Due to these findings, our study aimed to evaluate the inclusion of a small amount of straw (5%) with different particle sizes in calf starter on rumen pH, rumen fermentation, and development of the digestive system. A secondary objective was to observe rumen fermentation at an early age. We hypothesized that greater particle size of straw in the starter would enhance rumen development by providing abrasiveness to the rumen epithelium and a more stable rumen fermentation