Relative chlorophyll content (SPAD) is an important index for characterizing the nitrogen nutrient status of plants. Continuous, rapid, nondestructive, and accurate estimation of SPAD values in wheat after heading stage can positively impact subsequent nitrogen fertilization management strategies, which regulate grain filling and yield quality formation. In this study, the estimation of SPAD of leaf relative chlorophyll content in spring wheat was conducted at the experimental base in Wuyuan County, Inner Mongolia in 2021. Multispectral images of different nitrogen application levels at 7, 14, 21, and 28 days after the wheat heading stage were acquired by DJI P4M UAV. A total of 26 multispectral vegetation indices were constructed, and the measured SPAD values of wheat on the ground were obtained simultaneously using a handheld chlorophyll meter. Four machine learning algorithms, including deep neural networks (DNN), partial least squares (PLS), random forest (RF), and Adaptive Boosting (Ada) were used to construct SPAD value estimation models at different time from heading growth stages. The model’s progress was evaluated by the coefficient of determination (R²), root mean square error (RMSE), and mean absolute error (MAPE). The results showed that the optimal SPAD value estimation models for different periods of independent reproductive growth stages of wheat were different, with PLS as the optimal estimation model at 7 and 14 days after heading, RF as the optimal estimation model at 21 days after heading, and Ada as the optimal estimation model at 28 d after heading. The highest accuracy was achieved using the PLS model for estimating SPAD values at 14 d after heading (training set R² = 0.767, RMSE = 3.205, MAPE = 0.060, and R² = 0.878, RMSE = 2.405, MAPE = 0.045 for the test set). The combined analysis concluded that selecting multiple vegetation indices as input variables of the model at 14 d after heading stage and using the PLS model can significantly improve the accuracy of SPAD value estimation, provides a new technical support for rapid and accurate monitoring of SPAD values in spring wheat.

Drought during the overwintering period threatens the emergence rate and restricts the yield under the “winter-seeded spring wheat” cultivated model in the Hetao Plain Irrigation District in Inner Mongolia. To address this issue, from 2017 to 2019, six treatments were set up in the field to study the emergence rate, growth attributes, grain quality, yield and its percentage of winter-seeded spring wheat. These treatments were the (1) application of water-retaining agents under winter sowing (WRA), (2) soil amendments under winter sowing (SA) and wheat seed presoaking with amino acid water-soluble fertilizer under winter sowing (SP), (3) straw mulching under winter sowing (SM), (4) film-mulching hole sowing under winter sowing (FMHS), (5) blank control under winter sowing (CKW) and (6) conventional blank control under spring sowing (CKS). The results showed that the emergence rate of winter-seeded treatment was lower than CKS treatment, the emergence of WRA, SA, SM, and FMHS treatment increased by 5.4%, 2.3%, 6.5% and 10.8% compared with CKW treatment, respectively. The winter-seeded treatment is earlier than CKS treatment in the growth process, in which FMHS treatment is between 12 d and 16 d earlier in the emergence period, between 13 d and 15 d earlier in the maturation period, between 8 d and 12 d earlier than the CKW treatment in the emergence period, and between 8 d and 10 d earlier in the maturity period. Compared with CKW treatment, WRA, SA, SM, FMHS and CKS treatments increased yield by 13.49%, 11.42%, 14.75%, 21.61% and 28.15%, respectively. FMHS treatment significantly reduced the total water consumption and significantly improved water use efficiency. The protein content, wet gluten content, sedimentation value, dough ductility and maximum resistance in CKS treatment were significantly lower than other winter sowing treatments. The protein percentage and wet gluten percentage in FMHS treatment were the highest, and the difference with CKW treatment was significant. In summary, film-mulching hole sowing in winter improves soil water and the emergence rate, significantly accelerates the growth process of wheat, increases yield and promotes grain quality.