Genetic diversity and soybean yield: Finding the balance.

Based on national production statistics since 1924, average soybean yield in the U.S. has increased at a nearly steady rate of 22 kg ha^-1 year^-1.  It is possible to show some changes in this rate depending on how these past 85 years are divided, but two conclusions seem evident.  Soybean yield has not reached a plateau but the change in the rate of gain does not reflect the change in level of resources devoted to soybean breeding.  National average yield is influenced by many factors other than the genetic improvement of the cultivars grown, but this long term linear trend should be a concern. A critically important question for soybean breeders is how to manage genetic diversity to increase this rate of gain.  An essential, but sometimes overlooked, component of that process is the expansion of the current commercially used gene pool.  In the U.S., fewer than 100 ancestral lines have made any contribution to currently grown cultivars and more than 50% of that contribution comes from only five lines.  With more than 18,000 genetically compatible germplasm accessions in the USDA Soybean Germplasm Collection, there are abundant genetic resources available, but simply increasing genetic diversity is not a useful goal.  From a production perspective, it is not reasonable to accept a yield penalty to increase genetic diversity.  The challenges are to identify the accessions that can make unique genetic contributions to yield improvement and to derive effective measures to incorporate that diversity into the highest yielding cultivars.  Partnerships between public institutions and commercial companies are critical in this process.  There is much work left to do but we have seen enormous progress in the past 30 years.  There are released soybean cultivars and germplasm lines derived from exotic germplasm that yield competitively with the best cultivars available.  The number of exotic parents used to develop these lines is approaching the number of major ancestors for the current gene pool.  Technology that can be used to utilize genetic resources is improving rapidly.  We currently have opportunities to exploit genetic diversity to improve yield as never before but it will take a concerted effort by the research community to achieve what is possible.