The aim of this study was to reveal the effect of whey powder (WP) supplementation on chemical composition, silage fermentation, in vivo digestibility and net energy lactation (NEL) value of high dry matter (DM) alfalfa silage (Haylage). Alfalfa treated with different doses WP (0, 2, and 4%) were ensiled in plastic cans for 60 days. Nine Kıvırcık rams were used to estimated digestibilities of haylages in 3x3 Latin Square design with fifteen days experimental period. According to the results, the addition of WP significantly increased the DM of alfalfa haylage compared to the control whereas decreased ether extract, crude protein and crude ash. But, cell wall fraction rates of all groups were similar. Relative feed value and physical characteristics other than color improved in WP supplemented groups, and lactic acid rates increased in these groups. In terms of silo acids, the best values were found in the group with 4% WP. On the other hand, the addition of WP increased the organic matter digestibility of haylages, however the effect on other in vivo digestibility of crude protein, crude fiber, nitrogen free extract and neutral detergent fiber was insignificant. Moreover, total digestible nutrients (TDN) and NEL values of haylages were found to be improved with the addition of WP. As a result, it was concluded that in order to obtain quality haylage in the ensiling of high DM and long chopped alfalfa, WP generally can be used as an alternative source of carbohydrates and a 4% whey powder dose can be recommended in the production of haylage especially in the sense of improving silage fermentation.