Use of saponified tall oil fatty acid

A method of modulating a microbial population of an animal digestive tract comprising:

• forming a feed supplement comprising a saponified tall oil fatty acid comprising resin acids, wherein the saponified tall oil fatty acid comprises 2-10% (w/w) resin acids and 90-98% (w/w) fatty acids, wherein the saponified tall oil fatty acid is formed by combining a tall oil fatty acid with a base forming a mixture, and wherein the mixture is heated to at least 90° C. thereby forming the saponified tall oil fatty acid,
• and
• administering the feed supplement to the animal, wherein the concentration of the feed supplement is 0.005 to 1 kg/ton dry weight of the total amount of the feed.