Abstract: This experiment was conducted to investigate, using an in vitro gas production (GP) technique, the associative effect (AE) when corn cob (CC) was added as a supplement to an alfalfa (A) and commercial concentrate (C) feed mix. A series of rations was formulated with concentrate-roughage ratio (C:R) of either 40:60 (n=5), or 30:70 (n=6). Specifically, the C:CC:A ratios for the 11 ration formulations tested were: 40:60:0, 40:45:15, 40:30:30, 40:15:45, 40:0:60, 30:70:0, 30:55:15, 30:40:30, 30:25:45, 30:10:60 and 30:0:70. Gas production (GP) was recorded at 0, 2, 4, 6, 9, 12, 24, 36, 48 h. After incubation, the incubation fluid and residues were used to determine pH, volatile fatty acids (VFA), ammonia nitrogen (NH₃-N), dry matter digestibility (DMD) and organic matter digestibility (OMD). The single factor AE index (SFAEI) was defined as the difference between the observed value and the predicted value for an individual feed fermented alone. Also, the multiple-factor AE indexes (MFAEI) of the 11 rations were calculated. It was found that when C:R was 40:60 those rations with 15% or 30% CC had a higher GP, DMD, OMD, NH₃-N, and TVFA, as well as higher SFAEI of GP, DMD, OMD, NH₃-N, and total VFA (P<0.05 or P<0.01). When C:R was 30:70, then rations with 10%, 25%, 40%, or 55% CC generally performed well. Meanwhile, the ration formulations C:CC:A of 40:30:30 and 30:10:60 had a significantly higher MFAEI (P<0.01) than other rations. It was therefore concluded that the greatest AE occurred when C:CC:A ratios were 40:30:30 and 30:10:60.

Key words: associative effects, corn cob, alfalfa, concentrate, in vitro gas production