Grape Stalks as a Sustainable Feed Supplement for Dairy Cows: A Preliminary In Vivo Study on Milk Microbiota and Cheese Quality

In the Trentino Alto Adige region, disposal of grape stalks (GS) represents a major cost for wineries, despite their content of phenolic and tannin-rich compounds with potential functional value in ruminant nutrition. This study evaluated whether dietary GS supplementation could influence milk microbiota and cheese quality, supporting local circular-economy strategies. GS from three red cultivars (L-GS, CS-GS, M-GS) were dried, milled, and assessed for safety; their average total polyphenol content was approximately 15 g/kg DM. 3 Holstein cows underwent a 7-week trial consisting of alternating control (CTRL) and GS-supplemented periods (2% DM). Weekly milk samples (n = 21) and corresponding mini-cheeses (n = 21) were analyzed. GS supplementation did not affect milk coliforms (2.3–2.5 log CFU/mL), while total mesophilic counts were significantly lower in M-GS (2.8 ± 0.46 log CFU/mL) than in CTRL (4.5 ± 0.71; p < 0.05). Acinetobacter dominated the milk microbiota but decreased from 34.0% in CTRL to 18.0% in L-GS. Cheese total polyphenols were highest in CS-GS (224 ± 34 mg/kg). 1H-NMR and VOCs profiling indicated cultivar-dependent shifts in carbohydrate-related metabolites and short-chain fatty acids. Overall, GS supplementation subtly modulated cheese biochemical and aromatic traits without impairing technological performance, supporting the valorization of winery by-products within integrated dairy-viticulture systems.