Assessing Corn and Soybean Populations and Replanting Decisions

Penn State Extension

Replanting decisions can be difficult to make. This article explains how to go about making the call objectively.

This article is the Agronomy Highlight for Monday, June 1 at 8:00 a.m. The Agronomy Highlight is an opportunity to ask the author questions about the article, get updates from around the commonwealth, and share observations from your part of the state. Click here to ask a question or make a comment about the article.

 





 

You can join us on zoom or by calling +1 646 876 9923, and when prompted enter the webinar ID: 210 945 454. The discussion will also be stored for later viewing on the Field and Forage Crops Facebook page.

This year, cooler-than-usual temperatures from April into May slowed early planted corn and soybean emergence, and left seed and seedlings vulnerable to diseases and pests like Pythium and slugs. Part of the state also experienced chilling rains in early May, just when planting started to ramp up. Now and for the next few weeks, it is imperative to scout fields and assess whether replanting will have a positive effect on your net income.

Assessing Corn Stands
For corn on 30” rows, measure 17 feet 5 inches—this represents 1/1000 of an acre. Count the plants in that distance, then multiply the number you get by 1,000 to determine your plants per acre. For example, if you count 21 plants, that equals 21,000 plants per acre. Do this in multiple areas of the field to determine your average for the field.

Know the key planting times and relation to yield. This will come into play as stands are deemed inadequate and a replant is in question. Using Table 1.4-8A   from the Penn State Agronomy Guide , we know that planting corn on April 30th gives us 100% of our yield potential with a final stand of 30,000 plants per acre. However, if our average stand count is 17,500 plants per acre, the same chart for corn planted April 30th still has 87% of its optimum yield potential. If we replant the same field on May 29th and achieve a stand of 30,000 plants per acre, our yield potential is now down to 81%. We see nearly a 20% loss just by delaying the planting date by a month. Knowing this allows us to say, "If I want to replant today May 29th I will lose 20% of my yield, and if I don't replant I will only lose 13%, so it probably will not pay." It is possible that there are areas in fields that will have less than half a stand and would benefit from replanting. If replanting is necessary, it would be better to plant a shorter season hybrid to aid in a more uniform field dry down.

Assessing Soybean Stands
The soybean plant has the ability to branch and fill in sparse stands, however, there are limits to the lowest population establishment without losing top end yield. The other consideration is that the yield penalty for planting soybeans in late May and early June isn’t as severe compared to corn. However, do not be too quick to replant a field with reduced emergence. Using Table 1.6-3  from the Penn State Agronomy Guide , a 60,000 plants per acre soybean stand still has the potential to yield 92% of its full yield potential. Compare that 92% potential from those soybeans planted within the ideal planting window to the field being replanted on May 30th and having 95% of full yield potential, or June 10th at only 88% of its full yield potential.

So how do you assess soybean stands? Soybeans are more difficult than corn since there are multiple row width options. To determine stand count on 30” rows it is just like the example above for corn. For soybeans planted on 15” rows, you double that distance and measure off 34 feet 10 inches and count the plants. The number of plants x 1000 = plant population in plants per acre. Take multiple counts of adjacent rows in different areas of the field to get an overall stand for a field. For drilled soybeans, you can use the hula-hoop method: randomly toss the hoop and count the plants inside the circle. Convert plants per hoop to plants per acre by multiplying the number of plants by the appropriate factor. A 28 inch hoop is the easiest to calculate since the multiplication factor is 10,000, so 13 plants in the circle x 10,000 = 130,000 plants per acre. If you don’t have a hula-hoop, you can make your own. Cut a 1/2 inch piece of pex pipe to a length of 88 inches and attach the two ends with a ½ inch hose mender.

The Agronomy Guide Soybean replant worksheet offers details on determining the relative benefit of replanting.

  1. Identify the cause of the stand reduction and whether it will be a problem when the field is replanted. Take steps to correct the problem before replanting.
  2. Estimate the yield of a full stand at the original planting date.
  3. Determine the population and distribution of the existing stand.
  4. Estimate the yield potential of the reduced stand. Often, the stand is not uniform and the impact of patches or gaps in the rows would likely be greater than the population effect itself. Based on Midwest data, yields will not decline appreciably until stands drop below 60,000 plants per acre Penn State Agronomy Guide  . In one study, the impact of gaps lowered the yield potential of a 70,000-plants per-acre stand from 95 to 73 percent of normal (Table 1.6-3)  . One technique suggests estimating the number of gaps (at least one-half-pace long) encountered in several row sections consisting of 20 typical paces each. Add up the number of paces in half- or full-pace gaps and estimate the percentage of the total number of paces in your sample. Then refer to (Table 1.6-4)  for your yield loss estimate.
  5. Estimate any additional costs associated with keeping the stand. A thin stand may require an additional post emergent herbicide application.
  6. Estimate the yield potential of a replanted full stand (Table 1.6-5) later in the season.
  7. Estimate the cost of replanting.
  8. Compare the value of reduced stand to replanted stand. For example, assume the following:
    -- Estimated yield of full stand at original planting date is 50 bushels/acre.
    -- Average percent of row lost to gaps is 50 percent.
    -- The average plant population is 70,000 plant per acre.

From (Table 1.6-4)  , the estimated yield from the reduced stand would be 39 (50 x 0.78) bushels per acre. The estimated yield from planting can be obtained from Table 1.6-4  . Assume the original planting was on May 12, and the stand was evaluated on June 4 and could be replanted on June 5. The estimated yield would be approximately 45 bushels per acre, or 6 bushels per acre more than not replanting. Remember that the cost of replanting must be considered, and there is no guarantee that replanting will give a full stand. Another alternative is to fill in an existing stand to bring it up to an ideal population. This would require less seed. Be sure to include some consideration of the plants lost in the replanting process. Soybeans planted into an existing stand have far fewer negative effects than corn planted into and existing stand. Repairing a planting with a planter, if possible, rather than a drill may cause less damage to the stand.

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