Stand Assessments - Corn

Encyclopedia Article

As corn emerges, take time to evaluate corn stands. This involves taking stand counts and visually assessing seedlings to see growth and development appears normal. This can help alert us to potential issues that may due to planting, insects, diseases, and/or environmental conditions.

Group doing stand assessments in corn
As corn starts to emerge, take time to assess stands and do stand counts. Photo by Meaghan Anderson.

Taking Stand Counts

  1. Measure out 1/1000th of an acre based on your row width. For example, with a 30-inch row spacing, measure out a length of 17 feet, 5 inches.
  2. Count the number of live plants in the measured area. Watch this YouTube video to learn more about taking stand assessments.
  3. Repeat this process multiple times across the field to provide a more accurate estimate of the number of plants per acre.
  4. Average the counts and multiply the average number of plants by 1,000 to obtain the plant population per acre. 

Row lengths for 1/1000th of an acre at different row spacings
Table 1. Length of row representing 1/1,000 of an acre at different row widths. Source: Corn and Soybean Field Guide, IPM 1.

When taking stand counts, randomly select these locations and do not intentionally avoid areas that have missing plants or gaps. Try to avoid selecting rows representing the same planter row unit. If one count does not seem to fit the other counts, keep that count separate and make note of where that area is at in the field. Additionally, be looking at emergence uniformity (differences in crop stages), plant spacing, and seedling vigor.

Investigating poor stand assessments

If an area has a lower plant stand, uneven emergence, variable plant spacing, or poor seedling vigor, investigate what may be causing that to occur and how widespread it appears to be in the field. While some issues may not be remedied this growing season, determining the cause of poor stands assessments can help prevent or minimize that issues from happening again in the future.  

Dig up seedlings and check the plants for symptoms of the following:

  • Seed rots and seedling blights (rotting seed and brown discoloration of mesocotyl and seminal roots or missing plants)
  • Anhydrous ammonia burn (uneven emergence, plant wilting and discoloration, brown roots that appear burnt, missing plants)
  • Herbicide injury
  • Insect issues (seedcorn maggot, wireworms, cutworms, armyworms)
  • Planter issues (seed depth, sidewall compaction, seed drop and seed spacing)

Steps in determining if replanting is warranted

  1. Determine the yield potential of the existing stand using the estimates in Table 2 below. For example, if the original planting date occurred on April 15, a seeding rate of 35,000 plants/acre is expected to provide a maximum yield (100%). If the final population is only 24,000 plants/acre, the yield potential is still 94% of the maximum yield potential. Also, consider stand uniformity in regard to evenness in emergence and the presence of large gaps.  A few comments regarding stand uniformity impact on crop yield:
    1. Gaps present in rows can reduce yield potential. The size of the gap impacts how much yield potential could be lost. Yields can be reduced by an additional 5% if larger gaps (4 to 6 feet) within the row are present and by 2% if smaller gaps (16 to 33 inches) are present.
    2. If half or more of the plants in the stand emerge three weeks later than the first emerged plants, replanting could increase yields by about 10%.
    3. If the delay in emergence is less than two weeks between the early and late emerging plants, replanting may increase yield by 5% or less. Replanting would likely not be economical in this situation.
    4. If uneven emergence varies from row to row (i.e. most rows are emerged but some are not), replanting will likely not increase yield.
    5. More information on uneven emergence in corn can also be found in the publication NCR 344 “Dealing with Uneven Emergence in Corn”.
  2. Estimate the replant yield with Table 2 using the expected replant planting date and target plant population. For instance, replanting on May 25 at 35,000
    plants/acre is expected to have a yield potential of 87% of maximum yield. Note that there is no guarantee of getting a good stand with replanting.
  3. Compare the yield potential of the replanted crop with the yield potential of the existing crop. If the yield potential of the existing stand is less than that of the replanted crop, replanting may be warranted.
  4. In our example above, the yield potential of the original crop planted on April 15 with an existing stand of 24,000 plants/acre is 94% of the maximum. Replanting on May 25 with a targeted planting population of 35,000 plants/acre would give a yield potential of 87% of the maximum yield. The yield potential of the original stand is greater than the yield potential of the expected replanted stand.
  5. In addition to comparing yield potentials, also consider the cost of replanting. These costs include any tillage, seed, fuel (for tillage and planting), additional pesticides, labor, etc. 
  6. Other factors to consider when debating to replant or not:
    • It is important to remember that if corn requires replanting, the original stand needs to be destroyed as planting into the original stand will further complicate management. Check herbicide labels to be sure replant of corn is allowable.
    • Evaluate the chance of fall frost occurring prior to the physiological maturity of replanted corn. The Useful 2 Usable corn growing degree day (GDD) tool allows for users to input planting dates and hybrid relative maturity to graph estimated black layer (physiological maturity) date given projected GDD accumulation for the season. This tool shows the average first freeze date as well.
    • In regards to hybrid maturity, research has shown that later-planted corn can adapt to being planted later by requiring fewer growing degree days (GDDs) to reach maturity (Nielson et al., 2002). Therefore, even with a later planting date, it is recommended to plant a well-adapted hybrid for your specific area. An exception to that may be if planting is delayed past June 1, in which it is recommended to switch to a hybrid that falls within the early to middle end of the well adapted hybrid maturity’s for a given area. For more information on hybrid maturity considerations with delayed or late planting, check out this article from Purdue University “Hybrid Maturity Decisions for Delayed Planting.”

Relative yield potential of corn by planting date and population table
Table 2. Relative yield potential of corn by planting date and population. Note: Values based on the APSIM crop model and validated by field research conducted at seven locations across the state. Source: Guide to Iowa Corn Planting, CROPR 3161.

Replanting is not an easy decision, and numerous factors determine a crop’s yield potential. Note that actual yield losses could be greater or less than what is shown in Table 2 because weather conditions are the major driver of corn yield potential.