assessment of required growing degree days for phenological stages of four clover species in different planting dates

to assess required growing degree days (gdd) for phenological stages of four clover species in different planting dates a field experiment was conducted in 2010-11 and 2011-12 cropping seasons in seed and plant improvement institute in karaj, iran. three planting dates (14 september, 28 september and 13 october) and six clover varieties were assigned to main plots and sub-plots, respectively, as split plot arrangement in randomized complete blocks design with four replications. results showed that delaying planting date decreased the required growing degree days for forage production in clover species by 8.2% and 14.7% in second and third planting dates, respectively. average growing degree days showed that for three cutting of forage in karaj region about 2437 gdds are required. result also revealed that the greatest gdds (612.7) was required from emergence to vegetative growth stage and the least gdds (82.1) belonged to from emergence to the appearance of first simple leaf stage. comparison of clover species showed that red clover with 2994.9 gdds and crimson clover with 1107.7 gdds had the greatest and least thermal requirements for forage production, respectively.