The objective of this study was to evaluate the effect of plant population, hybrid relative maturity, and cutting height on yield, nutrient composition and ruminal in vitro starch (ivSD; 7 h incubations on dried and 4 mm ground samples) and NDF (ivNDFD; 30 h incubations on dried and 1 mm ground samples) digestibility in whole-plant corn forage. Eight hybrids (4 early [ER; 94–99 d] and 4 late [LT; 105–108 d] maturity) were each planted at 4 different plant populations (64,000 [64K], 79,000 [79K], 94,000 [94K] or 109,000 [109K] plants/ha), in triplicate, and harvested at 2 cutting heights (15 [LO] or 60 [HI] cm). Data were analyzed as a split-plot design using Proc Mixed of SAS with the fixed effects of maturity, plant population, cutting height and their 2- and 3-way interactions. Hybrid and plant population × maturity × hybrid interaction were random effects. Except for greatest CP content for 64K and lowest for 109K (P = 0.001) or a trend (P = 0.09) for 1.3%-units greater ivNDFD for 94K than 64K, no effects of plant population on nutritive value were observed (P > 0.10). Yield of DM (t/ha) was (P = 0.001) greatest for 109K and 94K, and lowest for 64K. Milk production was reduced (P = 0.001) by 2248 kg/ha, on average, for 64K than other treatments. Hybrid maturity did not affect (P > 0.10) any parameter. Reduced (P = 0.001) ADF (21.4 vs. 24.6%), NDF (33.8 vs. 37.7%) lignin (3.2 vs. 3.7%) and undigested NDF after 240 h of incubation (10.5 vs. 12.4%) concentrations and corresponding increased ivNDFD (52.7 vs. 49.6% of NDF) were observed for HI. Furthermore, HI had greater starch content (P = 0.001; 41.7 vs. 37.5%) but not digestibility (P = 0.13). A hybrid maturity × cutting height interaction was observed (P < 0.05) for DM yield/ha and milk production/ha. Although HI had lower DM yield than LO for both maturities, the magnitude of the difference was exacerbated in LT hybrids (2.56 vs. 1.54 t of DM/ha). This led to a reduction of 1203 kg of milk/ha for HI in LT but not ER hybrids. Plant population affected yield but not quality of whole-plant corn forage. In addition, cutting height increased nutritive value but at the expense of reduced yield.