Making Soybean Replant 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. Tables 1.6-3 and 1.6-4 in the Penn State Agronomy Guide offer details on determining the relative benefit of replanting.

 

Table 1.6-3. Yield potential of reduced stands. (From Penn State Agronomy Guide 2019-2020)

 

Table 1.6-4. Yield response of full-season soybean to deficient stands. (From Penn State Agronomy Guide 2019-2020)

 

We are midway in the planting of soybeans, but I have seen several stands planted back in early April that are beginning to be rowed and population estimates can be conducted now. Table 1.6-5 in the Penn State Agronomy Guide presents 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.

 

Table 1.6-5. Approximate yield response of soybean to changes in planting date. (From Penn State Agronomy Guide 2019-2020)

 

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 (PPA). At 70,000 PPA, yields will still be in the range of 90-95% of optimum. The PA On-Farm Soybean Network has studied the impact of reduced soybean stands over the past three years. The table below represents the impact of reduced populations on stands and supports replant decisions that if a uniform stand that is above the 80,000 PPA population yields will be maintained without replanting. The key in those plots were uniform stands.

 

 

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 PPA, we’d estimate the yield to be 83% of optimum. Where large gaps are or in fields where large patches occur, 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 a 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 had just shy of 60,000 PPA in the field. He chose to replant with an additional 100,000 seed drop per acre and at the end of the season he had about 120,000 PPA final stand and 70 bu/A yields.

 

 

Conducting a stand assessment in the field.

 

You will first need to determine the population as it stands in the field. 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.

For an example. 30-inch rows divided by 12 is 2.5 feet. 43,560 square feet in an acre divided by 2.5 gives us 17,424 linear feet. Now if you get 3 plants in a foot you have 17,424 plants per acre. If you get 2 then you have 34,848.

 

To simplify this method, you could now take 17,424 linear feet and divide by 1,000 to get the number of feet you need to represent 1/1000th of an acre. In this case it would be 17.4 feet. You could go to the field, measure 17.4 feet, count the plants, and multiply by 1,000 to calculate plants per acre. Do this in several places and you could find the average of the field.

 

Table 1.6-1 in the Penn State Agronomy Guide provides estimates of plant populations necessary to achieve optimum yield based on planting timing. 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. As of this writing it is May 21st of the year and our data suggests this is a later planting and narrowing rows and increase plant populations is a management tactic to avoid losses as the season progresses populations should approach the 200,000 PPA to ensure adequate yields. 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.

 

Table 1.6-1. Seeding rates and plant population estimates for soybean in full-season, full-season and low-risk, and double-crop scenarios. (From Penn State Agronomy Guide 2019-2020)

 

For early plantings, it is particularly important to treat the seed to avoid potential pest problems. Placing seed in a cold environment “naked” with 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 germinate 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 chloronicotinyl compounds.

 

In many cases, 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 instances 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. When the beans are 6 inches tall, replanting issues are likely.