Replacing conventional magnesium oxide with high-solubility magnesium oxide while reducing sodium bicarbonate...

High-starch diets increase dietary energy density for high-producing dairy cows but also increase ruminal acid production and the risk of ruminal pH depression. High-solubility magnesium oxide has emerged as a potential ruminal alkalinizing agent; however, whether it can replace conventional low-solubility magnesium oxide while reducing sodium bicarbonate inclusion under different dietary starch concentrations remains unclear. The objective of this study was to evaluate the effects of partially replacing sodium bicarbonate with high-solubility magnesium oxide on ruminal pH, fermentation, nutrient digestibility, nitrogen metabolism, and bacterial community composition using dual-flow continuous culture fermenters. Treatments were arranged in a 2 × 2 factorial design with 2 dietary starch concentrations [low starch (21% of DM) and high starch (32% of DM)] and 2 pH-modulation strategies: a high dose of sodium bicarbonate (0.66% of dietary DM) plus low-solubility magnesium oxide (0.40% of dietary DM), or a low dose of sodium bicarbonate (0.30% of dietary DM) plus high-solubility magnesium oxide (0.40% of dietary DM). No pH-modulation strategy × starch concentration interactions were detected. Across starch concentrations, the low-dose sodium bicarbonate plus high-solubility magnesium oxide treatment increased ruminal pH and improved OM, NDF, ADF, and hemicellulose digestibility, whereas SCFA production, nitrogen metabolism, and bacterial community composition were largely unaffected. Increasing dietary starch reduced ruminal pH and nutrient digestibility while altering SCFA profiles, nitrogen metabolism, and bacterial community structure. Overall, partially replacing sodium bicarbonate while replacing conventional low-solubility magnesium oxide with high-solubility magnesium oxide improved ruminal pH and fiber digestibility across contrasting dietary starch concentrations, indicating that this strategy can reduce sodium bicarbonate inclusion without compromising ruminal function.