The accuracy of several in vitro methods in estimating in vivo digestibility of the tropical dairy ration

In vitro digestibility methods have been developed to overcome problems in the in vivo digestibility measurement, but its accuracy should be tested in a local setting. In vitro methods developed by Tilley and Terry (T2), Theodorou (T3) and Sutardi (T4) have been compared to in vivo method (T1) in a block randomized design study. Four heifers FH (337.50 ± 45.87 kg BW) were used in T1, and two fistulated FH bulls (510 ± 20 kg BW) were used as inoculant sources in the in vitro methods. Dairy cattle ration consisted of 54.0% Napier grass and 46.0% concentrate with 58.8% DM, 12.1% ash, 10.0% CP, 3.3% EE, 26.5% CF, and 61.1% TDN. The observed parameters were ration fermentability (pH, NH3, and VFA concentration) and digestibility (DMD and OMD). The data were analyzed using analysis of variance (ANOVA) followed by the Tukey test. The correlation was made before regression analysis to estimate the in vivo parameters from the in vitro. The results showed that pH values are in the normal range (6.7 – 6.8), and insignificantly different between treatments (P>0.05). The concentration of NH3 and VFA were significantly different between the treatments (P<0.05), but T2 produced similar NH3 and VFA concentrations to T1. Similar results were also found in the DMD and OMD. Correlation analysis showed that pH value of T3 correlated significantly with T1, while DMD value of T4 correlated to T1. The T1 DMD (Y) could be estimated from T4 DMD (X) using formula Y (%) = y = -0.091x² + 9.1632x - 168.4. It is concluded that tropical dairy feedstuffs in vitro digestibility using Tilley and Terry's method produced similar result to in vivo digestibility method, but in vivo dry matter digestibility can be estimated accurately by in vitro dry matter digestibility using Sutardi method.