Abstract: We investigated the effects of supplementing an alfalfa diet with fennel straw, fennel shells, sunflower straw, sunflower plates, cotton straw, and cotton shells, and their associative effects (AE), in an in vitro experiment. The concentrate∶straw/shell∶alfalfa ratios were 30∶70∶0, 30∶63∶7, 30∶56∶14, 30∶49∶21, 30∶42∶28, 30∶35∶35, 30∶28∶42, 30∶21∶49, 30∶14∶56, 30∶7∶63, 30∶0∶70 in diets with a concentrate∶roughage ratio of 30∶70. Gas production (GP) was recorded at 0, 2, 4, 6, 9, 12, 24, 36, 48, 72, and 96 h. The AE was defined as the difference between the observed in vitro GP and the predicted value from the individual feed fermented alone. In total, there were 74 groups that included straws or shells: (11×6)+8=74. The AEs of the groups with 14%, 7%, and 0% fennel straw were 58.74%, 54.18%, and 77.91%, respectively, significantly higher than the AEs of the groups with 70%, 63%, and 56% fennel straw (P<0.01). The AEs of all groups with fennel shell were positive. The AEs were also high for the groups containing 7%, 14%, 21%, and 28% sunflower straw, with the highest AE (124.84%) for the group containing 7% sunflower straw. The AE of all groups containing sunflower plates were positive, and the highest AE was in the group with 7% sunflower plates (143.84%). The AE of the groups containing 0%, 70%, and 7% cotton straw were 77.91%, 57.76%, and 54.56%, respectively. The highest AE among the groups containing cotton shells was 139.05% in the group containing 7% cotton shell. The AE of the groups containing 14%, 21%, and 0% cotton shells were 100.09%, 86.22%, and 77.91%, respectively. We conclude that addition of straw or shells at 7%, 14%, or 21% is an effective strategy to extend alfalfa.