Yield potential of hail-damaged crops depends largely on the remaining plant population, the type and severity of damage, and the growth stage when damaged. Information for evaluating recovery and management of damaged crops is available in:
Corn Damage and Replant Guide: http://z.umn.edu/cornhaildamage (48 KB PDF)
Soybean Damage and Replant Guide: http://z.umn.edu/soybeanhaildamage
To determine whether a corn plant will survive and regrow following damage, split stalks and examine the growing point. In the mid- to late vegetative stages, the growing point is at the base of the tassel. A healthy growing point will be firm and white to yellow. If damaged, the growing point will be watery and orange to brown. Plants with damaged growing points or stalks broke below the growing point will not recover.
When soils are saturated, strong winds can cause corn plants to lean over due to pulling of shallow roots. Within a few days, root-lodged plants typically straighten upright and stalks have a curved appearance.
The impact of root lodging depends largely on the growth stage when it occurred. In Wisconsin, researchers simulated root lodging in corn by saturating the soil and pushing stalks to a 20 to 30 degree angle from the soil (Carter and Hudelson, 1988, J. Prod. Agric.). Most plants straightened upright within three days and yield loss was dependent on the growth stage when damaged. Yield was reduced by less than 5% when damaged at the V10 to V12 stage, by 5 to 15% when damaged at the V13 to V15 stage, and by up to 30% when damaged at V17 or later. Grain moisture at harvest was unaffected.
Stalk breakage, often referred to as green snap, also can be caused by strong winds. It is most common between the V10 stage and tassel emergence when plants are undergoing rapid vegetative growth and stalks are brittle. Yield loss due to stalk breakage depends on the percentage of stalks broken and the location of the break on the stalk. Grain can be produced on plants with stalks broke above the ear, and unbroken plants next to broken plants can partially offset yield losses because there is less competition for light.
Concerns about inadequate pollination arise when tassels are lost due to stalk breakage. However, individual tassels generally produce over two million pollen grains. Assuming 800 silks per ear, this corresponds to 2,500 pollen grains per silk, indicating great potential for adequate pollination of neighboring plants with lost tassels.
Yield potential for corn at various plant populations is listed in Table 1. When gaps of two feet or more are present throughout the field, assume an additional 5% reduction in yield.
Table 1. Typical relationship between corn plant population and yield in Minnesota.
|Population||Grain yield potential|
|plants/A||Percent yield (%)|
Loss of leaf area reduces yield potential. Only consider leaf area lost if it is removed or brown, as any green leaf area remaining on a plant contributes to yield. Typically, loss of 50% of leaf area in corn reduces grain yield by 6% if lost at V10, by 10% if lost at V13, by 18% if lost at V16, and by 24% if lost at V18.
Bruising of stalks by hail limits the plant's ability to translocate water and nutrients and also reduces standability. Plants with stalk bruising should have their stalks split to determine the severity of the bruising. Plants with damage extending beyond the leaf sheaths and into the pith either will not recover or likely will have large reductions in yield. Fields with severe stalk bruising should be harvested early to avoid significant losses from stalk lodging.
Plants with severe stem bruising are not likely to produce a harvestable yield and are likely to fall over late in the season if they are able to produce seed. Reduced stands can amplify the losses from leaf area alone.
Estimating yield loss
Soybean can tolerate low plant populations well, with only small reductions in yield potential across wide ranges of stand loss. For instance, populations near 100,000 plants per acre are likely to produce maximum yields, and those around 80,000 will yield about 90% of the maximum. However, expected yields drop more rapidly in stands below 50,000, with 39,000 plants per acre likely to produce about 75% of the normal yield. For soybean, leaf loss itself through the V4 stage (4 fully-developed trifoliolate leaves) has little effect on yield, but the effects of leaf loss increase through late seed-filling.
Results of a 3-year study in Lamberton, MN and Ames, IA are shown below. The study examined defoliation at R1 and R3 at 33%, 66%, and 100% levels. In addition, a second 66% defoliation treatment was created by thinning the existing 100,000 plant stand by 50% and defoliating by 33%. This allowed for a contrast between pure leaf loss and leaf loss with reduced stands.
As expected, more severe leaf loss led to greater yield losses, and defoliation at R3 produced a greater yield loss than at R1. Leaf loss at 66% resulted in only a 6% yield loss at R1, but the same level of leaf loss combined with a reduced stand increased yield losses to 21%. A total loss of leaves at R1 resulted in only a 25% yield loss. The yield penalty from 100% leaf loss ballooned at the R3 stage, where we noted a 69% yield loss.
Hail events are always painful, but late-season hail tends to reduce yields more than hail occurring earlier. Early July hail events will likely lead to yield responses similar to those seen in the R1 treatments from our study. Significant thinning of stands at this time should be considered as an additive form of yield loss at this stage of development.
Table 2. Soybean yield response to leaf defoliation and population thinning.
|33% defoliation @R1||52||+1%|
|66% defoliation @ R1||48||-6|
|50% thin and 33% defoliation @ R1||41||-21|
|100% defoliation @ R1||38||-25|
|33% defoliation @ R3||48||-6|
|66% defoliation @ R3||39||-24|
|50% thin and 33% defoliation @ R3||34||-34|
|100% defoliation @ R3||16||-69|
Replanting corn at this time is not feasible as the crop would not be expected to reach maturity before the first killing freeze.
Soybean may be replanted, but yields will be severely limited by the short season remaining. Soybeans planted beyond the first of July routinely produce a yield less than half of that of normal planting dates. Experience with soybeans after peas has shown that planting a soybean variety that is one maturity group earlier than adapted to the region before July 4 will occasionally produce reasonable yields. Producers are seldom content with yields from crops planted a week or more after July 4. By this time, yield potentials fall to 40% of normal or less. However, producers may choose to gamble on late-season replanting for weed control or other appropriate reasons.
Seed availability may be the primary determinant for replanting or not. Producers should contact their seed suppliers as soon as possible to confirm that early-maturity seed is still available. Soybeans with maturities of more than one full maturity group shorter than adapted will be required at this late date.
Managing damaged fields
While it is crucial to avoid further stress to your damaged crop, foliar fungicides are not likely to improve crop recovery and yield (see Do foliar fungicides provide a benefit to corn damaged by hail?). The most damaging diseases affecting corn and soybean after hail are bacterial in nature. Fungicides have no effect on these bacterial diseases.
Planting a cover crop may be a viable option for areas where the original crop was completely lost. Additional considerations for such fields include weed control and avoiding fallow syndrome (see Drowned-out or hailed-out crops and prevented planting – What to do now?)