Phenology of Dent Corn and Popcorn. III. Improved Crop Development Models

Simple temperature-dependent phenological regression models have been used for many years to predict seasonal development for dent corn (Zea mays L.) and sweet corn. However, only limited data are available for popcorn. More complex models have taken into account the effects of photoperiod but seldom planting date variation with respect to additional environmental variables. A recently established network of automated climate stations, reporting daily to centralized locations in the states of Nebraska, Kansas, Colorado, and South Dakota, has facilitated modeling corn phenology using hourly rather than daily data or long-term temperature normals. Soil temperature data is also recorded by these weather stations. An interactive agroclimatology program and service (AGNET) offered by the University of Nebraska, Lincoln (UNL), utilizes some of these data as daily thermal units accumulated from the time of planting. The objectives of this research were to evaluate, and where necessary, modify models used by AGNET after studying the influence of (i) using hourly thermal units accumulated from 50% seedling emergence, (ii) photoperiod effects associated with variations in planting date and seedling emergence, and (iii) using a modified universal growth staging procedure and growth curve analysis. A 2-yr phenology study incorporating three planting dates, one dent corn hybrid (B73 × Mo17), and three popcorn hybrids (P410, P609, and Iopop 12) was conducted in eastern Nebraska during 1982 and 1983. Statistically significant differences were detected among hybrids, among planting dates, and between years for regression coefficients derived from thermal and photoperiod/thermal regression models. Heat sums calculated over a range of temperatures between 5 and 28°C corrected for heat stress tended to be more consistent over the years than those calculated using a range of 10 to 30°C. The maturity rating of some corn hybrids was not constant across all planting dates modeled in terms of thermal units accumulated prior to the crop reaching a particular stage of phenological development. Iopop 12 matured proportionately earlier as planting date was delayed (i.e., fewer thermal units were accumulated prior to physiological maturity). As planting date was delayed the maturity rating of P41O remained unchanged while P609 matured later. The dent corn hybrid B73 × Mol7 first became earlier maturing then later maturing as the date of planting was delayed. A need to account for daylength at crop emergence as well as additional environmental variables associated with changing planting dates and the maturity rating of cultivars was demonstrated.