The invention provides an additive for livestock feed and ingredients for 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 providing the additive in the livestock feed, which includes monosodium L-glutamate and L-tryptophan, and wherein the 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.