The aim of this work was to evaluate in vitro the effect of replacing 0%, 50%, 75% or 100% of cereal-based concentrate in diets based on lucerne hay with feed blocks containing barley grain or 650 g · kg^–1 fresh matter of greenhouse waste fruits (tomato, cucumber, or a 1:1 mixture of tomato and cucumber) on ruminal fermentation, methane production, and bacterial and methanogen population sizes. The type of feed-block showed no effect (p ≥ 0.25). The level of concentrate replacement with blocks did, however, affect (p ≤ 0.042) the pH, CH₄ concentration, organic matter degradation rate, total gas, CH₄ and total VFA production, acetate/propionate and CH₄/total VFAs ratios, and molar proportions of acetate and butyrate, without changing (p ≤ 0.082) methanogen and total bacteria abundance. Increasing levels of concentrate replacement with feed blocks modified ruminal fermentation, dry matter and neutral detergent fibre digestibility, and had an antimethanogenic effect.