CURRENT CHALLENGES IN AGRICULTUREA large proportion of food is composed of crops and meat besides vegetables and fruits.The production of crops and meat needs a large area of land and other resources,such as water,chemical fertilizer,and pesticides.The growing world population increases the demand for food security.Moreover,developing countries with highly dense populations have dramatically changed food composition from mostly crops to more meat and milk,which has forced the adjustment and optimization of agricultural production structure to supply the demand caused by these dramatic changes.For example,after 40 years of economic reform in China,the living standard has considerably increased and is reflected by the stark changes in food demand.From 1990 to 2012 human consumption of crops in China declined by 38％,while consumption of meat and milk increased by 226％ (Louie et al.,2015).Considering that the Chinese population totals 1.4 billion,one-quarter of the worlds population,this rapidly increasing demand for animal-derived foods and products increases the need for more grains used as feed.Over the last several years,total grain output in China reached 0.6 billion tons per year,more than half of these being used as animal feed.