To maintain and promote the health of racehorse or domestic livestock by improving and stabilizing the intestinal bacterial flora of an animal such as domestic livestock, especially racehorse to promote the improvement in the stool nature and promoting the absorption of energy source such as calcium.

A purified gelatinous feed having weakly acidic nature of pH 4.5 or thereabout is produced by mixing lactofructo-polysaccharide with glucomannan and adding a pH modifier to the mixture. The purified feed is administered as an assistant feed by mixing with a main feed such as hay.