This study evaluated the effects of ensiling selected rice straw varieties with or without microbial additives (molasses, yeast, and a combination of both) on their nutritional composition and the subsequent effects on dairy cattle performance in Uganda. The rice straw varieties included Supa, Nerica 4, and K85, common in lowland and upland cultivation. Crude protein (CP), neutral detergent fiber (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL), dry matter (DM), and metabolizable energy (ME) were assessed through chemical analyses. Ensiling with molasses and yeast significantly improved CP (up to 7.54%), lowered fiber fractions, and increased ME to 8.5 MJ/kg. Feeding trials on lactating dairy cows demonstrated increased feed intake, milk yield, and feed conversion efficiency for treated straw diets compared to untreated controls. Notably, milk composition, particularly fat and protein content, were enhanced with microbially treated straw. The study concludes that ensiling rice straw with microbial additives enhances its nutritive value and can effectively replace more expensive conventional feeds in Uganda’s smallholder dairy systems.