The purpose of the present invention is to reduce the amounts of nitrate nitrogen and ammonia, which can be absorbed through the digestive tract and are factors for causing mastitis in livestock animals, thereby reducing the incidence rate of mastitis. A composition for preventing and treating mastitis for livestock animals comprises a rock powder containing silicic acid and aluminum oxide as active ingredients. When the composition is mixed with feed and the feed is given to livestock animals, nitrogen compounds including ammonia nitrogen and nitrate nitrogen generated in the body of the livestock animals can be chemically adsorbed by aluminum oxide and silicic acid, whereby the over-absorption of the nitrogen compounds through the digestive tract can be suppressed. At the same time, minor elements contained in the rock powder act to fully elicit the activity of a nutritional component capable of suppressing mastitis, e.g., vitamin A, vitamin E, copper, zinc and manganese obtained from the feed. In this manner, the onset of mastitis can be suppressed in a nutritional manner, and a therapeutic effect can be achieved on mastitis that has already been developed.
