Cooperative Extension Service
West Lafayette, IN 47907
Effects of Uneven Seedling Emergence in Corn
P. R. Carter, University of Wisconsin
E. D. Nafziger, University of Illinois
D. R. Hicks, University of Minnesota
G.O. Benson, Iowa State University R.R. Johnson, Deere & Co.
D.G. Coffman, Texas A&M University R.L. Nielsen, Purdue University
W.J. Cox, Cornell University
Corn seedlings may emerge unevenly when soil or weather conditions are
not ideal at planting. A full stand may eventually be achieved, but
plants emerge at different times. This publication discusses why corn
emerges unevenly, describes research that measured how much uneven
emergence reduces corn yields, provides recommendations for managing
uneven emerging stands, and gives suggestions for obtaining uniform
Why Corn Emerges Unevenly
Soil Moisture Variability
Uneven corn emergence is most commonly caused by variable soil
moisture levels in the seed zone at or shortly after planting.
Moisture at seed-depth may be adequate for seed germination and
emergence in some areas of a field but not in others. Soil moisture in
the seed zone can vary within a field because of differences in soil
type and topography. Cloddy seedbeds caused by working the field too
wet can create uneven contact between seed and soil, allowing some
seeds to absorb enough moisture to germinate while others are too dry.
In many cases, some seeds planted in dry soil do not germinate and
emerge until after rainfall occurs, which could be several weeks after
planting. As a result, a field can have a mixture of taller and
shorter plants, with plant size differences depending on time from
planting to rainfall. Emergence time may vary between parts of fields,
from one row to the next, or from one plant to the next.
Soil Temperature Variability
Uneven soil temperature also causes uneven corn emergence, especially
when planting early or under reduced tillage conditions. Soil
temperatures at seed-depth can vary if crop residues from reduced
tillage systems are unevenly distributed, if seed depths vary, or if
soil within fields varies in type and topography. Seeds planted under
heavy residue or somewhat deeper than normal are exposed to cooler
soil, and generally emerge a few days later than seeds planted under
bare soil, or at shallower depths. The negative effects of variable
soil temperatures are particularly important when temperatures at the
seed zone are near the critical 50 degree F threshold, where minor
fluctuations in temperature can significantly influence germination.
Corn may also emerge unevenly because of variable soil crusting,
herbicide injury, and insect or disease damage. Finally, uneven corn
emergence occurs when corn growers replant damaged fields by "filling
in" the existing stand, rather than tearing up the field and starting
How Uneven Emergence Affects Grain Yield
Competition from larger, early-emerging plants decreases the yield of
smaller, late-emerging plants. Research was conducted in Illinois and
Wisconsin to measure the extent of yield loss and to help answer the
- What is the yield loss under various patterns of uneven emergence?
- Should you replant stands with unevenly emerging plants?
- What are the benefits of "filling-in" a poor stand compared to
tearing up the field and starting over?
- Should you protect late-emerging plants during cultivation, or
are these late plants just "weeds"?
Details of Field Study
To answer these questions, the researchers simulated uneven emergence
by creating stands of corn with different emergence dates within rows.
Figure 1. How Uneven Emergence Affects Grain Yield. Grain yields are
shown as percentages of the maximum yield of 187 bu./a.
obtained with even emergence of a full stand (26,000
plants/a.) with early planting. Yields are averages of
studies with two corn hybrids in seven environments in
Illinois and Wisconsin.
The stands shown in Fig. 1 include the following planting patterns:
- Full stands of 26,000 plants/acre with even emergence at early,
medium, and late planting dates.
- Full stands of 26,000 plants/acre with various combinations of
uneven emergence within rows with 1/4, 1/2, and 3/4 delayed plants.
- Reduced stands with 1/4, 1/2, and 3/4 stand loss.
To imitate emergence delays, corn was planted either 1 1/2 weeks
(medium) delay) or 3 weeks (late delay) after the early date
(no-delay) (see Fig. 1). To assess within-row uneven emergence,
repeating patterns of in-row planting time were used for each of the
two delayed plantings. These patterns produced 1/4, 1/2, and 3/4
delayed plants within the stand.
For stand loss, the same repeating patterns were used but with blanks
rather than delayed plantings, producing stand losses of 1/4, 1/2, and
3/4 (Fig. 1). Grain yield and growth responses were similar for the
seven environments in which the study was conducted; the combined
results are presented in Fig. 1.
Effects of Uneven Emergence
When the planting delay was 1 1/2 weeks (medium delay), mixed early
and delayed plantings within a row decreased yield by 6 to 8% (Fig.
1). Similarly, a 1 1/2 - week delay in planting the entire stand
decreased yield by 5%. A 3-week delay (late delay) in planting 1/4 of
the plants within the row decreased yield by 10%. Again, a 3-week
delay in planting the entire stand decreased yield by about the same
(12%). A 3-week delay in planting 1/2 or 3/4 of the plants within the
row resulted in a yield loss of 20 to 22%.
Effects of Stand Loss
Stand losses of 1/4, 1/2, and 3/4 decreased grain yields 10, 30, and
51%, respectively (Fig. 1). The benefit of late-emerging plants within
a stand can be assessed by comparing yield percentages of uneven
emergence vs. stand loss. For example, when 1/4 of the plants were
planted 3 weeks late, yields were 90% of maximum, the same yield
obtained under a 1/4 stand loss. This indicates that the presence of
late plants did not contribute to overall yields. However, when 1/2 of
the plants were planted 3 weeks late yields were 80% of maximum, 10%
higher than the yield under a 1/2 stand loss. This indicates that the
presence of late plants helped overall yields.
Recommendations for Dealing with Uneven Emergence
The first step in following the recommendations is to determine the
general pattern of non-uniform emergence. Patterns will vary from
field to field as well as within parts of fields. You should manage
particular fields or parts of fields according to the most prevalent
Should Late-Emerging Plants Be Protected During Row Cultivation?
- If the delayed plants emerge only 1 1/2 to 2 weeks late, use
shields and avoid burying them during cultivation.
- Protect plants emerging 3 weeks late if 1/2 or more of the plants
in the stand are late-emergers.
- If less than 1/4 of the stand emerges 3 weeks late or later, it
probably will not pay to encourage their survival. Yields will be
about the same whether or not these delayed plants are buried
Should You Replant Stands with Uneven Emergence?
- If the delay in emergence is less than 2 weeks, replanting will
have minimal effect on yield, regardless of the pattern of
- If 1/2 or more of the plants in the stand emerge 3 weeks late or
later, then replanting may increase yields by up to 10%. To
decide whether to replant in this situation, estimate both the
expected economic return of the increased yield compared to your
replanting costs and the risk of emergence problems with the
Should You Fill in a Poor Stand?
- When replanting a poor stand (1/2 stand loss or greater), you can
either tear (up the stand and replant the whole field, or fill-in
the existing stand and accept the resulting uneven emergence.
- If you replant within 2 weeks of planting the original stand,
filling-in the existing stand may be an option. Yields will be
similar to those from a uniform-emerging, replanted stand, if you
can get relatively uniform plant spacing within the row between
old and new plants. However, within 2 weeks of planting, it
probably will be too early to determine what the final stand will
- If you replant 3 weeks after the initial planting, yield
potential is about 10% greater if you tear up the field and start
over with an even-emerging stand, rather than just fill-in the
original stand. Balance this possible yield increase against the
additional cost of tillage, seed, and dryer fuel.
- It may be more accurate to assess non-uniform emergence by
comparing growth stage differences between early and delayed
emerging plants rather than comparing time differences. The 1 1/2
and 3-week planting delays described in this publication resulted
in similar time delays in emergence. However, emergence delays
may vary with different environments, plus the actual time delays
may not be known. In the study described, at emergence of plants
delayed in planting by 1 1/2 there were 4 to 5 visible leaves (2
to 3 visible leaf collars) on early plants. When plants were
delayed 3 weeks in emergence, there were 7 to 9 visible leaves (4
to 5 visible leaf collars) on early plants.
- Plant populations can also influence the relative yield of
late-emerging plants. A Minnesota study found that when 1/2 of
the plants emerged 2 weeks late within final plant populations
greater than 30,000 plants/acre, the late-emerging plants made a
much smaller contribution to total yield than those occurring in
final stands of 24,000 plants/acre or lower.
- Uneven stands typically yield lower than even stands due to
direct competition of plants at two different stages of growth
next to one another. Older plants generally out-compete younger
plants for light, water, and nutrients. In some cases,
late-emerging plants are more vulnerable to silk clipping by corn
rootworm beetles. Severe silk clipping that occurs early in the
pollination process can interrupt pollination and reduce kernel
set on the ears.
- Late-emerging plants had higher grain moisture content at
harvest. This could result in harvested grain with varying
moisture levels, which would increase kernel damage and drying
costs. Late plants also lodged more due to smaller stems, weaker
stalks, and fewer brace roots. During harvest adjusting settings
on combines for variable ear sizes between early and late plants
is difficult. These problems would be minimal with a 1 1/2 -week
delay, but could be serious with a 3-week delay.
Recommendations for Avoiding Uneven Emergence
Corn sometimes emerges unevenly because of environmental factors that
corn growers cannot control. Nevertheless, the following management
practices can help you avoid uneven stands.
- Avoid excessive tillage trips that dry or compact the seedbed.
- Remember that tilling wet soils often creates cloddy seedbeds, a
major cause of uneven stands.
- Check your seed depth during planting in several areas of the
field. Increasing seed depth as little as 1/2 inch can often
eliminate most uneven emergence within a field, when seed-zone
soil moisture is marginal. If contact between seed and soil is
poor or seeding depth isn't uniform, adjust seed openers and/or
press-wheel pressure. A change in secondary tillage operations
may improve soil conditions for more uniform planting.
- Under conditions of substantial surface crop residue, adjust
harvest, tillage, and planting equipment so that residue cover
over the row area is distributed uniformly after planting.
- Use recommended herbicide application rates to avoid injuring
- After planting, closely monitor corn emergence and use a rotary
hoe if a soil crust prevents uniform corn emergence.
 Carter, P. R., and E. D. Nafziger. 1989. Uneven Emergence in
Corn. North Central Regional Extension Pub. No. 344.
 Ford, H. J. 1987. ``Uniform Stands: How Important Are They?''
Crop Soils 39(7):12-13.
 Johnson, R. R., D. R. Hicks, and D. L. Wright. 1985. ``Guidelines
for Making Corn Replanting Decisions.'' National Corn Handbook
 Nafziger, E. D., P. R. Carter and E. E. Graham. 1991. ``Response
of Corn to Uneven Emergence.'' Crop Science 31:811-815.
Cooperative Extension Work in Agriculture and Home Economics, State of
Indiana, Purdue University and U.S. Department of Agriculture
Cooperating. H.A. Wadsworth, Director, West Lafayette, IN. Issued in
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of the Cooperative Extension Service of Purdue University that all
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