Effects of partial replacement of alfalfa hay with alfalfa silage on fiber degradation and microbial protein synthesis in lactating dairy cows

This experiment aims to study the effects of replacing dietary alfalfa hay (AH) with alfalfa silage (AS) in TMR on nutrients digestibility, lactation performance and rumen fermentation characteristics of dairy cows, and to investigate the association of dissolved H 2 (dH 2 ) with fiber digestibility and microbial crude protein (MCP) biosynthesis through multiple linear regression analysis. In the first experiment, both forages were incubated to determine fiber degradation, rumen fermentation and gases accumulation through 48-h in vitro batch cultures. In the second experiment, 30 lactating Holstein dairy cows (716.7 ± 13.25 kg of body weight) were randomly assigned into two dietary treatments including 8.17% (DM basis) AH or 12% AS. AS-fed animals exhibited higher levels of total-tract dry matter digestibility (DMD), organic matter digestibility (OMD) and acid detergent fiber digestibility (ADFD), and greater milk fat and protein contents. For rumen fermentation characteristics, AS-fed animals observed lower pH and dissolved gases (both dH 2 and dissolved methane [dCH 4 ]) concentrations, as well as higher MCP production, acetate molar percentage and acetate to propionate ratio. Multiple linear regression analysis showed that ruminal dH 2 was positively correlated with dCH 4 (R 2 = 0.778, P < 0.001), and negatively correlated with OMD (R 2 = 0.498, P = 0.004), ADFD (R 2 = 0.535, P = 0.032), ammonia nitrogen (NH 3 -N) (R 2 = 0.522, P < 0.001), and MCP/NH 3 -N (R 2 = 0.373, P = 0.010). In summary, our results indicated that replacing dietary AH with AS enhanced nutrient digestibility and MCP synthesis efficiency. These improvements were significantly correlated with reduced ruminal dH 2 concentrations, suggesting that dH 2 may serve as a potential correlative indicator of enhanced fermentation efficiency.