Updates from an agronomist and weed nerd in central Iowa.
In this issue:
Hail on corn and soybeans
Fungicide application to hailed crops
Corn rootworm hatch
Reminder about frogeye leaf spot resistance in soybean
Hail on corn and soybeans
After hail storms came through some counties on Wednesday evening/Thursday morning, this week will be a good time to evaluate how the crops are coming out of this injury and determine what concerns may exist for yield or stand for the rest of the season. As I'm not yet in central Iowa, I'm relying on others to help me figure out when issues arise. I always appreciate a quick phone call if anything of concern has come up in your area for the crops!
Corn - The growing point of corn reaches ground level around V6 and most of the corn I've seen is between V5 and V8 (usingleaf collar staging method). Corn cut off at or slightly above the ground level is likely no longer a viable plant if corn plants were this large. Corn cut off higher on the plant should continue producing new leaves and trying to outgrow old tissue.
Corn with an intact and healthy growing point will survive, but there are considerations for the rest of the season.
Evaluate corn yield potential from original stand and planting date - this is not what you seeded in the field, it is what the stand was. Take several stand counts across the damaged field to evaluate stand loss and use resources listed below to determine potential loss in yield potential.
Some plants may be in a "questionable" category for survival and production of a harvestable ear - this might include plants caught up in old tissue or those with deep bruises on stems.
Plants with exposed stalks that are damaged (NOT shallow leaf sheath bruising) may suffer from stalk rot or standability issues later in the season and should be monitored closely to determine whether early harvest is necessary
Evaluate defoliation of surviving plants - this means tissue dead or lost from plant; if it is still green, it's still photosynthesizing!
NOTE: when using Figure 6 in "Hail on Corn in Iowa", add approximately 2 leaves to your "leaf collar estimate" of stage (i.e. if your corn had 7 collars (V7), use V9 for your potential yield loss estimates in Figure 6)
This is because those figures use crop insurance staging which counts more leaves on a plant than the collar method.
9-leaf corn (~V7 by collars) should withstand up to 25% defoliation without yield loss
Additional resources on assessing hail damage in corn:
Soybeans - The growing point of soybeans is above ground as soon as the plant emerges. At each location on the stem where a leaf attaches, an axillary bud exists that can grow if the top bud is broken off the plant. This means soybeans have an incredible ability to recover from lost tissue, as long as an axillary bud still exists on the plant. If soybean plants are cut-off below their cotyledons, they will not survive.
Significant damage to soybean stands often makes the replant/fill-in question more difficult than with corn. With significant defoliation, plants are slower to recover, making stand evaluation more difficult without giving them some more time to resume growth of axillary buds; 7-10 days should be enough to see if regrowth is occurring or plants are dead.
Evaluate potential maximum yield from original final stand and planting date. Again, this isn't the 150,000 seeds planted per acre, it is the final stand in the field.
Soybean yield loss can come from stand loss, defoliation on plants, and loss of nodes or breaking of stems on individual plants
Concern is about significantly thinned stands and nodes lost from plants
Damage to stems is difficult to quantify, but may result in lodging later this season
To evaluate stand: count plants in a known length of row in several areas (at least 5) across the field or hail damaged area. You could do a small area like 3 feet of row or evaluate the full length of 1/1000th of an acre. More counts = more accurate assessment. Separate plants into "will survive", "won't survive", and "questionable" categories. These categories can help in estimating a % stand loss from the original final stand in the field.
Plants should begin producing new leaves at axillary buds that will be visible within the next week.
Soybean stands need to be below 75,000 evenly-spaced plants in mid- to late-May and 50,000-60,000 evenly-spaced plants in mid- to late-June to receive a yield potential increase from replant. Soybeans have an incredible ability to make up for stand loss.
If many of the surviving plants fall into the "questionable" category, replant may pay from a yield perspective
Again, resources below are helpful for estimating potential yield loss from stand loss and node loss on soybean plants, but cannot easily assess potential lodging issues from stem damage
Check stems carefully when evaluating plants to look for bruising or potential for lodging later in the season
The value of canopy to help suppress late-season weed pressure is also not easily quantified. Soybean fields should be monitored for weed emergence that requires control.
If replanting, don't bother tearing up the current stand, but do try to plant across rows (at an angle or perpendicular to rows) and try to increase the stand closer to 140,000 plants per acre (i.e. don't plant a full rate).
We often use 'mid-June' as a cutoff for switching to a shorter maturity group soybean for replant. We're likely "OK" sticking with a full-season bean unless replant occurs after June 15-20, then switching to a shorter maturity variety, perhaps ~0.5-1.0 MG shorter, that is well-adapted for the area may be a good idea.
Additional resources on assessing hail damage in soybeans:
Some may be interested in whether the hail damage on the corn and soybean crops should alter the decision-making process for fungicide application. In short, the answer is, "No." Fungicide application decisions should be made in the same manner as if no leaf tissue had been damaged. Also, it's important to remember that while the cost of fungicide does not change, but the yield potential of the crop has likely been reduced.
With the exception of smut on corn, fungal diseases are not designed to enter the plant through open wounds; the diseases enter the plant in other ways. Also note that no fungicides claim efficacy on smut. ISU has done several hail/fungicide studies in both corn and soybeans. Check out thisvideoto learn more about the research and results. Foliar diseases will influence the yield response to fungicides much more than hail damage. So make fungicide application decisions in the same manner as if the tissue had not been damaged.
However, IF a fungicide is applied to corn, waiting at least a week after the weather event may be more beneficial than an immediate application. Read more about the study results in the ICM Blog "Would a fungicide benefit hail-damaged crops?", written by Adam Sisson, Daren Mueller, and Alison Robertson.
Corn rootworm hatch underway
Research shows that 50% egg hatch for corn rootworms generally occurs between 684 and 767 degree days (base 52 F, soil), which means hatch is underway and either has reached 50% or will be there very soon for all of central Iowa. Once hatched, rootworms look for corn roots to feed upon and can be best found by digging roots and doing a float test. Look for root feeding 10-14 days after peak hatch.
Erin Hodgson has more information about corn rootworm feeding and associated yield loss in a recent ICM news article.
Frogeye leaf spot resistance in soybean
Last summer, we reported that the first cases of frogeye leaf spot (Cercospora sojina) fungicide resistance to strobilurin (Qol, FRAC group 11) fungicides. It's likely that this pathogen is resistant in more fields than we're currently aware of and appropriate management of frogeye leaf spot to slow development and spread of fungicide resistance in other fields is important. Read more about frogeye leaf spot in this recent ICM news article.
While fungicides can be an important management tactic for problem pathogens, we need to take care to avoid prophylactic use of these whenever possible to reduce risk of resistance development in fields. Doing on-farm trials with fungicide applications is one way to determine if they are providing a yield benefit that pays for the application. Iowa State University research farm agricultural specialists are available to help set up and analyze results from on-farm strip trials.
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