The objective of this study was to evaluate the effect of plant population and hybrid relative maturity on yield, nutrient composition and ruminal in vitro NDF digestibility at 30 h (ivNDFD) in whole-plant corn forage. Five hybrids varying in relative maturity (115 [M115], 118 [M118a, M118b], 124 [M124] and 130 [M130] days) were each planted at 2 different plant populations (60,000 [60K] or 70,000 [70K] plants/hectare) during the summer, in quadruplicate. Samples were analyzed via NIRS at Dairyland Labs (Arcadia, WI). Data were analyzed as a completely randomized design using Proc Glimmix of SAS with the Fixed effects of hybrid, plant population, and their interaction. A plant population × hybrid interaction was observed (P = 0.001) for DM content, with greater values observed for 70K than 60K on M118a (33.9% vs. 32.1%) but the opposite on M118b (28.6% vs. 31.1%). Despite the greater (P = 0.02; 8.9% vs. 8.3%) CP content for 60K than 70K, plant population did not affect (P > 0.10) ADF, aNDFom, and starch concentrations. Greater starch (40.0% vs. 26.3% on average) but reduced CP (7.5% vs. 8.0% on average), ADF (22.7% vs. 28.8% on average) and aNDFom (36.0% vs. 44.7% on average) concentrations were observed (P < 0.01) for M118a than other hybrids. Increasing plant population from 60K to 70K tended (P = 0.08) to increase ivNDFD by 1.5%-units. This is related to reduced lignin (P = 0.02; 4.5% vs. 4.0%) and a trend for decreased uNDFom240 (P = 0.09; 14.3% vs. 13.0) concentrations for 70K. Yield of DM, milk/t and milk/ha were unaffected by plant population and averaged 13.2 t of DM/ha, 1263 kg of milk/t of DM, and 13952 kg of milk/ha, respectively. In addition, M118a tended to have greater ivNDFD (P = 0.06; 51.1 vs. 47.4% of NDF on average). This is related to the 1.1%- and 4.0%-unit lower (P < 0.01) lignin and uNDFom240 concentrations, and led to enhanced (P = 0.01) milk/t and milk/ha estimates. Yield of DM was 1.2 t/ha greater (P = 0.01) for M130 than other hybrids, on average. Plant population slightly improved NDF digestibility whereas hybrids varying in relative maturity affected yield and quality of whole-plant corn forage.