The invention provides an additive for livestock feed and constitutions of a feed composition for livestock to improve the feed conversion ratio and the body weight gain efficiency by increasing the feed intake of livestock. The feed intake of livestock can be increased by an additive for livestock feed, which comprises monosodium L-glutamate and L-tryptophan, wherein a mass ratio of free monosodium L-glutamate (provided that all converted into monosodium L-glutamate monohydrate) and free L-tryptophan (GLU/TRP ratio) is from 0.5 to 30.
