Jeff Coulter, Extension Corn Agronomist
Strong winds and hail on July 31 affected numerous corn fields throughout southern and central Minnesota (Figs. 1 and 2). Damage included loss of leaf area and stem bruising from hail stones, along with root lodging and stalk breakage from high winds. When the damage occurred, tassels were just starting to become visible in some fields, while pollination was partially or completely finished in other fields.
Unfortunately, corn is more susceptible to loss of leaf area from hail at tasseling and silking than at any other time in its life cycle. In general, complete loss of leaf area at this stage results in nearly 100% yield loss (Ritchie et al, 1997). This is because:
- Once the tassels have emerged, the plant has completed its vegetative growth. This means that all leaves have emerged and that there are no more leaves left to emerge if the exposed leaves are damaged.
- There has not been any grain fill at this point, and loss of leaf area prior to grain fill means that there will be less leaf area available to intercept the sunlight that is needed to fill the grain. In contrast, yield reductions associated with leaf loss after grain fill has begun will be less extreme because the leaf area that was lost would have already contributed to grain fill.
Hail stones can also result in significant stem bruising when hail is accompanied by strong winds. Stem-bruised regions on the stalk are an entry point for bacterial infection (Nielsen, 2008) and leave the plant more susceptible to stalk breakage in the future. As a result, fields with considerable stem bruising should be harvested early to minimize the amount of stalks that break below the ear prior to harvest.
Since stem-bruised plants are susceptible to secondary bacterial diseases (Nielsen, 2008), some have questioned whether a foliar fungicide should be applied to a damaged crop for disease control. While we do not know the answer to this question, we do know that fungicides are used to control fungi and not bacteria. In addition, many have difficulty justifying increased production costs for a damaged crop with a highly uncertain yield potential.
Research from 2007 in east-central Illinois by Dr. Carl Bradley simulated hail damage in corn before tasseling, followed by various foliar fungicides applied at tasseling (Bradley, 2008). While simulated hail injury reduced grain yield by approximately 30 bushels per acre, there was no yield advantage to foliar fungicide in corn that did or did not receive simulated hail damage. On-farm trials are currently being conducted by University of Minnesota Extension scientists to determine the effects of foliar fungicide on hail-damaged corn that received actual (not simulated) hail damage about two weeks prior to pollination.
Two days after the July 31 storm, I visited my corn planting date study at Waseca, MN, located about 20 miles southwest of Faribault, MN where a semi was reported to have been blown over on Interstate 35. In this study, corn was planted on April 30, May 14, and May 28, with plant densities ranging from 16,000 to 44,000 plants per acre within each planting date. Regardless of plant density, plants in the first two planting dates were standing straight. However, wind-induced root lodging (plants leaning more than 45 degrees) was considerable in the corn planted on May 28, especially in the plots with high plant densities. This was likely due to a lack of adequate brace roots for support.
While root-lodged plants are expected to "knee up" and result in stalks with a goose-neck appearance, there will likely be some reduction in yield due to root lodging alone. In 1985 and 1986 at Arlington, WI, researchers simulated root lodging on three corn hybrids by saturating the soil and pushing stalks over at the base so that the stalk angle was 20 to 30 degrees from the soil surface (Carter and Hudelson, 1988). Averaged over the three hybrids, grain yield was reduced by 14 to 24%, but grain moisture at harvest was unaffected.
Stalk breakage, often referred to as green snap, was also present in the planting date study at Waseca, MN. As with root lodging, it was only present in the May 28 planting date, and was most noticeable in plots with high plant densities. Stalk breakage is common following strong winds around tasseling because the plants have just completed rapid vegetative growth and the stalks are brittle.
Yield reduction associated with stalk breakage depends primarily on the location of the break on the stalk and the percentage of broken stalks (Hicks et al., 1999). Grain can still 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. However, yields are reduced on average by 0.28% for each percent of stalks broke above the upper ear. For plants with two ears that are broke between the upper and lower ear at tasseling, yields are reduced by 0.56% for each percent of plants broken, as the plant is unable to adjust the number of potential kernels on the lower ear at this stage.
Concerns about inadequate pollination arise when tassels are lost due to stalk breakage. While such concerns are justified, individual tassels generally produce two to five million pollen grains (Hoeft et al., 2000). Assuming 800 silks on each ear, this would be 2,500 to 6,250 pollen grains per silk, clearly more than enough pollen to pollinate neighboring plants that had lost their tassels. However, damage to ear shoots by hail can affect silk emergence and promote the formation of smut galls on ears. Silks that emerge after pollen shed has completed will not be fertilized, resulting in poor pollination and kernel set (Fig. 3).
Last year in southwest Minnesota, poor kernel set was reported in corn fields that were laid flat due to severe root-lodging just prior to pollination. Some agronomists in the area hypothesized that poor pollination in these fields occurred because the ear shoots were lying near the soil and the silks were covered up by other plants, making it difficult for them to intercept pollen.
Growers with damaged fields should contact their crop insurance agents as soon as possible. Some producers may even consider cutting the crop for silage. Corn silage from plants with few ears will likely be wetter and have higher nitrate levels than normal (Hicks et al., 1999). Since the highest nitrate concentrations will be found in the lower stalk, it is best to cut such plants a few inches higher than normal.
Bradley, C.A. 2008. Making profitable fungicide applications in corn. Available at http://ipm.uiuc.edu/bulletin/article.php?id=976 (verified 4 Aug. 2008). Univ. of Illinois, Urbana.
Carter, P.R., and K.D. Hudelson. 1988. Influence of simulated wind lodging on corn growth and grain yield. J. Prod. Agric. 1:295-299.
Hicks, D.R., S.L. Naeve, and J.M. Bennett. 1999. The corn growers field guide for evaluating crop damage and replant options. Available at http://www.soybeans.umn.edu/pdfs/CornGuide.pdf (verified 4 Aug. 2008). Univ. of Minnesota, St. Paul.
Hoeft, R.G., E.D. Nafziger, R.R. Johnson, and S.R. Aldrich. 2000. Modern corn and soybean production. MCSP Publications, Champaign, IL.
Nielsen, R.L. 2008. Recovery from hail damage to young corn. Available at http://www.agry.purdue.edu/ext/corn/news/timeless/HailDamageYoungCorn.html (verified 4 Aug. 2008). Purdue Univ., West Lafayette, IN.
Ritchie, S.W., J.J. Hanway, and G.O. Benson. 1997. How a corn plant develops. Iowa State Univ. Coop Ext. Serv. Spec. Rep. 48. Iowa State Univ., Ames.