Abstract: To evaluate the amount and proportion of added energy feed and protein feed,the effects of concentrate-roughage ratio (C∶R) and rapeseed meal supplementation level on associated effects of diets in vitro were studied. Wheat straw (WS) or ammonium bicarbonate treated wheat straw (ABWS) basal diet was supplemented with a mixture of rapeseed meal (RSM) and corn. The ratios of RSM and corn to WS or ABWS tested were 70∶30,60∶40,50∶50,40∶60,and 30∶70,with the proportion of RSM at 6%,12%,18%,24% in the whole diet. Gas production (GP) was recorded at 0,2,4,6,9,12,24,36,48,72,and 96 h. AE was defined as the difference between the observed in vitro GP and the predicted value from individual feed fermented alone. 1) Negative AE was observed under the WS basal diet in 15 groups. When the concentrate-roughage ratio was 70∶30 and the RSM content was 18% and 24%,a positive AE was observed. When the concentrate-roughage ratio was 40∶60 and the RSM content was 6% and 12%,positive AE were observed. The AE of the high C∶R group 70∶30 was significantly higher than that of the low C∶R groups 40∶60 and 30∶70 (P<0.01). 2) A positive AE was observed in ABWS basal diets in all mixtures except one group. The AE of the C∶R group of 30∶70 was significantly higher than that of the C∶R group of 70∶30 (P<0.01). The AE proportion at RSM 24% was significantly greater at RSM 12% (P<0.05). It is concluded that the AE is largest in the group with low C∶R and a high proportion of RSM. ABWS is better than WS in saving concentrated feed.