Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Planting soybeans between early May and early June provides the highest yield. Planting either before or after this optimal time can significantly reduce yields. Most of the yield from determinate soybeans comes from the branches. Branch development and yield are reduced at nonoptimal planting dates, while main axis yield is not. The relatively short photoperiods of nonoptimal planting dates appears to cause the reduction of both the vegetative and reproductive growth periods resulting in reduced development and yield. The objectives of this dissertation were to: Characterize the development of branches in relation to that of the main axis at different planting dates, determine the relative influence of pre-flowering and post-flowering photoperiods on branch development and determine methods to increase the development of branches at late planting dates. Data were collected on the main axis and on each individual branch biweekly from prior to R1 through R5 for two growing seasons. Results indicated that each branch develops differentially from the others. Total branch nodes develop in a linear fashion from approximately 32 days after planting. With a late nonoptimal planting date, branches fail to develop at the lowermost and uppermost main axis nodes. Those branches provide over 40% of the total branch pods at an optimal planting date under good growing conditions. Studies conducted in environmentally controlled chambers with combinations of optimal and nonoptimal photoperiod simulations showed that both pre-flowering and post-flowering photoperiods affect branch development. The combination of an optimal pre-flowering and post-flowering photoperiod is necessary for maximum branch development. Branch development is positively correlated with a long reproductive photoperiod (R1 to R5). Nonoptimal photoperiod simulations provided tall plants yet branches failed to develop on the lower main axis nodes. In a two year field study, flowers were removed from two sections of the main axis in an attempt to release the control of branch development from the main axis. The method either was unable to release control of branch development or branch development is not completely under the control of the main axis.