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降水量仅３００ｍｍ左右的旱农区发展农田草业，草种的更新周期受土壤水分平衡规律的影响，多年生豆科牧草苜蓿和沙打旺种植时间不得超过３年，第３年产草量明显下降；地下根量呈递增趋势，水分平衡规律失调，部分植株因干旱而死亡，牧草的更新周期以３年为好。
The precipitation cycle is only about 300mm in the development of arable land in arid farming areas. The renewal cycle of grass species is affected by the law of soil moisture balance. The planting time of perennial leguminous crops of alfalfa and alfalfa can not exceed 3 years, and the output of grass in the third year drops obviously ; Underground roots showed an increasing trend, imbalanced water balance law, some plants died due to drought, pasture regeneration cycle to 3 years as well.