When will you start planting corn and soybeans?

By Greg LaBarge, CCA, Ohio State University Extension

Every agronomist and farmer knows the connection between timely planting and crop yields. The Ohio Agronomy Guide has a recommended Ohio corn plant date range of April 15 to May 10 for northern and April 10 to May 10 for southern regions. Recommended soybean planting is a similar timeframe. RELATIVE grain yield potential is indeed reduced after May 1. However, we need to remember more factors than planting date influence ACTUAL YIELD at harvest. Soil conditions at and after planting are more important than the calendar date to answer when to start planting.

At the Conservation Tillage and Technology Conference (CTTC), we had research agronomists from around the Midwest share their research on planting date and management interactions. Dan Quinn, Purdue Corn Agronomist, shared these corn planting observations. 1) Early planting favors higher yields but are no guarantee. 2) Statewide averages for planting date and yield are not strongly related. 3) Planting date is only one factor influencing harvested yield. Dan concluded with the following recommendations for corn planting.

Chase conditions, not calendar dates. 

The sins of planting will haunt you all growing season. For example, planting too wet can result in seed opener sidewall compaction or open seed slot. Both situations impact root growth and are a problem if conditions turn dry. Planting into cool soil can result in imbibitional chilling of the seed or corkscrew mesocotyle of a seedling. Imbibitional chilling occurs with the first drink of water the seed takes after planting. Corkscrew mesocotyle results from cold damage during early seedling growth.

Faster emergence equals less seedling stress. 

The Growing Degree Days (GDD) threshold for corn emergence is 115 GDD when planted 1.5 to 2 inches deep. Soil temperature will be more critical than the air temperature in determining how soon plants emerge. We calculate GDD with the following equation (Daily high temperature + Daily low temperature divided by 2) – 50. On a day with an average daily temperature of 55 degrees F, 5 GDD accumulate, resulting in 23 days (115/5) to emergence. A day with 66 degrees F average daily temperature, 16 GDD accumulate, and emergence takes 7 days (115/16).

Our earliest corn and soybean planting completion year was 2012, while 2020 planting was the latest during the past 10 years. Note I did not consider 2019. We reached 94% of corn planted on 5/20/2012, while in 2020, 94% was reached 19 days later on 6/7/2020. State average corn yield in 2012 was 120 bushels per acre, while in 2020, we hit 171 bushels per acre. For soybean, we hit 93% planted on 5/27/2012 and 6/14/2020 with state average yields of 45 in 2012 and 55 bushels per acre for 2020.

Emerson Nafziger, University of Illinois, was also a speaker at CTTC. Emerson observed, “Planting current hybrids in mid- to late-May carries less risk of serious yield decreases than older hybrids.” I think there would be general agreement that current genetics gives us improved yield potential, better nutrient use efficiency, and more flexibility — some things to consider when we pull the trigger on planting.

Planting soybean early, before corn, has been a heavily discussed topic in the past few years. Soybean yield potential is improved with more time in vegetative growth. Having a plant with more leaf area will support greater photosynthesis. Early planting risks from wet and cool soils with soybean are similar to corn. The added concern for early planting soybean is freezing injury or death. The growing point for soybeans is exposed immediately at emergence. If the emerged seeding is frosted and killed, that plant will not recover. Corn’s growing point is protected underground until after growth stage V5.

Early planted soybean will also attract a few more pests. For example, bean leaf beetle often congregates in the neighborhood’s earliest planted field. In addition, an aggressive slug population can clip off plants and reduce population. 

I say all that to have you realize if you plant soybeans early, prepare yourself for two potential outcomes. First, you will likely have an ugly-looking field for most of the summer. Second, be prepared to accept a thinner stand. Laura Lindsey, OSU Extension Specialist, Soybean and Small Grain, shared at CTTC results from two soybean studies planted on April 5 and 6, 2021. She had 80,000 plants per acre at both sites with a respectable yield of 72 and 77 bushels per acre. So an ugly stand does not equal a low-yielding stand.

Answering when to start planting ultimately becomes a balance between risk-reward for each farm. We know we have the best result planting into soils at 50 degrees F or higher with a forecast for rising temperatures. Unfortunately, recent years have offered an early planting period followed by two-three weeks of cold, wet weather before we can finish planting. Use your experience this spring. Many have regretted planting into cool soils, followed closely by a cold rain. 

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