There is evidence that only small increases in yield have resulted from soybean breeding in the United States during the past 30 years and that some major limitations to yield have been reached (Frey, 1971). During this period the main advances have been in developing resistance to pests and diseases, and improving agronomic traits such as resistance to lodging and shattering. Considerable increases in yield have been achieved during this period in programs conducted for the development of soybeans in the tropics and subtropics. through adaptability to short days and high temperatures.