Phenology of Dent Corn and Popcorn. II. Influence of Planting Date on Crop Emergence and Early Growth Stages1

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Abstract
Date of planting and date of crop emergence have been used as reference points for calculating heat sums used in modeling corn (Zea mays L.) phenology. An interactive agroclimatology program and service (AGNET) offered by the University of Nebraska, Lincoln (UNL), accumulates daily heat sums from the time of planting using data generated by an expanding automated network of meteorological stations. The objectives of this research were to evaluate current UNL‐AGNET procedures for modeling corn phenology with special reference to early development and, where necessary, develop and evaluate modified procedures. Time of planting and crop emergence were used separately as reference points for accumulating heat sums calculated using different procedures thereby addressing the impact of slow crop emergence and poor early seedling vigor associated with early planting. Daylength at emergence was included in regression models to investigate the likelihood of bias resulting from a delay in the date of planting. A 2‐yr phenology study incorporating three planting dates and daily and hourly heat sums was conducted in eastern Nebraska during 1982 and 1983. Data generated using staging criteria and regression models currently used in the UNL‐AGNET service were unsuitable for modeling early crop development; alternative procedures were developed using a modified staging criteria and hourly temperature data accumulated from the time of crop emergence. The phenologies of one dent corn cultivar (B73 X Mol7) and three popcorn cultivars (P410, P609, and Iopop 12) were studied using growth curve analysis. Response profiles (growth curves) differed significantly (P = 0.01) among cultivars and among planting dates. Growth curves derived using heat sums calculated using a base temperature of 5°C varied less between years when compared with other procedures for calculating hourly heat sums. A need was demonstrated to (i) account for the effect of planting date in corn phenology models, (ii) accumulate heat sums from either the time of crop emergence or preferably after stem elongation rather than from the time of planting, (iii) develop separate regression coefficients for different cultivars of corn, and (iv) develop hourly rather than daily heat sums when data are available from automated agro‐meteorological networks. Plant development response to day length variation across planting dates and the post‐emergence accumulation of thermal units did not fully account for differences in response profiles developed for early phenological development of four corn cultivars. Photoperiod effects appeared to be confounded with the impact of soil temperature on seedling emergence and growth prior to elongation of stem internodes.