Microbial caecal fermentation in Iberic or Landrace pigs given acorn/sorghum or maize diets estimated in vitro using the gas production technique

The caecal microbial fermentation in pigs from Iberic (IB) or Landrace (LD) breed fed on an acorn/sorghum (A) or a maize (M) diet was studied in vitro using the gas production technique. Freeze-dried ileal contents from four pigs (133 kg live weight) from each treatment were used as substrate and pooled caecal contents from other two pigs from each treatment were used as inoculum. Ileal substrates showed a higher glucose proportion in LD than in IB in both the soluble (78.5 versus 35.6 g/kg DM; P<0.001) and fibrous (92.9 versus 39.4 g/kg DM; P<0.001) fractions. Differences in soluble glucose were more marked with diet A (interaction breed×diet; P<0.01). Gas production did not differ between breeds, but diet A yielded higher gas volumes throughout the incubation period (P<0.001). The cross-incubation of ileal substrates and caecal contents (i.e. from different diets) within breeds showed that up to 12 h of incubation gas production in IB from substrate M/inoculum M (sM/iM) was the lowest, and from sA/iA highest (P<0.05). No differences were observed when crossing substrates and inocula. In LD, inoculum from diet A produced larger gas volumes than from M up to 12 h. Since gas volumes from the self-fermentation of caecal inocula differed among treatments, fermentation from the cross-incubation was also studied by subtracting the caecal contribution. There were no differences between breeds, except for a higher gas production in IB after 4 h of incubation (P<0.01). Within breeds, both ileal substrate and caecal inoculum from diet A promoted higher gas volumes than M at 4 and 8 h (P<0.01), especially in IB. The in vitro gas production technique may be a useful procedure to study microbial fermentation in the pig caecum, but both the composition of ileal content, largely affected by feed composition, and the contribution of self-fermentation of caecal contents, must be considered.