May 22, 2012
Rootless Corn and Other Emergence Problems
There are fairly widespread problems with the emergence and early growth of corn in the area. One problem is associated with the corn planted around April 23-27, which went through several days of soil temperatures below 50F right when the corn was germinating. This has lead to a lot of leafing out underground and other abnormal growth which has reduced corn stands. We have also had some crusting in areas that received the heavy rains.
One problem that is now showing up across much of the area is rootless corn. Corn is flopping over and dying because the nodal roots which emerge about an inch below the soil are not developing. In some cases this is due to shallow planting and/or soil erosion, resulting in the roots emerging at or above the soil surface. In some cases it is where the corn was planted when the soil was too wet so the planter caused sidewall compaction, resulting in the seed slot not closing and the soil becoming hard and dry so the roots cannot penetrate. However, many of the fields that are starting to show this problem were planted no-till under near ideal soil moisture conditions. With the dry weather we have had in the last 2-3 weeks, our silty clay loam soils have the tendency to shrink and crack and the easiest place for them to crack is anywhere the soil has already been fractured. Unfortunately with no-till this occurs where the seed furrow is and can result in poor nodal root development. A rain would do a lot of good in overcoming this problem The only other option to consider would be to try to cultivate later (when corn is 8+ tall) to try to mound some soil up against the corn to encourage root growth. Unfortunately this will also tend to dry out the soil even more, so hopefully well get a rain in a few days to solve the problem.
Some corn herbicides can also lead to poor nodal root development. This occurs most often when 2,4-D is applied with an amide herbicide (such as acetochlor, metolachlor, or dimethenamid) around planting time and it is followed by a heavy rain. This is part of the reason for the 10 day window (7 days prior to 3 days after planting) when 2,4-D should not be applied to corn. The easiest way to tell this from other factors associated with rootless corn is to look for a sprayer pattern. Roger Elmore has a nice article on rootless corn at http://www.agronext.iastate.edu/corn/production/management/early/rootless.html.
Making Replant Decisions
Stands of 30,000+ will result in maximum yields. Half a corn stand does not mean half the yield, but yield a yield reduction of 20-25% would be typical. The cost of re-planting and yield loss from late planting needs to be compared to any yield loss from stand losses to make a good decision. The following table can help with re-plant decisions:
Influence of planting date and plant population on corn grain yields in Iowa
------------------ Corn Yields (% of maximum) -----------------
Stand April 20 - May 5 - May 15 - May 25 - June 5 -
X 1,000 May 5 May 15 May 25 June 5 June 15
35 100 96 87 70 54
30 99 95 86 69 53
25 95 91 83 67 51
20 89 85 77 63 48
15 81 78 71 57 44
10 71 68 62 50 38
This table comes from the latest Iowa research and modeling which is found on page 12 of the new Corn Field Guide (CSI001).
Numerous gaps of up to 4-6 feet can reduce yields by an additional 5-6%.
The usual method to check corn populations is to measure off 1/1000 of an acre in a row. That is 262 in 20 rows, 175 in 30 rows, 146 in 36 rows, and 139 in 38 rows.
Pythium Damping Off
Many fields are also showing some disease problems, although it is likely that in many of the fields, it is a combination of several factors in addition to disease that is leading to the stand reductions. If the mesocotyl (structure that goes from seed up to nodal roots) rots before the nodal roots have developed, the corn essentially then has no root system so it dies. Many fields are being replanted in SE Iowa where damping off is at least part of the problem. There are many strains of the fungus and the fungicide seed treatments are not effective on all of the strains. For more information on damping off including pictures see http://www.extension.iastate.edu/CropNews/2012/0516robertson.htm.
Time to Scout for Black Cutworms
Cutworms are a sporadic problem and not likely to be an issue in most fields, but fields should continue to be scouted until they reach the V5 growth stage so they can be treated if needed. A general rule of thumb is to treat if you find 2-3% of the plants cut and the worms are less than ¾ inch long. However, higher corn prices may alter that rule. An Excel decision-aid and more information are at http://www.extension.iastate.edu/CropNews/2012/0425sissonjessehodgson.htm.
Check Nitrogen Status With Late Spring Soil Nitrate Test
With the heavy rains in some areas we have likely lost some nitrogen again this year, but it least so far we havent had the long stretch of wet weather which should help to reduce the losses compared to the last few years. A good way to check the N status of the soil is to take one-foot depth soil samples when the corn is 6-12 tall. At least 16 soil cores (24 is better) should go into each sample and about a cup of this (soil bag full) sent to a lab for analysis. Cores should be pulled in a systematic way going across corn rows (ie first core pulled in the row, next one-1/8 the distance between rows, next ¼ distance between rows, etc.). For more information on the process see http://www.agron.iastate.edu/soiltesting/pm1714.pdf. An information sheet for sending samples to ISU is at http://www.agron.iastate.edu/soiltesting/LSN.pdf. The cost for analysis is $5/sample.
Bean Leaf Beetles are Back
We havent seen much of a problem with bean leaf beetles for several years now, but with the mild temperatures this winter, we are having to contend with them again. There have been several reports of beetles flocking to the earliest emerging soybeans. Fortunately young soybeans can tolerate a lot of leaf loss without much of an effect on the final yield, but if stands are being reduced and/or about 2 beetles per plant are found on seedling (VE-V1) soybeans, an insecticide seed treatment may pay off. Seed treatments are also effective on bean leaf beetles. The beetles do have to feed on the soybeans before the seed treatment can kill them, so also expect a little damage even where a seed treatment has been used. The beetles can do a lot more damage during pod fill in August, regardless of whether a seed treatment was used..
FOR YOUR CALENDAR
SOUTHEAST IOWA RESEARCH and DEMONSTRATION FARM, Crawfordsville
SPRING FIELD DAY and 25th Anniversary Celebration (afternoon) &
SPECIAL SESSION FOR CCAs (morning)
JUNE 21, 2012
Details will be posted at http://www.extension.iastate.edu/Pages/eccrops/meetserc.html.
NORTHEAST IOWA RESEARCH and DEMONSTRATION FARM, Nashua
SPRING FIELD DAY
JUNE 28, 2012, 1 4:30 p.m.
Details are posted at http://www.extension.iastate.edu/Pages/eccrops/meetnerf.html.
TECHNIQUES and TECHNOLOGIES TO AVOID SPRAY DRIFT
July 17, 2012, Field Extension Education Laboratory (FEEL) near Boone, IA
Two half-day sessions (morning session repeated in the afternoon) (no cost to participants) will be conducted, focusing on:
- Nozzle selection/use with demonstration on spray table
- Balancing efficacy and drift
- Environmental factors, adjuvants and limitations, field demonstration, etc.
More information and registration will soon be available at http://www.aep.iastate.edu/.
If you have any questions, please feel free to contact the Iowa State University Extension Office.
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