An extruder (20) is specifically designed for the production of animal feed products (e.g., aquatic feeds) containing substantial quantities of low-cost fibrous materials, such as rice byproducts, at high production rates. The extruder (20) includes an elongated barrel (22) with a screw assembly (24) within the barrel and an endmost extrusion die assembly (34). The screw assembly (24) includes an inlet screw assembly (42) and a processing screw assembly (44). The assembly (44) includes screw components (50-56) of differential pitch to present a long pitch inlet section (64) and a tight pitch discharge section (68). Materials passing through the screw assembly (24) are successively subjected to high levels of steam injection (STE) followed by high levels of friction and shear (SME), so that the STE/SME ratio is at least about 6/1.