With fieldwork just around the corner, now is the time to evaluate decisions related to corn planting. Corn planting-date studies from 1988 through 2003 conducted at the University of Minnesota Southwest Research and Outreach Center in Lamberton show that on average grain yield was maximized with an April 28 planting date, but planting dates ranging from April 21 to May 6 produced yields within 1% of the maximum. In central and northern Minnesota, optimum planting-date windows generally begin a few days later. When corn planting is delayed beyond mid-May, yield potential is reduced rapidly.
Corn requires soil temperatures of 50° F or higher for germination, and the amount of time from planting to emergence has been shown to be reduced from 24 days to 13 days when average soil temperature in the seed zone from planting to emergence increases from 51° to 54° F (Nielsen, 2009). A greater amount of time for emergence increases the potential for stand establishment problems, but in very late April most agronomists agree that growers should ignore soil temperature and plant corn as soon as soils are fit, since warmer temperatures are expected soon afterward.
Timely planting also increases the amount of time for in-field grain drying prior to harvest, but the advantages of timely planting can be lost if planting occurs when soils are too wet. Sidewall smearing can occur on heavy soils when double-disk openers on the planter cut through wet soil, resulting in compacted soil around the seed that is difficult for seedling roots to penetrate. Seed furrows can also open up after heavy soil dries following wet conditions at planting. In general, a field is fit for seedbed preparation if soil from the surface 3-4 in. breaks apart when pressed between your fingers rather than forming a ribbon or ball.
A firm seedbed at planting is critical for obtaining good seed-to-soil contact and establishment of roots that are located near the soil surface. A seedbed is too loose if your boots sink over 1 in. when walking through the field. At the same time, avoid excessive preplant tillage, which can result in surface crusting and emergence problems if heavy rainfall occurs prior to emergence. Excessive preplant tillage can also enhance wind erosion and injure corn seedlings with blowing soil.
Research funded by the Minnesota Corn Growers Association over the last two years shows that optimum final corn stands typically range from 32,000 to 34,000 plants/acre, and that this is consistent regardless of the planting date or row width. Over-seeding by 5% is common, but the necessary amount could be slightly higher or lower based on surface residue amounts and soil temperature. Thus, it is important for growers to document their seeding rates and final stand for each field, and adjust over-seeding rates for future years as needed based on their experience. Under most conditions in Minnesota, seeding rates ranging from 33,000 to 35,000 seeds/acre are economically optimal.
Planting depths of 1.75-2 in. are optimal for corn in most situations, but could be as deep as 2.5 in. under very dry conditions to ensure that adequate soil moisture is present in the seed zone for germination. Planting depths shallower than 1.75 in. increase the risk of poor establishment of the nodal roots that develop between the seed and the soil surface, which increases the potential for root lodging. Finally, keep an eye on planting speed, as speeds above 5.5 mph can result in excessive fluctuation in planting depth and within-row seed placement. Pay attention to details this spring, as mistakes with planting last the entire season. More information on corn production from the University of Minnesota is available at www.extension.umn.edu/corn.