Grass silage is the main forage source during indoor feeding periods in many temperate regions. The digestibility and energy concentration of silage are mainly determined by the maturity at harvest, but changes in carbohydrate and nitrogen fractions during fermentation process can markedly modify the profile of nutrients absorbed from the digestive tract and also influence intake potential. This chapter reviews research on factors affecting nutrient supply from the digestive tract in animals fed a grass silage–based diet. The chapter examines the effects of energy and protein supplementation on production responses, taking into account nitrogen and methane emissions. The chapter highlights recent developments in the big bale system that has become popular especially in Northern Europe and looks ahead to future trends in this area.