Sustainable Valorization of Grape Pomace in Sheep Through Systemic Health, Metabolic Safety, Milk and Meat Quality

Grape pomace (GP), a by-product of winemaking, is rich in polyphenols and fiber, making it a promising and sustainable feed supplement for ruminants. This study evaluated the safety and productive impact of a 5% GP-supplemented diet (GP5) including non-lactating end-cycle (EC) ewes regularly destined for slaughter and human consumption, and lactating (LAC) ewes, over a 30-day period. Control (CTRL) animals received a standard pellet diet with no GP inclusion. Sampling was performed at four time points (T0, T10, T20, and T30), corresponding to days 0, 10, 20, and 30 of the experimental period. The study assessed clinical status, hematology/biochemistry (T0 and T30), milk composition (T0, T10, T20, and T30), meat quality traits and oxidative stability in EC ewes (T30). Since no significant differences were observed in the CTRL animals, the effects were evaluated within the GP5 group by comparing T0 vs. T30. Meat quality was assessed by comparing EC-GP5 to CTRL at T30. The GP extract showed a high total phenolic content (254.02 ± 20.39 mg GAE/g DW). No clinical or hematological alterations were observed, and most values remained within physiological ranges. Biochemical analysis revealed significant increases in albumin, bilirubin, creatinine, and triglycerides (p < 0.05), with significant decreases in plasma urea and glucose (p < 0.05). In LAC-GP5 ewes, milk urea and lactose concentrations decreased (p < 0.05), while pH increased (p < 0.05), with no significant changes in fat or casein content. These findings are consistent with reduced ruminal propionate availability, leading to decreased hepatic gluconeogenesis and lactose synthesis, with secondary effects on nitrogen metabolism and the acid-base profile of milk. In EC-GP5 ewes, meat quality traits were unaffected, and DPPH scavenging activity did not differ from CTRL (p > 0.05). GP5 was metabolically safe, induced adaptive changes in milk composition, and had no negative effects on meat quality, supporting the valorization of grape pomace as a sustainable feed resource. This trial was designed as a metabolic safety assessment, representing a preliminary step toward future mechanistic and molecular investigations.