This study examined the effects of alfalfa silage versus alfalfa hay in a total mixed ration (TMR) on milk yield, rumen fermentation, and nutrient digestibility in dairy cows. Hydrolyzed tannins (HT) were supplemented individually to assess changes. Thirty-two multiparous Holstein cows (DIM: 94 ± 8 d; milk yield: 41 ± 2 kg) were assigned to four treatments in a 2 × 2 factorial design: basal diet (alfalfa hay, H, or alfalfa silage, S) and additive (control, C, or 100 g/d HT, T). Production performance, rumen fermentation, nutrient digestibility, and blood metabolites were evaluated. Compared with group H, group S had a 0.16% higher milk protein percentage and significantly higher fat-corrected milk yield, milk fat percentage, fat-to-protein ratio, total solids, and milk urea nitrogen. After feeding, the ST group had increased ruminal pH. HT supplementation significantly decreased ruminal NH₃-N levels (p < 0.05) and increased microbial crude protein (MCP) content (p < 0.05). Group H showed no significant changes, and the effects of HT were less evident in hay-fed cows than in silage-fed cows. In summary, alfalfa silage feeding increased ruminal microbial populations, while HT supplementation mitigated the post-feeding decline in ruminal pH. Considering the relatively small sample size (n = 32), the results should be viewed as indicative rather than conclusive, and future studies with larger cohorts will be valuable to confirm and extend these findings.