Improving efficiency and reducing environmental impacts pose challenges to Mediterranean livestock systems, with few viable alternatives to conventional forages. This study evaluated cucumber–straw silage (SIL) as an alternative forage to oat hay (HAY) for lamb production using integrated in vitro , in vivo , and life-cycle assessment approaches. An in vitro fermentation study was conducted by progressively replacing concentrate with SIL or HAY, and rumen fermentation, and CH 4 production were m