Effects of sowing dates, sowing rates, and nitrogen rates on growth and spectral indices in winter wheat

: To clarify the effects of sowing dates, sowing rates, and nitrogen rates on growth and spectral indices in winter wheat, a two-year winter wheat field experiment under different sowing dates, sowing rates, and nitrogen rates was conducted. We studied systematically the effects of three factors and their interactions on yield, leaf area index (LAI), and normalized difference red edge (NDRE) of winter wheat at critical growth stages. Furthermore, to facilitate real-time monitoring of winter wheat growth dynamics, we also established the appropriate time-series curves of winter wheat canopy NDRE under different yield levels. The results indicated that the change patterns of NDRE and LAI at critical growth stages were very consistent, and the response of three factors to the two indices at critical growth stages was basically the same in winter wheat. In 2018 and 2019, the yield, LAI, and NDRE of winter wheat at each growth stages decreased with the delay of sowing date. In 2019 and 2020, the yield, peaks of LAI and NDRE under late sowing date were the largest except for the filling stage. The LAI and NDRE of winter wheat at different growth stages in the two years increased with the increase of nitrogen rates. However, there was basically no significant difference among sowing rates. Among the three factors, the sowing dates and nitrogen rates had a significant influence on the time-series curves of winter wheat canopy NDRE. The NDRE time series curves of winter wheat were stretched with the increase of nitrogen rates, the descending part of NDRE time series curve shifted to the left with the delay of the sowing date. In 2018 and 2019, the peak values of NDRE time series curves of winter wheat were declined with the delay of the sowing date. In 2019 and 2020, the peak values of the NDRE time series curves of late sowing and over-late sowing winter wheat were higher than that of suitable sowing wheat. The data of two years were merged to establish suitable time-series curves of winter wheat canopy NDRE under three yield levels, and the yield levels were less than 6.75 t hm –2 , 6.75–8.25 t hm –2 , and higher than 8.25 t hm –2 , respectively. The peak values and width of the NDRE time-series curves increased with the increase of yield level. In summary, winter wheat should be sown early at an appropriate date, but if the accumulated temperature before winter was higher, the sowing date should be postponed appropriately. And the growth of late sowing winter wheat could be improved by increasing a certain amount of sowing rates and nitrogen rates. At the same time, these results could provide a technical support for monitoring the growth of winter wheat under different sowing dates and different yield levels.