Alfalfa inclusion rates in dairy cow diets are decreasing, often with a concomitant increase in corn silage. This article discusses how soy-based ingredients fit into diets containing lower levels of alfalfa and higher levels of corn silage. The data and conclusions in this review are derived from peer-reviewed journal articles. In many areas, corn silage is becoming the primary forage in dairy diets by replacing alfalfa. Replacing alfalfa with corn silage increases the need for supplemental protein. Soy ingredients are efficient sources of MP. The RDP fraction in soy is almost exclusively amino nitrogen, which is efficiently used by microbes, and the RUP is highly digestible. As the concentration of alfalfa decreases and the concentration of soy protein increases, less CP should be needed to meet the MP requirements. As corn silage replaces alfalfa, negative responses to greater starch concentrations increase. Soyhulls are highly digestible and contain little starch, which makes them a good replacement for starch in diets high in corn silage and low in alfalfa. Diets with little or no alfalfa that include soy products can produce similar amounts of milk and milk components using less starch and CP than diets with greater amounts of alfalfa.