The metabolizable energy (ME) content, digestibility in vivo and chemical composition of fermented and urea‐treated whole crop wheat (WCW) forage harvested were measured at different maturities. Over 2 years, twenty‐four wheat forages (cv. Slepjner, Hussar and Cadenza) were harvested at 376, 516 and 632 g dry matter (DM) kg−1 in Year 1 and 341, 467 and 544 g DM kg−1 in Year 2 (Cuts 1, 2 and 3 respectively). Forages were conserved in 200−‐ι barrels with or without a formic acid‐based additive and with urea in Cuts 2 and 3 (20 or 40 g kg−1 DM). Chemical compositions, digestibility in vivo and energy losses in faeces, urine and methane were measured in wethers fed 12 g DM kg−1 live weight. Respective ranges in pH, crude protein, water‐soluble carbohydrates, starch and neutral detergent fibre plus amylase (NDFA) contents were 3·8–8·5, 89–394, 2–43, 23–424 and 306–655 g kg−1 DM. ME content, digestible organic matter content (DOMD) and digestibilities of starch and NDFA ranged between 8·4 and 15·7 MJ kg−1 DM, 558 and 708 g kg−1 DM, 0·901 and 0·999 and 0·362 and 0·693 respectively. Predicted methane energy losses were poorly correlated with measured values (r2 < 0·45) in both years, but this accounted for less than 0.1 MJ ME kg−1 DM. Significant (P < 0.05) effects of maturity on the ME/DOMD ratio were observed and ascribed to differences in the extent of fermentation. Therefore, treatment application and maturity exert considerable influence on the ME content of WCW.