Reasonable Planting Density and Chemical Regulation Can Improve the Plant Morphological Characteristics of Grain Maize, Enhance Lodging Resistance and Increase Yield in the Hexi Oasis

To investigate the effects of planting density and chemical regulation measures, as well as their interactions, on the plant morphological characteristics, stem mechanical properties, leaf anatomical structure, dry matter accumulation and allocation, and yield and its components of grain maize in the Hexi Oasis irrigation area, a field experiment was conducted from 2024 to 2025. Planting density was set as the main factor, with five density levels: 82,500 plants·ha−1 (M1), 97,500 plants·ha−1 (M2), 112,500 plants·ha−1 (M3), 127,500 plants·ha−1 (M4) and 142,500 plants·ha−1 (M5). Chemical regulation measures were set as the secondary factor, consisting of two treatments: spraying 30% aminoethyl hexanoate·ethephon at the 10-leaf stage (T1) and spraying an equal amount of water as the control (T2). The results revealed that, as planting density increases, the maize plant height, ear height and stem breakage rate rise continuously, whilst stem diameter, stem breaking resistance, rind puncture strength, leaf thickness and epidermal tissue thickness showed a downward trend. The leaf area index, ear length, kernel number per ear, kernel weight and yield all exhibited a trend of first increasing and then decreasing, reaching their peak at the M3 planting density. Compared with conventional planting patterns, spraying chemical regulators significantly reduced plant height by 10.66~13.99% and ear height by 16.12~19.57%, increased stem diameter by 2.12~13.79%, and enhanced stem breaking resistance by 7.71~23.11% and rind puncture strength by 5.17~12.65% at 30 days after silking. Additionally, it delayed leaf senescence, increased the leaf area index by 4.37~10.03% during the filling stage, and increased yield by 1.99~4.06%. The synergistic effect of moderately increasing planting density combined with chemical regulation can effectively coordinate the ‘population–individual’ contradiction in maize, reduce plant height and ear height and increase stem diameter and rind puncture strength, while maintaining a higher leaf area index after the silking stage and promoting dry matter translocation to grains, thereby achieving a synergy between lodging resistance and high yield. Among them, a planting density of 112,500 plants·ha−1 combined with spraying chemical regulators yielded the highest maize yield and harvest index, reaching 20.28~20.48 t·ha−1 and 0.52~0.53, respectively. Compared with other treatments, the increases ranged from 2.54~47.51% for yield and from 1.92~36.84% for the harvest index. Meanwhile, this treatment exhibited superior stem mechanical properties and a lower stem breakage rate. Taking into account factors such as lodging resistance, yield, dry matter accumulation and allocation, it has been determined that a planting density of 112,500 plants·ha−1 combined with spraying 30% aminoethyl hexanoate·ethephon at the 10-leaf stage is an effective strategy for achieving both lodging resistance and high yield in grain maize in the Hexi Oasis irrigation area.