by Matt Leavitt, Organic Lead/Agronomist & Margaret Smith, PhD, Agronomist
Organic farmers may face replant decisions this year.
Tempting planting conditions in April were followed by cooler-than-average temperatures. Organic growers who planted early should evaluate their corn stands. Although stand evaluation is the same for organic farmers as for conventional farmers, the replant decision is often more complicated, mainly due to weed-control considerations.
“Why do I have a reduced stand?”
Factors other than cold, wet soil conditions can result in suboptimal corn stands including:
- Dry soil
- Planting seed too deep or too shallow
- Poor furrow closure
- Damage to seeds from seed corn maggots or fungal diseases
- Damage to seedlings from cutworms, army worms, tine weeders, or cultivators
Should You Replant Your Organic Corn?
The decision to replant corn is based on a number of factors:
- Existing plant stand
- Distribution of the plant stand
- Evenness of emergence
- Calendar date
- Yield potential of the existing crop
- Seed availability of earlier maturing hybrids.
- Cost to replant
- Weed situation: potential for weed control this year and weed seed production (impacting future crops)
Adapted from: Corn Grower’s Guide for Evaluating Crop Damage and Replant Options, University of Minnesota
Replant guidelines have been well developed for conventional corn growers, but the picture is murkier for organic growers. Before deciding to replant, consider recommendations developed for conventional corn systems, but also keep in mind hard-earned wisdom from experienced organic growers like Paul Mugge, an organic grain farmer from Paulina, Iowa.
–Paul Mugge, Paulina Iowa
Corn Plant Populations
Uniformity in corn stand is exceedingly important, especially in certified organic production. Any open spaces in the row will be areas for weeds to thrive and potentially to set seed. A lowered plant population isn’t necessarily cause for replanting. Research from University of Minnesota shows you can capture nearly 100% of your yield potential with a final stand of 27,000 – 29,000 plants/acre.
Stands of <22-000 -25,000 plants/acre, however, are candidates for replanting.
Table 1. Corn Response to Plant PopulationFrom: Corn Grower’s Guide for Evaluating Crop Damage and Replant Options, University of Minnesota
Evaluate Your Existing Stand
To measuring your existing stand, use a tape measure, a notepad or phone for record keeping & calculation, and a pocketknife. When corn has emerged, find at least five (but more than five is better) representative parts of the field to take stand counts. If you have problem spots where you’re considering replanting, focus on those areas.
Measure and count all emerged plants in 1/1000th of an acre for your row spacing.
Table 2. Row length required to equal 1/1000 acre for various row widths.
From: Evaluating Corn Stands, University of Nebraska-Lincoln
Count the emerged plants on both sides of the tape measure, average the two together, multiply that by 1,000. Average the count from your five sites and bingo: your quick and dirty stand count.
Don’t count any plants that are two or more leaves behind the rest of the plants, as these will contribute little or no grain yield.
Distribution of Plant Stand
For conventional growers, several 4- to 6-foot gaps within the row, will reduce yields additional 5% relative to a uniform stand. Stand gaps of 16 to 33 inches will only reduce yield by 2%. For organic growers, this lack of crop competition means WEEDS and an even greater potential for yield reduction.
Evenness of Emergence
While counting corn plants, dig in some of the gaps to find seed. Did it germinate? Does it have insect damage or a mushy, water-soaked appearance (indicative of fungal disease)? This will help determine if some plants are yet to emerge or if you’ll have gaps in the row. Plants that emerge significantly behind early-emerging plants will not contribute much or at all to crop yield. They do provide shading and improved weed control over total gaps in a stand, however.
The greatest impact of uneven emergence for organic growers is the impact on weed control operations. If operations, particularly row cultivation, are based on the size of first-emerging plants, it’s likely you’ll bury late emergers, leaving gaps in the row. Again, gaps in the row = more WEEDS.
Table adapted from Response of Corn to Uneven Emergence, Nafziger, E D., P.R. Carter and E.E. Graham
Calendar Date and Yield Potential: Should I Replant Given the Date?
Paul Mugge pays close attention to the calendar date when evaluating his stands and uses information from Iowa State University (below) to weigh the estimated potential yield loss from later planting compared with current yield projections at this current stand. These data are based on uniform corn stands, so Paul adjusts his own situation from these guidelines based on additional consideration of gaps and uneven emergence in his fields, as well as the near-term weather forecast. He is MORE likely to replant, even if the data for conventional corn indicates a marginal yield return on replanting.
Table 3. Relative yield potential of corn by planting date and population.
From: Corn Replant Checklist, Iowa State University
Similar corn yield response tables have been developed for the growing conditions in:
For organic producers who planted corn before May 15, you’ll likely see no yield penalty for replanting with a full-maturity hybrid for your production area.
Sometimes keeping a slightly substandard stand is more advantageous than replanting based on the planting date and the maturity of the replant hybrid. That all depends though on being able to keep your fields reasonably clean and not creating a problem weed field for years to come.
If you must replant at a later date, plant shorter-season hybrid maturities.
Table 4. Recommended corn hybrid maturities for late planting
From: Corn Grower’s Guide for Evaluating Crop Damage and Replant Options, University of Minnesota
Availability and Cost of Seed
Albert Lea Seed currently has a good selection of early organic corn (<95RM). For producers who’ve experienced weather-related emergence problems we can offer replant corn seed at half the normal price. Consider the costs of replanting (including seed, machinery and labor costs) before making your decision.
Weed Issues and Management
Weed control is the most important management consideration for organic producers in deciding to keep a suboptimal corn stand.
The earlier you plant corn, the less available time you have for effective full-width, pre-plant weed control passes that catch weeds at a smaller, more vulnerable stages. Tine weeders, drags and rotary hoes are not effective in cold, wet soil conditions.
Without a doubt, keeping a poor stand of organic corn can cause more headaches throughout the season, including significant yield loss resulting from weed competition and increasing your weed seed bank. Most yield loss from weed competition in corn comes from crop stages emergence, VE, to vegetative stage, V5. If weed control is poor early in the season, there are few available rescue treatments later in the season.
–Paul Mugge, Paulina, Iowa
Agronomic Assistance from Albert Lea Seed
Our staff agronomists can walk you through your situation and help you assess your stand. Our capacity for on-farm visits will be limited due to COVID-19 but we’re available for remote consultations and limited travel on a case-by-case basis. And don’t hesitate to ask the advice of other experienced organic growers.
Corn Replanting, University of Wisconsin
Corn Yield Predictions for Late Planted and Replanted Fields, Iowa State University
How Can I Organically Control Cutworms, National Center for Appropriate Technology (NCAT) ATTRA
Early-Planted Corn and Cold Weather, Purdue University
Assessing Frost/Cold Temperature Injury to Early-Planted Corn, Purdue University
Emergence Failure of Corn, Purdue University
Response of Corn to Uneven Emergence, Nafziger, E D., P.R. Carter and E.E. Graham
Response of Corn Grain Yield to Spatial and Temporal Variability in Emergence, Weidong L., M. Tillenar, G. Stewart, and W. Deen
Effects of Missing and Two‐Plant Hills on Corn Grain Yield, Emerson Nafziger, University of Illinois
Uneven Corn Stands, Joe Lauer, University of Wisconsin