Under the background of mechanical grain harvesting, the purpose of this study was to determine the accumulated temperature requirements of maize in different planting regions after physiological maturity by dehydration to the suitable harvest date, so as to provide the theoretical guidance for selecting suitable grain harvesting varieties, and rationally arrange agricultural operations and improve mechanical utilization efficiency in each planting region.

From 2014 to 2018, 141 maize varieties with different maturity periods selected to observe the dynamic changes of grain moisture content at typical test points in the northwest maize region (NW), the north maize region (NM) and Huang-huai-hai maize region (HM). Combining with meteorological date, the accumulated temperature requirements of maize field stalk dehydration to 25% and 20% grain moisture content after the physiological maturity were analyzed in different production regions.

The grain moisture content was different at physiological maturity in different production regions. The average grain moisture content was 28.5%, 29.9% and 29.6% in HM, NW and NM, respectively. Correlation analysis showed that there was no significant correlation between the growth period of different varieties and the grain moisture content at physiological maturity. The accumulated temperature of grain moisture content from physiological maturity to 25%, 20% and grain moisture content at physiological maturity were used as indexes. By using the two-way average method, the tested varieties were divided into 4 types, including low accumulated temperature demand and high moisture content (I), high accumulated temperature demand and high moisture content (II), low accumulated temperature demand and low moisture content (III), and high accumulated temperature and low moisture content (IV). For the northwest China, north China and northeast China, III and IV could be selected, but IV varieties needed to reserve enough accumulated temperature to dehydrate in the field. While the summer maize with growing twice a year in the Huanghuaihai region, III varieties could better coordinate the production and allocation of wheat and maize, and make full use of the excess temperature that could be used for grain dehydration.

Because of different dehydrating conditions such as temperature, the days when grain moisture content from physiological maturity to 25%, 20% showed that the northwest maize region was longer than the north maize region and Huang-huai-hai maize region. Grain moisture content and harvest quality can be effectively reduced by selecting the accumulated temperature varieties suitable for different regions and scientifically setting the harvest date.

In this study, the date when the kernel moisture content of different types of maize variety dehydrates to a suitable level for mechanical grain harvesting was predicted for different sowing dates in the Ningxia Hui Autonomous Region of China, in order to provide a basis for the variety selection at the regional scale, the determination of a suitable harvest period, and the development of the mechanical harvesting of maize varieties with a low moisture content.

Using the average kernel moisture content at physiological maturity (30.1%) and the average accumulated temperature (3 274.3 °C·d) required for sowing to physiological maturity ≥0 ℃·d as indicators, 38 common maize varieties were classified into four types by the two-way average method: type I varieties were characterized by late maturity and slow dehydration; Type II were characterized by early maturity and slow dehydration; type III varieties were characterized by early maturity and fast dehydration; type IV varieties were characterized by late maturity and fast dehydration. According to the production practice in Ningxia, the varieties with medium kernel moisture content and accumulated temperature requirement at physiological maturity stage were selected as the representative variety of each type. Then, the Logistic Power nonlinear growth model was used to predict the dehydration of the 38 maize varieties base on 10 years of recent meteorological data (2008-2017). Based on these data, the kernel moisture characteristics of each type of variety were predicted for various regions of Ningxia with different heat resources for three different sowing dates (initial sowing, peak sowing, and initial sowing).

The results showed that, initial sowing could obtain an accumulated temperature of 162.2-229.8 °C·d in crop growth more than that of final sowing. The accumulated temperatures required for kernel dehydration to a moisture content of 25% from sowing for Type I, Type II, Type III, and Type IV cultivars were 3 615.2, 3 290.6, 3 138.0, and 3 426.6 °C·d, respectively. In northern and central Ningxia, all types of varieties could meet the requirement that the kernel moisture content be reduced to 25%, while in the southern regions, Type III varieties could meet the requirement that the kernel moisture content be reduced to 25% for sowing at the initial and peak sowing times. The predicted accumulated temperatures from sowing to dehydration required for kernel dehydration to a moisture content of 16% for Type I, Type II, Type III, and Type IV cultivars were 4 320.6, 3 816.4, 3 632.9, and 4 023.6 °C·d, respectively. For the Type III varieties, in northern Ningxia on the initial sowing date and final sowing date, the kernel moisture content of both could be reduced to 16%; in the central region on the initial sowing date and peak sowing date, the accumulated temperature required for dehydration to 16% could be satisfied.

The heat resources of Ningxia could be used rationally through the selection of maize varieties with appropriate dehydration characteristics and early planting, thereby help to achieve high-quality mechanical maize kernel harvesting in this region. In northern and central Ningxia, it was recommended to select early-maturing, fast-dehydrating (Type III) varieties in order to achieve the mechanical harvesting of maize with a low kernel moisture content and thus convert regional heat resources into economic benefits.