Digestibility of forage silages from grain, sweet and bmr sorghum types: Comparison of in vivo, in situ and in vitro data

Three forage silages from grain (G), sweet (S) and bmr (B) sorghum types were evaluated in terms of chemical composition: ash, CP, aNDF, ADF and lignin(pm), in vivo digestibility of DM, OM and aNDF, in situ DM disappearance at 0, 3, 6, 9, 12, 24, 48 and 72 h of incubation and in vitro apparent DM and aNDF digestibility at 24 and 48 h of incubation. Forage silages did not differ in chemical composition, except for lignin(pm) content, which was lower in S than in G and B. The in vivo digestibility of DM (0.517) and OM (0.575) did not differ among forage silages, but the in vivo aNDF digestibility was higher in S (0.533) than in B and G (average 0.453). The in situ degradable fraction and the DM degradation rate were also higher in S than in the other two forage silages. The in vitro apparent DM digestibility at 24 h of incubation was the only indirect methodology that matched the corresponding in vivo data in all silages. In situ DM disappearance agreed with the in vivo DM digestibility at 24 h of incubation in B and G, but in S this agreement occurred at 12 h of incubation. In vitro incubation for 24 h underestimated of the in vivo digestion of aNDF in all forage silages. In vitro aNDF digestibility at 48 h of incubation agreed with in vivo data in G and B, but overestimated it in S. Although forage silages did not differ in DM or OM in vivo digestibility, the forage silage from the sweet genotype had superior in vivo aNDF digestibility. Over all forage silages, only the 24 h in vitro apparent DM digestibility consistently provided accurate estimates of the in vivo DM digestibility. In general, the capability of in situ and in vitro methodologies to accurately predict in vivo data depended upon incubation length, which was variable across feedstuffs, suggesting that caution should be exercised when using these techniques to estimate the digestibility of different feedstuffs at fixed incubation lengths.