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 (LER_N) 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.

可持续生产高产优质草料（例如苜蓿（苜蓿））对于快速发展的畜牧业区（例如华北平原（NCP））至关重要。在7月至9月的炎热夏季，该地区的苜蓿产量受到限制，但这些条件可能使苜蓿春季收获后间作青贮玉米（Zea mays L.）的产量增加了总草料产量。在这项研究中，在2017年至2019年的田间试验中对苜蓿-青贮玉米（AM）间作系统进行了评估，以评估0、60、120和180 kg N ha的玉米氮肥施用量的影响^{– 1种}，玉米植物密度为30,000、45,000和60,000公顷饲草产量和氮素吸收量为^–1。将AM系统与苜蓿单作（AA）和青贮玉米单作（MM）对照进行比较。3年平均土地当量比（LER）和氮产量的土地当量比（LER _N相对于AA和MM，AM的AM值介于1.05至1.37之间，这表明间作对提高整体草料产量和氮素利用率具有优势。间作与单作玉米相比，在3年间，玉米间作使玉米产量降低了23％至30％，玉米氮素产量降低了26％至35％。增施氮肥和增加玉米植株密度通常会提高AM系统中玉米的产量和氮素的吸收。AM的苜蓿林密度受到玉米植株密度增加的负面影响，比AA降低了30-55％。在大多数情况下，AM和AA系统中紫花苜蓿的春季饲草产量相似，但是当玉米以最高氮素养分（180 kg N ha ^–1）和最高植株种植时，AM系统的产量在2018年和2019年下降。密度（60,000公顷公顷^–1）。从系统产量，氮素产量和LER的角度来看，以120 kg N ha ^{- 1}施肥的玉米植株密度为45,000株ha ^-1的AM系统是最有利的，因此可以推荐作为替代种植系统NCP以相对较低的N输入量提高草料产量。