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Issue No. 22, Article 6/September 4, 2009

Time and Heat Still Required for Soybean Yield

A month ago I was still optimistic that we could achieve average soybean yields, particularly for the late planting dates. But I am increasingly more nervous about the latest-planted soybeans due to the cool temperatures we've been having. In addition, numerous reports in some areas of sudden death syndrome (Fusarium solani f. sp. glycines) and white mold (Sclerotinia sclerotiorum) have people concerned, but personally I think that a warm, sunny, and relatively dry September is more important to positively influence state soybean yields at this point.

Yield in soybeans is determined over a long flowering and pod-filling period that usually extends from the first week in July to the middle of September in a "normal" year. Unfortunately, as readers of the Bulletin are well aware, this year a high percentage of soybean acres were still being planted in the first week of July. Furthermore, even for the soybeans planted earlier, July 2009 was the coldest July on record in Illinois, so we didn't accumulate the maximum amount of GDD possible to maximize early vegetative growth. Because soybeans flower based on photoperiod, they reduce their vegetative stages more than their reproductive stages, when they must mature in seasons with a reduced number of growing days. When the time devoted to reproductive stages must be shortened, like this year, it is still difficult to estimate how many days are absolutely necessary to achieve normal yields.

Normally there are about 50 days that soybeans use to develop and fill pods between growth stages R4 and R8 (maturity). The amount of energy from sunlight that is captured and converted to produce soybean seeds during that time period is, of course, heavily dependent on sunlight intensity, temperature, and moisture conditions. There is still not a lot known about what "triggers" the plant to stop filling soybean seeds and drop leaves as maturity approaches. I have heard that two nights in a row below 55 F after August 15 will do it, but I believe that is purely anecdotal. We do know that soybeans do not like cool nights and that growth is severely reduced below 60 F during the night. As I have driven around the state recently, most soybean fields look good from the windshield. The only problem is more closely comparing crop development to the calendar date and the uncertainty in what that means.

The USDA NASS Weather and Crops report noted that 63% of soybeans were blooming and 18% were setting pods on August 2. That advanced to 89% blooming and 58% setting pods as of August 17. The 11% not yet flowering and the 42% not yet setting pods by the second half of August were a concern. The number of acres setting pods jumped to 91% for the August 30 report, which is only 4% and 7% behind last year and the five-year average, respectively. This might appear good, but it still doesn't make up for the lost time devoted to setting pods during a normally hot, sunny late-August. This development time might be further shortened if the cold temperatures we've been experiencing speed up leaf drop and maturity.

Of course, estimating yield in soybeans is a challenging task, and using almost any method it results in an educated guess. One method of tracking used by many people is simply counting pods per area on an annual basis. I have received reports from several agronomists that pod counts measured in this manner are down compared with last year. What's still unknown is whether this is due to delayed development that will still be recovered in September, or if we're past the point of recovery. We know that in addition to pod set, seed fill is still critical; between 3 to 7 bushels of soybean seeds are accumulated per day between R5 and R7. So the number of "good" days in this period can still have critical (positive or negative) influence on final yield. This year a large majority of soybean acres will be between R5 and R7 during September, and some in late September.

There is not much we can do but hope for the favorable September weather plus a late October frost. For most of Illinois the first fall frost occurs in the middle of October, but the dates range from the first week in the north to the third week in the south. If the first frost is moved back a week this year, several extra days of warm weather could help quite a bit. On the other hand, if the first frost moves earlier on us, much yield will be lost in the latest-planted fields. One thing that can be done now is to scout fields to develop an efficient harvest plan. Take into account the most critical fields to harvest first based on expected maturity, yield potential, and crop integrity. Also make sure you use this time to review proper combine settings according to manufacturer recommendations to prevent yield loss due to the harvest operation. The most common such loss occurs at the cutter bar, so give it careful attention.--Vince M. Davis

Vince Davis

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