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Abstract 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