When to Switch Corn Hybrids with Planting Delays

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The optimum planting date for corn in southwestern Ontario is typically on or before May 7 and in central and eastern Ontario on or before May 10. When planting is delayed due to wet weather, the question arises, “When should a grower consider switching from a planned relatively full-season hybrid to a shorter-season hybrid?”

The simple answer to when to switch maturities, as shown in Table 1, is not until the later part of May for a large portion of eastern Canada. The dates presented are the result of long-term data taking into account the yields of hybrids of various maturity ratings and deductions for test weights and drying. The switch date presented is the date when earlier-maturing hybrids would surpass full-season hybrids in terms of net returns.
 
 
  
   

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One main factor to keep in mind when planting delays necessitate switching hybrids is the yield potential of the shorter-season hybrid. Ontario Corn Committee (OCC) corn test information suggests that yield advantages are often associated with full-season hybrids as compared to earlier-maturing hybrids. In terms of grain yield, making the switch to a shorter-season hybrid may be reasonable if that hybrid can produce within 10% of the full-season hybrid’s yield.

The goal of correctly positioning corn products is to gain full advantage of the available Crop Heat Units (CHUs) in a given geographical area. Corn hybrid maturities are typically selected on the basis of the expected number of CHUs that can be captured between normal planting and the corn’s kernels reaching black layer (physiological maturity) at a point in time before a season-ending frost or a hard freeze. A switch to earlier-maturing hybrids should result in drier grain at harvest, and thus, lower grain drying costs and a decreased chance of low test weight grain.

Research results indicate that corn hybrids reduce their CHU needs as their planting is delayed. In essence, late planted full-season hybrids mature with less than expected CHUs after the late planting. Research completed at Purdue University indicated that the number of U.S. Growing Degree Days (GDDs) needed from planting to maturity decreased by nearly 7 GDDs for each day of planting delay after May 1. While there is not a direct conversion factor from U.S. GDDs to CHUs, the same principle of requiring fewer CHUs would apply. This response of hybrid development to later planting demonstrates that full-season hybrids can be safely planted later than once thought possible. While this indicates that growers can delay switching to earlier-maturity hybrids, at some point on the calendar the switch must be considered to minimize the risk of fall frost damage.

If soil temperatures are at 10° C (50° F) with favourable soil conditions and average or above average temperatures are forecasted, early planting of fields is recommended. After April 26 (May 1 in areas with less than 2,700 CHUs), planting should be done as soil moisture conditions permit, paying less attention to soil temperature. At some point after prolonged delays, it may be necessary to begin to adjust hybrid maturities, by 100 to 150 less CHUs per week of additional delay.

A delay in planting may sometimes benefit yield potential if soil moisture and temperature are the reason for delay. Appropriate soil moisture and temperature promote rapid seedling emergence, which could mean a more uniform corn stand.

Determining when a soil is fit for planting is a judgment based on knowledge and experience with a particular soil. Typically, it is when the soil moisture is reduced to the point where the soil is friable and mellow with tillage or disturbance and soil temperatures are in the range to foster seed germination.

Patience as planting is pushed beyond the ’normal’ timeframe is needed if fields have not dried. Wet soils can become compacted when supporting the weight of farm equipment. Sidewall compaction may also be a problem as seed placement mechanisms are pulled through wet soils. Such damage to the seed zone can have negative effects on seedling root development.

Contact your DEKALB® brand seed agronomist for more discussion regarding maturity switching.

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Sources:
Nielsen, R. 2013. Hybrid maturities for delayed planting. Purdue University Department of Agronomy. www.agry.purdue.edu (verified 4/7/14); Nielsen, R. 2009. Late planting and relative hybrid maturity decisions. Pest and Crop No. 5. Purdue University; Recommended dates to switch from full season hybrids across various heat unit zones. OMAFRA Agronomy Guide, Pub 811.