The storm damage season for crops is underway in Iowa. 

In response to heavy early storms across the Midwest, Iowa State University Extension field agronomist Clarke McGrath offered information and guidelines to help farmers evaluate hail damage and decide whether or not they need to replant damaged fields.

McGrath, who writes the "Corn/Soybean Insight" column each month in Wallaces Farmer magazine, has updated a hail evaluation guide he originally wrote a few years ago. Here are his newly-revised guidelines for farmers and crop consultants to use:

Should you replant a hail damaged soybean stand?
For soybeans, McGrath advises farmers to start with the following three steps:

REPLANT DECISIONS: Hail damage and flooding have hit a number of fields, leaving many farmers to make replant decisions.

1. Check the number of live plants per foot of row. Lay a tape in the row and dig up all plants in a 3 foot or more length. For drilled beans, two rows could be dug. Repeat several times over the field, keeping track of number of live plants per foot of row.

2. Examine plants carefully and separate into three piles: a) live plants; b) questionable plants; c) dead plants

3. Add the number of live plants and one-half the number of questionable plants and divide by the length of row to get the number of live plants per foot of row. Plants cut off below the cotyledons (thick seed leaves on the bottom of the plant) will not regrow. If plants are broken off above the cotyledons, there is a bud in the axil between the cotyledon and stem and between the unifoliate and trifoliate leaves and the stem which will produce new growth. It takes about four to seven days to see regrowth on soybeans after hail. More detail on some experience with this is included in the notes on the last page of this article.

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Beans tend to branch, so the number of plants per acre can vary greatly with moderate effect on yield. If the stand loss is fairly uniform, it generally takes a population of less than 75,000 plants per acre to pay to replant in mid-to-late May and less than 50,000 to 60,000 in mid-to-late June. However, if most of the remaining stand is made up of "questionable" plants, it may pay to replant with a higher population.

Table 1. APPROXIMATE NUMBER OF SOYBEAN PLANTS PER  FOOT OF ROW TO GIVE VARIOUS POPULATIONS

PER ACRE

                                       Population

Also, look at the following two tables: Table 2 and Table 3. Table 2 shows the yields that may be expected when soybean populations are thinned at various stages of development. Beans start to lose yield potential when planted after mid-May in southern Iowa. Table 3 shows average yield that can be expected from delayed planting.

TABLE 2.  PERCENT OF SOYBEAN YIELD AT VARIOUS POPULATIONS WHEN THINNED AT VARIOUS STAGES OF DEVELOPMENT

                                         Thinned at

TABLE 3.  APPROXIMATE PERCENT OF YIELD AT VARIOUS SOYBEAN PLANTING DATES

Generally, full season adapted soybean varieties can be planted in southern Iowa up until late-June. That would be Late Group II/Early Group III through the end of June and then Mid Group II maturity beans in July.

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A small amount of leaf area loss, especially at early stages of growth does not usually result in much yield loss. Hail loss estimates on beans are complicated by bruising, and the effect of lower stem bruises is hard to evaluate. Deep bruises can result in lodging of the soybeans later in the season. There is a little more discussion of that topic in the notes on the last page.

How about corn? Should you replant a hail-damaged stand?
The growing point on corn is belowground inside the developing plant stem until around the V6 growth stage of the corn plant, which is when six leaves are fully emerged from the corn whorl, or when about eight leaves are visible. Prior to this time corn will generally regrow from hail damage. If the growing point is aboveground, the growing point may still be alive. Make a horizontal cut through the corn stem to find the growing point, and look for healthy white plant tissue in the center.

Sometimes even though the growing point was not cut off by the hail, diseases set in after the hail bruises the stalk and cause the growing point to rot. Generally, some regrowth of the corn plant from the growing point can be seen three to four days after the hail. If no regrowth is seen, cut the plant and see if the growing point is rotting. Take stand counts and determine whether replanting is desirable based on the information below and the cost of replanting. Remember, the numbers in Table 4 assume a uniform stand, says McGrath.

Table 4. Relative yield potential of corn by planting date and population

Note: Values based on preliminary Iowa research and modeling; 100% yield potential is estimated to occur with 35,000 plant population and early planting. From: Iowa State University Extension, Corn Field Guide, CSI 001. 2009.

Here are some good rules of thumb or opinions on how to evaluate hail damaged corn or soybeans "from our experience," says McGrath. "I also included a few links to help you find more information."

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1. Wait 3 to 5 days. It's best to wait 3 to 5 days before looking at a cornfield; sometimes soybeans need closer to a week to see what you have. Fields will either look much better or worse in that time making it easier to make a replant decision (it is never easy to decide, but watching plant conditions as they try to regrow will help a lot). Flag a bunch of plants so you can reference their progress day to day.

2. How old is the corn the hail hit? Corn younger than V5 growth stage is seldom damaged economically. Some notable exceptions are large direct hits from hail stones that can bruise or destroy the growing point, or hail stones that lay against plants -- as they melt they can cause frost injury to the growing point. Look for the hail "divot" or soil depression next to plants to check for this. Given the large hail sizes and ferocity/volume of the June 3 storms in Iowa, this will probably be more common than we see with most storms, says McGrath.

3. Check the health of the plants. Research studies and experience show that half of the bruised corn plants (bruising means damage to the growing point, nodes or actual stalk, rather than damage to leaf sheaths) will not make it if the corn is beyond the 5th leaf stage. "I haven't seen much, if any corn that big yet but there may be some out there," notes McGrath.

4. Yield loss depends on growth stage. Yield loss in corn is very dependent on growth stage of the plant, percent defoliation and the amount of bruising. Consult the National Corn Handbook for exact percentages on defoliation. ISU also has stand loss charts to aid with replanting decisions.

5. Where are the soybean plants cut off? If soybean plants are cut off below the cotyledons on the stem, the soybean plants will not recover. Some amount of leaf tissue is also important for regrowth to occur because leaves produce the fuel for plant growth. "Injured plants with the equivalent of one cotyledon or one leaf have a good chance of recovery," says McGrath. "If no cotyledons or leaves remain, regrowth will be very slow, even if growing points remain intact. I typically put these types of plants in the category of pile B or questionable plants." 

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6. Look closely at bruised stems. Soybeans with bruised stems may or may not produce a harvestable yield. It largely depends on the timing and severity of the stem damage which is difficult to judge. Generally, plants that survive stem injury will go on to produce normal yields, but will be more susceptible to lodging in the fall, possibly reducing harvestable yields. These plants can fall over onto the ground when the pods begin to fill.

Note: This is more the case for beans as they approach reproductive stages, says McGrath. But there can be significant yield loss from bruised/broken stems on soybean plants that are in the vegetative stage of growth. For example the charts indicate that vegetative stage beans with 50% broken over or cutoff nodes is approximately a 9% yield loss.

7. What about mowing damaged corn? Mowing corn that is tied up with twisted leaves may work (there is disagreement about this) as long as you mow above the growing point. "I have seen it work well but only a very few times. I've seen significant portions of a surviving corn stand killed by this more often," says McGrath. "Some researchers say the mowing can cause more entry points for pathogens or diseases. Or perhaps farmers will cut too low and kill the growing point. It seems that mowing is a last resort for corn that has its leaves wrapped up with no sign of the wind or plant growth unbinding it."

8. Be sure to control weeds. An additional herbicide application will likely be needed to control the weeds given the extended time without a crop canopy.

9. Should you switch from corn to beans? Should you switch to soybeans if it is getting too late to replant corn? If you lose a corn stand and decide to switch to soybeans, double check the herbicide replant issues with your ag chem dealer.

10. Does fungicide application help? Do foliar fungicide applications help post-hail corn and soybean stands yield more? Preliminary information says… maybe. Key points of ISU research and other trials are: 1) Application of foliar fungicide to corn usually resulted in increased yield over non-application across all treatment averages (both hailed and non-hailed); 2) In soybeans, fungicide application resulted in increased yield about 60% of the time across all treatments (both hailed and non-hailed); 3) This is limited data from one year, but it does suggest application of fungicides on hail-damaged crops merits additional research. Read more here.