For semen collection from stallions, the most common method used is an artificial vagina; after collecting semen, it is tested, diluted, then stored according to the intended use. Semen can be either liquid or frozen. There are many kinds of preservatives used in dilution with semen – most contain energy and antibiotics for liquid, and protection from freezing for frozen semen. Many studies are ongoing to improve preservation, increase liquid semen storage time, and reduce or remove bacteria from semen.