Effect of the length of various developmental periods on soybean yield and yield components

To investigate the relationship between the critical period of development (VC-R1، R1-R6 and R6-R8) with grain yield and yield components of soybean, a field experiment were conducted with a split plot arrangement in a randomized complete block design in three replications in Gorgan during 2011. Main plot were including five planting dates (April 20, May 13, June 3, June 29 and July 22), and sub plot were including three cultivars (Williams, Sahar and DPX). Photoperiod in each of the developmental stages was calculated by using of the PP-calc program. Then, In Order to eliminate the effects of temperature on growth and development, the number of Cumulative thermal days in each period of development were considered as duration of developmental stages. Decreasing in photoperiod that occurred as a result of the delay in planting, Reduced the duration of VC-R1, R1-R6 and R6-R8 in all three cultivars. Significant positive relationship observed between yield and yield components (except grain weight) and duration periods of VC-R1 and R1-R6. So that the reduce of this developmental duration in the late planting dates, reduced grain yield and yield components. However, The length of R6-R8 no effective on grain yield, because the Duration of this period it may be only effective through grain weight on grain yield. but, no observed a positive significant correlation between seed weight and seed yield. So, the cause of soybean yield losses that occured in delayed planting dates, were due to the reduce of duration of VC-R1 and R1-R6.