Effect of polyethylene-glycol on the chemical composition and nutrient availability of olive (Olea europaea var. europaea) by-products

Total extractable polyphenols (TEP), total extractable tannins (TET), total extractable condensed tannins (TECT) and total condensed tannins (TCT) were quantified in two very important olive by-products: the two-stage dried olive cake (TSDOC) and the olive leaves (OL). The effect of polyethylene-glycol (PEG) addition (0, 20, 60, 100 and 200 g/100 g by-product) on the chemical composition, in vitro digestibility and in situ degradability was studied. The effect of PEG addition (0, 2 and 20 g/100 g by-product) to TSDOC and OL on the in situ–in vitro intestinal availability of both rumen undegraded and dietary protein was also studied using different approaches: (i) apparent intestinal digestibility of ruminal undegraded protein (AIDUP) or amino acids (AIDUAA); (ii) apparent intestinal digestibility of dietary protein (AIDDP); (iii) ruminally degradable and intestinally digestible dietary protein (100−ADF-N). Results obtained in Granadina goats and Segureña sheep were compared. The nitrogen fraction attached to acid detergent fibre (ADF-N) decreased significantly (P<0.01 and <0.001 for TSDOC and OL, respectively) by PEG addition. A significant effect was observed in TEP ( P<0.05 and <0.01 for TSDOC and OL, respectively), TET (P<0.001 for both TSDOC and OL) and TECT (P<0.001 for both TSDOC and OL). A significant decrease in cell wall components was only observed in OL treated with PEG. The in vitro digestibility, which was significantly higher when rumen liquor from goats was used in comparison with sheep, especially for TSDOC, significantly increased by the PEG addition. This effect was observed in both, TSDOC (from 0.202 to 0.328 IVOMD) and in OL (from 0.349 to 0.446 IVOMD). The effect of the animal species on the characteristics of the ruminal degradation of TSDOC and OL was variable and concerning effective degradability (ED) it was significantly higher in goats than in sheep (0.490 and 0.424, respectively) for OL protein. The effect of PEG addition was more pronounced in TSDOC than in OL. All the parameters of the TSDOC rumen degradation profiles increased significantly with increasing amounts of PEG (from 0.435 to 0.765 for ED of TSDOC protein). On the contrary, only the ED of OL dry matter increased (P<0.01) by the addition of 2 g of PEG (from 0.535 to 0.582). The availability of ruminal undegraded and dietary protein in the small intestine, estimated in situ–in vitro with different approaches, significantly increased by the addition of two grams PEG/100 g by-product, while a further increase in the amount of PEG resulted in a decrease of intestinal availability. Although low tannin content in TSDOC and OL, addition of 2 g of PEG/100 g of by-product improved nutrient availability either at the rumen and intestine level. The effect was more important for sheep than goats.