Three cannulated, lactating Holstein cows were used to determine rumen protein degradability of steam-flaked soybeans, solvent extracted soybean meal, expeller soybean meal and raw, whole soybeans. The raw whole soybeans and steam-flaked soybeans were ground through a 2-mm screen and all feeds were ruminally incubated in Dacron bags for 0, 2, 4, 8, 16, 24 or 48 h. Rumen undegradable protein was least for raw soybeans, similar for steam-flaked soybeans and soybean meal and greatest for expelled soybean meal (20.7, 28.3, 30.1, and 52.5%; P < 0.01). Intestinally absorbable dietary protein ranged widely with whole raw soybeans being the least absorbable followed by steamed-flaked soybeans, soybean meal and the expeller soybean meal (12.6, 23.6, 29.0 and 51.5%; P < 0.01). Total digestibility of dietary protein was least for the raw whole soybeans followed by the steam-flaked soybeans and was greatest for the soybean meal and expeller soybean meal (92.0, 95.4, 98.9 and 99.3%; P < 0.01). These results indicated that steam-flaked soybeans and the two soybean meals had greater overall digestibility compared to the whole raw soybean.

A second in situ study was conducted to evaluate the effects of particle size of steam-flaked soybeans on rumen degradability. In this study, 4 particle sizes (2mm grind, 4mm grind, coarsely chopped, or whole) were compared. Results indicated that the rumen undigestible protein increased with particle size (30.3, 30.8, 40.8 and 42.2%; P < 0.01). Overall, this study demonstrates that increasing the particle size increases rumen undigestible protein.