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This experiment aimed to investigate the associative effects among two low-quality forages (crown daisy, milk thistle) and three agro-industrial byproducts (apple pomace, citrus pulp, tomato peel), by means of an automated gas production (GP) system. All feeds were incubated alone or as 50:50 mixtures of each forage with each byproduct. Samples (0.500 +/- 0.0010 g) of single feeds or mixtures were incubated for 96 h, in three replicates in individual bottles (310 mL), with 75 mL of buffered rumen fluid. Bottles were vented by an open-close valve when the internal pressure reached 3.4 kPa. The metabolisable energy content of single feeds and mixtures was computed from GP at 24 h and feed chemical composition. Feed substrates were ranked for GP in the following way: byproducts, mixtures, and forages. The two forages did not differ for GP and metabolisable energy content, although differences were observed among byproducts and among mixtures. Both forages interacted positively with apple pomace from 6 h (P < 0.001) to 24 h (P = 0.029) of incubation and with citrus pulp at 12 h (P = 0.005) and 24 h (P = 0.012), whereas no associative effects were detected when forages were incubated with tomato peels. Results suggest that in vitro fermentability of low-quality forages could be efficiently improved by combining these two forages with apple pomace or citrus pulp. These findings are relevant, because the use of low-quality forages and byproducts in ruminant feeding is considered important for improving the environmental and economic sustainability of forage systems in arid and semi-arid areas.