemical for better spray deposition. In our research, an unmanned aerial vehicle (UAV) (15 L ha⁻¹ and 30 L ha⁻¹) with EDAH dosages of 72 and 90 g a.i. ha⁻¹, and electric knapsack sprayers (EKS) (450 L ha⁻¹) with dosages of 90 g a.i. ha⁻¹ were used to compare the droplet deposition distribution, uniformity and maize growth. According to our research, EDAH applied by UAV had a higher droplet deposition rate than EKS; EKS had a higher droplet coverage rate, deposition density, droplet distribution uniformity. At the same dosage of EDAH, the UAV had a better effect on controlling maize growth than EKS, and almost equal effects were detected when the dosage applied by the UAVs was decreased by 20%. Considering the lodging occurrence and yield, we recommend that the dosage of EDAH applied by UAVs should be 72 g a.i. ha⁻¹ when there is weak lodging and 90 g a.i. ha⁻¹ when there is heavy lodging, with a spray volume of 15 L ha⁻¹.

1. Introduction

Maize (Zea mays L.) is one of the most widely cultivated crops globally, and the production of maize needs to increase with the increasing global population [1,2,3]. However, maize lodging is one of the main global challenges that can make maize yield loss by 5–20% [4]. Maize lodging occurs in root and stem [5], both of which cause less photosynthesis and substance transportation to ear in maize plants and are also difficult to harvest by agricultural machinery [4,6]. Many factors cause crop lodging, such as variety, nitrogen, pests, soil and extreme weather [7,8]. Therefore, it is imperative to take effective measures to control the occurrence of maize lodging.

Plant growth regulators (PGRs) are involved in the whole process of vegetative and reproductive growth of plants [9,10,11]. Ethephon is a widely used PGR to control maize lodging at the early stage of jointing. It can significantly shorten plant height, ear height, basal internode length and increase internode crushing strength [12,13,14]. The mechanism of ethephon has been reported; DNA methylation variation is induced after being converted to ethylene in maize plants [15] while reducing the accumulation of gibberellin and auxin by differentially expressing gibberellin synthesis genes and auxin transport genes [16,17]. However, some reports showed that ethephon causes yield loss by decreasing the number of grains per ear and the 1000-grain weight when no lodging [12] or slight lodging occurred [14]. The PGR Diethyl aminoethyl hexanoate (DA-6) can improve the photosynthesis performance of ear leaf and increase the grain filling of maize [18,19,20]. At present, EDAH (containing 27% ethephon and 3% DA-6) is used to decrease lodging and promote the development of kernels to increase maize yield in China [21,22,23,24].

In recent years, the increase in urbanization led to a huge decrease in the population of farmers, and cropland has become more concentrated by land transfer in China [25,26]. Consequently, there is an urgent need to improve the mechanization of pesticide application machines to replace electric knapsack sprayers (EKS), which still accounts for the majority in China [27]. At present, crop protection unmanned aerial vehicles (UAVs) have developed rapidly in China [28,29]. It has a higher working speed, less water consumption, higher deposition rate of pesticides, and is safer for farms than EKS [30,31,32,33,34]. Moreover, the droplet deposition distribution of UAVs and EKS, including droplet coverage rate, deposition density, deposition, deposition rate and distribution uniformity, has attracted the attention of scholars in recent years [32,35,36,37]. Furthermore, droplet distribution is influenced by flight height, flight speed, spray volume, dosage, and spray adjuvants [38,39]. Spray volume is an important factor; thidiazuron and diuron contents increased along with spraying volume at the range of 17.6–29.0 L ha⁻¹ applied by UAV on cotton leaf [40]. However, the spray volume of EKS was higher than UAV, and it was 225–450 L ha⁻¹ for the control of lodging in maize [21,22,23,24]. It was reported that UAV application can increase pesticide efficiency and the control effects of UAV increase by 5.3–17.5% than those of EKS in rice [33]. In addition, some reports also showed that UAV treatments can reduce 20% dosage with spray additives compared with EKS [41,42]. However, there are no reports on EDAH applied by UAV during the early jointing stage for maize plants, and it needs further research.

Based on the current research situation, we hypothesized that EDAH applied by UAVs had a better effect on maize growth, and dosage could be reduced by 20% in UAV treatments compared to EKS. We set different EDAH dosages and application methods for UAVs and EKS in our research. The objectives of our work were to (1) study whether UAVs application has the same maize growth control effect even if the dosage is reduced by 20% with the instantaneous concentration increased by 15–30 times (spray volume of UAV was 15, 30 L ha⁻¹, EKS was 450 L ha⁻¹); (2) investigate the droplet deposition applied by UAV and EKS (3) obtain the appropriate EDAH dosage and spray volume applied by UAVs in maize.

2. Materials and Methods

2.1. Experimental Site

Field experiments were conducted at the Jiyang Experimental Station (36°58′ N, 116°58′ E) of Shandong Academy of Agricultural Sciences, Jinan city, Shandong Province, China (Figure 1) from June to October 2019 and 2020. The cultivated land was the annual rotation of wheat–maize. The field soil was sandy clay loam with pH 8.5, 15.5 g kg⁻¹ organic matter, 1.23 g kg⁻¹ total N, 135 mg kg⁻¹ available K, and 42.6 mg kg⁻¹ available P in the 0–40 cm soil layer. The daily average temperature and precipitation during the growing seasons are shown in Figure 2.

2.2. Experimental Design

EDAH dosage (ED) and spray volume (SV) applied by UAV were set as experimental factors in our research. The recommended ED is 90 g a.i. ha⁻¹ for maize [43], and our hypothesis was that the application by UAV might reduce the ED by 20%. Hence, two EDs, including 72 and 90 g a.i. ha⁻¹ were set to obtain the appropriate ED in the field. For SV applied by UAVs, we used the common dosages of 15 L ha⁻¹ (UAV (15 L) and 30 L ha⁻¹ (UAV (30 L)). Moreover, the treatment applied by EKS with ED 72 g a.i. ha⁻¹ and a blank control (CK) were added. There was a total of 6 treatments (

Table 1. Experimental treatments from 10 June to 5 October in 2019 and 2020.

Treatment	Spray Equipment	EDAH Dosage (g a.i. ha−1)	Spray Volume (L ha−1)	Aviation Adjuvant (g ha−1)
CK	EKS	0	450	0
TK	EKS	90	450	0
T1	UAV	72	15	225
T2	UAV	72	30	450
T3	UAV	90	15	225
T4	UAV	90	30	450

), each treatment had 3 replications, and every replication covered an area of 750 m² (50 m × 15 m). In addition, all plots were randomized in a block arrangement by factorial design. ‘Deng hai 605′ (♀DH351×♂DH382, Shandong Denghai Seeds Co., Ltd., Laizhou, China) was planted at 75,000 plants ha⁻¹ as the experimental material. As to ensure the accuracy of the experimental results without the influence of drug drift, a 10-m buffer (two spray widths) was set at the boundary of each plot.

The UAV MG-1P (SZ DJI Technology Co., Ltd., Shenzhen, China) was used for the experiment, and the flying height was 2 m over the maize canopy (Figure 3A). The flow rate of the nozzle (XR11001VS) was 0.379 L min⁻¹. The EKS 3WBD-16L (Taizhou Gufeng Sprayer Co., Ltd., Taizhou, China) was used, and the flow rate of the single nozzle (M14X1. 5) was 0.5 L min⁻¹ at 3 bar pressure (Figure 3B). During EDAH spraying, the field temperature, wind speed, and relative humidity were measured by PM6252B Digital Anemometer (Peakmeter Instruments Co., Ltd., Shenzhen, China), and they were 35–37.5 °C, 0.5–1.3 m s⁻¹, and 37.5–42.5%, respectively. In order to accurately control the actual volume of the spray, the flow rate was measured first by calculating the volume of water flow per unit time, and then the application speed of the EKS and UAV was controlled. The flying speed of the UAV for 15 and 30 L ha⁻¹ was 2 and 4 m s⁻¹, respectively, and the speed of operation of EKS was 0.33 m s⁻¹. Three tests were carried out in the non-experimental plots before application, and the experiments were carried out after ensuring the spray volume. The forward direction of UAV and EKS was consistent with parallel direction to the plot lines. Moreover, EDAH (30% aqueous solutions, Shanxi Haozhida Biotechnology Co., Ltd., Yuncheng, China) was applied at the seven-expanded-leaf (V7) stage on 20 July in two years). Beidatong (Hebei Mingshun Agricultural Technology Co., Ltd., Shijiazhuang, China) was added to the solution at a dosage of 15 g L⁻¹ as an aviation adjuvant for UAV spraying.

2.3. Analysis of Droplet Deposition Distribution

The droplet analysis was performed by adding the droplet indicator Allura Red (85% purity, Zhejiang Jigaode Pigment Technology Co., Ltd., Wenzhou, China) to the T3, T4, and TK treatments, and the dosage was 15 g L⁻¹ of the solution. The corresponding absorbance of Allura Red was measured under 510 nm by an ultraviolet spectrophotometer (UV2550, Shimadzu Scientific Instruments, Kyoto, Japan).

2.3.1. Droplet Coverage Rate and Droplet Deposition Density

Copper plate label paper (4 cm × 6 cm) (Standard & Poor’s Office Co., Ltd., Hangzhou, China) was used as the droplet tracer card (Figure 3C). A total of 15 sample plants per plot were fixed, and the sample was chosen in parallel (five positions, three plants per position) and vertical (three positions, five plants per position) to the flight direction. In addition, these cards were placed on the upper (8th leaf), middle (6th leaf), and lower leaf (4th leaf) of each chosen plant. These cards were collected 30 min after application and scanned through the scanner (DCP-1608, Brothers (China) Commercial Co., Ltd., Shanghai, China) at 600 DPI pixels. The imaging software Depositscan program (Agricultural Research Service, U.S. Department of Agriculture, Washington D.C., USA) was used to analyze the droplet density and coverage rate.

2.3.2. Droplet Deposition and Droplet Deposition Rate

The upper, middle, and lower leaf area was measured. Moreover, the leaf was collected in a Ziploc bag; distilled water was added to collect the droplet tracer. After that sample solution was shaken for 30 s and filtered by a 0.22 μm membrane, the absorbance was finally determined by an ultraviolet spectrophotometer. Furthermore, the droplet deposition rate was characterized as the efficiency of the chemical deposited on the leaf surface. According to Lou et al. [44], we made a few minor modifications. The droplet deposition was calculated using Equation (1). All the remaining leaves were also placed into the Ziploc bag to collect tracer in the remaining plants. After the above steps were followed to measure their absorbance, the droplet deposition rate was measured by Equation (2).

β_dep = (ρ_smpl − ρ_blk) × F_cal × V_dil / (ρ_spray × A_col ×α),

(1)

Droplet deposition rate (%) = F_cal × D_plant × [V_dil1 × (ρ_upper + ρ_middle + ρ_lower − 3 × ρ_blk) + V_dil2 × (ρ_remain − ρ_blk)]/(ρ_spray × V_spray) × 100

(2)

In the equation, β_dep is the droplet deposition per unit area (unit = mL cm⁻²), ρ_smpl is the absorbance value of the eluent, ρ_blk is the absorbance value of the blank sample, F_cal is the coefficient between the absorbance value and the tracer concentration, V_dil is the volume of the eluent (unit = mL), ρ_spray is the tracer concentration in the spray solution (unit = g L⁻¹), A_col is the area of the sampled leaf (area = length × width × 0.75, unit = cm²), α is the ratio of droplet tracer added by T3, T4, to TK, and for intuitive comparison, α equals 1, 2, and 30, respectively. D_plant is the number of plants per hectare, and ρ_upper, ρ_middle, and ρ_lower are the absorbance values of the eluent for the upper, middle, and lower leaf, respectively. V_dil1 is the volume of the eluent for those three leaves (unit = mL), ρ_remain is the absorbance value of the eluent for the remaining leaf (unit = mL), V_dil2 is the volume of the eluent for remaining leaf in one plant (unit = mL), and V_spray is the spray volume per hectare (unit = mL).

2.3.3. The Uniformity of Droplet Distribution

The coefficient of variation (CV) was used to characterize the uniformity of droplet distribution in droplet coverage rate, droplet deposition density, and droplet deposition, with the distribution becoming more uniform when the CV is lower. The CV of two directions and three droplet indicators were compared. The CV calculation (Equation (3)) followed [35].

$$\mathrm{C}\mathrm{V} (\%) = \mathrm{S}/\overline{\mathrm{x}} \times 100,$$

(3)

where S is the sample standard deviation, and $\overline{\mathrm{X}}$ is the average of indicator.

2.4. Sampling and Measurement

2.4.1. Plant Sampling and Measurement

The heights of the plants, ears and the center of gravity of 20 plants per repetition in the middle of the plots were measured continuously. The CV of plant height and ear height use the same method with Equation (3). Five plants per plot were selected to measure the length of the basal internode (eighth internode) at the R3 stage. According to Xue et al. [45], the internode crushing strength of the eighth internode was measured using the internode strength tester DDY-1 (Shijiazhuang Ivors Technology Co., Ltd., Shijiazhuang, China). The lodging rate was measured by counting the lodging plants in four lines at the harvesting time.

2.4.2. Photosynthetic Characteristics

The net photosynthetic rate (Pn) of five selected ear leaves were measured from 10:00 am to 11:30 am at R3 (leaf temperature = 30 ± 1 °C, irradiation =1200 μmol m⁻² s⁻¹) by using Li-Cor 6400 (LI-COR Biosciences, Lincoln, NE, USA). At the same time, 15 ear leaves per plot were selected, and the relative chlorophyll content (SPAD) was measured using a chlorophyll meter (SPAD 502, Minolta Camera Co., Ltd., Osaka, Japan).

2.4.3. Grain Yield and Yield Components

Five-meter double row ears were harvested from the middle of each plot after physiological maturity, and ear numbers were counted. Twenty representative ears were used to measure the kernel number per ear and the thousand-kernel weight (TKW) (dry to constant weight under 70 °C). Finally, the grain yield was determined.

2.5. Statistical Analysis

Statistical analysis was conducted using the SAS 9.4 software package (SAS Institute, Inc., Cary, NC, USA). The Tukey’s honestly significant difference (Tukey) test was used for multiple comparisons between treatments at p < 0.05. One-way analysis of variance (ANOVA) was used for analyzing droplet distribution and plant growth control for all treatments. A significant difference was obtained in the results of the year, SV and ED were analyzed by using GLM for UAV treatments, and the interaction between SV and ED was analyzed.

3. Results

3.1. Droplet Coverage Rate, Droplet Deposition Density, Droplet Deposition and Droplet Deposition Rate

The droplet coverage rate increased along with SV (Figure 4A), EKS had a significantly higher coverage rate than the two UAV treatments in the upper, middle, lower, and average position (p < 0.05). The difference between the coverage rate of UAV (15 L) and UAV (30 L) was significant in lower leaf and average coverage rate, and UAV (30 L) treatment had a higher coverage rate. The average droplet coverage rate of UAV (15 L) was 95.6% lower than that of EKS and 86.31% for UAV (30 L). Furthermore, the droplet deposition density of all positions also showed the pattern: EKS > UAV (30 L) > UAV (15 L), the difference between droplet density of these treatments was significant (Figure 4B). EKS had 115 and 92 droplets cm⁻² greater droplet deposition density than UAV (15 L) and UAV (30 L) on the average position. Moreover, the droplet density in the upper leaf was higher as compared to the middle and lower leaf in all the treatments, but statistically, there was no significant difference. The ratio of the deposition density of the lower leaf to the total deposition density was 28.78%, 29.42%, and 37.99% for EKS, UAV (15 L) and UAV (30 L).

As for the droplet deposition, UAV (30 L) had higher droplet deposition in the upper and middle position compared with the other treatments (Figure 4C). There was a significant difference between droplet deposition of UAV (30 L) and EKS in the upper leaf. Moreover, the droplet deposition rate of UAV (30 L) was 51.74%, it was 7% higher than UAV (15 L) and significantly 10% higher than EKS, while there was no significant effect between droplet deposition rate of EKS and UAV (15 L) (Figure 4D).

3.2. The Droplet Distribution Uniformity

The CV of two UAV treatments was greater than EKS in both flight directions for three indicators except the upper position of droplet deposition in the vertical direction (Figure 5). For the droplet coverage rate, the CV of UAV (30 L) was significantly greater than EKS in parallel to the flight direction for all sample positions, and UAV (15 L) had a slightly higher CV than UAV (30 L) in the vertical direction for middle, lower and average positions (Figure 5A,B). Moreover, the CV of droplet deposition density also followed: UAV (30 L) > UAV (15 L) > EKS in the parallel direction, there was a significant difference between CV of UAV (30 L) and EKS on middle leaf and average droplet density. EKS had a higher CV in the lower than upper and middle positions, while UAV (30 L) had less CV in the lower position at both directions (Figure 5C,D). For the droplet deposition, UAV (30 L) had the most CV in the upper, middle and average positions, and there was a significant difference between CV of UAV (30 L) and other treatments on middle positions in the parallel direction, while in vertical direction there was a significant difference between CV of the treatments in the upper leaf. CV of UAV (15 L) was highest in the lower position at both directions, but statistically not different (Figure 5E,F).

3.3. Plant, Ear and Center of Gravity Heights and Lodging Rate

EDAH treatments significantly shortened plant height 5.36–13.08% and 2.47–6.29%, ear height 10.31–27.00% and 11.50–18.00%, the center of gravity height 9.37–13.14% and 6.47–12.80% compared with CK in 2019 and 2020 (Figure 6A,B). T3 and T4 had the strongest effect on controlling maize height. Compared with TK applied by EKS, plant and ear height of T1 and T2 were reduced in 2019 and 2020, but statistically not different. Furthermore, the plant height, ear height, and center of gravity height of T3 and T4 were reduced by 4.5–8.2%, 4.51–18.6% and 1–4.2% compared with the TK in two years. There was no significant difference between these indicators of the two spray volumes for the same concentration of UAV treatment. At the same time, all treatments significantly reduced the lodging rate (Figure 6C,D). The occurrence of lodging was relatively slight in 2019. In contrast, the lodging rates for CK, TK, T1, T2, T3, and T4 were 37%, 17.3%, 16.0%, 17.3%, 14.3%, and 13.3%, respectively, in 2020. Plant, ear, central of gravity height, and lodging rate were significantly reduced by increasing ED in all UAV treatments (

Table 2. The results of significance analysis of spray volume and EDAH dosage on plant indicators for UAV treatments in 2019 and 2020.

Year	Treatment	Plant Height	Ear Height	Center of Gravity Height	Lodging Rate	CV of Plant Height	CV of Ear Height	Internode Length	Stalk Crushing Strength	Pn	SPAD
		cm	cm	cm	%	%	%	cm	N cm−2	μmol m−2s−1
2019	ED (g a.i. ha−1)
	72	271.8 a	103.1 a	101.9 a	1.2 a	5.0 a	8.8 a	13.1 a	502.6 b	25.5 b	64.1 b
	90	264.0 b	97.7 b	96.5 b	1.0 a	4.5 a	10.2 a	11.5 b	557.0 a	28.5 a	66.8 a
	SV (L ha−1)
	15	270.4 a	102.9 a	100.0 a	1.3 a	4.3 b	7.2 b	12.2 a	534.3 a	27.6 a	65.1 a
	30	265.3 a	97.9 a	98.5 a	0.8 a	5.2 a	11.8 a	12.3 a	525.2 a	26.4 a	65.7 a
2020	ED (g a.i. ha−1)
	72	273.0 a	112.0 a	104.8 a	16.7 a	2.2 a	7.8 a	12.3 a	459.2 b	22.2 b	61.8 b
	90	265.3 b	105.5 b	99.4 b	13.8 b	2.1 a	7.9 a	10.7 b	492.5 a	24.5 a	65.1 a
	SV (L ha−1)
	15	269.3 a	109.0 a	103.0 a	15.2 a	2.1 a	7.0 b	11.7 a	474.5 a	23.2 a	63.6 a
	30	268.9 a	109.0 a	101.3 a	15.3 a	2.3 a	8.7 a	11.3 a	477.2 a	23.5 a	63.3 a
ANOVA
Year		ns	***	***	***	***	ns	**	***	***	***
ED		***	**	***	**	ns	ns	***	***	**	***
SV		ns	ns	*	ns	**	**	ns	ns	ns	ns
ED*SV		ns	ns	ns	ns	ns	ns	ns	ns	ns	ns

Note: *, **, *** indicates significant at p < 0.05, 0.01, and 0.001. ns means not significant. ED: ethephon dosage. SV: spray volume. ED*SV: interaction between ethephon dosage and spray volume. Different letters to right of the value indicate significant differences at p < 0.05 as determined by the Tukey test in different treatments in the same year.

). The center of gravity height of UAV (15 L) was significantly higher than that of UAV (30 L) in 2020, while there was no significant difference between center of gravity height of two SV. Furthermore, no interaction was detected between the two factors.

3.4. The Uniformity of Plant and Ear Height

For the CV of plant height (Figure 7A,B), all UAV treatments had higher CV than CK and EKS in 2019. CV of T2 and T4 were significantly higher than CK in 2019, while there was no significant difference in plant height between CV of all the treatments in 2020. Furthermore, the CV of ear height in T2 and T4 was significantly higher than CK in 2019 and CK, TK in 2020 (Figure 7C,D). CV of plant and ear height was significantly affected by SV for UAV treatments (p < 0.01), and CV was increased along with SV (Table 2).

3.5. Length and Internode Crushing Strength of the Basal Internode

All EDAH treatments significantly shortened the internode length and increased the internode crushing strength compared with CK (Figure 8). The internode length of T3 and T4 was significantly reduced by 12.97% and 12.21% in 2019 and 17.31% and 17.57% in 2020 compared to TK when ED was the same. In the same way, the strength of T3 and T4 was significantly higher than that of TK and increased by 12.47% and 10.43% in 2019 and 19.2% and 17.2% in 2020, respectively. There was no significant difference between T1, T2 and TK for these two indicators. SV had no significant effect on the length and strength of the internode (Table 2). However, the internode length was significantly reduced, and strength was increased (p < 0.0001) along with ED. These two indicators were significantly different in two years, and no interaction was detected between ED and SV.

3.6. Net Photosynthetic Rate and Relative Chlorophyll Content of Maize Ear Leaf

The Pn and SPAD of the ear leaf were increased by EDAH treatments during the filling stage, while it was significant for T3 and T4. (Figure 9). Compared with CK, T3 and T4 had the greatest increase in Pn by 45.76% and 45.41% in 2019 and 24.80% and 19.80% in 2020. At the same time, Pn of T3 and T4 were higher than TK in 2019 and 2020, while it was not a significant difference. T1 and T2 had no significant difference with TK in two years. Compared with TK, T3 and T4 significantly increased SPAD of ear leaf, but there was no significant difference between SPAD of TK, T1 and T2 in 2019 and 2020. Pn and SPAD were significantly affected by year and ED, and these two indicators increased along with ED (Table 2). SV had no significant effect, and no interaction was detected between these two factors.

3.7. Grain Yield and Yield Components

There was no significant impact of UAV treatments on grain yield, ear number, kernel number, and TKW (

Table 3. The results of yield, yield components, and significance analysis of spray volume and EDAH dosage for the UAV treatments.

Treatment	SV (L ha−1)	ED (g a.i. ha−1)	Ear Number (M ha−1)		Kernel Number (ear−1)		TKW (g)		Grain Yield (t ha−1)
			2019	2020	2019	2020	2019	2020	2019	2020
CK	450	0	6.5 a	6.4 a	537 b	539 a	351.1 a	348.8 a	10.0 a	10.3 a
TK	450	90	6.8 a	6.6 a	547 a	546 a	351.8 a	352.0 a	10.3 a	10.3 a
T1	15	72	6.7 a	6.5 a	571 a	555 a	353.0 a	354.3 a	10.6 a	10.8 a
T2	30	72	6.7 a	6.5 a	571 a	541 a	356.8 a	349.4 a	11.0 a	10.6 a
T3	15	90	6.6 a	6.5 a	563 a	542 a	354.9 a	353.6 a	10.6 a	11.5 a
T4	30	90	6.8 a	6.6 a	581 a	549 a	353.5 a	357.7 a	10.9 a	10.8 a
ED (g a.i. ha−1)
72			6.7 a	6.6 a	571 a	549 a	354.9 a	351.8 a	10.8 a	11.1 a
90			6.7 a	6.5 a	572 a	545 a	354.2 a	355.6 a	10.8 a	10.7 a
SV (L ha−1)
15			6.7 a	6.5 a	567 a	548 a	354.0 a	353.9 a	10.6 a	10.7 a
30			6.7 a	6.6 a	576 a	545 a	355.2 a	353.5 a	11.0 a	11.2 a
ANOVA
Year			ns		ns		ns		ns
ED			ns		ns		ns		ns
SV			ns		ns		ns		ns
ED*SV			ns		ns		ns		ns

Note: ns means not significant. ED: ethephon dosage. SV: spray volume. ED*SV: interaction between ethephon dosage and spray volume. Different letters to right of the value indicate significant differences at p < 0.05 as determined by the Tukey test in different treatments in the same year.

). Compared with the CK, the yield of TK, T1, T2, T3, T4 increased by 3%, 6%,10%, 6%, 9% in 2019, and 0.5%, 4.8%, 2.9%, 11.65%, 4.85% in 2020, but there was no significant difference between the yield of all the treatments. The average TKW of TK, T1, T2, T3 and T4 increased by 0.15%, 0.34%, 0.18%, 0.51%, and 0.90% in 2019 and 2020. T4 had the highest kernel number, which averagely increased by 5.0% than CK. Moreover, all treatments significantly increased kernel number in 2019, while there was no significant difference in 2020. ED and SV had no significant effect on yield and yield components, and no interaction was detected between them.

4. Discussion

The droplet deposition is an important factor to increase the effectiveness of chemicals, and it is beneficial to reduce the use of PGRs. Different SV could cause the change of droplet deposition distribution, the droplet coverage rate and deposition density of all leaf positions increased along with SV, it showed a pattern: EKS > UAV (30 L) > UAV (15 L) in our work (Figure 4A,B). The reason for this phenomenon was that the droplet number increased along with SV, and it was also reported in [35,36,46]. Moreover, Xiao et al. [36] illustrated that upper layer > middle layer > lower layer of these two indicators applied by UAV and EKS in an experiment using pepper. It was also found that upper layer > lower layer of droplet coverage in wheat [35]. However, it was not a consistent trend in our research, the upper leaf had higher deposition density for EKS and UAV (15 L), but the lower leaf had higher for UAV (30 L). This was because the EDAH application was at the early jointing stage without a compact canopy structure. Higher spray volume had higher droplet density on the lower leaves, which resulted in higher droplet deposition on the lower plant canopy (Figure 4B).

The droplet deposition rate was the utilization rate of pesticide in a broad sense, and it was higher in UAV treatments than EKS; the UAV had greater penetration throughout the plant canopy with a strong downwash wind field [47,48,49]. This paper also revealed that the deposition rate of UAV treatments was increased by 3–10%, and the highest droplet deposition rate was recorded in the UAV (30 L) (Figure 4D). It was consistent with the fact that the UAV (30 L) had higher droplet deposition in upper, middle and average positions as compared to UAV (15 L) (Figure 4C). This indicated that higher SV could increase the utilization efficiency of agrochemicals applied by UAVs. Closed to our results, the effective concentration of cotton defoliant PGRs increased along with SV at the range of 17.6–29.0 L ha⁻¹ applied by UAV [39], control effect on cotton aphids was also increased when SV became higher with 18.0–30.0 L ha⁻¹ [50]. In brief, the utilization rate of EDAH was increased by the application of UAVs in this research.

In recent years, the droplet uniformity of UAV had been widely studied [31,35], and the UAV treatments had greater CV than EKS in wheat [31] and pepper [35]. However, the application of pesticides at the early jointing stage of maize has still not been reported. In our study, the CV of two UAV treatments was also greater than EKS in both flight directions for three indicators (Figure 5); it was demonstrated that EKS has a more uniform droplet distribution than UAV. The reason for this phenomenon was that the flying speed of UAV was 6–12 times that of EKS, which reduces the uniformity of the droplets with the increase in the CV. Furthermore, a non-standard application often leads to uneven spraying when farmers use EKS under their operation, which may lead to deviation from practical application. A previous study showed that the average CV of UAV (30 L) increased by 19% than UAV (22.5 L) in droplet coverage for cotton [50]; however, another report showed that the CV of UAV (22.5 L) was less than UAV (15 L) for cotton [51]. As for the droplet uniformity of two UAV SV in our research (Figure 5), the CV of UAV (30 L) was higher in all the positions at the parallel direction and several data at the vertical direction. It was revealed that the droplet distribution of UAV (15 L) was more uniform than that of UAV (30 L), and this was also verified by the CV in plant and ear height (Figure 7 and Table 2). The CV of plant and ear height for UAV (30 L) was higher than UAV (15 L) and almost all greater than EKS. These may be due to the higher flight speed of UAV (30 L), which leads to the uneven distribution of droplet deposition compared with UAV (15 L).

EDAH is an effective PGR for shortening plant height and length of basal internode [20,21,22,23]. In our work, all EDAH treatments shortened plant, ear, center of gravity heights by 2.47–13.08%, 10.31–27.00%, and 6.47–13.14%, respectively, in two years (Figure 6). Furthermore, all EDAH treatments significantly decreased the length and increased the crushing strength of the basal internode (Figure 8). Compared with TK, these indicators of T3 and T4 were better when the dosage of EDAH was the same, and T1 and T2 (20% dosage less) had no significant difference with TK. Previous studies have reported that pesticides applied by UAV have a better effect than EKS in rice [32] and wheat [52], while non-significant results were obtained by decreasing dosage by 20% as compared to EKS in cotton [40]. It was similar to our research that the application of UAVs could reduce the dosage of EDAH by 20% compared with EKS in controlling lodging. It was because of higher droplet deposition rate applied by UAV in the present study.

Ethephon was limited to single–use with the risk of yield loss in maize without lodging [11,13]. However, it showed that delayed use could increase yield, but the lodging control of maize was weaker [53]. When DA–6 and ethephon were simultaneously applied at the early jointing stage, grain development was promoted [20,21,22,23]. Our research also showed that EDAH increased Pn and SPAD of ear leaf (Figure 9). T3 and T4 had the highest Pn and SPAD, which were significantly increased compared to TK, while T1 and T2 had no significant difference. The kernel number of EDAH treatments was significantly higher, and UAV treatments had higher in 2019, and there was no big difference in 2020 compared with TK (Table 3). TKW was not significantly different in all treatments, while some treatments caused an increase. The grain yield of T2 and T4 was higher in 2019 and T3, T1, T4 in 2020. This showed that the application of EDAH by UAV could not cause loss to maize yield with greater instantaneous concentration. Analogously, it was also demonstrated that UAV application of cotton defoliant has no significant effect on cotton quality [39,41].

The ANOVA between ED and SV applied by UAV was analyzed (Table 2 and Table 3). It indicated that SV significantly influenced the CV of plant and ear height (p < 0.01). Although UAV (30 L) had a higher deposition rate, there is almost no difference in the effect on maize growth between UAV (15 L) and UAV (30 L), and UAV (30 L) reduced the uniformity of plant height and ear height. We speculate that the droplet difference caused by SV had a slight effect on plant growth. Hence, it was concluded that UAV (15 L) was appropriate. In addition, ED had a significant effect on the plant, ear and center of gravity heights, lodging rate, length and stalk crushing strength of the basal internode, Pn and SPAD of ear leaf, and more ED had a better effect. Considering the lodging occurrence and yield, we recommend that the dosage of EDAH applied by UAV should be 72 g a.i. ha⁻¹ when there is weak lodging and 90 g a.i. ha⁻¹ when there is heavy lodging.

It is reported for the first time that UAVs are used for the application of EDAH on maize in our research. The deposition and distribution of droplets at different leaf positions during the seventh leaf expansion period of maize were explored. Furthermore, the uniformity of plant height and ear height of maize affected by spray volume was investigated firstly. This research provides insight on how agrochemicals may be applied by UAVs to maize in the future. Maize lodging will be accompanied by maize planting, and the application of EDAH by UAV to control lodging will be widely used in China. The comparison between the application of UAV and EKS needs to be further studied, because there is a need for more in-depth research to increase crop yield with the continuous development of pesticide application machinery and PGRs.

5. Conclusions

EDAH applied by UAV had a higher droplet deposition rate than EKS, which was 10% higher in UAV (30 L) than EKS. EKS had a higher droplet coverage rate, deposition density, and uniformity of droplet distribution and plant height. At the same dosage of EDAH, UAV had a better effect on controlling maize growth than EKS, and equal effects were detected when the dosage applied by the UAV was decreased by 20%. In addition, there was no significant effect between the two spray volumes applied by UAV in different plant indicators. Considering the lodging occurrence and yield, we recommend that the dosage of EDAH applied by UAVs should be 72 g a.i. ha⁻¹ when there is weak lodging and 90 g a.i. ha⁻¹ when there is heavy lodging, with a spray volume of 15 L ha⁻¹.