Abstract: This experiment evaluated the nutritional quality, gas production characteristics and the fermentation products of mixed feedstuffs comprising cotton stalk and beet pulp. A three-factor, three-level orthogonal treatment combination and an in vitro fermentation method were used, in order to identify the optimal fermentation conditions. Beet pulp was added to cotton stalk in proportions of 10%, 30% or 50%, with 0.1%, 0.2% or 0.3% added urea, and 0.05%, 0.1% or 0.2% added bacterial liquid (lactic acid bacteria and yeast are the main components), with salt added 0.2% in all cases. From the 27 available combinations, 9 were chosen for inclusion in the experiment, designated A-I, and a control, K, as follows: A) 10% beet pulp, 90% cotton stalk, 0.1% urea, 0.05% bacterial liquid, 0.2% salt; B) 10% beet pulp, 90% cotton stalk, 0.2% urea, 0.1% bacterial liquid, 0.2% salt; C) 10% beet pulp, 90% cotton stalk, 0.3% urea, 0.2% bacterial liquid, 0.2% salt; D) 30% beet pulp, 70% cotton stalk, 0.3% urea, 0.05% bacterial liquid, 0.2% salt; E) 30% beet pulp, 70% cotton stalk, 0.1% urea, 0.1% bacterial liquid, 0.2% salt; F) 30% beet pulp, 70% cotton stalk, 0.2% urea, 0.2% bacterial liquid, 0.2% salt; G) 50% beet pulp, 50% cotton stalk, 0.2% urea, 0.05% bacterial liquid, 0.2% salt; H) 50% beet pulp, 50% cotton stalk, 0.3% urea, 0.1% bacterial liquid, 0.2% salt; I) 50% beet pulp, 50% cotton stalk, 0.1% urea, 0.2% bacterial liquid, 0.2% salt and control group K (100% cotton stalk, 0 urea, 0 bacterial liquid, 0.2% salt). It was found that for gas production at 48 hours, Group I was significantly higher than groups A, D, F (P<0.05), and group K (P<0.01), while, there were no significant difference between the other groups (P>0.05). Group I had the highest organic matter digestibility and metabolizable energy: 722.40 g·kg^-1 and 9.75 MJ·kg^-1, respectively. The slow degradation parameter, b, was significantly higher in group I than in groups A, D, F (P<0.05), and group K (P<0.01). Groups G and H, were also significantly higher than group K (P<0.05), while there were no significant differences between the other groups (P>0.05). The rapid degradation parameter (a), and the gas production rate (c), did not differ significantly between any of the groups. The concentration of ammonia nitrogen in group H was significantly lower (P<0.05) than in other groups, and group K was significantly lower than groups A, B, C and E. Soluble sugar was significantly higher (P<0.05) in groups H and I than other groups, and in group H was 44.06% and 31.23% higher than the lowest groups, B and K, respectively. No significant differences between groups for pH, lactic acid and microbial protein were detected (P>0.05). The highest total volatile fatty acid (VFA) concentrations occurred in groups was group H and G (100.31 and 99.73 mmol·L^-1, respectively), and these values were which was significantly (P<0.05) higher than those of other groups. For the volatile fatty acids, acetate and butyrate were highest in Groups G, H, and I and propionate was highest in Groups G and H, while acetate:propionate ratio was lowest in groups G, H, and I. Using a membership function analysis for multivariate ranking of the mixed fermentation products of cotton stalk and beet pulp, the average membership function values of each group were I (0.719)>H (0.692)>F (0.595)>G (0.591)>C (0.407)>E (0.400)>B (0.395)>K (0.374)>D (0.307)>A (0.243). In conclusion, a dry matter-based ratio of cotton stalk to beet pulp of 50∶50, with 0.1% urea, 0.2% bacterial liquid, and 0.2% salt (group I) is recommended based on this research. The mixed fermentation product of this mixture has the highest in vitro gas production, the highest digestible organic matter and metabolic energy content, the higher concentration of acetic acid and butyric acid, and the highest multivariate score of the fermentation products. It is the best among the fermentation combinations tested and is suitable as a roughage for ruminants.

Key words: cotton straw, beet pulp, gas production characteristics, fermentation parameters