Gentry Sorenson, Field Agronomist
Stand evaluation visually and with stand counts after emergence alerts us to possible planter malfunctions, insects, diseases, and environmental factors including wet soil conditions that could affect seed emergence and growth. Tools to evaluate stands include a flat spade or trowel for digging up plants, a tape measure, tables that are referenced in this article, as well as field flags for marking areas of the field to reference later.
Stand counts are a good way to evaluate populations and allow an opportunity to investigate reasons for stand loss. The first step of stand evaluation is to measure 1/1000th of an acre. For growers using 30” row spacing, the length of 1/1000th of an acre is 17 feet, 5 inches. Growers that plant in 15” row spacing, 1/1000th of an acre is 34 feet, 10 inches. Tables can be found online for other row lengths or can be found in IPM1 Corn and Soybean Field Guide available from Iowa State University Extension and Outreach.
Use field flags to mark the distance of 1/1000th of an acre. Next evaluate the plants within 1/1000th of an acre. Count the live plants present in the single row and multiply the number of plants counted by 1,000 to obtain the plant population per acre. Repeat your counts across several areas of the field and average the counts that you have taken to gather the average plant population per acre. Take counts in random areas to eliminate bias in the sampling process.
After the plant populations have been gathered, evaluate why an area has uneven stands. Poor stand areas may include variable spacing, poor seedling vigor, seedling blights, herbicide injury, planter issues, and insect issues. It is recommended to dig up the seedlings in areas of poor stands to evaluate the cause for the stand reduction. These evaluations are valuable for understanding the underlying issues present.
Next, make an evaluation to understand if there is a need to replant or not. Soybean guides have been developed to assist growers. Soybean Replant Decisions (PM 1851) from Iowa State University Extension and Outreach is available as a free download from the Iowa State University Extension Store which provides tables on replant and stand evaluations.
Corn stand evaluation charts can be found in table 2 of CROPR 3161 Guide to Iowa Corn Planting which is also available from the Iowa State University Extension Store. Table 2 can assist in understanding the yield potential of corn compared to the planting date and final stand. In the case of uneven emergence of corn, the publication titled Dealing with Uneven Emergence in Corn is helpful to review.
When evaluating and considering replanting of corn, also evaluate the cost of replanting. Consider timing of fall frost when making a replant decision if replanting commences later in the growing season. Plant well adapted hybrids to the area and evaluate the date of planting in reference to the hybrid maturity.