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Guidelines for Late-Planted Corn and Soybean in Minnesota

Jeff Coulter, Extension Corn Agronomist and Seth Naeve, Extension Soybean Agronomist

May 24, 2011

Fig. 1. Late Planting.jpg
Figure 1. Heavy and frequent rainfall have made timely corn and soybean planting a challenge this year in Minnesota.

As of May 22, only 81% of the corn and 38% of the soybean in Minnesota were planted (USDA-NASS, 2011). This is well behind the 5-year average of 93% for corn and 68% for soybean. With significant amounts of rain this past weekend, planting in many fields will be further delayed. This is leading to several questions about late-planted corn and soybean that are addressed below.

What is the yield penalty for late-planted corn and soybean?

In corn planting date trials conducted annually from 1988 to 2003 at the Southwest Research and Outreach Center near Lamberton, grain yield was within 1% of the maximum on average when planting occurred between April 21 and May 6. In comparison, average yields were reduced by 8% when planting was delayed until May 20, and by 18% when planting was delayed until May 30 (Table 1). More recently, trials conducted at Lamberton and Waseca, MN from 2008 to 2010, which were funded by the Minnesota Corn Growers Association, found yield reductions of 3% and 15% when planting was delayed until mid-May and late May, respectively. The smaller yield reduction with late planting in 2008 to 2010 was likely related to the limited number of growing degree days lost when planting was delayed in these years.

This year, growing degree day accumulation from late April until now has been behind normal in most of Minnesota, so yield reductions due to delayed planting may be closer to our previously mentioned results from 2008 to 2010 than those reported in Table 1. For example, corn planted on April 15 this year at the St. Paul campus did not emerge until May 16.

Table 1. Relationship between corn planting date and yield loss. Data are from planting date trials at Lamberton, MN from 1988-2003 by Bruce Potter and Steve Quiring.

Planting date Grain yield loss (%)
April 25 0
April 30 0
May 5 1
May 10 2
May 15 5
May 20 8
May 25 13
May 30 18
June 4 24
June 9 24
June 14 39

Soybean planting date trials conducted by Steve Quiring and Bruce Potter at Lamberton between 1998 and 2004 show similar but not as severe yield losses with delayed planting. Results from these trials show that planting soybeans around June 1 will provide about 85% of a maximum theoretical yield. However, yield potential drops quickly as planting is delayed though June. With a July 1 planting, yields are expected to be about 50% of those of early-planted soybeans. This relatively linear decrease in yield potential though the month of June indicates that one loses around 1% (or about 0.5 bushels per acre) of the soybean yield potential per day of delayed planting.

Although it is important to finish corn and soybean planting as soon as possible, it is also important to avoid the temptation of planting when soils are too wet. Sidewall smearing can occur on heavy soils when double-disc 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, resulting in poor seed-to-soil contact and poor stand establishment. In addition, soil crusting can greatly hinder soybean emergence in fields planted under marginal conditions. In some fields, growers may need two planting dates: one for the majority of the field when it is dry enough, followed by a second planting date to fill in the remaining low areas after they have dried sufficiently.

What are the latest recommended planting dates for corn?

June 5 in central and northern Minnesota if corn is to be harvested for grain.

June 15 in southern Minnesota if corn is to be harvested for grain.

June 25 in southern Minnesota if corn is to be harvested for silage.

What corn and soybean maturities should be used when planting late?

When corn planting is delayed, growers should consider the guidelines in Table 2 for reducing the risk of corn being frozen in the fall before reaching maturity.

Table 2. Corn maturity guidelines for late planting in Minnesota. Source: Hicks, et al. (1999)

Planting date Relative maturity units earlier than full-season for the region
Prior to May 25 Plant normal seed choices
May 25 to 31 Plant hybrids
June 1 to 10 Plant hybrids that are 8 to 15 relative maturity units earlier than full season
June 11 to 15 Plant hybrids that are 15 or more relative maturity units earlier than full season

Soybean maturities should be adjusted when planting after about June 10. At this point, plant a soybean variety with a relative maturity rating of 0.5 units shorter than your original soybeans. For instance, if you live in central Minnesota and originally planted a RM 1.7 variety, by about June 10 switch to an early group I soybean (such as a RM 1.2). Late June replants may need to be switched to a variety that is one (whole) maturity unit shorter than you would normally plant. The only caveat here is that producers who normally plant very long-season varieties need to make the switch earlier. Likewise, producers who normally plant short-season varieties are in a position to hold with their current varieties a bit longer.

Should corn and soybean seeding rates change when planting is delayed?

Research conducted at Lamberton, Morris, and Waseca, MN from 2008 to 2010 that was funded by the Minnesota Corn Growers Association clearly shows that the optimum final stand does not change when planting is delayed. However, with warmer soils, growers may be able to slightly reduce the rate of over-seeding for a desired final plant population. In general, a 5% over-seeding rate is reasonable for normal to late planting dates.

On productive soils in southern and central Minnesota, planting rates of 33,000 to 35,000 seeds/acre are optimal in most situations. On productive soils in northwestern Minnesota, however, on-farm research from 2009 and 2010 indicates that planting rates of 34,000 to 36,000 seeds/acre or more may be needed. With high corn grain prices, it may be beneficial to be on the upper end of these ranges. The relationship between grain yield and final stand from trials at Lamberton and Waseca, MN from 2006 to 2008 is shown in Table 3.

Table 3. Relationship between corn final plant population and yield potential. Data are from research at Lamberton and Waseca, MN from 2005-2008.

Final stand (plants/acre) Grain yield potential (%)
36,000 100
34,000 99
32,000 99
30,000 97
28,000 95
26,000 93
24,000 91
22,000 88
20,000 84
18,000 80
16,000 76

Likewise, trials examining the interaction between planting date and row spacing and population in soybean in southern Minnesota have shown that row spacing and population need not be altered when planting soybeans at later than optimal dates. While narrow rows produced larger yields in the late-planted fields, this advantage was no larger than with early-planted soybeans. Similarly, increased seeding rates did not preferentially benefit later-planted soybeans.

Should corn and soybean planting depth change when planting is delayed?

Fig. 2. Late Planting.jpg
Figure 2. Late-planted corn and soybean still have good yield potential, especially if appropriate relative maturities are chosen and above-average air temperatures (coupled with timely rainfall) occur during the grain-filling period.

A corn planting depth of 1.75 to 2 inches is optimum for most fields and planting dates. In corn, poor stand establishment is often the result of insufficient moisture in the seed zone after planting. Thus, it is important to avoid the temptation to plant shallow. Emergence will occur quickly when planting is late and soils are warm. Shallow planting depths increase the risk for poor establishment of the nodal roots that develop between the seed and the soil surface.

While soybean does not require deep planting for proper root development, producers should continue to closely monitor soybean planting depth so that seeds are placed on top of the moisture and will have ample access to moisture prior to emergence so that a uniform stand can be established.

Additional information

Corn production:

Soybean production:


Hicks, D.R., S.L. Naeve, and J.M. Bennett. 1999. The corn growers field guide for evaluating crop damage and replant options. Available at Univ. of Minnesota, St. Paul.

USDA-NASS. 2011. Minnesota ag news - crop weather. Available at USDA-National Agricultural Statistics Service Minnesota Field Office, St. Paul.
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