Compositions and methods useful for processing animal semen for use in assisted reproduction. A composition, or colloid formulation, for separation of spermatozoa from a semen sample, the composition including at least one salt of an alkali metal and/or an alkaline earth metal, EDTA, a zwitterion buffer, silane-coated silica particles and water, the composition having a pH of 7.0-7.35 and an osmolarity of 300-345 mOsm. A method for preparing spermatozoa from a semen sample from a non-human animal by separating the spermatozoa from other semen constituents by centrifugation through a single layer of the composition, or colloid formulation. Also, a method for separating a sperm sub-population of interest from a semen sample from a non-human animal by providing a density gradient comprising at least two layers of the composition of the invention, each layer having a different density； separating the sperm sub-populations in the semen sample by centrifugation through the density gradient； and selecting the sperm sub-population of interest. Finally, spermatozoa prepared by these methods and the use of such spermatozoa in artificial insemination, in vitro fertilisation or intracytoplasmic sperm injection.
