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The most common cause of uneven emergence in corn is dry soil. If the soil is too dry at or shortly after planting, seedlings will emerge at different times. Emergence time may vary between parts of fields, from one row to the next, or from one plant to the next. Soil moisture can differ within a field due to differences in soil type, topography, or uneven distribution of moist and dry soils by secondary tillage. Cloddy seedbeds caused by working the ground when it's too wet can mean poor contact between seed and soil. As a result, some seeds absorb enough moisture to germinate, while others remain dry. In many cases, seeds placed in dry soil dont germinate and emerge until after rainfall. This produces a mixture of larger and smaller plants, depending on the amount of time between planting and rainfall.
Uneven soil temperature is another cause of uneven corn emergence. Seed-depth soil temperatures can vary if crop residues from reduced tillage systems aren't distributed evenly, if seed depths vary, or if soil within fields varies in type and topography.
Finally, uneven corn emergence occurs when corn growers, with stand loss or uneven stands, replant by "filling in" the existing stand, rather than tearing up the field and starting over.
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