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NCH-36

CROP MANAGEMENT

Purdue University
Cooperative Extension Service
West Lafayette, IN 47907

Reviewers

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 emergence.

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.

Other Causes

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 over.

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 following questions.

• 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 emergence pattern.

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 during cultivation.

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 unevenness.

• 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 replanted stand.

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 be.

• 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.

Other Considerations

• 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 corn.

• After planting, closely monitor corn emergence and use a rotary hoe if a soil crust prevents uniform corn emergence.

References

[1] Carter, P. R., and E. D. Nafziger. 1989. Uneven Emergence in Corn. North Central Regional Extension Pub. No. 344.

[2] Ford, H. J. 1987. ``Uniform Stands: How Important Are They?'' Crop Soils 39(7):12-13.

[3] Johnson, R. R., D. R. Hicks, and D. L. Wright. 1985. ``Guidelines for Making Corn Replanting Decisions.'' National Corn Handbook Publication NCH-30.

[4] 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 furtherance of the Acts of May 8 and June 30, 1914. It is the policy of the Cooperative Extension Service of Purdue University that all persons shall have equal opportunity and access to our programs and facilities.