Advances in Semen Extenders for Small Ruminant Sperm Preservation: Biological Mechanisms and Emerging Strategies

Semen preservation represents a fundamental component of reproductive biotechnology in small ruminants, supporting artificial insemination programs, genetic improvement, germplasm conservation, and sustainable livestock production. However, ram and buck spermatozoa are highly susceptible to cryoinjury induced by cold shock, oxidative stress, osmotic imbalance, and membrane destabilization during cooling, freezing, and thawing procedures. Consequently, semen extenders play a critical role in maintaining sperm viability, motility, membrane integrity, mitochondrial activity, and fertilizing capacity during preservation. This narrative review summarizes the biological basis of sperm preservation in sheep and goats and discusses the principal conventional and emerging extender systems used in small ruminant semen cryopreservation. Conventional extender components, including buffering systems, energy substrates, cryoprotective agents, egg yolk, and milk proteins, are reviewed with emphasis on their mechanisms of action and protective effects against cryodamage. Particular attention is given to species-specific limitations associated with buck seminal plasma enzymes and their interactions with egg yolk- and milk-based extenders. In addition, recent advances involving chemically defined extenders, soy lecithin formulations, antioxidants, synthetic membrane stabilizers, and nanotechnology-derived additives are discussed as promising alternatives aimed at improving cryosurvival, reducing oxidative stress, and enhancing extender standardization and biosafety. Practical applications of semen extenders in artificial insemination and semen banking are also discussed together with current methodological limitations, species-specific challenges, and future research priorities. Although numerous emerging strategies have demonstrated promising improvements in laboratory sperm-quality parameters, evidence supporting consistent fertility benefits under field conditions remains limited. Overall, this review critically evaluates current knowledge and highlights the need for species-specific, protocol-oriented, and well-standardized approaches to advance semen preservation in small ruminants.