Adequate kernel breakage and sufficient particle size are desirable in corn silage (CS). Most Brazilian dairy farms use one-row forage harvesters. For such machines, a kernel processor (KP) was developed (MU9000462–0 U2 INPI patent). We evaluated the effect of theoretical length of cut (TLOC) and KP on CS particle size and kernel damage. Corn (Dow 2B587 Hx) was harvested at half milk line maturity (36.4% DM, 52.1% NDF, 30.0% starch). Treatments were a factorial combination of TLOC (3, 4.5, 6, 8.5 mm) and KP (with or without), and 50 m rows were harvested in triplicate. Statistical model had effects of TLOC, KP, and their interaction. Particle size distribution was measured with the Penn State Particle Separator. Visible kernels were quantified and classified in extreme (E), poor (P) or intact (I) breakage. Kernel in situ degradation in 12 h was 24.4% for E, 12.6% for P, and 11.5% for I, in 24 h 39.5%, 26.2%, and 23.6% (P < 0.05 E vs. P and E vs. I), and the residue of 72 h 25.2%, 39.4%, and 48.3% (all differ at P < 0.05), respectively. The KP reduced visible kernels in forage (44.2 vs. 70.5 g/500 g forage, P < 0.01) and the proportion of P+I (23.2 vs. 41.0 g/500 g forage, P < 0.01). The TLOC 3 mm without KP (traditional in practice) had more visible kernels (56 g/500 g forage) than 8.5 mm with KP (44 g/500 g forage). The reduction in visible kernels with smaller TLOC was larger when KP was not used (P < 0.01 for interaction). The proportion of particles below the 8 mm screen was 22 and 28% of fresh forage for TLOC 6 and 8.5 mm with KP, respectively, and 35% for 3 mm without KP. The TLOC 3 mm without KP had smaller proportion of starch below the 8 mm screen (21% of starch) than all TLOC with KP (29 to 35% of starch). The TLOC 3 mm without KP had 72% of NDF greater than 8 mm and 6 and 8.5 mm with KP had 79% and 82%, respectively. Although KP reduced NDF particle size, the reduction was not as large as the reduction induced by small TLOC. Reducing TLOC was more effective in reducing NDF particle size when KP was not used (P < 0.01 for interaction). Large TLOC with KP resulted in forage with longer NDF and increased kernel damage than small TLOC without KP, which is nutritionally desirable.