Maize breeding programs have indirectly altered many plant traits; however, our knowledge of some important phenological traits remains unexplored. One such trait is leaf appearance rate, which is crucial for predicting maize development. We studied 40 short-season (103-day) and 38 long-season (111-day) hybrids released from 1980 to 2020 by Bayer Crop Science. Measurements included weekly counting of collared leaves across 13 experiments in the US Corn Belt. The progression of leaf number was expressed as a function of thermal time and described with a trilinear model. Results indicated that new 111-day hybrids produce leaves faster than old hybrids throughout the vegetative phase (7.4% and 3.1% faster before and after the ninth leaf stage, respectively), whereas new 103-day hybrids produce leaves faster only after the ninth leaf stage (9.4%). Thermal time to silking and anthesis decreased by about 1 and 0.56°C day year−1, respectively. Our data revealed that silking and anthesis can precede the final collared leaf by 96°C day (3.3 days under optimal conditions), which indicates an overlap between vegetative and reproductive phases. We concluded that maize breeding has indirectly altered the rate of vegetative development of maize hybrids without affecting the final leaf number. Present results expand our knowledge base on the genotypic variability in maize development traits, which can improve empirical and process-based models used for crop stage and yield prediction.