Abstract: An experiment was conducted to investigate the effects of the proportions of pomelo peel and alfalfa in different concentrate-roughage ratios (C:R) on the associative effects (AE) of diets in vitro. Pomelo peel:alfalfa proportions were 80:0, 70:10, 60:20, 50:30, 40:40, 30:50, 20:60, 10:70,0:80 when C:R was 20:80. Pomelo peel:alfalfa proportions were 70:0, 60:10, 50:20, 40:30, 30:40, 20:50, 10:60, 0:70 when C:R was 30:70. Pomelo peel:alfalfa proportions were 60:0, 50:10, 40:20, 30:30, 20:40, 10:50, 0:60 when C:R was 40:60. Gas production (GP) was recorded at 0, 2, 4, 6, 9, 12, 24, 36, 48, 72, 96 h. AE was defined as the difference between the observed in vitro GP_{24 h} and the predicted value from individual feed fermented alone. The results showed that the NDF content of pomelo peel was lower and its gas parameters were higher than either concentrate or alfalfa. The AE of all groups were negative and the AE of the group of pomelo peel:alfalfa at 80:0 was significantly greater than the 0:80 group (P<0.05) when C:R was 20:80. The AE of all groups were not significantly different when C:R was 30:70 (P>0.05). The AE of the groups 30:40 and 10:60 were 14.74% and 3.61% respectively and the AE of the 30:40 group was higher; the other groups were negative when C:R was 30:70. The AE of the 0:60 group was significantly greater than the 60:0 group (P<0.01) and significantly greater than the 30:30 group (P<0.05) when C:R was 40:60, and the AE of the 10:50 group tended to be greater than the 60:0 group (P=0.079). The AE of the 0:60, 10:50, 20:40 groups were greatest when C:R was 40:60. In conclusion, AE is the greatest when high levels of concentrate (C:R, 40:60) and low proportions of pomelo peel (0,10,20) are combined, when middle levels of concentrate (C:R, 30:70) and pomelo peel (30) are combined, and when low levels of concentrate (C:R, 20:80) and a high proportion of pomelo peel (80) are combined.

Key words: associative effects, concentrate-roughage ratio, pomelo peel, Medicago sativa, concentrate