Efficiency of Digestate as Inoculum for in vitro Digestibility of Feeds

This experimental study was carried out from march to may 2022 to study digestate as inncoulum source for invitro studies. Most of digestibility studies utilize the rumen fluid (RF) as inoculum. The in vivo methods use live fistulate animals, which make them laborious and difficult. The in vitro methods are cheaper and less time-consuming, and the RF can be sampled from slaughtered animals. However the microbial composition of RF is largely depending on the diet fed to the animal, and the sampling and the storage procedure of the RF can influence the viability of rumen microorganisms and affects the repeatability of results. For the in vitro studies, many laboratories use the Ankom DaisyII incubator, a specific apparatus with a thermostatically controlled chamber (at 39 °C) containing four rotating digestion jars that simulate the rumen environment. Rumen fluid (RF) or feces are generally used as inoculum in the Ankom DaisyII incubator. The use of the digestate (D) would be immensely beneficial for researchers who have limited or difficult access to RF; moreover, its increasing availability, easiness in sampling, a more constant chemical composition than the rumen fluid and the possibility to store it for long times could be interesting for future in vitro digestibility trials. Aim of this study was to perform a preliminary test on the potential efficacy of the digestate as inoculum for digestibility trials with the Ankom DaisyII incubator. In this trial, the D has been tested at various incubation times and the results were compared with those obtained with the rumen fluid. The Apparent Dry matter digestibility (ADMD), in vitro True Digestibility (IVTD) and Neutral Detergent Fibre digestibility (NDFD) were determined on 9 Total Mixed Rations (TMR) samples, following the procedure of Tassone et al. (2020). The incubation time with the RF was 48 hours, while with the digestate 4 times have been tested: 48 h (D48), 60 h (D60), 72 h (D72) and 96 h (D96). TMR samples were analyzed for dry matter (DM), ash, crude protein (CP), neutral detergent fiber (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL) and ether extract (EE) according to AOAC methds. Based on the NDF (%DM) amount, samples were divided into 3 groups: A (NDF < 41%), B (42% < NDF < 61%) and C (NDF > 62%). Two digestibility runs were performed; for each run, samples of TMRs (0,05 g) were weighted in triple into F57 Ankom filter bags. The data were analyzed with the SAS - GLM procedure and the Tukey-Kramer test for the comparison between different inoculum and incubation times. Results showed that, for ADMD, no differences were observed between RF or D at different times for Groups A and Group C samples, but for Group B, the RF values were statistically lower from D60, D72 and D96. Regarding IVTD, no differences were observed between RF ad D, with the exception of D96 of Group B. The same result was observed for NDFD of Group B. In conclusion, the digestate proved to be an efficient substitute of the rumen fluid, and a 48 hours incubation time with the digestate or with the rumen fluid generally showed similar results, suggesting that longer times of incubation (60, 72 and 96 hours) with the digestate might not be necessary. However the exceptions observed in Group B for ADMD, IVTD and NDFD (probably due to the amount of and type of fiber of this diets) need a deeper investigation.