Soybean yield potential--a genetic and physiological perspective

Soybean [Glycine max (L.) Merr.] yields in the USA have risen 22.6 kg ha(-1) yr(-1) from 1924 to 1997, but in the last quarter century (1972-1997) have risen 40% faster, 31.4 kg ha(-1) yr(-1). This upward trend in on-farm yield is fueled by rapid producer adoption of technologies emerging from agricultural research. Published estimates of the annual gain in yield attributable to genetic improvement averaged about 15 kg ha(-1) yr(-1) prior to the 1980s, but is now averaging about 30 kg ha(-1) yr(-1) in both the public and proprietary sectors. Periodic advances in agronomic technology, and a relentless rise in atmospheric CO(2) (currently 1.5 microliters L(-1) yr(-1)), also contribute to the upward trend in on-farm yield. In Nebraska, irrigated yield averages 800 kg ha(-1) more than rainfed yield, and is improving at a 40% faster annual rate (35.1 vs. 24.9 kg ha(-1)). About 36% of the annual variation in the irrigated-rainfed yield difference is attributable to annual variation in absolute rainfed yield. Inadequate water obviously limits absolute crop yield, but also seems to be an obstacle in terms of the rate of yield improvement. Several physiological traits changed during six decades of cultivar releases in Ontario that led to a genetic gain in yield of about 0.5% yr(-1). Changes in some traits were obvious (improved lodging), but more subtle in others (greater N(2)-fixation, greater stress tolerance). In terms of photosynthate supplied to sinks across a wide range of environments, recent cultivars seem to be superior to obsolete ones. To sustain and enhance soybean yield improvement in the future, technological innovation must be continually injected into the agricultural enterprise.