Considerations for Soybean (Re)Planting in July

Carrie Knott, Extension Agronomist-Princeton, University of Kentucky

Considerable rainfall has accumulated throughout most of Kentucky.  Since June 1 the average rainfall for the Commonwealth is more than 3 inches above normal, with almost 2 inches of that falling in the last 7 days.  There are areas of KY that have received 10 or more inches in the last 3 weeks. Given these conditions many producers must decide whether to plant and in some cases re-plant soybeans once fields dry. 

When considering whether to plant late or re-plant two things come to mind. 

Profitability Analysis of Late Planted Corn Compared to Soybeans

Greg Halich, Department of Agricultural Economics, University of Kentucky

May 2, 2011

Background:
Although new-crop corn and soybean prices are near record highs, planting conditions remain terrible throughout Kentucky as of early May 2011.  It is estimated that only around 5% of the corn crop has been planted.  Grain farmers throughout the state are now worrying whether they will get their crops out in a timely manner this year.  An associated question being asked is at what point should they stop planting corn and switch to soybeans?  In order to answer this question, we need to estimate potential yield losses for late planted corn and compare the associated profitability against soybeans.

Wet Conditions Create Weed Management Challenges in Corn

 Jim Martin and J.D. Green, Extension Weed Specialists, University of Kentucky, April 27, 2011

The prolonged wet conditions this spring have created additional challenges in regards to weed control. The following are possible scenarios that corn farmers may encounter and factors to consider as we progress through the next several days.

Later Corn Planting Means Faster Development

Chad Lee, Extension Agronomist, University of Kentucky

Corn planted later will germinate and emerge more quickly, and go through crop stages faster than corn planted early.  Typically, corn requires about 100 growing degree days (GDDs) to emerge and about 200 GDDs to reach the V2 growth stage. An accumulation of about 475 GDDs is required to reach V6, the stage when the growing point moves above ground.

Wet Soils and Corn Seedling Disease Risks

Paul Vincelli, Extension Plant Pathology, University of Kentucky

Soggy soils certainly are favorable for important seed rots and seedling diseases of corn, especially Pythium diseases. However, two things are working in the producers’ favor:

1. Pythium species attack stressed corn. Soils in May are generally warm enough that corn is better able to fight off infections than it would be in late March or early April; 
2. Common seed-treatment fungicides provide a fair amount of protection against seed and seedling diseases. 

It’s always a good idea to monitor corn stands for emergence and stand establishment, but I am less worried about widespread seedling problems than I am about leaf diseases. Continued sogginess could eventually result in major delays in planting. Corn crops planted in early to mid-May and beyond are at greater risk from a variety of destructive diseases, especially gray leaf spot, northern leaf blight, and southern rust. We’re not at that stage yet, but it is worth being aware of this potential risk.

Worrying about Small Soybeans

Some farmers are concerned that the smaller growth of soybeans could result in reduced yields. Some are questioning the use of foliar fertilizers and/or fungicides to help make up the difference.

Soybeans were planted late across most of the state. According to the latest Crop and Weather Report from NASS, soybean flowering is at 35%, behind the five-year average of 47%. Most farmers' "internal clock" says that soybeans should be larger by now. Most years, that is correct. This is not most years.

Some soybeans are getting to flowering (growth stage R1) and may be a little smaller than in previous years. The cooler temperatures combined with later planting dates will cause smaller plants. The smaller plants could be a concern if rows are not closed in shortly after flowering. If the soybean rows are closed, then height is less of an issue. As long as the rows are closed, tall plants do not automatically equal high soybean yields.

If the rows are not closed and the soybeans begin to flower, then yield potential is likely lost. As the soybeans move into pod development and the rows are not closed, yield potential is likely lost. If the soybeans get to seed fill and the rows are not closed, yield potential is lost. This brings us to the main question: will a foliar fertilizer or a foliar fungicide help? The short answer...probably not.

Fungicides will not improve the speed at which soybeans grow and will not help with canopy closure, in the absence of a disease. Fungicides will help soybeans retain leaves, if a disease is present in the field. However, the cooler night temperatures and the smaller soybean plants both contribute to less of a threat from diseases this season.

Foliar fertilizers will not compensate for lower temperatures. They will not increase the speed of growth, assuming P2O5 and K2O levels are adequate in the field. They will make the plants greener and that might make someone feel better.

If your, or your neighbor, is absolutely set on spraying something, then consider the foliar fertilizer. It will likely make the plants greener and it should cost a little less than the fungicide. Or, take that money you would have spent on the foliar product(s) and take a trip someplace warm. Someplace where you don't have to see the soybeans for a couple weeks. It just might make everyone happier, including your friends! For others, keeping that money in the bank may be the best stress reliever right now.

Bottom Line:
Small soybeans or late-planted soybeans that do not reach full canopy by flowering probably have lost some yield potential. Cooler temperatures also reduce the chances of soybeans reaching full canopy by flowering. In hindsight, the best management practice would have been to plant in 7.5-inch rows. The narrow rows would have improved the chances of getting complete canopy closure by flowering. Foliar fertilizers and fungicides will not make up the difference in temperatures, planting date or row spacing.

Corn, Late Planting and Management

Photo: Possible Mn defiency in corn, which is most likely due to weather and not lack of Mn in the soil.

Believe it or not, corn has emerged on 96% of the acres in Kentucky which is right in line with the five-year average, according to the Kentucky Crop and Weather Report. While emergence is on track, planting was not, and management decisions need to be adjusted.

Preplant nitrogen fertilizer and preplant herbicides may have been lost to the heavy rainfalls. Farmers may need to sidedress additional N and apply postemegernce herbicides. Because of the late planting, farmers will have a smaller window to make these postemergence applications.

Corn planted April 1 will likely reach V6 growth stage in 35 days, while the same hybrid planted May 15 will reach V6 in about 22 days. Corn planted even later will reach V6 earlier.

Many postemergence herbicide labels have restrictions for V6 corn. Corn will need about 20 to 40 lbs of N/acre through about V6. If additional N has not been applied by V6, then yield losses can be expected.

Determining how much N fertilizer or herbicide was lost from the heavy rains is not easy. A calculation for N fertilizer lost is available in the Corn and Soybean Newsletter from April. Assessment of herbicide losses are more of a case-by-case assessment.

In addition, the rapid growth and wet weather can result in some transient nutrient deficiency symptoms...symptoms where weather has more to do with nutrient deficiency that availability of the nutrient. Normally in these cases, the best cure is sunshine and warm weather. Often farmers will spray a foliar fertilizer over the top and a couple days later the crop turns greener, but it may not yield any more. We always suggest to leave a test strip, and take both the treated and non-treated areas to yield.

As always, consult your county agents for more information.

Planting Soybeans Late and Target Populations

The USDA Crop Progress and Condition Report for June 7, 2009 indicates that soybeans planting is 48% completed, well below the 5-year average of 71%. About 32% of the soybean plants had emerged.

Normally, for full season soybeans, we suggest that a final stand of 100,000 plants per acre is sufficient for maximum yield. However, we are getting later into the season. For full season soybeans planted today or later, farmers may want to increase the goal of a final stand to 140,000 plants per acre. Double-crop soybeans (soybeans planted into wheat stubble) should attempt to get a final stand near 160,000 plants per acre. In addition, any soybeans being planted today or later should be in row widths of 15 inches or less.

The recommendation for the higher target population and narrow rows, is because a fundamental principle for higer yields is getting full canopy closure when the soybeans flower. These later planting dates require more soybean plants per acre and narrow rows to get to full canopy closure by the time the soybeans flower.

Some things to consider when planting corn in May

Jim Herbek and Chad Lee, Extension Agronomists, University of Kentucky

The following was taken from the most recent Corn and Soybean Newsletter on corn planting dates.

Use a corn hybrid with the Bt trait for corn borer. University of Kentucky research shows a yield benefit and an economic benefit to Bt corn borer hybrids planted in May. The later the planting date, the greater the yield benefit for the Bt hybrid.

Possibly switch to an earlier-maturing hybrid. Based on calculations of growing degree days remaining in an average season, hybrids with relative maturities of 118-day (and less) will reach physiological maturity (black layer) before frost in west Kentucky even if planting is delayed until late May. For late May plantings in central and eastern Kentucky, hybrids of 116-day relative maturity or less are likely needed. If planting occurs in early to mid-June, then a switch to hybrids of 113 to 116-day relative maturities will likely be needed. For extremely late plantings (after mid-June), hybrids of 110 to 113-day relative maturities or less are needed.
Keep the seeding rates and row spacing the same. Two exceptions: if someone convinced you to try 38,000 seeds per acre, try it in a small area instead of a large field. Your odds of getting decent yields from late plantings are good, but your odds of getting excellent yields are not.

Keep the seeding rates and row spacing the same. Two exceptions: if someone convinced you to try 38,000 seeds per acre, try it in a small area instead of a large field. Your odds of getting decent yields from late plantings are good, but your odds of getting excellent yields are not.

If you have the ability to plant narrow rows or twin rows, the late planting date could make for some excellent comparisons. We know that going to narrow rows in late-planted soybeans improves yields. Perhaps the same could occur in corn. Some on-farm comparisons would be excellent. One caution here: setting up a really good comparison takes time, and when fields are suitable for planting again, you won't have much time.

For more information on corn planting date and hybrid options, refer to publication AGR-195: Replanting Options for Corn that includes a table on Kentucky location, planting date, and expected date to reach black layer (physiological maturity) for three hybrid maturities.

For more questions about corn planting dates, contact your local county extension agent.

Late Corn Planting Usually Results in Faster Corn Development

Chad Lee, Grain Crops Extension
Plant and Soil Sciences, University of Kentucky

Corn planting in May will germinate, emerge, grow and develop in warmer temperatures compared to corn planted in April. Two critical stages of growth for early season management of corn are at growth stage V6 and V8. The growing point normally moves above ground on V6 corn (corn with six visible collars). If possible, N fertilizer should be applied by V8 (see related article in this newsletter). Many postemergence herbicide labels have applications restrictions for corn at either V6 or V8.

Based on data from the National Corn Handbook, corn should reach V6 in about 475 Growing Degree Days (GDD) and V8 in about 610 GDD. Using data from the University of Kentucky Ag Weather Center, over the last five years at Princeton, corn seeded on May 1 would have reached V6 and V8 about 9 and 12 days earlier, respectively, than corn seeded on April 1. Corn seeded on May 15 would have reached V6 and V8 about 13 and 16 days earlier, respectively, than corn seeded April 1.

As you plant corn late, and try to get soybeans planted as well, your postemergence herbicide applications and sidedressing of N fertilizer will need to occur earlier, relative to planting.

More information about late planting of corn is available in the Corn and Soybean Newsletter and on the Grain Crops Extension website at the Late Corn, 2009 page.

Fertility Considerations/Strategies for Late Planted Corn

Lloyd Murdock and Greg Schwab, Extension Soils Specialists and John Grove, Soils Research
Plant and Soil Sciences, University of Kentucky

At this late stage in the season, priorities should be on planting the corn crop, first, then applying needed fertilizers after planting. Fertilization after the crop is planted is not yield-reducing and, in some cases, can actually cause nitrogen (N) fertilizer use to be more efficient.

Phosphorus (P) and Potassium (K)
There may be a few situations where P and K were not applied prior to corn planting. If the soil test P and K levels for a field were in the medium to high range, any fertility added this year would generally maintain these soil test levels and would have little, if any, effect on yield. This would probably be the situation in a majority of Kentucky corn fields. Therefore, adding P and K fertilizers after planting would not be a problem and, if the field's P and K soil test levels are in the upper part of the medium range or higher, the P and K could be delayed until this fall. If a field tests in the low soil test range, the P and K should be added this season, but can be delayed until after planting.

P and K fertilizers are soluble in water, so they dissolve with rain and move into the soil fairly quickly. They will only move into the top 2 to 3 inches, but this will be enough to make them available to the plant, in most situations. If the field was no-tilled or has a lot of residue on the surface, many corn roots will be close to the surface and will take up added P and K. A clean tilled field might result in reduced uptake. This would more than likely only be a problem if P and/or K were in the low soil test range and there was an extended period of dry weather that resulted in the top few inches of soil becoming very dry.

Nitrogen (N) Management Strategies for Late Planted Corn

Once the optimum planting date has passed, the yield loss from further delayed planting far exceeds the yield loss associated with post-planting N applications. Your goal should be to get corn planted as soon as soil conditions permit. Your corn-N management situation, field by field, falls into one of two categories: a) some or all N has already been applied; b) no N has been applied. Your N loss situation has also shifted. Early N applications should be managed to avoid denitrification loss, while later applications need to be made so as to avoid volatilization loss. Because of the wet spring, the soil will contain less residual N, or N mineralized from organic matter, than normal.

For the full article, go to the Corn and Soybean Science Newsletter.

Avoiding Sidewall Compacting During Late Corn Planting

Lloyd Murdock, Extension Soils Specialist
Plant and Soil Sciences, University of Kentucky

Sidewall compaction can result from planting a crop when the soil is a little too wet. This damaging effect can be even greater on soils with a relatively high clay content at the surface. It occurs when the double disc opener leaves the side wall of the planting furrow smooth and compacted (slick as opposed to shattered) as it pushes the soil aside. The trailing press wheel then increases the compaction with its downward force. If the soil stays very moist or wet, the roots may be able to penetrate the compacted mud at the sidewall and expand further into the soil. However, if the weather turns dry after planting, the sidewalls then harden, and roots are not able to push through since there are no pores or cracks. This causes the roots to grow within the planting furrow, along the direction of the row. Although plants may look normal at emergence, they will begin to show nutrient and drought stress after the corn is several inches high. This problem may be more common in no-tillage because no-tillage soils have better structure, and it is easier to traffic them in a wetter condition. The old adage of "waiting on no-till" is a good one. Sidewall compaction can also occur with conventional tillage. If you can mold the soil into a ball in your hand and the soil ball will not easily crumble apart, it is too wet to plant.

Increased Risk of Foliar Diseases in Late-Planted Corn

Paul Vincelli, Extension Plant Pathologist
Plant Pathology, University of Kentucky

Rainfall patterns have delayed corn planting throughout much of the state. Late planting increases the risk of damaging levels of certain foliar diseases, particularly gray leaf spot, southern leaf blight, and northern leaf blight.

Several factors can contribute to this increased risk:

1. When there is a mix of fields planted early near fields planted late, the early fields can be a source of spores for late-planted field. The early fields act a bit like “Typhoid Mary”.


2. Compared to early fields, late-planted corn is often at an earlier stage of crop development during periods of spore release and leaf blighting. Since leaf blighting early in plant development is more harmful to yields than late-season blighting, the late-planted fields have the potential to be hit harder than earlier fields.

Fields not planted until the last week of May or into June have the highest risk of foliar disease. Producers planting corn late this spring should use hybrids with adequate levels of resistance to gray leaf spot. Selecting hybrids with good resistance to gray leaf spot is especially important if the field is under conservation tillage (30% or more residue cover) and has had corn anytime in the last two years. Also, if a field has a recent history of southern leaf blight or northern leaf blight, consider those diseases in hybrid selection.

Of course, many producers have already purchased seed for this spring. If a field is sown late and the hybrid doesn’t have substantial resistance to the diseases mentioned above, a fungicide application is more likely to be cost-effective. Figure 1 lists the factors that increase the likelihood of getting a positive economic return from a fungicide application in corn. The more of those that are in place, the more likely a corn field is to benefit economically from a fungicide application.

If you do choose to use fungicides, it is always a good idea to leave at least one untreated strip in the field in order to see if the fungicide provided any benefit. Sometimes it will but often it won’t, and getting on-farm evidence helps in making future farming decisions.

Figure 1. The more of these factors are in place, the higher the probability of getting a positive economic return from a foliar fungicide application.

Planting Dates Affect Corn Insect Management

Ric Bessin, Extension Entomologist
Entomology, University of Kentucky

Planting date is one of the more important factors determining the potential for insect problems in field corn. Many may not realize it, but the time at which the seed goes into the ground determines which insect pests are more likely to be encountered. Some pests are favored by early planting dates while others are more likely to show up in damaging numbers in late planted corn.

There are two reasons for differences in pest problems due to planting date. First, for many corn pests, the plant has specific vulnerable stages. For example, with cutworms, the plant is susceptible to cutworm damage anytime between emergence and when the plant reaches 12 to 18 inches in height. Second, pest population numbers tend to run in cycles. Usually a pest reaches damaging levels only during certain periods of the season. With black cutworm, because it is a migratory pest, usually it will only reach damaging levels in the late spring. Fields with early and normal planting dates usually escape significant cutworm damage because the plants have grown out of the vulnerable stage by the time the damaging populations arrive.

Wireworms and the damage they cause on the other hand are favored by early planting. The reason for this is that the cool soil slows germination, emergence and growth. Late planting into warmer soils speeds germination, emergence and growth thusly limiting wireworm damage. With treated seed and later planting dates, wireworm is less likely.

There are two generations each of European and Southwestern corn borer each year. The first generations attack in late June and early July, and the second generations begin their assaults in early to mid August. Corn borers prefer whorl-stage corn for egg laying. Studies at the UK Research Center in Princeton Kentucky have shown that corn planted after May 10 is at much greater risk to yield loss from late season European and Southwestern corn borers. Growers using Bt corn often save this seed for later plantings to protect against corn borer attack.

Fall armyworm is another pest that migrates from southern states each spring and is favored by late planting dates. This pest also tends to prefer to lay eggs on whorl-stage corn. Corn which planted after June 1 and that is still in the whorl stage in late July and early August is more likely to be damaged by fall armyworm. Not all Bt corn for corn borer provide effective control of fall armyworm. Insecticides used for fall armyworm control in whorl stage corn need to be applied before the larva forms a frass plug in the whorl of the plant, as the larva feeds under the plug and the plug limits penetration of the spray into the whorl.

Unfortunately, we often have little control of planting date and are at the mercy of the weather. But understanding what your potential problems are should help you manage your crop more effectively.

Avoiding Anhydrous Ammonia Seedling Injury

Greg Schwab, Extension Soil Specialist
Plant and Soil Sciences, University of Kentucky

The extremely wet spring experienced over much of Kentucky has delayed anhydrous ammonia applications. When the soil finally is dry enough for field work to begin, corn producers likely will not be willing to further delay planting to allow anhydrous ammonia applications to become safe. The standard recommendation is to wait 7 to10 days after injection before planting corn, but delayed planting may also reduce grain yield. A second option is to plant corn and then sidedress anhydrous after corn emergence. If anhydrous ammonia is applied before planting, then care must be taken to avoid seedling injury.

The Problem
Free ammonia (NH3) in the soil is extremely to toxic emerging seedlings and their roots. When anhydrous is injected into the soil, it immediately begins to convert into ammonium (NH4+) by associating with hydrogen ions. Hydrogen ions can come from soil cation exchange sites, but most come as a result of splitting water molecules, leaving behind hydroxyl ions (OH-). This reaction dramatically increases the soil pH at the injection site, which allows some free ammonia to exist in the soil for some time. Several soil factors, including moisture content, texture and temperature, affect the length of time that ammonia persists in the soil. Low soil moisture causes the ammonia to persist longer in the soil, as does cooler soil temperatures. However, shot-term ammonia concentrations are higher in warm soils. Excessive soil moisture increases chances of ammonia loss.


For more on this issue, go the Corn and Soybean Newsletter.


Many anhydrous ammonia applications were delayed because of the wet weather.


A handheld pH meter can help you quickly determine the soil pH, and thus, the risk of ammonia injury to germinating and seedling corn.