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Think You Need to Replant Corn? Consider These Factors

Monday's USDA-NASS Crop Progress report showed farmers spent last week getting after it, bringing corn and soybean planting up to schedule. Farmers in parts of the Corn Belt have all their corn in the ground and have moved on to soybeans, many well ahead of last year's slow, soggy progress.

But weekend storms ushered in a cooldown; now, overcast skies and temperatures in the low 60s instead of the 70s or even 80s are far from what those corn seeds need as they work to germinate and poke young corn plants through the soil's surface. In other words, it may already be time to start checking your fields and get a feel for whether you'll need to replant any of your acres, a team of Iowa and Nebraska agronomists say.

Checking for early seedling populations and potential "attrition losses" is critical at this stage in the growing season, taking things like seed viability, pest and disease pressures into account to determine whether you'll need to replant any acres. And, growing degree days (GDD) are of high importance in getting young plants off to a good start; falling short on GDD can sometimes make it easier for things like pests and disease to hit seedlings early.

"Compare the plant populations calculated from your fields to what you intended to plant; calculate your attrition losses. Typical losses from planting to final stands in our University research trials ran between 4% and 7%. That's probably mostly due to poor germination. But, final stands are within 1% to 4% of seeding rates for top managers. If your attrition losses range up to 10% or more, investigate the reasons. You'll need to dig for seeds or seedlings to determine what happened," according to a university report led by University of Nebraska Extension plant pathologist Tamra Jackson and Iowa State University Extension agronomist Roger Elmore. "Check the seed tag and/or run your own germination test. Cutworms, wireworms, and white grubs are the most common insects affecting seedlings and reducing stands. Seedling diseases can be caused by any of several common soilborne organisms, such as Pythium, Fusarium, Rhizoctonia, or plant parasitic nematodes. Seedling diseases are often difficult to diagnose because their symptoms are very similar. But diagnosis may actually be of limited value at times because pathogen management is often the same for several seedling diseases."

It takes between 90 and 120 GDD to get a corn seed to germinate and sprout. Most points in Iowa, for example, are there, ranging from 91 to 154, according to the Iowa Environmental Mesonet. Yet there are other points in the Midwest that may be struggling to reach that range, and if that continues, it can add to the pressure to replant.

All these factors contribute to uneven emergence and inconsistent plant populations, two major factors to consider when reaching a corn replant decision. However, it's when those conditions are present that's more important than the fact they're present alone.

"Replanting may be necessary if populations are low but not necessarily if plants have emerged but are not uniform.  Although we know that variable plant heights or development will likely reduce yield potential, it's not a reason to replant," according to the report. "In most cases, losses due to lack of plant uniformity will be more than outweighed by losses due to delayed planting. Plant height differences may reflect lack of uniformity in emergence timing or other issues the plants faced. In any case, we can't justify replanting simply based on uneven emergence, variable plant heights, or lack of similar developmental stages. However, replanting may be necessary with reduced plant populations."

Then, there's the time and cost to get your planter out again. Is it worth the price?

"Consider costs of tillage, seed, fuel, additional pesticides, and labor. Consider also that delayed planting certainly means higher grain moisture at harvest and the possibility of fall frosts before physiological maturity. You may want to consider planting a shorter season hybrid seed," according to Jackson and Elmore. "Caution: Before replanting, contact your crop insurance agent, Farm Service Agency, and others with an interest in your crop."

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