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CropTalk: Sorting Out Soybean Replant

April 2019

Published on Monday, April 01, 2019

Today, let’s talk soybean replant and discuss best practices for making replant decisions. No matter how much time and effort is put into planning, Mother Nature seems to have plans of her own, and she trumps all.

Plants/A. or Nodes/A.?

Corn is king!! And in the case of many farmers, a corn mentality often transfers over to their management of that crop in between corn crops: soybeans. However, soybeans are a whole other animal, and need to be managed differently, especially during stand establishment and when determining whether or not to replant. You see, maximum corn yields come from plant to plant consistency. Ears can flex, however, breeders have selected for plants that produce a very consistent ear, plant after plant after plant. Therefore, an even stand and final population are crucial for maximizing yield in corn. Put another way, corn is a very population dependent crop.

Soybeans operate differently. Soybeans can produce axillary branches, which result in more nodes per plant, in response to environmental conditions like low plant density, longer growing seasons, and abundant resources. This greatly decreases the impact of a reduced stand. For example, you have likely seen a single soybean plant growing by itself in a barn lot or corner of a field. Picture it: a big bush with multiple branches and hundreds of pods. Ultimately, soybeans are much less population dependent than corn. I bring this differentiation up because it has a significant impact on replant decisions. From today forward, I want you to think about potential nodes/A. opposed to potential plants/A. when evaluating soybean stands.

This phenomenon is demonstrated in Beck’s 2017 Practical Farm Research (PFR)® Population Study - 15 in. rows (figure 1). Yields were equivalent with emerged populations ranging from 81,675 to 126,506 plants/A. The lower populations allowed for additional branching and, therefore, a greater number of nodes per plant. This resulted in similar yield per acre.

Planting Date DISCLAIMER!!!

Because soybean reproduction is influenced by photoperiod, the above recommendation is only valid with early planting dates. In the case of later planting, there is less vegetative growth prior to the reproductive stages, ultimately capping the potential number of nodes produced per plant. Put another way, we can get away with thinner stands (<100,000 plants/A.) when early planted, but later planting requires more plants per acre in order to maximize the number of nodes/A. Additionally, it would not be recommended to tear up a stand of 70,000 plants/A. or greater that was planted early in order to replant later in the year. Evidence of this can be seen in the five-year, multi-location Economic Optimum Seeding Rate (EOSR) Study results shown in figure 2.

Can I “Thicken Up” a Thin Stand?

Often, a thin stand is sore on the eyes at first, even if it is fairly even. The human eye is drawn to nice, thick rows of crops, and most often, stands look much less desirable before you get out and count plants. This is even further exaggerated in narrow rows, due to the increased plant to plant spacing within the row. It is not uncommon to hear of thickening thin soybeans stands up, but is this the best economic decision for your farm?

Figure 3 shows the effects of filling in thin soybean stands (7.5 in. rows) with a 30 in. row planter at the V2 growth stage1. The teal bars indicate the yields from the original soybean plants, the gray bars indicate yield from the laterplanted soybeans used to "thicken up" the stand. In this study, a drilled stand of 66,000 seeds/A. was supplemented by adding three different rates with a planter (120,000, 80,000 and 40,000 seeds/A.). They were able to measure the contribution of the replanted plants by using black soybean seeds, which always produce black soybeans (a recessive trait), and then sorting the harvested soybeans with a color sorter. The data clearly indicates that supplementing stands with a 30 in. planter likely did as much damage during the planting operation as it contributed in additional yield.

I hope that 2019 is the year when no farmer has to replant. But if Mother Nature has other plans, remember that a thinner stand planted early is often times better than a thicker stand planted late.

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Author: Travis Burnett

Categories: CropTalk, 2019

Tags: CropTalk, soybean, Replant

Travis Burnett
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