By Roger Elmore, Jenny Rees, and Robert Klein
According to the
USDA-NASS report, 35% of Nebraska’s corn had been planted as of May 5, similar to 38% last year but behind the five-year average of 47%. Emergence was at 2%, similar to last year but behind the 9% average. The April 26 report indicated 16% of corn had been planted compared with 15% last year, and 23% for the five-year average. Corn already planted has no doubt been subjected to
cool soil temperatures. Corn planted into soil with temperatures below 500F may experience difficulties or fail to emerge. Bob Nielsen, Purdue University corn specialist, covered these issues to some depth―no pun intended―in his
recent article.
The following is a list of things to think through as seedlings emerge from cool/wet soils. This is revised from articles Lori Abendroth and Roger Elmore wrote a few years ago.
1. Wait five to seven days after emergence begins then determine the current plant population.
Dig in the gaps to check the status of “missing” plants.
- 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 as you can. Scout in a zig-zag pattern across the field to best represent its overall condition.
- To estimate surviving plant stands, count plants in at least three areas in affected fields. Increase your accuracy by counting plants in 1/100 of an acre in contrast to just those in 1/1000 of an acre―the more the better! We recognize that taking ten 1/1000 of an acre samples in random areas is certainly easier and will likely provide better information than one 1/100 of an acre sample. Use Table 1 to determine the length of row necessary to achieve either a 1/100 or a 1/1000 of an acre sample. Multiply by either 100 or 1000 (depending on which area you counted) to determine the number of live plants per acre.
- Determine whether the seedlings you counted will survive. Were there plants included in the count that appear to be damaged due to insects, disease, frost, hail, flooding, soil crusting, or other factors? Estimate whether severely injured plants will survive by checking the growing point and recovery. If the growing point is discolored and soft, the plant won’t survive. If it is firm and new green growth is emerging from it, it will most likely survive. With recent rain and flooding, one may need to wait a few days to a week to determine subsequent recovery.
- Consider plant stand uniformity (if there is uneven emergence). If uneven emergence is row to row, that is, most rows have emerged but some are not, replanting will likely not increase yield significantly.
- 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%.
Table 1. Row length necessary to equal 1/100th and 1/1000th of an acre in various row widths| Row Width (inches) | Row length in feet to equal 1/100th of an acre | Row length in feet to equal 1/1000th of an acre |
|---|
| 15 | 348’6” | 34’10” |
| 20 | 261’4” | 26’2” |
| 30 | 174’3” | 17’5” |
| 36 | 145’2” | 14’6” |
| 38 | 137’7” | 13’9” |
2. Calculate the expected yield from the existing stand using Table 2.
- 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 89% of the maximum for the April 30 planting date.
- 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 with Table 2. Use planting date and target plant population to estimate the yield potential of the replanted field. Replanting on May 20 at 35,000 plants/acre will result in approximately 87% of the maximum yield. Compare the replanted crop to the original crop which was planted on April 30 and now has a 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 though, there is no guarantee of getting a good stand with replanting. Insect and disease pressure may be greater in replanted fields.
- Remember the concept of planting windows. (See CW articles with Nebraska data and Corn Belt data.) With that in mind, consider that the yield reductions listed in Table 2 may be greater than what may actually occur in 2019.
- Crop competition is an effective weed management practice. Good, uniform stands provide competition for weeds. You may need to improve your weed management plans to compensate for less competitive stands.
3. Estimate replanting costs.
- The cost of replanting a field is often the deciding factor.
- Make sure to include tillage, seed, fuel (for tillage and planting), additional pesticides, labor, etc. Don’t forget that the chance of fall frost is higher for late-planted corn.
- Check with your seed dealer to see what hybrid seed is available and if there is any rebate or price reduction for replant situations. In general, there is not much of a reason to switch to earlier-season hybrids until early June. (See new CW article on when to change hybrids.)
Table 2. Relative yield potential of corn by planting date and populationPopulation
(Plants/Acre) | Planting Date
April 20–May 5
% Max Yield | Planting Date
May 5–15
% Max Yield | Planting Date
May 15–25
% Max Yield | Planting Date
May 25-June 5
% Max Yield | Planting Date
June 5–15
% Max 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 |
|---|
For example, if there are 25,000 plants per acre and the field was initially planted on April 25 and you cannot replant until May 20, it would be better to leave your present stand, which has 95% yield potential, than to replant on May 20 when the yield potential for a stand of 30,000 would be 86%. Make sure you consider replant costs in your decision.
Note: Values based on preliminary Iowa research and modeling; 100% yield potential is estimated to occur with 35,000 plant population and early planting.
From: Iowa State University Extension, Corn Field Guide, 2nd edition. CSI 001. 2013.