Abstract: The high yield of Chinese traditional agriculture leads to land overused and farmland reclaimed. So it is meaningful to change the unreasonable use pattern of farmland. Gansu province is studied as a case to find out the necessity of farmland converted into grassland and its impact on food security by analyzing the relevant of grain sown area, per unit yield and total yield, the grain supply and demand of Gansu and the output of different proportions of farmland converting into grassland. The result shows the necessity of returning farmland to grassland whether in aspect of limited grain production or in changes of food consumption structure. When 15% of farmland converted into grassland, the total food equivalent of the land decrease 2.3%, increase 2.6% and 16.8% under lower (3 t·hm^-2), secondary (6 t·hm^-2) and higher (10 t·hm^-2) productivity level, respectively. Therefore, returning farmland to grassland and developing grassland agriculture can make full use of land and contribute to food security.

Key words: Food security, Returning farmland to grassland, Grassland agriculture, Food equivalent

摘要： 我国传统农业对谷物高产的追求导致土地的过度使用、许多不宜农用的土地被开垦为农田，因而改变耕地不合理的利用方式对食物安全具有重要意义。因此，以甘肃省为例，通过分析粮食播种面积、单产和总产量三者的相关性、甘肃省粮食供给和需求情况以及不同比例的耕地转变为牧草地时的食物产出效益，来探索耕地转化为牧草地的必要性及其对食物安全的潜在影响。结果表明：退耕还草具有必要性，一是因为传统农业粮食增产的空间十分有限，二是因为食物需求结构的变化导致口粮消费量递减。当15%的耕地转变成牧草地时，较低生产力（3 t·hm^-2）、中等生产力（6 t·hm^-2）、较高生产力（10 t·hm^-2）水平下的总食物当量变化分别为：减少2.3%、增加2.6%、增加16.8%。因此，退耕还草发展草地农业可以实现土地的最大效用并持久地服务于食物安全。

关键词: 食物安全, 退耕还草, 草地农业, 食物当量