May 2002
Field Crops 28.42-35

Corn Replant / Late-Plant Decisions in Wisconsin

Joe Lauer, Corn Agronomist

PDF Version

Farmers are faced with corn replanting or late-plating decisions every year. Cold temperatures, wet or crusted soils, and/or pesticide or fertilizer injury may reduce seed germination and seedling emergence. After emergence, stands may be further reduced from insects, diseases, wind, frost, hail, and/or flooding. Stands too dense or non-uniform because of planter malfunctions or variable seeding depth may warrant replanting. Machinery breakdowns or wet soils may delay planting to where corn may not be economically produced and an alternative crop must be grown.

The major decision facing the corn farmer is whether it is more profitable to keep the original stand using a full-season hybrid or replant. Replanting may result in an optimum stand, but it would be planted at a later than desired date using a shorter-season hybrid. This publication describes how to make economically sound replanting or late-planting decisions.

Replanting - The Decision Making Process

Relationships between planting date, plant population and hybrid maturity must be known and this information used quickly to make the best of a less than optimal situation. Reasons for low stands must be determined so the problem can be avoided in future plantings. Observing stand uniformity, plant health, and potential pest problems, and knowing seed availability, alternative crops, and herbicide rotational restrictions enter into the decision. Base your replant decision on proven agronomic facts rather than emotion.

When stands after planting are less than ideal, compare the yield potential of the reduced stand to the yield potential of a late-planted stand. Observe and measure the existing corn stand plant population, their health, and the distribution or "eveness" of remaining live plants.

How To Determine Stand

To make this comparison, first take plant counts in several areas to accurately determine the existing corn stand population. Table 1 presents row length equivalents to 1/1000 acre for various row widths. For example, to determine the average number of corn plants in a 40 acre field planted at a 30 inch row spacing, measure 17 feet, 5 inches along a length of row and count the number of plants. If the average number counted in 15 areas of the field was 26.5 plants, then the average population for the field is 26.5 x 1000 = 26,500 plants per acre. If the stand is poor in spots, treat these areas separately from the rest of the field.

Determining Plant Health

It is easier to judge a reduced stand of healthy plants than one with weakened or partially damaged plants. When hail, frost or other damage occurs, wait 2 to 4 days with temperatures above 70 F before assessing the living stand. If the growing point is not damaged, plants will usually recover and perform better than replanted corn. The corn plant growing point remains protected below ground 2 to 3 weeks after emergence. To determine growing point location and condition, split the stalk down the center with a knife. For normal, viable plants, the growing point will have a yellowish-white color and firm texture. Decayed, discolored tissue indicates a dead plant. Count plants of questionable health as a half-plant in stand assessments.

Uneven Stands

Yields in Tables 2 and 3 are based upon uniform within-row plant distribution, but this seldom occurs in reduced stands. Yields are reduced about 2 to 10 percent if the stand includes several small gaps of 1.5- to 3- feet when 25 percent of the plants are missing. Numerous 4- to 6-foot gaps reduce yields by 10 to 20 percent.

Yield Potential of Reduced Stand Versus Replanted Stand

After taking a living plant population count, you can determine the yield potential of the remaining stand and compare it to replanting at a full stand rate (Tables 2 and 3). Note that full-season hybrids produce best yields at early planting dates, but with late plantings, which would usually occur in a replant situation, shorter-season hybrids perform better. Use Table 4 to determine the adapted Relative Maturity of corn hybrids that apply to your location and planting date.

Example:

Here's an example of how to use Tables 1-3 to make replanting decisions: Jane and John Farmer planted a full-season corn hybrid May 10 near Oshkosh, Wisconsin, with a desired harvest population of 28,000 plants per acre. Emergence appeared complete by late May, but only 14,000 living plants per acre were present, and the stand was somewhat uneven. While it would be possible to rework the field and have it ready to replant by June 1, would it be worth it?

Using Table 3, a full-season hybrid planted May 10 in the Oshkosh area (95- to 100-day Relative Maturity) with a desired harvest population of 28,000 plants per acre should produce 95% of a maximum possible yield. In this case, a remaining stand of 14,000 plants per acre would result in 72% of maximum yield minus 2% for some unevenness, equaling a 70% yield potential. If the farmer used an earlier hybrid (80- to 85-day Relative Maturity, Table 4) and replanted to a stand with 28,000 plants per acre on June 1, the yield potential is 71% (Table 3).

It is unlikely that the extra 1% gain (71% minus 70%) in yield potential would make it worth the time and effort to replant. For example, if the Farmer's expected yield potential is 130 bushels per acre, this amounts to an increase of only 1.5 bushels per acre.

Remember the information in Tables 2 and 3 are only guidelines to help you make decisions about replanting. It will not apply to all situations. Each case must be evaluated individually.

Replanting Costs

Replanting decisions must incorporate the costs of extra tillage (equipment, fuel, and labor), planting, seed and additional pesticides, if required. This often amounts to $20 to $40 per acre. Late-planted corn for grain will also have the cost of extra drying. Extra interest on borrowed money will be required for replant expenses. Replanting costs can be reduced by replanting at a low seeding rate alongside or over the original row to "fill in" the stand without tearing it up. However, although this option saves costs, uneven within-row plant spacing and maturity differences are a problem.

Replanting Risks

There is no guarantee that replanting will result in a full stand. Diseases, insects or herbicide injury that reduced original stands may again cause reductions in replanted corn. Rain may further delay replanting after the field is re-worked and less than average growing conditions for the remaining season may result in grain yields much lower than those indicated in Tables 2 and 3.

Late Planting

The following management practices apply to late planting -- either a delayed initial planting, or a replant situation.

Hybrid Maturity

When planting corn later than May 15 to May 20, use shorter-season hybrids. Table 4 lists alternate hybrid Relative Maturities for delayed planting dates for the standard Relative Maturity belts shown in Figure 1. For additional information on hybrid selection and performance, check Extension publication A3265 - Selecting Corn Hybrids.

With average growing conditions corn planted after June 1 to June 5 in northern and central Wisconsin and after June 10 to June 15 in southern Wisconsin, will probably not mature with reasonable grain yield and moisture content, even with very early hybrids. However, corn silage from shorter-season hybrids may still have acceptable quality when corn is planted until June 20. Corn planted after June 20 will likely contain little or no grain, and only stover (stems and leaves) will be produced.

Pest Control

It is usually easier to control weeds in late corn plantings than in early plantings. Late tillage kills many germinated weeds and crop seedlings are more competitive due to warmer temperatures. For replant situations, weed control must take into account any previous herbicide applications. If herbicides were applied pre-emergence or pre-plant incorporated, their effectiveness may be reduced by the time corn is replanted, especially if the field is tilled before replanting.

Insects normally are a greater threat to late plantings than weeds. Later plantings may have more feeding from second-generation European corn borers, and silk feeding by corn rootworm beetles may also be more severe. Soil rootworm insecticide will need to be applied if the field was tilled since the initial planting application.

Effects of Early Freeze on Yield Potential

Earlier than normal autumn frosts can devastate late-planted corn. Yield is decreased if late-planted corn does not reach physiological maturity before plants are damaged by a freeze. Grain from corn plants killed by a freeze before maturity may be slow to dry down, and it tends to be brittle after artificial drying -- making it more likely to break during handling. Test weight also will be lower when corn is prematurely killed.

If late-planted corn does mature ahead of frost, grain will be wetter and probably have to dry down in weather less favorable for drying. The following lists grain characteristics and appropriate management considerations for corn killed at various growth stages:

Corn Killed in Dough Stage

  1. Kernels contain about 70% moisture.
  2. About one-half of mature kernel dry weight accumulated.
  3. Grain will unlikely achieve maximum yield potential unless stalk, ear and some lower leaves survive.
  4. Corn can be used for good quality silage, but entire plant must be allowed to dry to about 65% moisture.

Corn Killed in Dent Stage

  1. In early dent, kernels contain about 55% moisture; are 3 to 3½ weeks from maturity; and about half of mature dry weight has accumulated. In late dent, kernel moisture is decreasing and yield is within 10 percent of final mature dry weight when kernels are past half milkline.
  2. Corn will make good silage when harvested at a whole plant moisture content of 65%.
  3. Can be harvested for grain after long field-drying period.
  4. Grain yields will be reduced and test weights low.
  5. If plant is only partially killed or the crop is close to physiological maturity before the freeze (kernel milk line half-way or closer to tip), yield loss will be only 5 to 20 percent, and test weight will be lower.
Corn Killed When Physiologically Mature (Black Layer)
  1. Kernel moisture is 28 to 35% depending on hybrid.
  2. Killing freeze will not affect grain yield or quality.
  3. Dry-down rate of grain depends on hybrid and environment.
For a more detailed description of the growth stages of corn, see Special Report 48 - How a Corn Plant Develops.

Crop Choice

If planting is delayed past the time acceptable corn production can be expected, consider planting an alternative crop. Compare the relative yield potential and current price of an alternative crop for a given date with that of late-planted corn.

For example, corn yield potential of a late planting declines at a faster rate than the yield potential loss of soybeans. After June 1, it may be advantageous to plant soybeans, instead of corn, if this fits your rotation. Sunflowers and buckwheat are other grain crops that can be planted very late. Forage sorghum, sorghum-sudan crosses or sudangrass can help boost forage supplies and be planted into July. For more information on late-planted forage crops, see Extension publication A1119 -- Supplementary and Emergency Forage Crops.

You must consider prior herbicide and fertilizer applications, desired rotation, livestock feed requirements, and the possibility of erosion on slopes when you are choosing a crop to plant late. For more information on herbicide rotational restrictions, see UW Extension publication A3646 -- Field crops pest management in Wisconsin.

Summary

A corn replant or late-plant decision is often difficult. Decisions need to be based on sound agronomic and economic principles and the farmer's ability to utilize the crop as silage, if it doesn't reach maturity. The original stand must be accurately counted and evaluated for uniformity and overall plant health. The expected yield for the original stand is then compared to potential replant yield from the later-than-optimum replant date. Identifying stand problems early will help minimize yield reductions from late plantings. Figure 1. Relative maturity zones for full-season corn hybrids planted before May 15.

Table 1. Length of row equal to 1/1000 acre at various row widths.
Row width Row length for 1/1000 acre
15 34' 10"
20 26' 1"
22 23' 10"
26 20' 1"
30 17' 5"
36 14' 6"
38 13' 10"
40 13' 1"

Table 2. Expected corn grain yield for various planting dates and harvest populations in Relative Maturity zones of 70 to 95 days.
Harvest Planting date
population April 20 May 1 May 10 May 20 June 1 June 10 June 20
  percent of expected yield
36000 96 82 100 89 97 89 86 82 63 65 39 46 5 18
34000 95 81 99 88 96 88 85 81 63 65 39 46 5 18
32000 94 80 98 87 95 87 85 80 62 64 38 45 5 18
30000 93 79 97 86 94 86 83 79 61 63 38 45 5 18
28000 91 78 95 85 92 84 82 78 60 62 37 44 5 18
26000 89 76 93 83 90 83 80 77 59 61 37 43 5 17
24000 87 75 91 81 88 81 79 75 58 59 36 42 5 17
22000 85 73 89 79 86 79 76 73 56 58 35 41 5 16
20000 82 70 86 76 83 76 74 70 54 56 34 40 4 16
18000 79 68 83 74 80 73 71 68 53 54 32 38 4 15
16000 76 65 80 71 77 70 69 65 50 52 31 37 4 15
14000 73 62 76 67 74 67 65 62 48 49 30 35 4 14
12000 69 59 72 64 70 64 62 59 46 47 28 33 4 13
10000 65 55 68 60 66 60 58 56 43 44 27 31 3 13
Figures for shorter-season hybrids are in italics. The actual Relative Maturities of short-and full-season hybrids vary with location and soil type. See Table 4 for more specific Relative Maturity values.

Table 3. Expected corn grain yield for various planting dates and harvest populations in Relative Maturity zones of 95 to 115 days.
Harvest Planting date
population April 20 May 1 May 10 May 20 June 1 June 10 June 20
  percent of expected yield
36000 96 91 99 95 95 93 85 87 63 71 40 55 8 32
34000 97 92 100 96 96 94 85 87 63 72 40 56 8 32
32000 97 92 100 96 96 94 86 87 63 72 40 56 8 32
30000 96 92 100 96 96 94 85 87 63 72 40 56 8 32
28000 96 91 99 95 95 93 84 86 63 71 40 55 8 32
26000 94 89 97 93 93 92 83 85 62 70 39 54 8 31
24000 92 87 95 91 91 89 81 83 60 68 38 53 7 31
22000 89 85 92 88 89 87 79 81 58 66 37 51 7 30
20000 86 82 89 85 85 84 76 78 56 64 36 49 7 29
18000 82 78 85 81 82 80 72 74 54 61 34 47 7 27
16000 78 74 80 77 77 76 68 70 51 58 32 45 6 26
14000 73 69 75 72 72 71 64 65 47 54 30 42 6 24
12000 67 64 69 66 67 65 59 60 44 50 28 38 5 22
10000 61 58 63 60 60 59 54 55 40 45 25 35 5 20
Figures for shorter-season hybrids are in italics. The actual Relative Maturities of short-and full-season hybrids vary with location and soil type. See Table 4 for more specific Relative Maturity values.
 

Table 4. Relative maturity of adapted corn hybrids for different planting dates and relative maturity zones in Wisconsin.
Full-season relative maturity zone Relative maturities** for late planting
May 20 June 1 June 10 June 20
(planting before May 15) Days
80 and earlier 75-80 75-80 (silage) -- --
85-90 80-85 75-80 (silage) -- --
90-95 85-90 75-80 75-80 (silage) --
95-100 90-95 80-85 75-80 (silage) --
100-105 95-100 85-90 75-80 75-80 (silage)
105-110 100-105 90-95 80-85 75-80 (silage)
110-115 105-110 95-100 85-90 75-80 (silage)
* To determine the Relative Maturity belt for your location, see Figure 1.
** These Relative Maturities are for grain unless silage is indicated. Relative maturities for silage can be 5 days longer than those listed.

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