Effects of ensiling on ruminal degradability and intestinal digestibility of Italian rye-grass

The effective degradability (ED) and the intestinal effective digestibility (IED) of dry matter (DM) and crude protein (CP) of a green Italian rye-grass (GRG) crop and its silage (ERG) were determined using in situ and particle passage techniques on three wethers, cannulated in rumen and duodenum. Two rumen incubations with duplicate bags filled with 3 g of freeze-dried samples were performed for each feed at times of 0, 2, 4, 8, 16, 24, 48 and 72 h. On each incubation, one series of bags was used to determine rumen degradation and the other was freeze-dried and pooled for each incubation time. Then two sub-samples (0.2 g) were incubated into mobile nylon bags through the entire intestine of each animal. The microbial contamination of rumen incubated residues (determined with 15N techniques) was fitted to an exponential function. The asymptotic values (m) for DM contamination showed that silage particle reached a greater colonisation (P=0.006) but with a slower rate of microbe accumulation (P=0.03). Values of m for CP also showed that the undegradable CP fraction was mainly of microbial origin (0.745 and 0.870 for GRG and ERG, respectively). Two different ED estimates were established: EDp, considering the particle rumen outflow rate (kp) of the ERG, and EDc,p, considering additionally the fractional rate of conminution and mixing (kc) of these particles. Mean values (and S.E.) of both rates were: 0.0497 ± 0.0113 and 0.181 ± 0.0165 (h−1), respectively. The microbial contamination implied an under-evaluation of EDp and EDc,p for CP: 12.7 and 13.5% in GRG and 11.0 and 12.4% in ERG, respectively. These errors were higher for the associated values of undegraded CP. Thus, a proportion of 0.380 and 0.490 in GRG and 0.429 and 0.538 in ERG of these estimates will be really of microbial origin. No differences for EDp or EDc,p values were shown between CP and total analysed amino acids (assumed as representative of true protein). The interference of microbial contamination of feeds in the rumen was also evident in intestinal disappearance values, which showed an irregular and inconsistent evolution with the increase of the rumen residence time. Ensiling reduced the ED and IED of DM as well as the protein undegraded fraction and its IED.