Introduction
The limited availability of groundwater in Hamedan Province, along with the short growth period and lower water requirements of sweet corn varieties (compared to late-maturing grain corn), has encouraged the expansion of sweet corn cultivation in the region. The super sweet corn produced in Hamedan is considered one of the most suitable in Iran for industrial processing such as canning or freezing, due to the region’s favorable summer climate – characterized by warm days and moderate nights-which contributes to a high grain extraction percentage from green ears. Planting date significantly affects the phenological development of super sweet corn, thereby affecting the yield components and ultimately the grain and green ear yields. This study aims to evaluate the effects of different planting dates on the phenological and agronomic traits of various super sweet corn cultivars, with a focus on grain and green ear yields as well as their suitability for industrial food processing.
Materials and Methods
A two-year field experiment (2020-2021 and 2021-2022) was conducted at Ekbatan Station of Agriculture and Natural Resources Research and Education Center in Hamedan province, Iran. The experimental design was a split-plot arrangement. Planting dates (9 May, 4 June, and 30 June) were assigned to the main plots, while cultivars (Basin, Messenger, Sheba, Passion and Shaker) were allocated to the subplots. At harvest time (milky grain stage), the green ear yield per unit area was recorded. After separating the grain from the cob, the canned grain yield and the grain-to-green ear weight ratio (percentage of grain extraction) were calculated. In addition, the percentage of Total Soluble Solids (TSS) in the grain was measured using a refractometer. A combined analysis of variance (ANOVA) was performed across the two years, and the effects of planting date and cultivar on the mean of each trait were compared using the Least Significant Difference (LSD) test at the 5 % probability level.
Results and Discussion
According to the results, the highest values of Growth Degree Day (GDD) to pollination were observed in the cultivars Shaker (713.4 °C), Messenger (698.2 °C) and Basin (688.7 °C).  Among the planting dates, the earliest date (9 May) recorded the highest GDD to pollination with 708.5 °C. However, the highest GDD values ​​ to ear harvest were observed from the 4 June planting date. In this planting time, the cultivars Messenger, Shakar and Basin demonstrated superior performance with 1212 °C, 1195 °C and 1162 °C, respectively. Regarding the number of rows per ear, the second planting date (4 June) showed a significant advantage with an average of 15.17 rows, outperforming the other planting dates. Among the cultivars, Messenger (14.95), Shaker (14.74), and Basin (14.59) exhibited significantly higher values than the others. Similarly, in terms of the number of grains per row, ​all cultivars achieved their highest values under the second planting date. The Basin cultivar was significantly superior with 44.58 grains per row. The high values for both the number of ear rows and grains per row - key components of grain yield – contributed to the Basin cultivar achieving the highest green ear and grain yields (38854 and 15922 kg ha^-1, respectively) under the second planting date. Additionally, the highest TSS content (23.11%) was recorded under the third planting date (30 June), although this value was not significantly different from that of the second planting date (22.32%). Among cultivars, Messenger (24.37), Sheba (22.38) and Basin (22.17%) showed significant superiority in TSS content compared to the other cultivars.
Conclusion
In the Hamedan region, where late spring and early autumn frosts are common, selecting an appropriate planting date for sweet corn is crucial. Based on the results, the most suitable planting period for super sweet corn in Hamedan province is the first ten days of June. Delaying planting until the third ten-day period of June is not recommended, as it leads to a significant reduction in both green ear and grain yields. Furthermore, due to its high green ear and grain yields, along with acceptable levels ​​of TSS content, the Basin cultivar is recommended for cultivation in Hamedan and other areas with similar climatic conditions.