Iowa Corn, Soybeans: Delayed Planting – How Will It Affect Yields?

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Planting conditions in northern Iowa, especially north of highway 20, is experiencing delays due to abundant rainfall. Corn planting progress is currently at 40% across the northern crop reporting districts compared to 65% and 70% for the state and 5-year average. Soybean planting progress is at 13% across the northern crop reporting districts compared to 33% and 28% for the state and 5-year average.

There is still time to get both the corn and soybean crop planted to achieve good yield outcomes. Here are some considerations and resources to help make decisions.

Corn planting date effect on yield and crop maturity

Corn yield begins to decline after May 15 planting dates with rapid declines after June 1. In data from a recent study from 2014-2016, corn relative maturity was an important factor for April and May planted corn where at northwest, north central, and central Iowa locations a full season hybrid achieved higher yields than shorter season hybrids.

At all seven locations, the early and late June planting dates did not result in hybrid relative maturity yield differences.

Plant Density April 20 – May 5 May 5 – 15 May 15 – 25 May 25 – June 5 June 5 – 15
  relative yield (%)
45,000 97 93 85 68 52
40,000 99 95 86 69 53
35,000 100 96 87 70 54
30,000 99 95 86 69 53
25,000 95 91 83 67 51
Adapted from Corn and Soybean Field Guide, ISU Extension, IPM 0001.

Recommendations: Deciding on what hybrid relative maturity to plant is an important consideration when planting is delayed. The time at which corn reached physiological maturity is about a 10-day difference between a full season and short season hybrid. This is an important factor to consider to reduce fall frost risk ahead of physiological maturity as well as post-physiological maturity grain dry-down.

April and early May planted corn should mature in mid-September while June planted corn would reach maturity in early to mid-October. The potential for grain dry-down in the field is lower the later corn reaches physiological maturity. This could decrease crop profitability because of higher grain drying costs.

  1. Plant the originally planned well-adapted corn hybrid through May 31.
  2. After May 31, consider planting a ~5-day earlier maturing corn hybrid.
  3. After May 31, consider crop insurance prevented planting provisions.
  4. After June 15, consider switching to soybean with consideration to already applied herbicide and nitrogen programs as well as impact on crop rotation and plans for future years.

Soybean planting date effect on yield and crop maturity

The highest yield potential for soybean production in Iowa is achieved with April to mid-May planting dates. Soybean yields begin to decline after May 20 with larger declines the later planting becomes. Soybean maturity selection is often discussed as a means of achieving higher yields, however, in late planting situations soybean grain yield is not influenced by maturity selection.

Planting Date Northern IA Central IA Southern IA
  relative yield (%)
Late April 100* 96* 98*
Early May 96* 100* 100*
Mid-May 99* 96* 98*
Early June 81 93 89
Mid-June 61 59 82
Early July 33 45 47
Adapted from Soybean Replant Decisions, ISU Extension, PM 1851.

Recommendations: Soybean typically reach maturity in mid to late September regardless of planting date. Soybean physiological maturity is minimally delayed by either planting date (5- to 10-day delay) or maturity group selection (up to 5-day delay). This minimizes the need to change soybean maturity group selection when faced with late planting situations.

  1. Plant the originally planned well-adapted soybean varieties through June 30.
  2. After June 15, consider crop insurance prevented planting provisions.

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