Lori Abendroth and Roger Elmore
14 May 2010
Determine the remaining plant population. Calculate the plant population in several "random" areas in the affected part of the field to help estimate the existing population. Do not be tempted to go to the worst area and start counting there, ignoring the better parts of the affected area. The idea is to characterize the field as best you can. Try to make a zig-zag pattern across the field to best represent its overall condition.
To estimate surviving plant stands, you will want to count plants in at least three places in the affected field. Your accuracy will be greater if you count plants in 1/100 of an acre. Use Table 1 to determine the length of row necessary to achieve 1/100 of an acre. You can also measure 1/1000 of an acre; more than three measurements should be taken though to increase accuracy.
Table 1. Total linear feet of row required to make 1/100 and 1/1000 of an acre at different row widths. Row spacing (inches) Row length for 1/100 acre (linear feet) Row length for 1/1000 acre (linear feet) 7 747 74.7 10 523 52.3 15 348 34.8 20 261 26.1 22 238 23.8 28 187 18.7 30 174 17.4 32 163 16.3 34 154 15.4 36 145 14.5 38 138 13.8
Determine whether the population you counted will completely survive. Were there plants included in the count that appear to be damaged due to insects, disease, frost, hail, flooding, soil crusting, etc? You will need to estimate whether any plants are severely injured or will soon die. These considerations may reduce the number of plants in the field that will actually contribute to final yield.
Consider plant stand uniformity (if there is uneven emergence).
- If uneven emergence is row to row, that is, most rows are emerged but some are not, replanting will likely not increase yield.
- If the delay in emergence is less than two weeks between the early and late emerging plants, replanting may increase yields, but by only 5% or less. Replanting would likely not be economical.
- Yet if one-half or more of the plants in the stand emerge three weeks later than the initial plant emergence, replanting may increase yields by about 10%. Several tools are available for estimating yield loss from variable stands, such as the worksheet found in Tools to calculate yield loss from uneven heights.
Calculate expected yield from the existing stand.
Table 2. Relative yield potential of corn by planting date and population. Note: Values based on preliminary Iowa research and modeling; 100% yield potential is estimated to occur with 35,000 plant population and early planting. This table is from the ISU Corn Field Guide (CSI 001 publication) on page 12.
Planting Date Population
April 20-May 5 May 5-May 15 May 15-May 25 May 25-June 5 June 5-June 15 Percent Maximum Yield 45,000 97 93 85 68 52 40,000 99 95 86 69 53 35,000 100 96 87 70 54 30,000 99 95 86 69 53 25,000 95 91 83 67 51 20,000 89 85 77 63 48 15,000 81 78 71 57 44 10,000 71 68 62 50 38
The potential yield associated with the original planting date with the actual population will give a fair estimate of the actual yield of the problematic field. Table 2 summarizes planting date and plant population (final stand) relationships. For example, if the original planting date was April 30, a population of 35,000 plants/acre is expected to provide maximum yield, based on Table 2. If the population is only 20,000 plants/acre, yield potential is still 89% of maximum.
If several 4- to 6-foot gaps occur within the row, yields will be reduced an additional 5% relative to a uniform stand. Stand gaps of 16 to 33 inches will only reduce yield by 2%.
Estimate replant yield. Planting date and target plant population are used to estimate the yield potential of the replanted field. Use Table 2 for this too. Replanting on May 20 at 35,000 plants/acre will result in approximately 87% of maximum yield. Compare the replanted crop to the original crop which was planted on April 30 which has an existing population of 20,000 plants/acre, and consider the costs of replanting. Expected yields are 89% for retaining the old stand versus 87% for a replant. Remember also that there is no guarantee of getting a good stand with replanting. Insect and disease pressure may also be greater in replanted fields.
Estimate replanting costs. The cost of replanting a field is often the deciding factor. Costs include tillage, seed, fuel (for tillage and planting), additional pesticides, labor, etc. Moreover, the chance of fall frost is higher for late-planted corn. Check with your seed dealer to see what hybrid seed (shorter maturity) is available and if there is any rebate or price reduction for replant situations.
This article has been updated on 14 May 2010. Portions of this text were also taken from a Crop Watch article (University of Nebraska extension newsletter) written by Roger Elmore and Lori Abendroth on 20 May 2005.