Metabolizable energy content of feeds based on 24 or 48 h in situ NDF digestibility and on in vitro 24 h gas production methods

Abstract: The true dry matter (DM) digestibility (TDMd) and metabolizable energy (ME) contents of feeds are frequently assessed from the amount of digestible neutral detergent fibre (dNDF) measured in situ or in vitro after a fixed time of incubation using a theoretical summative approach. Feed ME content can also be predicted from the gas produced after 24 h of in vitro incubation (GP24) and its chemical composition. Aims of this study were to evaluate the validity of reducing the in situ incubation time from 48 to 24 h for determining TDMd and to study the relationship between the ME values estimated from dNDF and from GP24. Eleven feeds (aNDF: 101-768 g/kg DM) were simultaneously incubated in the rumen of two cows for 24 and 48 h with two replications. Following National Research Council (NRC) guidelines, dNDF measured after 24 and 48 h of incubation was used to compute true DM digestibility (TDMd24 and TDMd48, respectively) and ME based on 48 h (ME(NRC)). From the same feeds incubated in vitro, GP24 was measured by an automated GP system with 4 replications. The in situ and the in vitro incubations were repeated twice. Estimates of ME from GP24 were computed using the Hohenheim (ME(Menke)) and UC Davis (ME(UCD)) equations developed using in vivo digestibility measurements. The ME(NRC,) ME(Menke) and ME(UCD) estimates were compared by regression. The repeatability of TDMd24 and TDMd48 was comparable (SED = 12 g/kg DM) and these values were related with a RMSE of only 21.5 g/kg DM suggesting that, for in situ evaluations, the incubation time can be reduced from 48 to 24 h. The measures of GP24 and TDMd48 were also well correlated (R(2) = 0.97), but only for feeds with < 160 g CP/kg DM. The repeatability of ME(NRC,) ME(Menke,) and ME(UCD,) expressed in terms of SED, were 0.35, 0.46 and 0.46 MJ/kg DM, respectively. The ME(NRC) was related to ME(UCD) and ME(Menke) as: 1.04 x ME(UCD) (RMSE = 0.63 MJ/kg DM) and 1.11 x ME(Menke) (RMSE = 1.16 MJ/kg DM), suggesting that ME(NRC) overestimates ME compared to the GP24 methods developed from in vivo digestibility data. The precision of ME predictions based on GP24 depends on availability of standardized procedures to measure GP, reliable equations relating GP and feed chemical composition (with special regards for crude protein) to predicted or in vivo measured ME content of feeds, and reliable equations which predict ME from feed digestible nutrient contents. (C) 2011 Elsevier B.V. All rights reserved.