Lebanon Crop Management Video

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11 May 2016

Soybean Replanting Decisions



As the soybeans begin emerging growers might find some stands might need to be replanted due to a number of factors.  The soybean plant has the ability to branch and fill in however there are limits to the lowest population established without losing top end yield.   The Agronomy Guide Soybean replant worksheet offers details on determining the relative benefit of replanting.

We are early in the planting of soybeans but I have seen several stands planted back in early April that are beginning to be rowed and populations can be conducted now.  To illustrate this here is a table with the relative impact on yield by planting date as the season progresses. This illustrates the ability of the soybean to respond to varying planting date.
                                              1.       May 10 – 100%
2.       May 20- 98%
3.       May 30 – 95%
4.       June 10- 88%
5.       June 20 – 76%

Replanting is one of those thankless jobs in farming, so it pays to take time to evaluate stands carefully before replanting.  Consider some of these issues in deciding to replant.

1.  Soybeans have a tremendous capability to recover from reduced stands.  Data from various states would indicate that near maximum yields are possible with stands at 100,000 plants per acre.  At 70,000 plants, yields will still be in the range of 90-95% of optimum.  A recent Purdue study showed that across four locations, final stands of 46,000 plants per acre averaged 66 bushels /acre compared to 79.0 bushels /acre for 171,000 stands. That’s 83% of the maximum yield with a fairly sparse stand.

2. Soybeans cannot compensate for large gaps in the stand.  Many Pennsylvania fields with varying soil types are prone to uneven soil conditions which could lead to many gaps in the rows.  More gaps in the stand lead to a lower yield potential.  In a field with 40% gaps, at 70,000 plants, we’d estimate the yield to be 83% of optimum. Where large gaps are or in fields where large patches occurr replanting might be the best option.

3. It is important to identify the cause of poor emergence before replanting.  Seed depth, crusting, dry seedbeds, cold wet seedbeds, seedling disease insects, hail or seed quality are possible culprits. Stands with significant number of injured seedlings may have limited yield potential.
4. The one great aspect of soybean replanting is that one does not have to eliminate the existing stand. It can be replanted directly with only slight differences in seed set at the end of the season.  I had a grower a few years ago that we had just shy of 60,000ppa in the field so he chose to replant with an additional 100,000 seed drop per acre and at the end of the season he had about 120,000ppa final stand 70bu/acre yields.  

So what should you do to conduct a stand assessment?

First you will need to determine the population as it stands in the field. To do this utilize the following table below. Otherwise there is a simple method I use to estimate populations.  To determine populations you must first know the width of planting. 6”, 7”, 8” for small grains 7”, 15” or 30” for beans and 15” or 30” for corn.
  1. Now to determine population you simply convert the row width from inches to feet by dividing by 12.
  2. Then divide the square feet per acre(43560) by the foot of row. This gives you linear feet
  3. Then, take the number of plants you find per foot in the field and multiply by the linear feet number to get the ppa.
  4. For an example. Thirty-inch rows divided by 12 is 2.5 feet. 43560 square feet in an acre divided by 2.5 gives us 17424 linear feet. Now if you get 3 plant in a foot you have 17,424 plants per acre. If you get 2 then you have 34,848. To simplify this you could now take 17424 and divide by 1000 to get the number of feet you need to represent 1/1000th of an acre. In this case it would be 17.4 feet. So you could go to the field measure 17.4 feet count the plants multiply by 1000 and you will have your plants per acre. Do this in several places and you could find the average of the field.
Table 1.6-1. Seeding rates and plant population estimates for soybean.
Row width
(inches)
Number of seeds planted/ft row¹
Number of plants/25ft row

Full Season²
Double Crop
Full Season
Double Crop



7
2.5
3.5
50
65

10
3.5
4.5
70
95

14
5.0
6.5
100
135

15
5.0
6.5
110
145

21
7.0
*
150

24
8.0
*
170

30
10.0
*
215


Based on 85% germination (full season = 176,000 seed/A, double crop = 235,000/A.




  • ²Full season = 150,000 plants/A; double crop = 200,000 plants/A
  • * Double-crop beans should be planted in row widths of 15 inches or closer.

  • The number of seeds planted per foot of row is based on 85 percent germination and an optimum population of approximately 150,000 plants per acre for full-season beans and 200,000 plants per acre for double-cropped beans.

    Know that Final Stands for high level yields need to be between 100-120,000 ppa.  If you count a stand and it is below 100,000 as of today it may be advantageous to replant.


    For early plantings it is particularly important to treat the seed to avoid potential pest problems.  Placing seed in a cold environment “naked” no insecticide or fungicides will open numerous doors for pest issues to surface most notably disease organisms.   My advice here is that at this late planting the soybeans could remain untreated since most likely the seed will germinating rather quickly.  It would not be wasted money to treat these soybeans as well particularly for soybean aphid and other insect management benefits of systemic treatments that contain chloronicotynil compounds.  Many times growers can add more soybeans to the field without terminating the existing soybeans.  The planter will terminate some soybeans and that will need to be accounted for when no tilling more soybeans into the existing stand.    In many cases in the past growers have replanted with roughly half the amount of seed per acre to thicken thin stands of soybeans.  Now is the time to be looking at these stands and not when the beans are 6 inches tall and will cause issues with replanting.

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