The efficiency of irrigation water use is improved if irrigations are scheduled only when soil water depletion exceeds a chosen crop-specific threshold percentage of field capacity (FC). Further improvement may be possible by accounting for seasonal variance in yield sensitivity to water stress. In soybean [Glycine max (L.) Merr.], the vegetative and bloom stages are notably less sensitive to water stress than are the pod-set and seed-fill stages. A 2-yr experiment was conducted to determine the yield impact of deferring irrigation until the beginning-pod stage (R3). Eight soybean cultivars were evaluated in subplots within four main plot irrigation treatments: (i) full-season control, with a 35% FC soil-water depletion trigger to schedule irrigation that replenished 100% of the crop evapotranspiration (ET) loss since the prior irrigation; (ii) a defer-irrigation-to-R3 strategy with the 35% FC depletion trigger ignored until the onset of stage R3; (iii) a deficit irrigation strategy with irrigation events on the same dates as those in the full-season control, but limited to two-thirds of the control amount; and (iv) rainfed control. Below-normal spring rainfall in each year resulted in soil water depletion greater than 35% FC before irrigation commenced in the R3-deferred strategy but did not impact yield versus the full-season control yield. A season-long deficit irrigation strategy significantly reduced yield below the full-season control, though it still produced significantly more yield than the rainfed control. These findings suggest that differentials in sensitivity of the crop developmental stage should be incorporated into irrigation scheduling models.
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