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  • Follow a logical, step-wise process to make soybean replant decisions.
  • Measure standing soybean populations; determine adequacy and evenness of stand; evaluate plant health and determine reasons for poor emergence.
  • Evaluate yield potential of original planting against replant yield potential. 
  • Decide whether to fill-in versus completely replanting the field. 
  • Consider additional factors, such as residual herbicide, replant economics, and crop insurance.
Soybeans planted ahead of very heavy rains this spring have led to some agonizing replant decisions.  A less than ideal stand of soybeans can sometimes prompt a knee-jerk reaction to replant, but a logical, step-wise assessment process will lead to more sound and profitable replant decisions.  
Step 1.  Measure population.  The process starts with an accurate measurement of standing soybean populations.  For soybeans planted in rows, the 1/1000th acre method will provide good results.  For the 1/1000th acre method, simply count the number of plants in a length of row equal to 1/1000th of an acre, based on row width (see Table 1).  Repeat the measurement process for randomly selected rows in at least 5 different locations in the field, then calculate the average and multiply by 1000 for final population.  Example:  With 15-inch rows, the 1/1000th acre row length is 34’10”.  If the average number of plants in 5 locations is 98, multiply by 1000 to get 98,000 plants per acre.  If beans are solid-seed or drilled, use the hoop method.  Measure the diameter of the hoop, randomly toss it into the field and count the number of plants inside the hoop.  Repeat the measurement in at least 5 different locations in the field, calculate the average, and multiply times the table factor, based on hoop diameter (see Table 2).  Example:  The average number of plants from 5 locations was 8 plants in a hoop diameter of 28.25”, so multiply by 10,000 to get 80,000 plants per acre.              
Step 2.  Determine adequacy and evenness of stand.  Research in Illinois and elsewhere has determined that maximum soybean yields are achieved at plant stands equal to or slightly greater than 100,000 plants per acre.  Soybean plants are responsive to low populations and adjust size and/or branching to compensate, when given more space.  In some reported cases, stands of only 50,000 plants per acre have produced as much yield as the optimum populations.  However, as populations fall below 100,000 plants per acre, some yield is usually sacrificed, especially if stands have large gaps or are very uneven.  Uneven stands sometimes fail to close canopy and can lead to more weed problems, which competes for resources that lead to yield.  The amount of yield reduction is variable and difficult to predict with low and uneven stands.  
Step 3.  Evaluate plant health and determine reasons for poor emergence.  Only viable plants that are expected to survive should be counted when measuring population.  Check for herbicide injury, insect feeding, and disease symptoms. Plants that have severe stem injuries and girdled stems are less likely to survive than plants with foliar injuries alone.  If stands have been reduced due to insect feeding, an insecticide strategy may be warranted on replant.  Failed emergence due to cool and wet soils, slow growth, and seedling pathogens may be corrected with seed treatment and warmer planting conditions when the replant occurs.  Seeds planted too deep on individual rows or across the planter may require some planter adjustments prior to replanting.  Finally, check viability of seedlings that have not yet emerged and estimate their potential to emerge, survive, and contribute to final population.
Step 4.  Determine yield potential of existing stand versus a replant scenario.  Highest yield potential is achieved with initial planting dates in late April and early May.  Yields may be reduced by 0.25 bu/ac/day between early May and late May.  Sharper yield reductions of 0.5 bu/ac/day start to accumulate as planting date slips into late May through mid-June.  A replant date in late May will reduce yield potential by about 7% from the optimal original planting date, whereas replant dates in mid-June generally result in about 80% of the maximum yield potential.  The planting date penalty tends to increase at more northerly latitudes.   
Step 5.  Determine whether to fill in or replant?  Recent research at University of Wisconsin found that highest yields were achieved with populations greater than 100,000 plants/ac in the original early planting date.  When initial stands were below the 100,000 plants/ac threshold, filling into the existing stand improved yields more than total replant, but still not to the levels of an optimal stand at optimal timing.  Using tillage and replanting the entire field at a later date greatly limited yield potential even at very high seeding rates, due to planting date impacts explained above.  These replant data trends held up over three different initial planting dates.  (See Figure 1.)
Step 6.  Consider other factors.  Excessive rains and the passage of time can carve into the window of residual herbicide efficacy, necessitating adjustments to the weed control program.  Label restrictions may limit the amount of additional active ingredient that can be applied.  Herbicide product choices may be more limited when filling in an existing stand.  A farmer should always consider the additional costs of labor, fuel, herbicide, and seed and offsetting revenues when evaluating the total economic decision.  Lastly, your crop insurance provider may require a visit by an adjuster prior to replant operations.  Insurance may not pay for the cost of filling in a sub-par stand.  Be sure to check with your crop insurance provider on requirements before replanting.  And of course, contact your FS Crop Specialist to assist with the replant decision process and any material or service needs associated with replanting. 


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Figure 1.  Yield (bu/ac) of twelve replant scenarios, averaged across three planting dates.  The number printed at the top of the bar represents the final the final plant stand (1000 plants/ac) after replanting or filling in.  (Source:  Think Twice Before Replanting Soybeans, University of Wisconsin, 2015)  

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