By David Nicolai, Extension Crops Educator
Maximum corn yield in Minnesota is typically achieved when planting occurs in late April or early May. In years when warm weather arrives earlier than normal, planting in mid-April can produce similar yield if young corn plants are not significantly damaged by a freeze in May according to Dr. Jeff Coulter, U of MN corn extension specialist. In addition to the calendar there are four simple requirements for achieving uniform germination and emergence of corn as indicated by Dr. Bob Neilsen at Purdue University and Bruce Potter, University of Minnesota Extension IPM Specialist:
Adequate and Uniform Soil Moisture in the Seed Zone: Uneven soil moisture at seed depth is the most frequent cause of uneven emergence. Uneven soil moisture in the seed zone can be caused by variable soil characteristics, tillage patterns, unusual weather conditions and uneven seeding depth. When seedbed conditions are dry, make sure you choose a seeding depth that ensures uniformly adequate soil moisture for germination and emergence. It is important to avoid more preplant tillage than necessary. Excessive preplant tillage reduces soil moisture in the seed zone. A planting depth of 2 inches is optimal for corn in most situations in Minnesota. Shallow seed placement increases risk of poor nodal root establishment. Nodal roots which develop closer to the surface when corn is planted shallow can be aborted if hot, dry conditions occur where roots are developing. Planting shallower than 1.5 inches increases the risk of poor or uneven germination if rapid soil drying occurs after planting due to warm, sunny, windy days. Even with the recent advances in planter and monitor technology, it is extremely difficult to plant a field uniformly at 1 ½ - inches. Loose, fluffy seedbeds can easily settle 1/2 inch or more. Often, an attempt at 1 ½ - inches creates a good chance of ending up too shallow. Seed depth that started out at 1½ inches can easily be an inch or less after a good rain. Bruce Potter’s recent IPM newsletter contains more information about planting and links to other pest management information.
Adequate and Uniform Soil Temperature in the Seed Zone: Seemingly minor variability in soil temperatures throughout a field can have large effects on germination timing when soils are hovering close to 50F or cooler. Soil temperatures at the Lamberton Research Center reported on April 20 were 48 and 46 degrees at 2- and 4-inch depth respectively. Dark-colored soils will typically warm more quickly than light-colored soils. If soils dry differently across the field, the drier areas will typically warm faster than the wet areas. Uneven residue cover (surface trash or cover crops) in reduced tillage systems results in lower soil temperatures beneath heavier residue areas than in more bare spots in the field. Uneven seeding depth risks exposing slightly deeper planted seeds to slightly cooler soil temperatures than slightly shallower planted seeds.
Adequate and Uniform Seed-to-Soil Contact: Good seed-to-soil contact is vital for the kernels to imbibe (absorb) soil moisture quickly and uniformly to begin the germination process. Seed-to-trash contact results from “hair-pinning” of surface trash into the seed furrow during no-till planting when soil and/or trash are too wet for adequate coulter cutting action. Seed-to-clod contact results from planting into cloddy fields created by working soil too wet. Seed-to-rock contact is, needless to say, not good for proper germination either. Seed-to-air contact results from open planter furrows when no-till planting into excessively wet soils. Germination of kernels lying in open planter furrows is dependent on rainfall keeping the open furrow environment moist.
Soil Surface Free From Crust: Severe surface crusting or compaction will restrict elongation of the seedling’s mesocotyl and emergence of the coleoptile at the soil surface, potentially leading to underground leafing out of the seedling or outright seedling death. Severe sidewall compaction, caused by the smearing action of double-disc openers in excessively moist soils, can also restrict elongation of the mesocotyl and emergence of the coleoptile. Avoid excessive tillage prior to planting the crop, especially if significant rainfall is forecast prior to emergence of the crop. This will help avoid the development of dense surface soil crust. Tillage in wet soil also creates a compacted layer below the depth of tillage, which can restrict root development. Sidewall compaction can occur when planter disc openers cut through wet fine-textured soil, resulting in compacted soil around the seed that is difficult for roots to penetrate. Seed furrows can also open after planting in such conditions, resulting in poor seed-to-soil contact.