Effects of nitrogen, variety and planting density on the mechanized harvest characters and yield of spring maize

Mechanized harvesting has been one of the vital measures to improve the efficiency of agricultural production. The popularization of mechanized harvesting can also be confined to the lodging resistance and grain moisture content. Particularly, it is difficult to realize the mechanized harvest of tall crops (such as maize), due to the large-area lodging and high grain moisture. Previous studies were focused mainly on harvesting machinery and breeding new varieties. However it is still lacking in the agronomic measures. There is a high demand to combine agricultural machinery and agronomy practices during maize harvesting. This study aims to investigate the effects of nitrogen (N) application rate, variety, and planting density on the lodging resistance and grain moisture content of spring maize. The field experiment was performed on the Loess Plateau in Changwu County, Shaanxi Province, China, from 2019 to 2020. Xianyu335 and Shaandan609 were also used as the test varieties. Three rates of nitrogen application were set as 0, 180, and 225 kg/hm², and two planting densities were 6.5×10⁴ and 8.5×10⁴ plants/hm². A series of parameters were assessed via field experiments, including the plant height, stem diameter, bending moment, bending strength, lodging index, grain dehydration rate at the end of the filling stage, grain moisture content at harvest, grain and biomass yield. The results showed that the lodging was only affected by the N application. The lodging index was significantly affected by the plant height and stem diameter without the N, whereas, the lodging index was closely related to the bending moment, bending strength, and plant height with the N (P<0.05). The N application enhanced the stem diameter and bending strength for a better lodging index of maize, indicating the different response of the lodging index of maize varieties to N. Specifically, the plant height and stem diameter of Xianyu335 increased by 13%-31% and 28%-38%, respectively, whereas, the bending strength and bending moment increased by 153%-203% and 133%-203%, respectively, under N treatment, compared with the without N. At the same time, the plant height and stem diameter of Shanndan609 increased by 8%-21% and 26%-45%, respectively, whereas, the bending strength and bending moment increased by 157%-277% and 72%-114%, respectively, under N treatment, compared with the without N. The stem diameter and bending strength of Shanndan609 increased more than that of Xianyu335, but the plant height and bending strength increased less than that of Xianyu335. N significantly decreased the lodging index of Shanndan609 (P<0.05), but there was no effect on Xianyu335. The lodging index of Shanndan609 decreased by 30%-47% under N treatment, compared with the without N. The grain moisture content was affected by N application rate, variety and planting density. N application decreased the grain dehydration rate and delayed the ripening, thus significantly increasing the grain's moisture content by 7%-9% at harvest (P<0.05), compared with the without N treatment. There was a delay of 10 d in the grain moisture content for the requirement of mechanized harvest, while the grain ripening was delayed by 4 d, due mainly to the nitrogen application. Compared with the lower density, the higher density treatment improved the grain dehydration rate at the end of the filling stage, where the grain moisture content decreased by 3%. In addition, the grain moisture content at the high density reached the requirement of mechanized harvest 10 d in advance. The grain moisture content of Xianyu335 was lower than that of Shaandan609 by 7% at harvest. Furthermore, the grain moisture content of Xianyu335 reached the requirement of mechanized harvest about 10 d earlier than that of Shanndan609. The yield and biomass of N treatment significantly increased by 92% and 63%, respectively, compared with the without N treatment (P<0.05). Furthermore, the higher-density treatment significantly increased the yield by 12% (P<0.05), compared with the lower density. Consequently, the lodging index, the grain dehydration rate at the late filling stage, and the grain moisture content of maize were significantly controlled by the variety, nitrogen, cultivation practices and climatic conditions. The varieties with fast grain dehydration, the optimal application of N fertilizer, and the high planting density can be expected to improve the suitability of spring maize for the mechanized harvest.