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１９９３年，在美国的密执安州建立了长期作物栽培系统试验，以测量轮作和作物覆盖在玉米产量方面的影响。轮作和作物覆盖目的都是为了作物在生育期内得到足够的氮。土壤氮含量用ＰＳＮＴ得到。试验包括玉米—玉米—大豆—小麦轮作系统和玉米连作两种种植系统。实验地设１个裂区，每个裂区有４次重复，设有两种肥料管理：有机肥及化肥。撒播的覆盖作物有黑麦草和红苜蓿。研究结果表明：有覆盖作物的玉米比无覆盖的玉米产量明显提高，轮作的玉米比连作的玉米产量高，轮作有覆盖的玉米产量明显高于无覆盖玉米。
In 1993, a long-term crop cultivation system trial was established in the state of Michigan in the United States to measure the effects of crop rotation and crop cover on maize production. Rotation and crop coverage are all aimed at getting enough nitrogen in the crop during the growing season. Soil nitrogen content is obtained using PSNT. The experiment included two planting systems: corn-corn-soybean-wheat rotation system and corn continuous cropping system. There was one split zone experimentally, with 4 replications in each split zone. There were two types of fertilizer management: organic fertilizer and chemical fertilizer. Cover crops covered with ryegrass and red clover. The results showed that the yield of corn with covered crops was significantly higher than that of uncoated corn, the yield of corn was higher than that of corn under rotation and the yield of maize covered by rotation was obviously higher than that of uncoated corn.