The Effect of Planting Date on Thermal Indices and Dry matter Yield of Different Clover Species

This study aimed to investigate the changes in thermal indices of various clover genotypes under cold stress. A field experiment was conducted to evaluate the effect of different planting dates (14 Sep., 28 Sep., and 8 Oct.) and clover genotypes (late-maturity Persian clover, mid-maturity Persian clover, early-maturity Persian clover, berseem clover, red clover, and crimson clover). The results showed that delaying the planting date from September 14 to October 8 caused a significant decrease in growing degree-day (GDD), photothermal index (PTI), and heat use efficiency (HUE) in all studied genotypes. Early-maturity Persian clover and crimson clover had the lowest thermal requirements, while red clover had the highest GDD in all the investigated planting dates. During the first cut, the early-maturity Persian clover demonstrated the highest HUE (4.09 kg ha-1 °C days), followed by crimson clover. In contrast, red clover recorded the lowest HUE (1.43 kg ha-1°C days) on the last planting date. Early-maturity Persian clover and crimson clover may be preferred for forage production under cold stress due to their higher HUE values. The highest dry matter yield of the first cut (6300 kg ha-1) was obtained on the first planting date and by mid-maturity Persian clover, while the lowest yield (2429 kg ha-1) was obtained on the last planting date and by red clover. Overall, delayed planting dates resulted in accelerated development and decreased thermal requirements in clover species. The early-maturity genotypes were found to be more suitable for forage production under environmental stresses such as water shortages.