The objective of this study was to investigate the effects of heat treatment (toasting) and particle size alterations (grinding; rolling) on nutrient utilization, ruminal fermentation, and supply of metabolizable protein (MP), and to study the interaction between processing conditions of fava beans and forage type. Six Danish Holstein dairy cows fitted with ruminal, duodenal, and ileal cannulas were used in a 6 × 4 incomplete Latin square design with 4 periods of 21-d duration. Cows were fed ad libitum with 6 experimental diets: diets high in either grass-clover silage or corn silage were combined with ground untoasted, ground toasted, or rolled untoasted fava beans. Samples of ruminal fluid, digesta from duodenum and ileum, and feces were collected, and nutrient digestibility was estimated using Cr2O3 and TiO2 as flow markers. Diets high in corn silage resulted in higher ruminal pH and higher proportion of propionate in ruminal volatile fatty acids compared with diets high in grass-clover silage. Diets high in corn silage resulted in higher apparent total-tract digestibility of crude protein and starch but lower apparent ruminal and total-tract digestibility of neutral detergent fiber compared with diets high in grass-clover silage. Rolling of fava beans decreased the in situ small intestinal disappearance of rumen-undegradable protein corrected for particle losses. Compared with grinding, rolling of fava beans reduced apparent ruminal digestibility of starch, true ruminal digestibility of organic matter, crude protein, and AA, and small intestinal digestibility of AA and starch. Grinding of fava beans increased apparent ruminal digestibility of neutral detergent fiber and reduced the proportion of propionate in ruminal volatile fatty acids compared with rolling of fava beans. In addition, rolling of fava beans had no effect on MP supply. Toasting of fava beans had no effect on in vivo nutrient digestibility except for an interaction with forage source on apparent ruminal dry matter and organic matter digestibility. Toasting of fava beans did not affect small intestinal digestion of individual and total AA, and therefore failed to increase MP supply. In conclusion, neither replacing grass-clover silage with corn silage, nor toasting nor rolling of fava beans had an effect on supply of MP.