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

Kernels contain about 70% moisture.
About one-half of mature kernel dry weight accumulated.
Grain will unlikely achieve maximum yield potential unless stalk, ear and some lower leaves survive.
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

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.
Corn will make good silage when harvested at a whole plant moisture content of 65%.
Can be harvested for grain after long field-drying period.
Grain yields will be reduced and test weights low.
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)

Kernel moisture is 28 to 35% depending on hybrid.
Killing freeze will not affect grain yield or quality.
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
Full-season relative maturity zone	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.