体外产气法评价不同分级指数玉米青贮—苜蓿干草组合发酵特性

为明确不同分级指数（Grading index，GI）下牦牛日粮粗饲料营养情况，本试验选取了5种不同GI（7.20，6.43，5.66，4.89，4.12）的玉米青贮—苜蓿干草组合，其质量比分别为70：30，60：40，50：50，40：60，30：70，进行牦牛瘤胃体外发酵试验，旨在探究不同组合下牦牛瘤胃体外发酵效果。试验结果表明：发酵48 h时，GI=7.20组合的累积产气量（Gas production，GP）显著高于或极显著高于其他组合（P<0.05或P<0.01）；各GI组合的干物质降解率（Dry matter degradation in vitro，IVDMD）差异不显著（P>0.05）；发酵3 h和12 h时，GI=7.20组合的中性洗涤纤维降解率（Neutral detergent fiber in vitro，IVNDFD）与GI=6.43组合差异不显著但极显著高于其他GI组合（P<0.01）；3 h，6 h，12 h，24 h时的氨态氮（NH₃-N）浓度随着GI降低呈现出减小趋势，48 h时的NH₃-N浓度随着GI降低呈现出先减小后增加趋势，GI=7.20时的NH₃-N浓度最大；发酵48 h时，各GI组合的微生物蛋白（Microbial protein，MCP）浓度差异不显著（P>0.05），但GI=7.20时的MCP浓度最大；发酵48 h时，乙酸、丙酸、总挥发性脂肪酸（Total volatile fatty acid，TVFA）浓度随着GI降低呈现出减小趋势，GI=7.20时的乙酸、丙酸、TVFA浓度最大；各GI组合发酵6 h~48 h均呈现了正组合效应，综合组合效应值（Multiply factors associative effects index，MFAEI）随着GI减小呈现先减小后增大的趋势，且各时间段GI=7.20组合的MFAEI高于其他GI组合。综上所述，GI值为7.20玉米青贮—苜蓿干草组合的牦牛瘤胃体外发酵效果最优。

Evaluation of Fermentation Characteristics of Different Grading Indexes Corn Silage and Alfalfa Hay Combination Using in Vitro Gas Production

The purpose of this experiment is to optimize the forage nutritional combination of yak diet with the grading index (GI). Five combinations of corn silage and alfalfa hay with different GIs (7.20,6.43,5.66,4.89,4.12) were selected to do yak rumen fermentation in vitro. After 48 h of fermentation,the cumulative gas production (GP) of the GI=7.20 combination was significantly higher or very significantly higher than the other GI values (P<0.05 or P<0.01). The dry matter degradation in vitro (IVDMD) of each GI combination was not significantly different (P>0.05). After 3 h and 12 h of fermentation,the neutral detergent fiber in vitro (IVNDFD) of GI=7.20 was not significantly different from that of GI=6.43 and was significantly higher than the other GI combinations (P<0.01). For 3 h,6 h,12 h and 24 h of fermentation,the concentration of ammonia nitrogen (NH₃-N) showed a decreasing tendency as GI decreased,at 48 h of fermentation,the concentration of NH₃-N showed a first decreasing and then increasing tendency with the decreasing of GI,and the concentration of NH₃-N at GI=7.20 was the largest. At 48 h of fermentation,the concentration of microbial protein (MCP) in all GI combinations was not significantly different (P>0.05) with the maximum MCP concentration at GI=7.20. After 48 h of fermentation,the concentrations of acetic acid,propionic acid and total volatile fatty acid (TVFA) showed decreasing tendencies as GI decreased. The concentrations of acetic acid,propionic acid and TVFA with GI=7.20 were the largest. Different GI forage combinations showed a positive combination effect from 6 to 48 hours of fermentation,the value of multiply factors associative effects index (MFAEI) had a first decrease and then increase tendency then increased trend as the GI decreases,GI=7.20 forage combination of MFAEI was higher than other GI combinations. Overall,in vitro fermentation of yak rumen with the GI=7.20 alfalfa hay and corn silage combination was optimal combination.