New Methods to Assess Corn Stands and Make Replant Decisions

Adverse conditions are causing many corn stand establishment issues this spring. Most issues are resulting from plantings which were planted in late March and early April prior to heavy rainfall beginning the first weekend in April. Cool soil temperatures slow the emergence process, which further increases vulnerability. Earlier plantings which were emerging from the soil near that time were not generally as sensitive to the saturated soil resulting from the rainfall. Corn seedlings are quite vulnerable to prolonged anaerobic conditions, particularly from the initiation of germination growth, when roots and the spike emerge from the seed coat, until emergence. The duration of anaerobic conditions is generally more dependent on the timing of rainfall and soil drainage potential, than the total amount of rainfall. Therefore, the when heavy rainfall persists over a 4-day period, as it did this year, aeration problems are likely, particularly on heavy soils, flat-planted fields and deep-planted corn seed. These adverse conditions can cause seedling mortality or stunt seedling development, leading to growth disparity.

A good plant stand is critical to productivity because corn is grown at a relatively low plant density and generally has less ability to alter its reproductive capability, compared to other crops. There is considerable data to characterize corn productivity relative to plant density.  We can generally sustain yields within 10% of optimum if plant population remains 15-20% of the intended goal. However, corn yield potential declines more rapidly as plant population drops beyond 20%. This graph illustrates how corn productivity varies relative to a uniform, vigorous plant population – which is not entirely representative of the marginal corn stands we normally encounter in the field.

It is well-documented that corn yield generally is responsive to higher plant population, but recent research sponsored by the Mississippi Corn Promotion Board and conducted at Mississippi State University has shown corn emergence uniformity is also very important to high productivity. Corn plants that emerge later than others reduce productivity, and thus, also need to be accounted for when assessing marginal stands. These late-emerging plants will lag in growth throughout the season creating disproportionate competition between plants for resources, which reduces crop yield potential. While these “late-emerging” plants do mimic weed competition in some ways, they do contribute some grain production, so they have more value than that analogy. However, corn yields will be collectively reduced, dependent primarily upon the degree of growth disparity representing the stand.

Plants which differ by two leaf stages when using the “Droopy” leaf staging method.

In order to consistently assess emergence disparity, we must use a method to characterize the disparity, regardless of planting date, time and other environmental variables that effect corn growth, such as temperature. Our research showed the best field method to characterize growth disparity is to measure differences using the “Droopy” leaf staging method. This method is based upon the number of leaves mostly emerged, where the top leaf is at least 50% emerged and/or the leaf tip is pointing down or drooping below horizontal. This method is different than the more universal “Leaf Collar” growth staging method, which is based upon fully emerged corn leaves which possess a visible leaf collar.

The reduction in corn productivity resulting from variable emergence is largely determined by the degree of growth disparity representing the stand.  Although various emergence disparity patterns evaluated in the research did not influence productivity, disparity present in one-third of the stand or more may reduce yield.  Corn yield was reduced 10% when growth disparity was at least 2 leaf stages and increased to 18% for at least 3 leaf stages. In summary, growth disparity can reduce corn productivity similar to losses associated with low plant density stemming from seedling mortality.

Of course, you should also realize replanted corn will likely have lower yields relative to delayed planting. Please refer to this article regarding corn yield response to late-plantings.

Replanting in a clean seedbed produced 11% higher yield than intra-planting.

If do you need to replant, the most important thing to remember is to resist the urge to “fill in” or “spot plant” a partial stand. Replanting into a partial corn stand will produce severe competition between the two drastically different ages of corn plants, severely reducing yield potential. Thus, it is absolutely imperative to destroy the remaining live corn plants with either tillage or herbicides, before you replant an unacceptable stand. We also suggest you spray any herbicide at least a day prior to replanting, so that herbicide activity is not hampered by tissue mangled by the planter.

Herbicide options for controlling failed stands of Roundup Ready corn include:

1. Select Max (6 oz/a) + 0.25% NIS + AMS: excellent control of RR or LL corn, can replant corn six days after application.
2. Gramoxone SL (40-48 oz/a) + 0.5% NIS + a photosystem II inhibitor (Direx, Linex, Atrazine, or Simazine @ 16 oz/a): good control of RR and/or LL corn when applied as specified with adequate coverage, no plant back restriction for corn.
3. Liberty 280 SL (32 oz/a): is not effective on Liberty Link (LL) corn hybrids, moderate control of RR corn, no plant back restriction for corn.