Abstract: The present invention pertains to reducing milk fat levels optionally to enhancing overall milk yields in ruminants by administration of trans-fatty acids, trans-fatty acid salts, amides, and triglycerides containing trans-fatty acid substituents. The invention also pertains to a method of identifying those trans-fatty acids, trans-fatty acid salts, amides, and triglycerides containing trans-fatty acid substituents which reduce milk fat levels and may also enhance milk yields, and designing ruminant diets containing such trans-fatty acids, salts, or triglycerides.