Compact maize canopy improves radiation use efficiency and grain yield of maize/soybean relay intercropping system

Maize/soybean relay intercropping system is a popular cultivation system to obtain high yields of both crops with reduced inputs. However, shading by maize decreases the photosynthetically active radiation, reaching the soybean canopy in maize/soybean relay intercropping system, which reduces soybean radiation use efficiency and competitiveness. Here, we reveal that compact maize in maize/soybean relay intercropping system enhances the photosynthetically active radiation transmittance, leaf area index, dry matter production, radiation use efficiency, and competitiveness of soybean and compensates the slight maize yield loss by substantially increasing soybean yield. In this experiment, soybean was relay intercropped with different maize types (SI, spreading maize; SII, semi-compact maize; and SIII, compact maize) in maize/soybean relay intercropping system, and all the relay intercropping treatments were compared with sole cropping systems of soybean and maize. Results revealed that SIII significantly enhanced the soybean radiation use efficiency (by 77%, from 0.35 g MJ^-1 in SI to 0.61 g MJ^-1 in SIII) and total radiation use efficiency (soybean radiation use efficiency + maize radiation use efficiency) of maize/soybean relay intercropping system (by 5%, from 3.53 g MJ^-1 in SI to 3.73 g MJ^-1 in SIII). Similarly, SIII improved the competitiveness (by 62%, from 0.58% in SI to 0.94% in SIII) of soybean but reduced the competitiveness (by 38%, from 1.73% in SI to 1.07% in SIII) of maize, which, in turn, considerably increased soybean yield by maintaining maize yield. On average, over the 2 years, in SIII, relay-intercropped soybean produced 89% of the sole soybean yield, and relay-intercropped maize produced 95% of the sole maize yield. Besides, treatment SIII achieved the mean highest land equivalent ratio value of 1.84 in both years. Thus, enhanced radiation use efficiency of soybean, especially during the co-growth period, was the primary factor responsible for the high productivity of the maize/soybean relay intercropping system.