Effects of different graded index alfalfa hay and wheat straw on yak rumen fermentation characteristics in vitro

The purpose of this study was to optimize the forage nutritional combination of yak diet with the grading index (GI). Five combinations of alfalfa hay and wheat straw with different GI (7.03, 6.21, 5.38, 4.55, and 3.72) were selected to study yak rumen fermentation in vitro. The fermentation time was 48 h and 3 replicates were performed for each sample. At 3 ~ 48 h of fermentation, the gas production (GP) of the GI = 6.21 combination was significantly higher than that using the other GI values (P < 0.05). At 3 h and 48 h of fermentation, the dry matter degradation in vitro (IVDMD) showed a decreasing trend with a decrease in GI. At 3, 12, and 48 h of fermentation, the difference between the neutral detergent fiber in vitro (IVNDFD) of GI = 7.03 and GI = 6.21 is not significant, but they were higher than other GI combinations (P < 0.05). At 3 ~ 48 h of fermentation, the concentration of ammonia nitrogen (NH₃-N) showed an initial increasing trend followed by a decrease as the GI decreased. At 48 h of fermentation, the microbial protein (MCP) showed a trend of an initial increase followed by a decrease as the GI decreased. At 48 h of fermentation, the concentrations of acetic acid, propionic acid, and total volatile fatty acids (TVFAs) showed an initial trend of increase followed by a decrease as the GI decreased. At 3 ~ 24 h of fermentation, the concentration of CH₄ and DE showed an initial trend of increase followed by a decrease as the GI decreased. The GIs of 7.03, 6.21, 5.38 showed a positive combination effect from 3 ~ 48 h of fermentation, whereas GI = 4.55 and 3.72 showed a positive combination effect beyond 48 h of fermentation. The results of multiple factor associative effects index (MFAEI) were first increased and then showed a decreasing trend as the GI decreased. The GI = 6.21 combination of MFAEI was higher than other GI combinations. Overall, in vitro fermentation of yak rumen with GI = 6.21 alfalfa hay and wheat straw combination were found to be optimal.