The objectives of this study were to quantify the effects on nitrogen metabolism and dairy performance of substituting soybean meal for proteaginous seeds or replacing raw with treated proteaginous seeds in dairy cow diets. This study was focused on three proteaginous seeds: faba bean, lupin and pea. Two databases were created, which gathered information on in vivo and in situ results, respectively. These were then used to analyze nitrogen ruminal degradability, nitrogen intestinal true digestibility, ruminal parameters, nitrogen partitioning and milk production and composition. A total of 32 and 36 articles were analyzed from the in vivo and in situ databases, respectively. Statistical analyses assessed the effects of substitution for each measured or calculated variable; t-tests were applied to compare the difference between the tested feed (i.e. raw or treated proteaginous seed, or treated proteaginous seed) and the control feed (i.e. soybean meal, or raw proteaginous seed, respectively). The proteaginous seeds contained less crude protein than soybean meal (on average -199 g/kg DM, P < 0.001). The use of raw proteaginous seeds instead of soybean meal led to higher nitrogen ruminal degradability (on average +16 g/100 g, P < 0.005), but treatment of these seeds led to a decrease in nitrogen ruminal degradability (-13 g/100 g on average) compared to raw seeds. Replacing soybean meal by raw faba bean, lupin or pea in the iso-crude protein diets of dairy cows led to an increase in NH3 in ruminal fluid (+20 mg/L, P < 0.040) and tended to decrease milk protein content. However, when those seeds were treated and their values compared to those of raw seeds, nitrogen in milk tended to increase, but milk fat content was decreased.