Abstract: This study aimed to determine the associative effects（AE） of 28 feed combinations of concentrate: elm leaves: alfalfa under different concentrate-roughage ratio（C:R） by in vitro gas production method. The 28 feed combinations were incubated. The GP at 0, 2, 4, 6, 9, 12, 24, 36, 48,72 h were recorded and the AE were calculated by GP_72h. After 72 h incubation, the incubation fluid and residues were determined pH, volatile fatty acids（VFA） and ammonia nitrogen（NH₃-N）, dry matter digestibility（DMD）, organic matter digestibility（OMD）. The single-factor AE index（SFAEI） and multiplefactor AE index（MFAEI） were computed. The results showed that groups 80:20:0, 60:40:0, 60:10:30, 50:20:30, 50:10:40, 40:40:20, 40:30:30, 30:55:15, 30:40:30, 20:20:60 had higher GP_72h, DMD, OMD, NH₃-N, in addition, higher AE of GP, DMD, OMD, TVFA and NH₃-N than those of the other group under the same C:R（P＜0.05 or P＜0.01）, especially groups 30:55:15 was the optimal. The SFAEI and MFAEI of groups 30:55:15, 40:40:20, 50:20:30, 20:20:60, 60:10:30, 80:20:0 were the greatest. It was concluded that the greatest AE occurred when concentrate: elm leaves: alfalfa ratio were 30:55:15, 40:40:20, 50:20:30, 20:20:60, 60:10:30, 80:20:0. Elm leaves can be used in ruminant diets according to the proportions.