Issue No. 4, Article 10/April 21, 2006
Soybean Planting Considerations
Corn planting is getting under way but at a slower pace than in recent years. For almost everyone, the best sequence is to finish planting corn and then move into planting soybean. One recent report from Iowa State University suggested that soybean yields can drop at a rate of almost a bushel per day of delay relative to "early" planting, which I believe was late April. Late April is typically a good time to plant soybean, but in our studies we have seen little loss in yield if planting is delayed to mid-May in northern Illinois and to late May in southern Illinois. The drop in yield with delayed planting begins earlier for corn than for soybean, and it accelerates at a faster rate. Thus it makes sense to complete corn planting before planting soybean.
In almost every case we've seen, 90,000 to 100,000 soybean plants per acre will produce as much yield as higher plant populations, regardless of row spacing, when soybean is planted at the normal time. With soybean, though, establishment is usually not as certain as it is in corn, and so it comes down to a question of what it takes to get 100,000 healthy plants in the field. If warm (standard laboratory) germination is above 90%, soils are above 55°F and warming, and soil moisture is good with no threat of crusting before emergence, we can often expect 85% to 90% of planted seed to produce plants, especially if planted with well-adjusted row units. On the other hand, if germination is 80%, soils are cold or wet, heavy rain and crusting occur after planting but before emergence, and seed is placed at variable depths or with variable soil conditions, 50% or fewer of the planted seeds might produce plants.
So how many soybean seeds we should plant involves some guessing, but fortunately we have some ways to improve our guesses. First, seed we purchase has warm germination listed, and in some cases we can even get cold germination scores, which are helpful if we're planting early. In general, seed quality from the 2005 crop has been reported as not being particularly good, and there are indications that fungicidal seed treatments might improve emergence more than usual. On the other hand, soybean seed with quality problems often loses germination and vigor more quickly, so storing it under good conditions and handling it gently during planting may be helpful. Using planters and planting units that handle seed gently and that distribute and drop seed uniformly is another way to help establishment.
Finally, soil conditions at and after planting usually make a difference in how successfully the crop establishes. Soybean has "delicate" seed, and as such it benefits when planted about 1½ inches deep, modestly firmed into the seed furrow, covered by relatively loose soil, and into soils with temperatures at 60° to 70°F. Remember that soybean seedlings need to pull their cotyledons through and out of the soil and that this feat is helped a great deal by good surface soil conditions. Of course, several inches of rain immediately after planting can quickly turn surface conditions from good to poor, especially in some soils. The lack of oxygen in saturated soils and the formation of a soil crust of even modest strength can almost eliminate soybean emergence.
Due to these factors, it's probably appropriate for producers to use soybean seeding rates ranging from 120,000 to as much as 200,000 per acre, though there might be a question about the wisdom of planting into conditions where we expect only 50% establishment. A question under such conditions is whether the 100,000 plants that might struggle up are going to be capable of producing full yields. In most cases, we think that plants have the ability to thrive and to produce full yields if conditions improve greatly after a tough start, but if planting conditions are too wet, soil compaction might be a problem later in the season, even if we get an adequate stand. We have also seen some cases in which soybean plants do not seem to recover fully after a tough start, though reasons for this are often not clear.
One of the more unusual things I've encountered is the technique of mixing non-Roundup Ready seed with Roundup Ready seed to increase the seeding rate, then spraying with glyphosate after emergence to kill the "extra" plants. The idea is that seeds planted closer together in the row will help one another emerge by providing more push to get through the soil surface, usually when it has formed a crust. It's also possible, if the stand is only adequate after using extra seed, to leave it without spraying to kill the extra plants.
In general, it's doubtful that this technique is very cost effective as an attempt to ensure against the need to replant. Saturated soils and hard surface crusts might mean very little emergence even with the additional seed, in which case replanting will be needed anyway. If the decision is to leave all the plants, then there will be a mixture of varieties (one of which might not be top yielding) and reduced herbicide options. And the seed that is mixed in is not without cost, even if it's taken from last year's crop. It also would need to be tested to make sure it has high germination, and it might need to be treated as well. For most people, making sure planting is done using good seed under good conditions is a better approach to getting adequate stands.
Finally, using a commercial soybean seed inoculant is a question that many people are dealing with. Specialists at both Ohio State and Purdue universities have reported a bushel or two of extra yield from using such inoculants, averaged over numerous trials. Other states have reported less response. We've conducted 23 inoculant trials in Illinois over the past three years, and averaged over all, which included several different commercial preparations, we have seen very little response. All of these trials are done following corn in a corn-soybean rotation. It's possible that soil organic matter, texture, and temperatures in Illinois simply make the crop here less likely to respond than it might be in cooler, wetter soils with less organic matter. Whatever the case, we have seen no reason to change our longstanding position that yield response to inoculant is unlikely in Illinois if soybean has been growing in the field in the past five years. Inoculant should definitely be used in fields coming out of CRP or continuous corn.
We continue to do some inoculant work in Illinois, including testing some inoculants that contain microorganisms besides those that help the plant fix nitrogen. There have been reports that some of these "growth-enhancing" microorganisms can increase yield, presumably by providing substances to the plant that are not provided by organisms already in the soil. We'll report on these results as they come in.--Emerson Nafziger