Drought Stress in a Wet Year - Soil Water Limitations to Soybean Growth and Yield in Ontario, Canada.

Theoretical considerations suggest that soil water deficits limit soybean yields in Ontario, Canada in most years, but there have been no published attempts to quantify these yield losses experimentally.  In 2009 we began a multi-year project to compare soybean yields under both rainfed and water-replete (irrigated) conditions.  The objectives were to i) quantify yield losses associated with naturally occurring water stress in this region, and ii) quantify the underlying yield loss components, to identify plant traits that would enhance soybean yield under these types of soil water deficits.   A replicated field study was conducted at Elora, Ontario in 2009 using a locally-adapted commercial cultivar and agronomic practices typical for the region.  The 2009 growing season was unusually cool, with daily high temperatures averaging 1.8°C below the 10-year mean.  Precipitation was slightly above the 10-year mean, and was very evenly distributed across the growing season.  At no time during the season did the rainfed plots exhibit visual symptoms of water stress.   Still, maintaining the crop water-replete using irrigation resulted in a 10% yield increase (P < 0.05) over the rainfed treatment.   It was difficult to definitively parse this small yield difference amongst underlying components, but numerical trends suggested that irrigation increased radiation use efficiency and dry matter accumulation, as well as pod numbers per unit ground area, and seed size.  In contrast, radiation interceptance and seeds per pod appeared unaffected.   Canopy reflectance spectroscopy indicated a significantly (P < 0.01) delayed end-of-season canopy senescence in the irrigated treatment.   In total, these results support the hypothesis that there is substantial yield loss associated with water stress in Ontario soybean, even in a season when the stress is not readily apparent.