Field Performance of Commercial Ontario Soybean Cultivars Differing in Water Use Efficiency.

For soybean crops in Ontario, Canada, crop water use during the seed-filling stage generally exceeds precipitation, leading to soil water deficits that limit growth and yield. Previous research has demonstrated that there is genetic variation for water use efficiency (WUE) among commercial soybean varieties grown in Ontario. We hypothesized that under naturally occurring water deficits in the field, varieties with higher WUE would be less susceptible to yield loss. The WUE of selected varieties with similar maturities was determined under greenhouse conditions. The same varieties were grown under two soil water conditions, rain-fed and water-replete, in a replicated field experiment during 2011 and 2012.  The growing season conditions differed between the two years; 2011 was wetter than average, but 2012 had an extended early-season dry period. In both years, across varieties, percent reduction in yield due to water deficit was strongly correlated with percent reduction in biomass.  In 2011 we found that the maturity date was delayed under water-replete conditions, and there was a significant variety x treatment interaction for this effect.  Varieties with high WUE were significantly less susceptible to early senescence under naturally occurring water stress.  In 2012, WUE was positively correlated with percent yield reduction under water stress, suggesting that high WUE was disadvantageous in that season.  Under water-replete conditions, WUE was either positively correlated with yield (2011) or unrelated to yield (2012).  The results so far therefore indicate that high WUE can be beneficial under some conditions (mild late season water stress, as typically occurs in Ontario), but may be detrimental under other conditions (strong, early season water deficits).  Results from the third year of the field trial will also be presented.