# Evaluating Corn Stands

May 22, 2009

 Table 1. Row length required to equal 1/1000 acre for various row widths. Row width (inches) Row length (feet) to equal 1/1000 acre 20 26.1 28 18.7 30 17.4 36 14.5 38 13.8 40 13.2

Now is a good time to evaluate corn stands. Getting a uniform stand at the right population is an important part of getting a good yield.

To determine the plant population, use Table 1. First, find your row width in the first column, then read across to the number in the second column. Count the plants in this row length for the row width that you have.

Do this several times in the field and calculate an average number of plants per row length. Since this is the average number of plants in 1/1000 of an acre, multiply that average by 1000 to get the population.

Iowa State University developed a table with final stand and planting date to estimate relative yield potential (Table 2). Iowa's rainfed production is similar to Nebraska's irrigated production, thus the populations are higher than in dryland Nebraska.

 Table 2. Influence of planting date and plant population on corn grain yields. (Iowa State) Planting Date April 20-May 5 May 13-19 May 26-June 1 June 10-16 June 24-28 Final Stand* Relative Yield Potential (%) 28,000-32,000 100 96 90 68 52 24,000 94 93 85 64 49 20,000 81 80 73 55 42 16,000 74 73 67 50 38 12,000 68 67 61 46 35 *Assumes uniform plant spacing. (Based on current research, this table potentially overestimates corn yields for later planting dates.)

For example, if you average 24,000 plants per acre and cannot replant until the May 26 to June 1 period, it would be better to leave your present stand, which has a 94% yield potential, than to replant at the later date when the yield potential for a stand of 28,000-32,000 would be 90%. In most situations it may be best to reduce the yield potential of the replanted area by about 10% to cover problems associated with replanting.

## Seedling Uniformity

Another important factor to assess when considering whether to replant is the uniformity of seedling emergence. Research has shown that if one out of six plants is delayed by two leaf stages, yields can be reduced by 4%. If one out of six plants is delayed by four leaf stages, yields can be reduced by more than 8%. Obviously, uniform timing of emergence is critical for maximizing yield, but little can be done to improve it after the crop has emerged.

Often plants that are more than three leaf stages behind will be barren and not contribute to yield. Don't count these plants when estimating your population. Try to determine the cause of any observed delays and take steps to correct the situation next year. Check out the uniformity of planting depth and seed-to-soil contact as they both effect germination and emergence uniformity.

After considering all of these factors, you also need to calculate and consider the cost of replanting. In many situations this could be about 10% of the potential yield. Each producer needs to calculate the cost and make an independent estimate for their operation. The numbers presented here are only guidelines.

Before proceeding, always contact your crop insurer, Farm Service Agency, and others with an interest in your crop production.

Bob Klein