Effect of Dietary Rumen-Degradable Starch to Rumen-Degradable Protein Ratio on In Vitro Rumen Fermentation Characteristics and Microbial Protein Synthesis

Simple Summary Synchronizing the energy and nitrogen supply in the rumen was reported to improve feed efficiency; however, synchronization indicators were rarely reported. The present study was conducted using a batch culture technique to evaluate the optimal rumen-degradable starch (RDS) and rumen-degradable protein (RDP) ratio (SPR) for synchronizing the energy and nitrogen supply for dairy cows. The in vitro disappearance of dry matter and neutral and acid detergent fiber, gas production and microbial crude protein synthesis quadratically increased with increasing the SPR and peaked at 2.3. Although an in vivo study could be further needed, our results provided an optimal SPR range for dairy cows to synchronize the energy and nitrogen supply. The objective of this study was to investigate the effects of dietary rumen-degradable starch (RDS, g/kg of DM) to rumen-degradable protein (RDP, g/kg of DM) ratios (SPR) on in vitro rumen fermentation characteristics and microbial protein synthesis (MCPS). Treatments were eight diets with SPR of 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5 and 2.6 and were formulated to be isoenergetic, isonitrogenous, and isostarch. Substrates were anaerobically incubated in sealed culture vials (100 mL) for 6, 24 or 48 h. Three incubation runs were conducted within two consecutive weeks. With the increase of the dietary SPR, the gas production (GP), in vitro dry matter disappearance (IVDMD) and concentration of MCPS and total volatile fatty acids (TVFA) linearly increased after 6 h of incubation (p <= 0.01), whereas they quadratically increased and peaked at the SPR of 2.3 after 24 and 48 h of incubation (p < 0.05). In response to dietary SPR increasing, the in vitro neutral detergent fiber disappearance (IVNDFD) quadratically increased (p < 0.01), and the ammonia nitrogen (NH3-N) concentration linearly decreased (p < 0.01) after 6, 24 and 48 h of incubation. Based on the presented results, an SPR of 2.3 is recommended for formulating a diet due to its greatest IVDMD, IVNDFD, GP, TVFA and MCPS. However, as the results obtained are strictly dependent on the in vitro conditions, further in vivo studies are needed to verify our findings.