Corn And Soybeans Bruised By Hail Not Easy To Evaluate

Corn And Soybeans Bruised By Hail Not Easy To Evaluate

Here's what ISU Extension agronomists are finding as they inspect fields hit hard by hail and flooding in southwest Iowa last week.

BRUISED STEMS: "While some hail charts may indicate very minimal damage from stem bruising, what we've seen in the fields the last several days tell us stem bruising may be a bigger issue given the severity of the storm and impacts to the stems," says ISU's Clarke McGrath.

A number of corn and soybean fields were clobbered by hail, wind and heavy rain as storms struck southwest Iowa and northern Missouri the evening of June 3. Clarke McGrath, Iowa State University Extension field agronomist at Harlan in western Iowa has updated the guidelines he is handing out to people regarding stand assessment, deciding whether or not you need to replant, etc. Following are a few other observations as he watches the crops try to recover from the storm. Many of the soybeans are showing new growth, even some that did not have the equivalent of a leaf or cotyledon left on them, says McGrath. "The new growth on these totally stripped plants is promising, but it is coming slowly and the trifoliates are still small and most have not opened," he notes. When making your decision and rating these plants for survivability, he suggests putting them in Pile B -- the questionable plants. "That's the middle classification that we consider half of to be viable plants, still looks like the best place for counting these plants," he explains.

Some bruised plants appear to be recovering, others aren't
After he and fellow ISU Extension field agronomist Aaron Saeugling have spent the past several days observing fields and helping farmers make replant decisions, McGrath offers some additional thoughts on stem bruising.

He says some of the bruised plants look like they are recovering well but others are more damaged than they first appear. Deciding if these plants go into the "live" plant or the "questionable plant" pile is subjective at best, but here is one approach they have discussed. If the stem is bruised but still feels rigid and resilient, "questionable plants" is probably a good category. If it bends over at the bruise easily or you can see that a majority of the stem tissue at the bruise is damaged, then the odds are this plant won't contribute to yield if it does survive. Putting those plants in the third class, "dead plants," is probably justified.

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Evaluating hail damage on soybeans and corn
First, here are the ISU agronomist's guidelines for soybeans.

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 the live plants per foot of row.

2. Examine plants carefully and separate into three piles: a) live plants; b) questionable plants; and 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 (the thick bottom seed leaves) 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 in the notes on the last page.

 Beans tend to branch, so the number of plants per acre can vary greatly with moderate effect on yield, says McGrath. 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

Row Width

150,000

125,000

100,000

75,000

50,000

 

 

 

 

 

 

30

8.6

7.2

5.7

4.3

2.9

20

5.7

4.8

3.8

2.9

1.9

15

4.3

3.6

2.9

2.2

1.4

10

2.9

2.4

1.9

1.4

1.0

Table 2 shows the yields that may be expected when populations are thinned at various stages of development. Beans start to lose yield potential when planted after mid-May in southern Iowa.

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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

Final Stand

VC

V3

V6

50,000

92

85

74

75,000

98

99

92

100,000

100

107

98

125,000

99

102

100

150,000

100

101

100

75,000 w/ 1 ft. gaps

97

97

89

75,000 w/ 2 ft. gaps

92

92

86

Source: University of Minnesota

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

Planting Date

Northern

Central

Southern

 

Iowa

Iowa

Iowa

Late April

100

96

98

Early May

96

100

100

Mid-May

99

96

98

Early June

81

93

89

Mid-June

61

59

82

Early July

33

45

47

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

A small amount of leaf area loss on soybean plants, 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 in the notes on the last page of this article," he adds.

What about replanting corn at this late date?
The growing point on corn is belowground until around V6, when six leaves are fully emerged from the corn whorl, or 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 and look for healthy white plant tissue in the center. Sometimes even though the growing point was not cut off, diseases set in after the hail and cause it to rot. Generally, some regrowth 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.

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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. Iowa State University Extension, Corn Field Guide, CSI 001. 2009.

McGrath adds: "Here are some good rules of thumb/opinions with hail damage, based on our experience and a few links to more information."

1. Wait for three to five days before looking at a cornfield, sometimes soybeans need closer to a week to see what we 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. 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, this will probably be more common than we see with most storms.

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

4.  Yield loss in corn is very dependent on growth stage, percent defoliation and bruising. Consult the National Corn Handbook-1 for exact percentages on defoliation.  ISU also has stand loss charts to aid with replant.

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5. Soybeans cut off below the cotyledonary leaf scars on the stem will not recover. Some amount of leaf tissue is also important for regrowth because leaves produce food for plant growth. Injured plants with the equivalent of one cotyledon or one leaf have a good chance of recovery. 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 "Pile B -- questionable plants," says McGrath.   

6. 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 slight-to-moderate stem injury will go on to produce normal yields, but will be more susceptible to lodging in the fall, possibly reducing harvestable yields. They 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. There can be significant yield loss from bruised/broken stems on Growth V stage beans, for example the charts indicate that the growth stage 5 beans with 50% broken over or cut off nodes is approximately a 9% yield loss.

7. 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 a few times, and I have seen significant portions of a surviving corn stand killed by this more often. Some researchers say it can cause more entry points for pathogens, or some farmers will cut the plants off too low and kill the growing point. It seems that this mowing tactic is a last resort for corn that has wrapped up its leaves with no sign of winds or growth unbinding it.

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

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9. If you lose a corn stand and decide to switch to soybeans, double check the herbicide replant issues with your ag chem dealer.

10. Do fungicide applications help post-hail stands yield more? Preliminary information

says "maybe." The key points are: 1) application of fungicide in 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.

To add more context to the fungicide question, most of the work done has been on reproductive stage crops; there is very little data on treating vegetative stage hail damaged crops. The researchers suggest that the positive results from R (reproductive) stage crops may not translate directly to the same results on V (vegetative) stage crops. "We are currently working to get fungicide plots planted and growing in these hailed fields this year to give us a clearer picture for future reference," says McGrath.

More information to consider: Tables 3 and 5 sourced from University of Missouri's replant guide.

Given the proximity of much of the storm damaged area to northern Missouri, we could factor their numbers with the other information in this document to get potentially more regionally relevant estimates for our hailed fields.

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