Replant Decisions for Field Corn
The Agronomy Team
(Richard Taylor, Joanne Whalen, Mark VanGessel, Nathan Kleczewski, Amy Shober, Phillip Sylvester, Cory Whaley, and Dan Severson, University of Delaware
The prolonged period of cold and wet weather this spring plus the usual culprits such as slugs have led to questions about the adequacy of corn stands this year. In addition, many growers have only recently or have not yet gotten their corn acreage planted. In this article, the UD Agronomy Team will outline considerations involved in making replant decisions as well as whether to plant another crop, assuming herbicides have not eliminated some choices.
The most important consideration when thinking about replanting is timing. How quickly you can make the final decision to replant and actually replant the crop? Waiting too long to assess a stand increases the potential yield loss if a decision is made to replant the field.
Potential yield loss percentages for delayed corn plantings were developed many years ago: advances in corn genetics and irrigation management have significantly improved hybrid performance. It is important to note that the loss per day of delay estimates may overestimate the impact of delaying planting. Yet, these estimates are useful as guidelines for both irrigated and dryland corn production systems.
In mid-May for irrigated corn, every day you delay making a replant decision and actually replanting the crop reduces the hybrid’s yield potential by 0.4 to 0.7 percent for short-season and full-season hybrids, respectively. Delaying planting into early June increases that per day yield loss to 1.3 to 1.7 percent of the hybrid’s yield potential for short-season and full-season hybrids, respectively.
In a dryland cropping situation in mid-May, daily delay in replanting can result in a loss of 0.4 to 0.9 percent of the hybrid’s yield potential for short-season and full-season hybrids, respectively; whereas by early June, a delaying replanting by one day results in a 2 to 1.3 percent loss of the hybrid’s yield potential for short-season and full-season hybrids, respectively. Dryland corn yields can be impacted even more by delayed planting than estimated by these average losses because pollination is also delayed to the hotter and drier portions of summer.
The first step is to determine the plant population to estimate the chances of obtaining the hybrid’s maximum yield potential. Estimate current corn stand by counting the number of plants in a 17 ft 5 inch row length. (For 30-inch rows, a row length of 17 feet and 5 inches is equal to 1/1000 of an acre.) Repeat this count in 6 to 8 random locations for each 20 acre block of a field. Average the number of plants in the 6 to 8 row lengths to determine an estimated population. During past field trials, we saw a 1 percent decrease in yield for each 1,000 plant per acre decline in harvest population. However, with many hybrids now planted at 32,000 to 36,000 or more plants per acre, our former trials determining yield losses with lower populations are questionable for reliability. We suggest that you start calculating the yield loss per loss of 1,000 plants once the population falls below 32,000 since the yield increase as you go above that target is small.
While counting the number of plants, also observe the unevenness of the stand. If the stand has a number of small gaps (1.5 to 3 feet in length), deduct 2 to 10 percent from the hybrid’s expected yield potential with a perfect stand. If there are numerous gaps between plants that measure 4- to 6-feet in length, deduct 10 to 20 percent from the field’s yield potential.
The next step in the process is to estimate the yield potential of the stand actually in the field. Use the stand reduction loss percentages (above) and the realistic yield goal to estimate the yield potential of the reduced stand. This is the expected yield without replanting. You then want to estimate expected yield if you replant. Deduct from that the expected percentage yield loss based on the date that you expect to be able to replant the field. If the initial stand was not planted around the ideal planting date, you may also need to adjust the realistic yield goal for the actual planting date. Make your best guess as to when you can prepare the field for replanting (killing the existing stand), obtain new corn seed, and get back into the field to replant. Keep in mind that the current weather pattern could easily force you to delay planting again, just like it did for the initial planting but it is best not to underestimate how long it will take to replant!
Next, you should calculate the replanting cost including extra tillage (equipment, fuel, and labor) if you plan on doing any tillage either to kill the remaining corn and/or to prepare the seedbed. Add in the planting cost; seed costs; any needed pesticide costs; and, if the corn will be planted late, add in a cost for drying the corn.
Compare the expected yield without replanting with the expected net yield (after you deduct those additional costs involved in reseeding the stand) with replanting and decide if it is worth the effort to replant.
One final consideration is that you should factor in the risks involved in replanting. Replanting corn does not guarantee that you will achieve any better a stand the second time around. If the weather stays bad, if slugs or insects attack the crop, if poor growing conditions continue for much of the remaining season, or a hurricane, hail, or other storm damages the crop later, you may expend a great deal of money for minimal to no benefit.
Other considerations when deciding to replant include:
Sometimes, seeding alongside the rows already in the field is suggested in lieu of a full replant. However, the plants often end up having more than a 2-leaf difference in their stage of growth and the younger plants will be at a competitive disadvantage. Yield will likely be a lot less than expected.
There have been a few places where replanting is necessary and existing plants need to be killed. The difficulty is that the corn is Roundup Ready (in additional many hybrids are also Liberty Link), so control will be difficult. If by chance the corn is not Roundup Ready, glyphosate is the best option. The herbicide options include Gramoxone plus atrazine, Select (clethodim), or Liberty (if not a Liberty Link hybrid). Check the clethodim label and follow the required time between application and replanting because clethodim can cause corn injury if planted too soon. A multi-state project conducted in this region found Gramoxone provided the most consistent control and it performed better on 5 inch corn and then corn that was 2 to 3 inches tall. No treatment consistently controlled all the corn plants. If complete control is necessary, tillage will be required.
If residual herbicides were used, 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 the replanting and do they need to be killed? Would delaying a herbicide application until the corn is up and then using an early postemergence application that includes a product that provides residual control be the best option for the replanted field?
If replanting occurs during May and early June, damage from cutworms, seed corn maggot, wireworms, and white grubs can continue to affect stand establishment. The most common insect problem in later planted corn is the black cutworm. If slugs were a problem on the first planting, weather conditions after planting will determine if they will continue to be a problem. Rescue treatments are only available for cutworms and slugs. The cool, wet conditions that resulted in reduced stands and poor plant growth have also slowed the development of white grubs and wireworms. In addition, wireworms can remain in the larval stage for up to six years, depending on the species. So you can expect them to be present when you re-plant, especially in fields with a history of wireworm problems.
