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Calculating the Impact of Hail On Sunflower

Wednesday, March 1, 1995
filed under: Fertility

One would not normally consider sunflower growers and new car dealers to be kindred spirits, but there’s at least one feeling with which both groups can identify: that knot in the stomach while watching a midsummer hailstorm pummel the objects of their livelihood.

While it’s an expensive proposition for the insurance carrier (News Flash: Premiums to Increase), car dents can be repaired and windshields can be replaced. Many sunflower growers carry hail insurance, too. In fact, it was insurance underwriters’ need for better hail injury documentation that stimulated the research which produced much of the information we have today on hail’s effects on sunflower and the ability of the plant to recover from it.

North Dakota State University agronomist A.A. Schneiter (currently interim chairman of the NDSU Plant Sciences Department) developed most of the current data on hail-induced defoliation, stand reduction and plant injury in simulated hail damage studies conducted during the 1980s. Schneiter’s findings comprise the basis for the hail insurance industry’s adjustment scales for defoliated and otherwise damaged sunflower fields.

The sunflower plant can, of course, be affected in various ways by hail. There can be defoliation, plant head and/or stalk injury or plant death. The type and degree of injury will depend on several variables: hailstone size and hardness; speed, density and storm duration; and plant status (e.g., whether the leaves are flaccid or turgid). Finally, stage of plant development is obviously an important factor.

The ultimate question for both farmer and hail adjuster — and the question which Schneiter sought to answer through his research — is: How much yield loss does the hail inflict on the sunflower crop?

Reduced yields due to defoliation will depend on the amount of leaf loss and the stage at which it occurs. Growers sometimes are surprised by the ability of sunflower plants to “rebound” and still produce well following substantial defoliation. Growth stages R-1 through R-6 (bud through flowering) are the most sensitive to defoliation since much of the energy produced by the plant — especially the leaves— during those stages is being directed into head and seed development. As Table 1 indicates, defoliation levels can be high during early and late stages of sunflower plant development, but still not have a major impact on final seed yield.

Plant death due to hail injury is a common occurrence — particularly if the hail strikes the sunflower field while the plants are small. The resulting yield losses would be similar to those incurred if the sunflower stand was lowered as a result of reduced seeding rates. “If the amount of stand reduction is significant and/or occurs when the plant has begun to develop and compete with neighboring plants, the remaining uninjured plants cannot compensate enough, and yields will be reduced,” according to Schneiter.

Table 2 presents approximate yield reductions from variable levels of random stand reduction at several stages of plant development. Based on studies at Fargo and Carrington, N.D., these values “represent direct stand reduction where the plants have been destroyed and are no longer competing with uninjured plants for light, water or nutrients,” Schneiter indicates.

Along with defoliation and stand reduction, plant injuries (such as bruising or breaking off of the terminal bud) also can contribute to reduced sunflower yields. Schneiter points out that injured plants that continue to “live on” may negatively impact total crop yield more than had they been killed, since they’ll continue to compete with healthy plants for moisture, nutrients and sunlight — yet will never produce an equal yield. For example, if the hail injury occurs near flowering or shortly thereafter, the plants often remain green and continue to live, but do not produce seed.

Determining the effect of bruising by hail stones is very difficult, Schneiter notes. If the sunflower stalk does not weaken or break prior to harvest, the effect on yield may be quite minimal. But if a stalk breaks prior to its plant head being combined, or if disease (e.g., Rhizopus head rot) sets in due to hail damage, there’s an obvious impact on the sunflower yeild.
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