** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** INCREASING RUMINANT PRODUCTION TO SATISFY THE DEMANDS OF THE GLOBAL POPULATION IS A GROWING CHALLENGE FOR PRODUCERS. THE SOLUTION IS NOT AS SIMPLE AS RAISING THE NUMBERS OF ANIMALS, GIVEN INCREASINGLY LIMITED RESOURCES AVAILABLE. INSTEAD, EFFORTS SHOULD BE DIRECTED AT ENHANCING PRODUCTION EFFICIENCY. IN THAT REGARD, DIFFERENCES IN THE MICROBIAL COMPOSITION AND ACTIVITY OF AN ANIMAL'S RUMEN ARE ASSOCIATED WITH RUMINANT PRODUCTION EFFICIENCY. NEVERTHELESS, MANIPULATION OF THE RUMEN MICROBIAL ECOSYSTEM REMAINS ONE OF THE MAIN CHALLENGES IN THE FIELD, AND THE FORCES THAT DETERMINE ITS COMPOSITION AND ACTIVITY ARE STILL POORLY UNDERSTOOD. IN THIS APPLICATION, WE WILL INVESTIGATE THE ROLE OF MILK SECRETORY IMMUNOGLOBULIN A (SIGA) ON THE RUMEN MICROBIAL ECOSYSTEM. OUR CENTRAL HYPOTHESIS IS THAT SIGA IS AN EVOLUTIONARY MECHANISM RESPONSIBLE FOR ASSEMBLING AND MAINTAINING THE RUMEN MICROBIOME. WE POSTULATE THAT THE MILK SUPPLY SIGA WILL ACCELERATE THE RUMEN MICROBIOME'S DEVELOPMENT, WHICH WILL LATER BE MAINTAINED BY SALIVARY SIGA. THE SUPPLY OF MILK SIGA DURING EARLY LIFE WILL ULTIMATELY RESULT IN A BENEFICIAL MICROBIAL STRUCTURE AND FUNCTIONALITY OF THE RUMEN LEADING TO SUPERIOR PRODUCTION EFFICIENCY. THIS HYPOTHESIS IS BASED ON OUR PRELIMINARY DATA. UNDERSTANDING HOW TO MILK SIGA IMPACTS RUMEN MICROBIAL ASSEMBLY, COMPOSITION, AND ACTIVITY IS EXPECTED TO HAVE A SUBSTANTIAL POSITIVE IMPACT ON RUMINANT PRODUCTION. THIS KNOWLEDGE IS LIKELY TO CONTRIBUTE TO DEVELOPING AND DISSEMINATING STRATEGIES TO MODULATE THE MICROBIAL ECOSYSTEM AND INCREASE RUMINANT PRODUCTION EFFICIENCY. THEREBY HELPING TO ENSURE FUTURE GLOBAL DEMANDS FOR SAFE AND NUTRITIOUS RUMINANT PRODUCTS ARE ADEQUATELY MET WITH AGRICULTURALLY SUSTAINABLE PRACTICES.