Effects of nitrogen and maize plant density on forage yield and nitrogen uptake in an alfalfa–silage maize relay intercropping system in the North China Plain

Sustainable production of high-yielding and high-quality forages such as alfalfa (Medicago sativa L.) is critically important for fast-growing animal husbandry regions such as the North China Plain (NCP). Alfalfa production in this region is limited during the hot-rainy summer period from July–September, but these conditions may allow the production of intercropped silage maize (Zea mays L.) following spring harvests of alfalfa to boost the overall forage production. In this study, an alfalfa–silage maize (AM) relay intercropping system was evaluated in a field experiment from 2017 to 2019 to assess the impact of maize nitrogen (N) fertilization rates of 0, 60, 120, and 180 kg N ha–1 and maize plant densities of 30,000, 45,000, and 60,000 plants ha–1 on the forage yield and N uptake. The AM system was compared with alfalfa monocropping (AA) and silage maize monocropping (MM) controls. The 3-yr average land equivalent ratio (LER) and land equivalent ratio for nitrogen yield (LERN) of AM ranged from 1.05 to 1.37 relative to AA and MM, indicating that intercropping was advantageous for improving the overall forage production and N utilization. Intercropping reduced the maize yield by 23–30 % and the maize N yield by 26–35 % compared with the monocultured maize across 3 years. Increasing N fertilization and maize plant density often enhanced the maize yield and N uptake in the AM system. The alfalfa stand density in AM was negatively affected by the increased maize plant density and was 30–55 % lower than that in AA. Spring forage production of alfalfa in the AM and AA systems was similar in most cases, but yields in the AM system were reduced in 2018 and 2019 when maize was grown with the highest N rate (180 kg N ha–1) and the highest plant density (60,000 plants ha–1). On the basis of the system yield, N yield, and LER, the AM system with a maize plant density of 45,000 plants ha–1 fertilized at 120 kg N ha–1 was the most advantageous and could be recommended as an alternative cropping system in the NCP to increase forage yields with a relatively low input of N.