Abstract: This study investigated the effects of mixing oat hay and alfalfa in different proportions, on in vitro fermentation rate when incubated with donkey cecum fluid, so as to provide a theoretical basis to determine the optimal use of roughage in donkey breeding. Oat hay and alfalfa were mixed in ratios of 80:20, 60:40, 40:60, 20:80, denoted as treatment groups Ⅰ, Ⅱ, Ⅲ, and Ⅳ respectively. Each mixture was cultured for 2, 4, 8, 12 and 24 hours using continuous fermentation in an artificial rumen, and in vitro gas production measured. There were 3 replicates at each time point. After termination of fermentation, fermentation parameters and nutrient degradation rate of forage were determined. The results showed: 1) With increased alfalfa proportion in the ration, the dry matter (DM) and acid detergent fiber (ADF) degradation rates increased gradually, and were significantly higher for group Ⅳ than for group Ⅰ (P<0.05); there was no significant difference in the degradation of neutral detergent fiber (NDF) (P>0.05). 2) With increase in alfalfa content, the pH increased gradually, and group Ⅳ was significantly higher than group Ⅰ (P<0.05); The gas production (GP) of group Ⅱ was significantly higher than that of the other three groups at each time point (P<0.05). 3) The concentration of NH₃-N in groups Ⅲ and Ⅳ was significantly higher than for groups Ⅰ and Ⅱ from 12 hours (P<0.05). At 24 h, the concentration of microbial protein in group Ⅱ was significantly higher than in group Ⅲ (P<0.05), and there was no significant difference between groups Ⅰ, Ⅱ and Ⅲ (P>0.05). 4) The combination of oat hay and alfalfa had significant effects on volatile fatty acids (VFA), the concentration of propionic acid in groups Ⅰ and Ⅳ was significantly higher than that in groups Ⅱ and Ⅲ at 24 h (P<0.05), while the total VFA content in groups Ⅰ and Ⅱ was significantly higher than that in group Ⅲ and Ⅳ (P<0.05). 5) After 24 h of fermentation, a positive interaction effect was observed in groups Ⅰ, Ⅱ and Ⅳ, with the interaction being largest in group Ⅳ. Conversely, a negative interaction effect was observed in group Ⅲ. We concluded that the optimal proportion of oat hay and alfalfa was 20:80 (group Ⅳ), as the degradation rate of DM, NDF and ADF was improved, and a higher interaction effect occurred.

Key words: donkey, cecum, oat hay, alfalfa, in vitro fermentation, combination effect