Richard Taylor, Extension Agronomy Specialist; rtaylor@udel.edu; Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu; Mark VanGessel, Extension Weed Specialist; mjv@udel.edu; Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu; Amy Shober, Extension Nutrient Management and Environmental Quality Specialist; ashober@udel.edu; Phillip Sylvester, Kent Co., Ag Agent; phillip@udel.edu; Cory Whaley, Sussex Co. Extension Ag Agent; whaley@udel.edu; Dan Severson, New Castle Co. Ag Agent; severson@udel.edu
The prolonged period of cold and wet weather this spring has delayed planting for many growers. Late planting dates (roughly after May 26) offer challenges that must be successfully met to ensure the minimum impact on yield potential. In this article, the UD Agronomy Team will outline adjustments and decisions needed to grow a successful corn crop when planting is delayed. We’ll cover some of the management decisions and options available to help late planted corn by practice category.
Soil Fertility: An important potential problem with delayed planting occurs when a portion of the required nitrogen (N) fertilizer has been applied in the weeks prior to when the corn is actually placed in the soil. During the delay, nitrate-N added can be lost via denitrification or leaching and nitrification of ammonium or urea can begin again resulting in the loss of N if the rainfall pattern continues. To give the process more time, ammonium or urea sources can be treated with urease and/or nitrification inhibitors such as Super U or Agrotain Plus and this can delay a significant loss of N through leaching or denitrification by three or more weeks. Losses that do occur will require the grower to apply additional N fertilizer at an additional cost and require changes to the nutrient management plan (NMP).
Also along these lines, the application of manure well before planting can also permit loss of any inorganic N present in the manure. Although the cold weather has delayed the process of mineralization there was a short period earlier this spring when air and soil temperatures rose enough to encourage mineralization and nitrification of organic N from the manure. With additional rainfall and a return to cold temperatures, any nitrate N formed will likely be lost before the crop can grow enough to reach the stage when N uptake accelerates. If N is lost, additional N fertilizer can be applied to the crop, but the NMP will need to be modified.
Although many of Delaware’s growers currently use a banded starter fertilizer and include at least some ammonium sulfate in the starter band, growers may be tempted to speed up the planting process by eliminating banded starter fertilizer. It is true that as we move into June and if soil temperatures finally warm up, phosphorus (P) and potassium (K) will become more available to the crop and may not be needed as part of the starter fertilizer. However, slightly higher than usual rates of starter featuring the soil mobile nutrients, N and sulfur (S), or planning on an earlier sidedress N application should help corn get off to a faster start and keep it growing rapidly during the critical V5 to V9 growth stages when kernel number and row number are being set.
Soil Considerations: Although soil temperature should be increasing rapidly at this time of year, the cooler, wetter conditions we face in 2016 are preventing that increase. The higher the soil temperature, the faster and more uniform seed germination and emergence. Rapid germination and emergence will translate into improved yield potential.
Some options for growers include the use of a turbo-till or similar tillage implement to help dry and warm the surface soil. Although more extensive tillage could be used as well, further delaying planting to complete preparing a fine-firm seedbed is counterproductive. In addition, extensive tillage especially on soil that is at the upper limit or past it for water content can lead to severe compaction issues. Even with a turbo-till, the key to using it successfully will be to avoid any tillage if the soil is too wet since compaction can translate to yield losses that will continue for years. Turbo-till and similar light tillage that warms and dries the soil surface without causing compaction issues will shorten the time until a field can be planted. Keep in mind that this type of tillage will incorporate some of the crop residue or disturb a killed cover crop and may not be acceptable in some situations.
Another option is the use of aggressive row sweeps or row cleaners to clear the top of the seed row and allow the soil to warm faster. This will allow the soil immediately over the seed to quickly warm up and dry if we receive some periods of sunny weather. Again, warmer soil translates to more rapid and uniform emergence and higher yield potential.
Hybrid Selection: Growers often start thinking of changing to shorter season hybrids as planting is delayed into early June. Dr. Peter Thomison from Ohio State University found that a hybrid planted in late May/early June will mature at a faster thermal rate (require fewer total heat units) than the same hybrid planted in late April or early May. He found that the required heat units from planting to kernel black layer decreased on average about 6.8 GDDs (growing degree days) per day of delayed planting so that a hybrid rated at 2800 GDDs planted at the normal time would require 204 fewer GDDs or about 2600 GDDs if planted 30 days late in late May or early June. Dr. Thomison does point out that other factors should be considered when deciding on whether to change from a full season to a short season hybrid. One of these considerations is that a full season hybrid, although yielding more could, have a significantly higher grain moisture at maturity than earlier maturing hybrids if fall weather conditions are not conducive to rapid drydown.
Another factor that relates to insect control is that European corn borer (ECB) damage and yield reductions are often greater even under low ECB pressure when corn hybrids are planted late. This warrants the selection of ECB Bt hybrids whenever possible for late planted corn situations.
Since late planting is most likely to occur on soils that are either warmer than the temperature seen at normal planting time or will warm up much quicker as we move into June, germination and emergence will be better than that seen at the optimum planting date. For early planting dates and optimum plant dates, we often plant 5 to 10 percent higher seeding rates than the target or desired harvest population since we expect greater seedling mortality. For late planting, seeding rates can be decreased to about 3 percent higher than the desired harvest population and this will reduce the production cost at least a little.
Weed Control: If the field has not received a burndown, you may need to adjust your standard burndown program to account for larger weeds. If residual herbicides were used ahead of the anticipated planting, you need to think about when the products were applied and at what rate. Most of the residual herbicides will not provide more than 3 to 4 weeks of activity. What do the labels allow regarding an additional application? Are weeds present at time of planting and do they need to be killed?
Disease Issues: Most issues with stand are caused by wet conditions. No seed treatment will save you from plants submerged in water or growing in standing water for prolonged periods of time. The presence of Pythium or Fusarium on roots of plants growing in wet cool soils does not mean stand loss was caused by these organisms. Rather, stand issues were likely a complex of issues related to poor plant growth and excess water.
Planting into cool soils can result in more issues with pre- or post-emergent damping off due to the seeds remaining in the soil for longer periods of time or delayed seedling growth. If you replant and stick with a 100-120 day hybrid you can end up with stalk rot or stalk strength issues later on, especially if growing unirrigated corn, because the corn may be exposed to more stressful growing conditions (hoy dry) during critical periods of plant growth.
Two other diseases that should be targeted for finding resistant hybrids are gray leaf spot and northern corn leaf blight. Whether you are choosing a corn hybrid to replant or choosing a hybrid for the normal planting time, hybrids with resistance to these two diseases should be high on your list.
Irrigation Practices: On late planted corn, any early moisture stress around V4 to V6 would be more critical and possibly contribute to a yield reduction. Late planted corn is growing faster than is normally seen since there are so many heat units (GDDs) accumulating in June and early July and the soil is probably already warm. Therefore, a grower might fail to recognize how rapidly corn roots are growing and how fast soil moisture is being depleted. This could lead to underestimating the need for additional irrigation. This is one aspect where moisture stress could have a larger impact on the yield of late planted corn.
If wet soil conditions continue into the rapid growth phase, it might become difficult to apply N via fertigation in a timely fashion. In case this is a concern, growers could set their irrigation system to run as fast through the field as possible so the water volume is kept as low as possible while applying N fertilizer to keep the grow growing and developing without causing excessive denitrification, leaching, or root suffocation from water ponding. This applies N almost as a foliar feed application but in a dilute enough solution that foliar burn is not likely to happen.
Insects and Slugs: One of the most common insect problems in later plated corn is the black cutworm. Conditions favoring cutworm out-breaks include a combination of late planted corn, poorly drained soil, heavy broadleaf weed growth, planting into soybean stubble, and reduced tillage. Even if an at-planting protection method is used, including at-planting insecticides, seed treatments or Bt corn, scouting after plant emergence will still be important. If conditions remain cool and wet, wireworms and white grubs can continue to be a problem. Although problems from annual grubs tend to decrease with the warming of the soil and development of grubs from the damaging larval stage to pupation, it will still be important to sample fields for grubs before planting to determine what level and species is present and if larvae have started to pupate. Wireworms can remain in the larval stage for up to six years, depending on the species, so you can expect them to be present in fields with a history of wireworm problems. Since slugs have already hatched, the potential for slug damage will be determined by weather conditions after planting. In wet years, we have seen economic levels of damage from slugs continue through June, so scouting as soon as corn is spiking is important to time a rescue treatment.