Influence of particle size of crude olive cake on in vitro rumen fermentation and gas production kinetics = Influence de la taille des particules des grignons d’olive sur la fermentation ruminal et la cinétique de production de gaz

By-products of olive oil industry can be used in ruminants feeding, but their nutritive value can be affected by the processing method. The aim of this research was to estimate in vitro the nutritive value of four samples of crude olive cake (COC) with different particle size obtained during the drying process from the same trommel at different stages of the screening procedure. Particles size was greater than 3 mm (COC3), 2 mm(COC2) and 1 mm (COC1) or smaller than 1 mm (COC0). Alfalfa hay and barley straw were included in the study for comparative purposes. Chemical composition was analysed in all samples. Gas production kinetics was determined in 144-h in vitro incubations with sheep rumen fluid as inoculum, and fermentation parameters and in vitro dry matter digestibility (IVDMD) were analysed after 24 h of incubation. COC3 and COC0 samples hadlower contents of neutral detergent fibre (NDF; 415 and 391 g/kg DM), but greater crude protein (96.6 and 114g/kg DM) and ether extract (220 and 242 g/kg DM) contents, than COC2 and COC3 (627 and 624; 42.4 and48.7; 102 and 124 g/kg DM, respectively). IVDMD of COC samples ranged from 39.4 to 58.7%, and was lower than that for alfalfa hay (67.5%) for all samples, but greater than that for barley straw (42.4%) for COC0 and COC3 samples. COC3 and COC0 samples had greater (P<0.05) potential gas production (103 and 66.2 ml/gDM, respectively) and total volatile fatty acids production (608 and 600 μmol, respectively) than COC2 andCOC1, but values were lower than those for alfalfa hay (202 ml/g DM and 1023 μmol, respectively) and barley straw (198 ml/g DM and 733 μmol). The results indicate that decreasing particle size to <1 mm increased the fermentation potential of COC, but its nutritive value was still slightly lower than that of barley straw. The similar quality observed for COC3 and COC0 samples was attributed to the high sugar content of COC3 samples.