Suitable sunshine and temperature for mechanical grain harvesting of summer maize in the Huang-Huai-Hai Plain

Mechanical grain harvest is a key technology in modern maize production. Low percent of mechanical harvest is the factor limiting fully mechanized production of summer maize in the Huang-Huai-Hai Plain. Grain moisture content is the key index of mechanical grain harvest, but measurement of grain moisture content is tedious and complicated, prone to error and destruction while sampling. Grain moisture content is closely related to weather conditions. Based on the relationship among dehydration rate after physiological maturity, sunshine duration and accumulated temperature, grain moisture can indirectly be calculated. Thus building sunshine and temperature indexes for the suitability of mechanical grain reaping of different varieties can overcome the above shortcomings. It also can guide timely mechanical harvest and promote the development of mechanical grain harvest technology. In 2015 and 2016, 24 main cultivated cultivars in the Agro-meteorological Experiment Station of Chinese Meteorological Administration in Hebi City, Henan Province were investigated. Controlled pollination was used in every cultivar. Grain weight and moisture content were measured every third day after milk line reached half kernels of middle ear grains. The accumulated temperature from sowing to sampling and sunshine duration from milking stage to sampling were set as the independent variables, and grain water content in different times as dependent variables for regression analysis. And the regression equations were validated by test of significance. The sunshine and temperature indexes suitable for mechanical grain harvest of different varieties were calculated based on the threshold value of 28% grain moisture content by fitting the equations. The results showed that the order of grain moisture content in the same ear was upper part < middle part < lower part for all the tested varieties, except 'Xindan 38'. The grain moisture content in different parts of ear differed greatly in 'Xianyu 335', 'Denghai 701', 'Dedan 5' and 'Xindan 61'. The suitable accumulated temperature from sowing to harvest was 2 941-3 147℃·d and the suitable sunshine duration during milking stage to harvest stage was 179-235 h for mechanical grain harvesting of different varieties of summer maize. Sunshine duration and accumulated temperature were sorted from small to big, which showed a good consistency of variety order between temperature index and sunshine index. T-test showed that sunshine duration and accumulated temperature were significantly positively correlated with grain moisture content at harvest (P < 0.01). This indicated that sunshine and temperature indexes established in the study accurately reflected the changes in grain moisture content in each variety, and could be used to guide mechanical grain harvest. Base on sunshine and temperature indexes, 'Xindan 65', 'Xindan 68', 'Denghai 618', 'Xindan 38', 'Longping 206', 'Denghai 3', 'Xianyu 335' and 'Xindan 80' varieties needed less accumulated temperature and sunshine duration to meet mechanization standards and therefore more appropriate for mechanical grain harvest. However, 'Yuhe 998', 'Zhengdan 958', 'Denghai 662', 'Denghai 518', 'Xindan 66', 'Denghai 605', 'Dedan 5' and 'Yifeng 29' varieties had slower dehydration rates after maturity and therefore relatively less suitable for mechanical grain harvest.