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Corn yield loss due to hail damage can result from leaf area reduction caused by defoliation, plant bruising, stand reduction, and direct damage to the ears.
Understanding the severity of each of these factors is important to accurately assess the extent of hail damage and its impact on yield potential. Evaluating the health of the growing point can be done soon after the storm, but making a decision regarding the yield potential of the field is premature because the plants have not been given enough time to recover. To accurately assess potential yield loss from hail, corn plants should be evaluated 7 to 10 days after the storm. At that time, it is easier to more accurately distinguish between living plants and plants that are unable to withstand the hail damage or subsequent disease infection.
Plant defoliation results in the loss of photosynthetically active leaf area. The severity of the loss depends not only on the amount of leaf area removed, but also the corn growth stage when damage occurs. Keep in mind that leaf damage usually looks worse than it really is, especially in the first few days after a storm. Shredded leaves that remain green and attached to the plant will often continue to produce photosynthates for the plant.
To avoid overestimating the amount of damage, be sure to look at all leaves and average the percent defoliation. Use the average percent defoliation for all leaves and the stage of plant growth and refer to Table 1 to estimate the reduction of yield potential from defoliation.
Assessing the extent of stem and whorl bruising and its effects on the health and productivity of the plant is difficult. Bruising may create an avenue for infection, which can increase the risk for stalk lodging later in the season. Fields that contain severely bruised plants may need to be evaluated again toward the end of the season.
Plant wounds, bruising, and reduced photosynthesis caused by hail events can stress plants and leave openings for pathogens. It is important to note that foliar diseases managed by fungicides including gray leaf spot, northern leaf blight, and southern rust do not require wounds for infection. Diseases such as Goss’s wilt, common smut, and stalk rots that are favored by wounding are not controlled by fungicides.1
Results from fungicide trials on hail-damaged crops have been mixed. A beneficial crop response from using a fungicide is more likely in years with disease pressure. Periods of rain and high humidity can accompany storms and create an environment favorable for foliar disease. A single-year study from Iowa State University suggested a fungicide application one week after a simulated hail event could result in greater yields compared to fungicides applied two days after a simulated hail event.2
By the V6 growth stage, the growing point is generally at or above the soil surface. Wind, hail, and/or driving rains may cause stalks to break below the growing point. If stalks are broken below the growing point, they will not recover to produce an ear. After V10, potential yield loss and stand reductions are at nearly a one to one ratio (for example: 80% stand = 80% yield potential). Yield losses due to stand reductions are additive to yield losses due to defoliation. Stand loss at this stage will likely result in considerably greater loss of yield potential compared to leaf defoliation.
Late-season hail damage often results in direct damage to the ear. Ears can be damaged below the husks by the force of hail storms. Examine corn ears and calculate the percent of damaged kernels. Bruised ears and stalks may also allow the entrance of disease to further reduce yields and grain quality. Corn experiencing hail damage later in the season will usually reach physiological maturity sooner, but will likely take longer to dry-down
The factors involved in estimating potential yield loss include: defoliation, stand loss, plant bruising, possible disease infection of damaged plants, stalk lodging later in the season, and environmental conditions during the remainder of the growing season. Growers should scout and monitor for stalk rot and lodging, increased nitrate levels in fields intended for animal feed, and late-season weed flushes due to increased light penetration in defoliated areas. Potential yield loss figures due to damaged or missing plants are only estimates. True yield loss from a hail storm cannot be fully determined until harvest.
Sources: 1Jackson-Ziems, T.A. 2014. Fungicide use in corn after hail or wind damage. University of Nebraska, Lincoln. CropWatch. 2Sisson, A. and Mueller, D. 2013. Hail and fungicides update 2013. Iowa State University. Klein, R. and Shapiro, C. 2011. Evaluating hail damage to corn. University of Nebraska Extension. EC126. www.ianrpubs.unl.edu Lauer, J. 2009. Late-season hail effects on corn. Field Crops 28.492-69. University of Wisconsin Agronomy Department. http://corn.agronomy.wisc.edu . Nielsen, R.L. 2012. Late-season hail damage to corn. Purdue University Cooperative Extension. www.agry.purdue.edu Nielsen, R.L. 2012. Recovery from hail damage to young corn. Purdue University Cooperative Extension. www.agry.purdue.edu Vorst, J. 1995. Assessing hail damage to corn. NCH-1. National Corn Handbook. Web sources verified 04/24/15. 140731161514