Corn plants emerged unevenly in some fields at the Northwestern Illinois Agricultural Research and Demonstration Center (NWIARDC) in 2014. The picture above shows uneven emergence in a field that was cultivated on April 16 and seeded 1.75 inches deep on April 22 (Figure).
Yield costs associated with uneven emergence. Yield loss can occur when some plants emerge significantly earlier than their neighbors. Those plants that emerge earlier than their neighbors are often taller, have larger root systems and more photosynthetic capacity due to their later stage of development. Later emerging plants must compete with those that have already emerged for finite sunlight, moisture and nutrient resources and are often barren.
To determine how much yield loss may be associated with uneven emergence of corn plants, over seven location-years researchers in Illinois and Wisconsin (Nafziger et al., 1991) hand planted seeds at different times within rows to force plants to emerge at different times. Seeds were sown at three different times: 1) early: late April to early May, 2) middle: 10 to 12 days after early, and 3) late: 21 to 27 days after early (Figure).
Uneven emergence, 1 ½ week delay. When a portion of the plants emerged 1 ½ weeks later than others yield was reduced by 6 to 9 percent compared to a uniformly emerging early stand. Yields of these uneven stands were comparable to planting the entire stand 1 ½ weeks later.
Uneven emergence, 3 week delay. Delaying emergence of some plants in a field by 3 weeks resulted in significant yield reductions. Yield was decreased by approximately 10 percent when 25 percent of the plants emerged 3 weeks later than the rest of the stand. Yield loss reached 20 to 22 percent when 50 to 75 percent of the stand emerged late. These yield losses were greater than had the entire stand been planted 3 weeks later (Carter et al., 1989).
Causes of uneven emergence. There are many environmental conditions that can cause corn plants in a field to emerge unevenly.
Inadequate soil moisture. Non-uniform soil water availability within a field can contribute to uneven emergence as seeds exposed to adequate water can grow and emerge while the growth and development of seeds without adequate water may be delayed.
Poor seed to soil contact. Planting into wet soils or soils with considerable plant residue can affect seed to soil contact. Planting when the seed bed is not too wet and adjusting both planting depth to reach moisture and planters to assure good seed to soil content are essential.
Soil crusting prior to emergence. On April 28 the NWIARDC received 1.91 inches of rain. Following this large rain event air temperatures rose and soils dried and formed a hard crust (Figure).
Soil temperature variability. According to Purdue University Agronomist Dr. Bob Nielsen, drops in soil temperatures below 45 to 50 degrees can "decrease the uniformity of germination". At the NWIARDC, soil temperatures measured at 4 inches below the soil surface after planting were at or below 50 degrees for 12 of the 17 days between planting and when the pictures above were captured. One can speculate that soil temperatures at a 2 inch depth may have been lower than at the 4 inch depth as daily low air temperatures after planting ranged reached as low as 34.
Preventing uneven emergence or dealing with an uneven stand. The best way to deal with uneven emergence is to avoid the environmental conditions that favor uneven emergence. A good resource for useful recommendations for how best to adjust planters to ensure uniform plant establishment is a Purdue University document called "Stand establishment variability in corn".
One must weigh the costs of replanting with the potential benefits of establishing a uniform stand by replanting. Visit the North Central Regional Extension Publication 344, "Uneven emergence in corn" for a candid discussion on whether it pays to replant.
Carter, P.R., Nafziger, E.D. and Lauer, J.G. 1989. Uneven emergence in corn. NCR-344.
Nafziger, E.D., Carter, P.R., and Graham, E.E. 1991. Response of corn to uneven emergence. Crop Science. 31:811-815.
Nielsen, R.L. 1991.. Stand establishment variability in corn. Agronomy Dept. Pub. No AGRY-91-01., Purdue Univ. (online: http://www.agry.purdue.edu/ext/pubs/AGRY-91-01_v5.pdf).