# Optimization of Topdressing for Winter Wheat by Accurate Growth Monitoring and Improved Production Estimation

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

_{HP}) is proposed that uses unmanned aerial vehicle (UAV)-based remote sensing to accurately acquire the growth status and an improved model for growth potential estimation and optimization of N fertilizer amount for topdressing (NFT). The method was validated and compared with three other methods by a field experiment: the conventional local farmer’s method (T

_{LF}), a nitrogen fertilization optimization algorithm (NFOA) proposed by Raun and Lukina (T

_{RL}) and a simplification introduced by Li and Zhang (T

_{LZ}). It shows that when insufficient basal fertilizer was provided, the proposed method provided as much NFT as the T

_{LF}method, i.e., 25.05% or 11.88% more than the T

_{RL}and T

_{LZ}methods and increased the yields by 4.62% or 2.27%, respectively; and when sufficient basal fertilizer was provided, the T

_{HP}method followed the T

_{RL}and T

_{LZ}methods to reduce NFT but maintained as much yield as the T

_{LF}method with a decrease of NFT by 4.20%. The results prove that T

_{HP}could enhance crop production under insufficient N preceding conditions by prescribing more fertilizer and increase nitrogen use efficiency (NUE) by lowering the fertilizer amount when enough basal fertilizer is provided.

## 1. Introduction

^{7}ha between 2001 and 2016 and has shown an increasing trend in recent years [1]. In winter wheat cultivation, topdressing is a widely adopted operation to boost production in the area. Currently, the nitrogen fertilizer amount for topdressing (NFT) is still mostly decided through past experience or arbitrarily without quantitative consideration of the crop growth status [2]. Topdressing usually works well to maintain high production by providing additional nutrition for the following growing stages when the basal fertilizer is insufficient. However, there is a risk of fertilizer waste and low nitrogen use efficiency (NUE) when enough basal fertilizer has already been applied because farmers tend to apply excessive NFT to lower the risk of yield reduction due to fertilizer deficiency [3].

^{TM}(Trimble Navigation Limited, Sunnyvale, CA, USA) to obtain normalized difference vegetation index (NDVI), and then developed matched topdressing methods based on the monitored crop parameters [6,7,8,9,10]. These methods greatly improve the efficiency of topdressing management, but when applied to a large area, intensive monitoring is needed to obtain detailed regional data. It is therefore labor-intensive [11].

^{2}of 0.51 at the beginning of August, which solves the problem of NDVI saturation in the late growing stages. Therefore, it is a promising way for the wide application of topdressing optimization methods to adopt crop growth data acquired by UAV-based remote sensing [11].

## 2. Materials and Methods

_{HP}) with modified assessment of the growth potential of winter wheat was proposed. T

_{HP}was validated and compared with the local farmer’s method (T

_{LF}), the NFOA method (T

_{RL}), and the simplified method (T

_{LZ}) in a field experiment using real-time, high- resolution UAV-based information.

#### 2.1. Topdressing Methods

_{HP}is a modified topdressing method which uses a new parameter, the relative volume (RV), to estimate the growth potential. RV is a direct indicator of above-ground biomass (AGB) that makes it possible to predict the accumulation of AGB. The T

_{HP}method provides more basis for the management of topdressing than methods using NDVI alone.

_{LF}, 76 kg∙N∙ha

^{−1}, fertilizer is applied to the whole field regardless of the winter wheat growth status, while for T

_{HP}, T

_{RL}and T

_{LZ}, the recommendations of NFT are based on the growth status monitored by UAV-based remote sensing.

#### 2.1.1. T_{HP}- Method Based on Improved Growth Estimation

_{HP}used RV to estimate AGB of winter wheat at topdressing period and for further prediction of AGB at harvest, where RV is the product of plant height (H) and coverage (C). The acquisition of plant height and UAV-based information will be introduced in Section 2.3 and Section 2.4, respectively. The calculation processes are as follow. Additionally, the formulas used in the calculation processes were derived from an ancillary experiment which will be introduced in Section 2.2.

- (1)
- Calculation of the AGB-related parameter RV (cm):$$\mathrm{RV}=\mathrm{H}\times \mathrm{C}/100$$
- (2)
- Estimation of AGB of winter wheat in the topdressing period (AGB
_{t}, Mg∙ha^{−1}) from RV:$${\mathrm{AGB}}_{\mathrm{t}}=0.90\times \mathrm{ln}\left(\mathrm{RV}\right)-0.58$$ - (3)
- Estimation of nitrogen concentration in the aboveground part (NCA) of the topdressing period (N
_{t}, g∙kg^{−1}):$${\mathrm{N}}_{\mathrm{t}}=7.08\times {\mathrm{e}}^{1.31\times \mathrm{NDVI}}$$ - (4)
- The forage N uptake of the topdressing period (FNUP, kg∙ha
^{−1}) was calculated as follows:$${\mathrm{FNUP}}_{\mathrm{AGB}}={\mathrm{AGB}}_{\mathrm{t}}\times {\mathrm{N}}_{\mathrm{t}}$$ - (5)
- Determination of AGB of harvest time (AGB
_{h}, Mg∙ha^{−1}):$$\{\begin{array}{c}{\mathrm{AGB}}_{\mathrm{h}}={\mathrm{AGB}}_{\mathrm{hm}}\times {\mathrm{AGB}}_{\mathrm{t}}/{\mathrm{AGB}}_{\mathrm{m}}\left({\mathrm{AGB}}_{\mathrm{t}}\ge {\mathrm{AGB}}_{\mathrm{th}}\right)\\ {\mathrm{AGB}}_{\mathrm{h}}={\mathrm{AGB}}_{\mathrm{t}}+{\mathrm{AGB}}_{\u2206}({\mathrm{AGB}}_{\mathrm{t}}{\mathrm{AGB}}_{\mathrm{th}})\end{array}$$_{hm}(13.00 Mg⋅ha^{−1}) is the AGB at the harvest time of winter wheat, which received adequate nitrogen during the entire growing period, AGB_{m}(2.15 Mg⋅ha^{−1}) is the AGB during the topdressing period of winter wheat, which received adequate nitrogen before topdressing, AGB_{th}(2.03 Mg⋅ha^{−1}) is a threshold to judge whether winter wheat is under nitrogen deficiency during the topdressing period and AGB_{∆}(10.50 Mg⋅ha^{−1}) is the maximum increment of AGB from topdressing to harvest:$${\mathrm{AGB}}_{\u2206}=0.9\times \left({\mathrm{AGB}}_{\mathrm{hm}}-{\mathrm{AGB}}_{0}\right)$$_{0}(1.33 Mg⋅ha^{−1}) is the AGB during the topdressing period of winter wheat that received no nitrogen fertilizer. - (6)
- Estimation of NCA of the harvest period (N
_{h}, g∙kg^{−1}):$${\mathrm{N}}_{\mathrm{h}}=0.15\times {{\mathrm{AGB}}_{\mathrm{h}}}^{2}-3.27\times {\mathrm{AGB}}_{\mathrm{h}}+27.88$$ - (7)
- Calculation of aboveground N uptake of harvest period (NUP
_{h}, kg∙ha^{−1}):$${\mathrm{NUP}}_{\mathrm{h}}={\mathrm{AGB}}_{\mathrm{h}}\times {\mathrm{N}}_{\mathrm{h}}$$ - (8)
- Recommendation of NFT (RN, kg∙N∙ha
^{−1}):$$\mathrm{RN}=\left({\mathrm{NUP}}_{\mathrm{h}}-{\mathrm{FNUP}}_{\mathrm{AGB}}\right)\times 0.88$$

#### 2.1.2. T_{RL}- Method Based on Localized NFOA

- (1)
- Calculation of INSEY:$$\mathrm{INSEY}=\frac{\mathrm{NDVI}}{\mathrm{DAT}}$$
_{max}and T_{min}represent daily ambient high and low temperatures [20]. In this study, the DAT = 88d. - (2)
- Estimation of PGY (Mg∙ha
^{−1}) based on current nutritional status:$$\mathrm{PGY}=0.66\times {\mathrm{e}}^{235.43\times \mathrm{INSEY}}$$ - (3)
- Estimation of FNUP from NDVI (FNUP
_{NDVI}, kg∙ha^{−1}):$${\mathrm{FNUP}}_{\mathrm{NDVI}}=1.03\times {\mathrm{e}}^{4.50\times \mathrm{NDVI}}$$ - (4)
- Calculation of the aboveground biomass at harvest (AGB
_{h}, Mg∙ha^{−1}):$${\mathrm{AGB}}_{\mathrm{h}}=2.00\times \mathrm{PGY}$$

_{h}[19,20]. The following steps are the same as those in T

_{HP}. We used NUP

_{h}to replace the grain N uptake (GNUP) used in NFOA because the straw was removed in this cropping system. The nitrogen in straw should also be considered.

#### 2.1.3. T_{LZ}- Method Based on Simplified Estimation of Nitrogen Uptake

- (1)
- Estimation of the N uptake from topdressing to harvest period (∆NUP, kg∙ha
^{−1}):$$\u2206\mathrm{NUP}=67.75\times {\mathrm{NDVI}}^{2}+55.51\times \mathrm{NDVI}+80.45$$ - (2)
- Recommendation of NFT (RN, kg∙N∙ha
^{−1}):$${\mathrm{RN}}_{\mathrm{LZ}}=\u2206\mathrm{NUP}\times 0.88$$

#### 2.2. Field Experiment

_{HP}, T

_{LF}, T

_{RL}, T

_{LZ}methods and a control (T

_{0}) with 0 kg∙ha

^{−1}topdressing N fertilizer, and the three preceding nutrient conditions were achieved by applying controlled N basal fertilizers: B

_{0}(0 kg∙N∙ha

^{−1}), B

_{1}(57 kg∙N∙ha

^{−1}) and B

_{2}(114 kg∙N∙ha

^{−1}), where B

_{2}basal rates describe approximate rates used by many farmers in the region. Each treatment had 4 replications, and in total, there were 60 experimental units in the experiment. The units were 16 m

^{2}(4 m × 4 m). Phosphorus and potassium fertilizers were applied without treatments at 130 kg∙ P

_{2}O

_{5}∙ ha

^{−1}in the form of Ca(H

_{2}PO

_{4})

_{2}and 120 kg∙ K

_{2}O ∙ha

^{−1}in the form of K

_{2}SO

_{4}as basal fertilizers. Nitrogen fertilizers were applied in the form of urea for basal and topdressing. In the entire growing stage, we carried out irrigation in four time periods—before sowing (October 21st), after topdressing (March 24th), booting (April 17th) and milk ripening (May14th). The irrigation amount was 60, 30, 50 and 50 mm, respectively (Figure 3).

^{−3}, porosity of 0.46 cm

^{3}∙cm

^{−3}, 6.09 g∙organic C∙kg

^{−1}, 0.55 g∙total N∙kg

^{−1}, 0.81 g∙total P∙kg

^{−1}, 18.08 g∙total K∙kg

^{−1}, and 8.01 cmol cation exchange capacity (CEC)∙kg

^{−1}[21,22,23,24]. Winter wheat (Bainong Aikang 58) was sown on 24 October 2018 and harvested on 05 June 2019, and topdressing was applied on 24 March 2019.

_{0}: 0, N

_{1}: 150, N

_{2}: 190, N

_{3}: 230, N

_{4}: 270 kg∙N∙ha

^{−1}fertilizer throughout the entire growing period (60% as the basal fertilizer and 40% as topdressing) was conducted. As the 190 kg∙N∙ha

^{−1}fertilizer is approximate rates used by many farmers in the region, fertilizer levels of 190, 230, and 270 kg∙N∙ha

^{−1}are considered adequate. The size of the experiment unit was 48 m

^{2}(8 m × 6 m) and units were separated by concrete slabs to prevent lateral exchange of soil solutions. The other field managements were the same as those for the topdressing experiment.

#### 2.3. Plant Sampling and Measurement

_{HP}.

^{2}were harvested. The grain and straw were separated, dried and weighed to measure grain yield and straw biomass [24].

#### 2.4. UAV-Based Remote Sensing

_{NIR}is the reflectance of near IR band, R

_{red}is the reflectance of red band [21].

_{green}is the digital number of the green band and DN

_{red}is the digital number of the red band.

#### 2.5. Evaluation of NFT Use Efficiency

_{t}) was used to evaluate the profit and return from N applied as topdressing, and it refers to the calculation formula of partial factor productivity from applied N (PFP

_{N}) [30]:

^{−1}) is the grain yield and N

_{T}(kg∙N∙ha

^{−1}) is the total N fertilizer amount applied, including basal and topdressing:

_{0}(Mg∙ha

^{−1}) is the grain yield of the treatment that received a certain amount of nitrogen as basal fertilizer without topdressing. NFT (kg∙N∙ha

^{−1}) is the nitrogen fertilizer amount of topdressing under a certain basal fertilizer level.

## 3. Results

#### 3.1. Growth Status of Winter Wheat Before Topdressing

_{2}represented the growth of winter wheat under local farmer’s growing conditions. Compared to B

_{2}, the plant height, AGB, chlorophyll concentration, and NCA of B

_{0}decreased by 22.73%, 24.01%, 16.70% and 21.28%, respectively; for B

_{1}, these parameters decreased by 8.32%, 14.08%, 7.74%, and 8.71%, respectively.

_{0}and B

_{1}was 34.58% and 13.65% lower than that under B

_{2}, respectively. The unit average NDVI under B

_{0}and B

_{1}was 9.41% and 2.35% lower than that under B

_{2}, respectively.

#### 3.2. Recommendations of NFT

_{HP}tended to increase NFT when N deficiency was present in the early growing season, and recommended a moderate NFT reduction when N was sufficient in the early growing season. When N was severely deficient during the early growing stage (B

_{0}), T

_{HP}recommended the same NFT as T

_{LF}to compensate for the nutrient deficiency, while T

_{RL}and T

_{LZ}recommended reducing NFT by 14.42% and 9.94%, respectively, due to the low expectations of growing potential. Under the B

_{1}condition, T

_{HP}, T

_{RL}and T

_{LZ}recommended decreasing NFT by 4.61%, 10.81% and 6.39% compared to T

_{LF}, respectively, due to the difference in the evaluation of the nutritional conditions of crops. When sufficient basal fertilizer was provided (B

_{2}), the T

_{HP}method followed the T

_{RL}and T

_{LZ}methods to reduce NFT by 4.2%, 3.6% and 4.9%, respectively, compared to T

_{LF.}

#### 3.3. Responses of Winter Wheat after Topdressing

_{HP}ensured the accumulation of biomass under different early nutrient conditions, especially for B

_{0}. The accumulation of AGB went through two periods after topdressing: a rapid growing period from booting to anthesis, and the accumulation ranged from 3.58~4.42 Mg∙ha

^{−1}for B

_{0}; 3.79~4.46 Mg∙ha

^{−1}for B

_{1}and 4.44~5.13 Mg∙ha

^{−1}for B

_{2}; and a slow accumulation period from anthesis to milk ripening with an accumulation ranged from 1.72~2.83 Mg∙ha

^{−1}for B

_{0}; 2.41~3.56 Mg∙ha

^{−1}for B

_{1}and 2.46~2.75 Mg∙ha

^{−1}for B

_{2}. With the growth of winter wheat, the differences in AGBs between different topdressing methods gradually increased, so we focused on the AGBs at milk ripening stage. It was found that under the B

_{0}condition, T

_{HP}achieved an average AGB of 12.55 Mg∙ha

^{−1}, which was close to T

_{LF}(12.59 Mg∙ha

^{−1}) and significantly higher than T

_{RL}(11.55 Mg∙ha

^{−1}) and T

_{LZ}(12.31 Mg∙ha

^{−1}). Under B

_{1}and B

_{2}conditions, the average AGBs of T

_{HP}, T

_{LF}, T

_{RL}, and T

_{LZ}were not significantly different but were significantly higher than that of T

_{0}.

_{2}condition, the increment of height for each topdressing methods in this stage only reached 2.40~4.00 cm. We focused on the plant height of each topdressing method at the milk ripening stage for the same reason as for AGB. Under the B

_{0}condition, T

_{HP}achieved an average plant height of 69.94 cm, which was close to that of T

_{LF}(70.08 cm) and significantly higher than those of T

_{RL}(64.09 cm) and T

_{LZ}(67.50 cm). Under B

_{1}and B

_{2}conditions, the average AGBs of T

_{HP}, T

_{LF}, T

_{RL}, and T

_{LZ}were not significantly different but were significantly higher than that of T

_{0}.

_{HP}slowed the decreasing trend compared to other topdressing methods, but there was no significant difference between T

_{HP}, T

_{LF}, T

_{RL}and T

_{LZ}at the milk ripe stage. The mean NCA of T

_{HP}at the milk ripe stage was 8.14, 10.80 and 11.87 g.kg

^{−1}under B

_{0}, B

_{1}and B

_{2}conditions, respectively.

_{HP}slowed the reduction in chlorophyll concentration and maintained the chlorophyll concentration at 357.11 μmol∙m

^{−2}, 481.45 μmol∙m

^{−2}and 526.36 μmol∙m

^{−2}for B

_{0}, B

_{1}, and B

_{2}, respectively, at the milk ripe stage.

_{HP}performed well in terms of winter wheat development among all topdressing methods in multiple growing stages regardless of the nutritional conditions during the early stages.

#### 3.4. Harvest Index and Nitrogen Use Efficiency

_{HP}achieved high yield under different basal fertilizer levels. The average grain yields of T

_{HP}were 5.41, 6.17 and 6.58 Mg∙ha

^{−1}under B

_{0}, B

_{1}and B

_{2}level, respectively. The grain yields of T

_{HP}were not significantly different from those of T

_{LF}(B

_{0}: 5.42 Mg∙ha

^{−1}, B

_{1}: 6.23 Mg∙ha

^{−1}, B

_{2}: 6.59 Mg∙ha

^{−1}), and T

_{LZ}(B

_{0}: 5.29 Mg∙ha

^{−1}, B

_{1}: 6.16 Mg∙ha

^{−1}, B

_{2}: 6.59 Mg∙ha

^{−1}) under all three basal fertilizer levels but were significantly higher than that of T

_{RL}when the early-stage nutrition was insufficient (B

_{0}: 5.17 Mg∙ha

^{−1}and B

_{1}: 5.93 Mg∙ha

^{−1}).

_{HP}, T

_{LF}, T

_{RL}, and T

_{LZ}. Only when nitrogen deficiency occurred in the early stage (B

_{0}) did the average straw biomass of T

_{HP}(7.77 Mg∙ha

^{−1}) was slightly higher than that of T

_{LF}(0.77 Mg∙ha

^{−1}), T

_{RL}(0.73 Mg∙ha

^{−1}) and T

_{LZ}(0.75 Mg∙ha

^{−1}).

_{HP}presented high PFP

_{t}s under different basal fertilizer levels (Table 3). Under the B

_{0}and B

_{1}levels, the PFP

_{t}of T

_{HP}was higher than that of T

_{RL}and T

_{LZ}, and under the B

_{2}level, it was higher than that of T

_{LF}and T

_{RL}. However, the difference between each topdressing method was not significant.

## 4. Discussion

#### 4.1. Difference in NFT

_{HP}, T

_{RL}and T

_{LZ}) tested in this experiment varied in NFT for winter wheat under the three preceding nutritional conditions, as the predictions of growth potential were different. Under B

_{2}conditions, T

_{HP}, T

_{RL}, and T

_{LZ}recommended similar NFTs, which were lower than that of T

_{LF}because the basal fertilizer was excessive and the additional fertilizer for topdressing had no contribution to yield. With the decrease in basal fertilizer, T

_{RL}and T

_{LZ}gradually reduced NFTs for B

_{1}and B

_{0}due to the diminishing expectation of production, while the circumstance was inverse for T

_{HP}, as it increased NFT for B

_{1}and B

_{0}aiming to compensate for the deficiency of nitrogen.

#### 4.2. Differences in Responses of Winter Wheat to NFT Recommended by Each Topdressing Method

_{0}) [24]. Topdressing could slow down the decreasing process of NCA, and the NCAs of T

_{0}were therefore significantly lower than those of other topdressing methods. It was supposed that the NCAs of winter wheat with low NFTs should decrease faster than those with high NFTs. The fact, however, is that no significant difference was found in NCA between the different topdressing methods. This is because well-nourished wheat tended to achieve higher AGBs, and according to the N dilution theory, the NCA decreases with the accumulation of AGB. Due to the influence of many factors, the variation of NCAs with NFT did not show obvious regularity, and as a result, the NCA at the milk ripe stage of T

_{HP}was not different from that of T

_{RL}, even though 15.35 kg·N·ha

^{−1}additional NFT was applied under the B

_{0}condition. However, the additional NFT retarded the breakdown of chlorophyll, so the chlorophyll concentrations of T

_{HP}and T

_{LF}were significantly higher than those of T

_{RL}and T

_{LZ}. This support the photosynthesis and material accumulation of winter wheat and presented a significant difference in AGB and plant height at the milk ripe stage. The additional material accumulation is transferred to the grain at late growing stages and manifested as grain yield differences. Under the B

_{1}basal fertilizer level, the AGB and plant height were not different among the four topdressing methods, while the N, chlorophyll concentration, and grain yield of T

_{RL}were significantly lower than those of the other topdressing methods, implying that the N and chlorophyll concentrations are more sensitive to the nutrient status. Under the B

_{2}basal fertilizer level, all tested growth parameters and the grain yield were not significantly different among the 4 topdressing methods, demonstrating that excessive NFT is unable to increase production.

#### 4.3. Limitations of the Study

_{HP}to different topdressing times. In addition, this work is based on one cultivar and conducted at one experimental site, and as such, large-area applications and evaluation under diverse farm-field conditions are needed to improve this preliminary method.

## 5. Conclusions

_{HP}) and compared the effect of this method with other topdressing methods in a field experiment. The results showed that T

_{HP}has the potential to ensure grain yield and avoid excessive N fertilization. T

_{HP}reduced the nitrogen application amount of topdressing by 4.18~4.61% over T

_{LF}, with no significant decrease in grain yield, when the early-stage nutrition condition was sufficient, and maintained a relatively high grain yield of 5.41 Mg∙ha

^{−1}, which was 17.83% and 20.68% higher than that of T

_{LZ}and T

_{RL}when the early-stage nutrition condition was deficient, as it was recommended to increase the nitrogen application amount of topdressing when nitrogen deficiency occurred, and moderately reduce the topdressing amount when the nitrogen levels were sufficient. The adaptation of early-stage nutrition conditions was achieved by adjusting the NFT based on estimation of the maximum growth potential. More studies are necessary to further improve this topdressing method for a wider range of applications and obtain information acquired by UAV-based remote sensing.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Calculation steps of 4 topdressing methods for winter wheat. (T

_{HP}, T

_{RL}, T

_{LZ}and T

_{LF}are different topdressing methods; RN: recommended nitrogen fertilizer amount for topdressing, RV: relative volume, AGB: above ground biomass, N

_{t}: nitrogen concentration in aboveground part at topdressing period, N

_{h}: nitrogen concentration in aboveground part at harvest, INSEY: in-season estimates of grain yield, PGY: predicted grain yield, FNUP: forage nitrogen uptake, NUP: N uptake in the aboveground part; the subscripts represent different times, i.e., t represents topdressing and h represents harvest, except for AGB

_{th}, which represents a threshold of AGB at topdressing).

**Figure 2.**Study site and the experimental design. (

**a**) The location of the Fengqiu Agro-ecological experimental station, CAS. (

**b**) Experimental design (T

_{HP}, T

_{LF}, T

_{RL}, T

_{LZ}and T

_{0}are different topdressing methods, and basal fertilizer levels (B

_{0}, B

_{1}and B

_{2}) are represented by different colors).

**Figure 3.**Rainfall and irrigation and mean temperature recorded during the 2018 to 2019 wheat growing seasons in Fengqiu Agro-ecological experimental station, CAS.

**Figure 4.**UAV-based images of winter wheat under different basal fertilizer levels. (

**a**) RGB. (

**b**) NDVI. (

**c**) Coverage.

**Figure 5.**Illustration of formulas used in T

_{HP}, T

_{RL}and T

_{LZ}obtained from the auxiliary experiment. (

**a**) Predicting potential grain yield (PGY) from INSEY (in-season estimate of grain yield). (

**b**) Predicting forage nitrogen uptake (FNU

_{NDVI}) from the normalized difference vegetation index (NDVI). (

**c**) Predicting nitrogen concentration in the aboveground part at topdressing (N

_{t}) from NDVI. (

**d**) Predicting aboveground biomass at topdressing, AGB

_{t}from relative volume.

**Figure 6.**Recommended nitrogen fertilizer amount for topdressing for each unit. (T

_{HP}, T

_{LF}, T

_{RL}, T

_{LZ}, and T

_{0}represent different topdressing methods).

**Figure 7.**Recommended nitrogen fertilizer amount for topdressing under different basal fertilizer levels. (B

_{0}, B

_{1}and B

_{2}represent basal fertilizer levels of 0, 57 and 114 kg∙ha

^{−1}, respectively; T

_{HP}, T

_{LF}, T

_{RL}, and T

_{LZ}represent different topdressing methods; different letters indicate significance using Fischer’s protected least significant difference at p < 0.05).

**Figure 8.**Growing status of winter wheat after topdressing. (

**a**–

**c**) Cumulative graphs of above ground biomass. (

**d**–

**f**) Cumulative graphs of plant height. (

**g**–

**i**) Decreasing graphs of nitrogen concentration in above ground part. (

**j**–

**l**) Decreasing graphs of chlorophyll concentration. (B

_{0}, B

_{1}and B

_{2}represent basal fertilizer levels of 0, 57 and 114 kg∙N∙ha

^{−1}, respectively; T

_{HP}, T

_{LF}, T

_{RL}, T

_{LZ}, and T

_{0}represent different topdressing methods; different letters indicate significance using Fischer’s protected least significant difference at p < 0.05 at the milk ripe stage).

**Figure 9.**Grain yield and straw biomass of winter wheat under different topdressing methods. (

**a**–

**c**) Grain yields of different topdressing methods under three basal fertilizer levels. (

**d**–

**f**) Straw biomasses of different topdressing methods under three basal fertilizer levels. (B

_{0}, B

_{1}, and B

_{2}represent basal fertilizer levels of 0, 57 and 114 kg∙N∙ha

^{−1}, respe (B

_{0}, B

_{1}, and B

_{2}represent basal fertilizer levels of 0, 57 and 114 kg∙N∙ha

^{−1}, respectively; T

_{HP}, T

_{LF}, T

_{RL}, T

_{PGY}, and T

_{LZ}represent different topdressing methods; different letters indicate significance using Fischer’s protected least significant difference at p < 0.05).

Growth Parameters | Plant Height (cm) | Above Ground Biomass (Mg·ha^{−1}) | Chlorophyll Concentration (μmol·m^{−2}) | Nitrogen Concentration in Above Ground Part (g·kg^{−1}) |
---|---|---|---|---|

B_{0} | 19.33 ± 1.81 (c) | 1.59 ± 0.06 (c) | 540.79 ± 11.39 (c) | 18.05 ± 0.55 (c) |

B_{1} | 22.94 ± 2.85 (b) | 1.80 ± 0.05 (b) | 598.94 ± 13.77 (b) | 20.93 ± 0.47 (b) |

B_{2} | 25.02 ± 2.04 (a) | 2.10 ± 0.06 (a) | 649.18 ± 15.01 (a) | 22.93 ± 0.66 (a) |

_{0}, B

_{1}and B

_{2}represent basal fertilizer levels of 0, 57, and 114 kg∙N∙ha

^{−1}, respectively; different letters indicate significance within the same column using Fischer’s protected least significant difference at p < 0.05.

Remote Sensing Information | NDVI | Coverage |
---|---|---|

B_{0} | 0.77 ± 0.04 (c) | 49.99% ± 9.76% (c) |

B_{1} | 0.83 ± 0.02 (b) | 65.98% ± 6.99% (b) |

B_{2} | 0.85 ± 0.02 (a) | 76.41% ± 6.62% (a) |

_{0}, B

_{1}and B

_{2}represent basal fertilizer levels of 0, 57 and 114 kg∙N∙ha

^{−1}, respectively; different letters indicate significance within the same column using Fischer’s protected least significant difference at p < 0.05.

PFP_{t} | B_{0} | B_{1} | B_{2} |
---|---|---|---|

T_{HP} | 11.86 ± 1.69(a) | 14.90 ± 2.12 (a) | 9.84 ± 1.65 (a) |

T_{LF} | 12.01 ± 1.63(a) | 14.72 ± 2.03 (a) | 9.56 ± 1.72 (a) |

T_{RL} | 10.92 ± 2.56(a) | 12.56 ± 1.74 (a) | 9.37 ± 1.39 (a) |

T_{LZ} | 11.59 ± 1.85(a) | 14.72 ± 1.80 (a) | 10.12 ± 1.87 (a) |

_{0}, B

_{1}and B

_{2}represent basal fertilizer levels of 0, 57, and 114 kg∙N∙ha

^{−1}, respectively. T

_{HP}, T

_{LF}, T

_{RL}, and T

_{LZ}represent different topdressing methods. Different letters indicate significance within the same column using Fischer’s protected least significant difference at p < 0.05.

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## Share and Cite

**MDPI and ACS Style**

Ji, J.; Liu, J.; Chen, J.; Niu, Y.; Xuan, K.; Jiang, Y.; Jia, R.; Wang, C.; Li, X.
Optimization of Topdressing for Winter Wheat by Accurate Growth Monitoring and Improved Production Estimation. *Remote Sens.* **2021**, *13*, 2349.
https://doi.org/10.3390/rs13122349

**AMA Style**

Ji J, Liu J, Chen J, Niu Y, Xuan K, Jiang Y, Jia R, Wang C, Li X.
Optimization of Topdressing for Winter Wheat by Accurate Growth Monitoring and Improved Production Estimation. *Remote Sensing*. 2021; 13(12):2349.
https://doi.org/10.3390/rs13122349

**Chicago/Turabian Style**

Ji, Jingchun, Jianli Liu, Jingjing Chen, Yujie Niu, Kefan Xuan, Yifei Jiang, Renhao Jia, Can Wang, and Xiaopeng Li.
2021. "Optimization of Topdressing for Winter Wheat by Accurate Growth Monitoring and Improved Production Estimation" *Remote Sensing* 13, no. 12: 2349.
https://doi.org/10.3390/rs13122349