Effects of olive leaf extracts on nutrient digestibility, rumen fermentation, blood parameters, and antioxidant capacity in sheep: an in vitro and in vivo study

The study aimed to investigate the effects of olive leaf extract (OLE) on ruminal fermentation and nutrient digestibility in Akkaraman sheep. In the in vitro phase, ruminal inoculum was incubated with various doses of OLE containing from 100 to 400 ppm of oleuropein. In in vivo phase, sheep were assigned to three treatments and fed basal diet without oleuropein (control), basal diet was supplemented with 8.2 g and 16.4 g of OLE per sheep per day to provide 200 and 400 ppm of oleuropein (OLE200 and OLE400), respectively. Rumen and blood parameters were measured at various intervals post-feeding in the in vivo trial. Increasing doses of OLE significantly affected in vitro nutrient digestibility, though values fluctuated between doses. The in vivo study showed that OLE200 and OLE400 had no effect (P > 0.05) on overall nutrient digestibility compared to the control, except for a reduction in crude protein digestibility. The acetate-to-propionate ratio increased in both OLE200 and OLE400. In addition, OLE400 led to a significant rise (P < 0.05) in butyrate concentration. Although ammonia-nitrogen (NH3-N) were lower in OLE200 compared to the OLE400, but ruminal pH and protozoa counts remained unchanged. Triglyceride, total antioxidant status and malondialdehyde were increased in both treatments. The OLE supplementation did not improve (P > 0.05) overall nutrient utilization and negatively affected some fermentation parameters at higher doses. These results suggest that although OLE possesses potential as a feed additive, its effective dosage must be carefully optimized to prevent negative impacts. Further studies are needed to establish effective inclusion levels that ensure beneficial outcomes.