No. 8 Article 4/May 28, 2010

More on Soybean Stands and the Decision to Replant

There is little to report regarding atypical crop development concerns this week with a move to warmer, and in some areas drier, conditions. The state's soybean crop is almost half planted, and I suspect most producers are either working hard or waiting for wet soils to dry to finish the other half. Given heavy rains in some of the southern, western, and northwestern regions, I suspect there are still a few situations where there are considerations of the need to replant thin soybean stands.

If you are facing this decision, first consider the data that has been presented regarding seeding rates for the initial planting. My article in issue 4 is a good place to start. From the information there, it is clear that good yields can be achieved in many environments at final plant populations between 45,000 and 50,000 plants per acre in 30-inch rows. Those final stand numbers assume at least 95% emergence of the target seeding rate; though emergence was likely not that good in all cases, on average there's an expectation of a 5% to 7% yield penalty for seeding at 50,000 versus 150,000 seeds per acre.

The difference with this scenario versus a replanting scenario may be in the uniformity of plant distribution. It stands to reason that a stand of 50,000 plants that is not uniform will yield less than 50,000 plants spaced uniformly. The degree of severity in yield difference would be difficult to quantify in relation to the number of factors that would interact with this scenario, including weed control.

So if you are left with a partial stand and it is still early in crop development (<V1-V2), you have three options: keep it as-is; no-till plant additional seeds into the existing stand ("repair-plant"); or terminate the existing stand and replant to establish an entirely new stand.

Data from an experiment to determine soybean yield from these three approaches was published in the updated 2010 Illinois Agronomy Handbook (Table 1). The experiment was conducted for three years, at DeKalb by Lyle Paul and at Perry by Mike Vose. Either a no-till drill or a 30-inch row planter was used to make initial soybean stands either low or high in density. The low stands were then either repair-planted with the two different planting methods or tilled up and replanted with a narrow-row drill.

Averaged over six site years, the worst-case scenario of low initial stands in 30-inch rows yielded 45 bushels/acre from only 37,000 final plants per acre when no repair planting was done. This was 77% of the highest yield achieved (58 bushels/acre) for the drilled, high-initial-populations treatment. However, an additional 8 bushels/acre was achieved when those low stands in 30-inch rows were repair-planted with either a no-till drill or a 30-inch planter. In contrast, 54 bushels/acre was achieved when the initial stand was tilled up and an entire new population was established with a narrow row drill.

In summary, a soybean stand with less than 50,000 plants per acre is likely to experience decreased yield. How dramatic a decrease will depend on how far the population is below 50,000 and how uniformly plants are distributed. Keep the following points in mind for the three options: If you choose to keep a field as-is, pay particular attention to controlling weeds with early postemergence herbicide applications to prevent more yield from being lost to weed competition. If you choose to repair-plant, calculate enough extra seeds to replace initial plants that will be injured or killed during the second planting operation. If you choose to terminate an initial stand and start over, realize that you will likely still experience a yield penalty for planting later, and of course there is no guarantee on establishing a better stand from the second planting.--Vince M. Davis

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