By Jeff Coulter and David Nicolai
With corn planting wrapping up, it is time to assess germination and emergence. On average, 120 growing degree days (base 50 degrees) are required for corn emergence. Before emergence, it is a good idea to dig up some seeds to gauge seed quality and germination. Seedlings that have been subjected to stress prior to emergence will often show symptoms when dug up, and these can be indicators of problems to come. As long as seeds are not soft and rotting, they can produce healthy plants. In 2002, much of the corn planted in Minnesota did not emerge until 30 to 40 days after planting, but the state average yield was still 157 bushels per acre.
Before scouting, it is helpful to understand the process of germination and emergence. After seeds take on about 30 percent of their weight in water, the radicle (root) breaks through the seed coat, followed by the coleoptile (spiked protective sheath that protects young leaves during emergence). Shortly afterwards, lateral seminal roots arise from the seed. It is desirable to see a healthy, white root system. The coleoptile reaches the soil surface through elongation of the mesocotyl, a white, tubular structure located between the seed and the coleoptile. An excellent set of pictures illustrating the germination process have been prepared by Bob Nielsen at Purdue University, and are available at http://www.agry.purdue.edu/ext/corn/news/timeless/germinationgallery.html.
With wet conditions this spring, it is likely that portions of some fields were planted when they were too wet, which can result in seed furrows opening up after the soil dries out. If the seed furrow opens, seeds can have difficulty imbibing enough water for germination. Open slots prior to emergence can also prematurely expose the coleoptile to sunlight, resulting in the first leaf breaking through the coleoptile too early and problems with successful establishment of the primary roots. Corn planted when it was too wet can also be subject to compaction in the seed zone caused by planter disk openers slicing through wet soil, and this can result in uneven emergence.
Pounding rain on fine-textured soil with little surface residue to intercept raindrops can result in surface crusting. This makes it difficult for the mesocotyl to elongate and push the coleoptile through the soil surface. Seedlings having difficulty emerging through crusted soil often have a thickened coleoptile, as storage reserves in the seed are moved to the shoot to help emergence. When a severe crust is present, the first leaf can prematurely split through the coleoptile below ground. This can result in death unless the leaf can somehow break the soil surface. Leafing-out underground can also be associated with herbicide injury and prolonged exposure to cold temperatures. Rotary hoes are a good tool for breaking a surface crust, but they can be ineffective when a severe crust is present. However, modern corn hybrids have a tremendous ability to push through crusted soil, and surface crust is of little concern if you can push through it with your index finger.
Authors: Jeff Coulter is a corn specialist with University of Minnesota Extension. David Nicolai is an Extension educator in crops management.