Soybean yield loss due to hail depends on the growth stage, type of damage and severity. Currently, we rely on soybean recovery data from the U.S. as there is no data available for Western Canada. However, research is underway to quantify the effect of stem breakage and defoliation on soybeans in Manitoba. The project was initiated in 2015 and is planned to be complete in 2019. To date, 3 out of 8 site-years have been lost due to actual hail.
Based on preliminary results from four site-years, if hail damage is limited to leaf defoliation at V2 to V3 (2nd to 3rd trifoliate), yield loss is likely minimal (e.g. Figure 2), although some yield loss has been detected at 100% defoliation (average of 15%).
If main stem breakage occurs, yield loss can be high depending on the percentage of main stem nodes that are left. If only the cotyledons remain (100% stem breakage), re-growth can occur from the axillary buds (Figure 3, 4), but an average yield loss of 45% should be expected (Table 1). New growth at the axillary buds will be visible in a few days.
If the cotyledons and unifoliate leaves remain (60-80% stem breakage), you can expect 18-25% yield loss (Table 1). Lastly, if only the first trifoliate or second trifoliate is broken (20-40% stem breakage), minimal yield loss is expected (Table 1). Growth will resume from the uppermost leaf node. If plants are cut below the cotyledons, re-growth is not possible and stand reduction has occurred.
The yield loss data reported here is based on a full plant stand. It is important to acknowledge that hail damage is usually not this clear cut; typically, it is a combination of defoliation and stem breakage that occurs at a range of severity levels. The best you can do is survey the field and come up with a good average. In addition to yield loss, expect a delay in maturity if 100% defoliation or stem breakage occurs (up to 5 days).
To assess hail damage:
- Identify the growth stage prior to hail. Most soybeans in Manitoba were at V1 to V3 (1st to 3rd trifoliate), which corresponds to 2-4 nodes on the main stem (excluding the cotyledons). To identify the number of nodes on the main stem, add 1 to the V-stage, which counts trifoliate leaves.
- Assess multiple plants in multiple areas of the field. For each plant, identify how much leaf defoliation or stem breakage occurred. Measure plant stand.
- Identify the average % defoliation and/or % node loss and plant stand (# plants/ac) and apply yield loss data.
In other work, late seeded soybeans have been successful in the central region of Manitoba, maintaining about 80% yield potential through the 2nd week of June. Yield loss and maturity risk increased significantly through the 3rd week. In the shorter season area, late seeding was not successful beyond the 1st week of June due to late maturity and yield reduction.
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