Soybeans on a farm near Towanda in central Illinois Sept. 12

Soybeans on a farm near Towanda in central Illinois Sept. 12. By Sept. 1, nearly 10% of the crop was still not setting pods.

Editor’s note: The following was written by Emerson Nafziger, University of Illinois Extension crop production specialist, for the university’s crop development Bulletin Sept. 5.

After the worst start to a cropping season in decades, mid-season lack of rain in parts of Illinois, and season-long low crop ratings, it’s time to take a look at what comes next as 2019 moves into its final stages.

With 80% of the 2019 Illinois soybean crop planted after June 1 and some 10% planted after July 1, we set a new record for late planting of soybean in Illinois this year. With such late planting, the flowering and pod setting took place at least two weeks later than normal (average of the last five years). By Sept. 1, nearly 10% of the crop was still not setting pods.

Two main factors will combine to limit soybean yields in much of Illinois in 2019. One is that late planting has, at least in many areas, resulted in lower numbers of pods that are filling. Reasons for this are complex, but include:

  • late canopy formation, which likely limited the supply of sugars needed to set pods;
  • lighter green canopy color, at least in some fields;
  • lower than normal numbers of nodes with pods, especially in dry areas where plants are short; and
  • low pod numbers per node.

We see very little of the three-to-six pods per node (in the central part of the stem) that we saw in the 2018 soybean crop, even in fields that appear to have made fairly good vegetative growth.

Another factor that is likely to lower soybean yields in 2019 is the late start of podsetting followed by the late start of seedfilling. This isn’t a problem at the beginning of September, but it is by the end of the month. Based on temperature and daylength changes, we would expect the amount of daily photosynthesis (on a day with full sunlight) in central Illinois to drop by about 55% from Sept. 1 to Sept. 30. Most of this is due to lower temperatures.

One of the difficult-to-predict differences between early- and late-planted soybeans is the timing of crop maturity. We’re now starting to see the loss of leaf color in early-maturity varieties, even those that were planted late.

Based on the low pod numbers we’re seeing this year, it appears unlikely that early-

maturing soybeans are going to produce high yields. That can indicate fields that mature later may not have great yields, either.

We’ll need to wait to find out how well seeds fill before leaves lose their color, but we should keep in mind that soybean yields are more closely tied to seed numbers (per acre) than to seed weight, and in the parts of Illinois most affected by late planting and dry weather, we aren’t seeing the high seed numbers we’d need for high yields.

At 120,000 plants per acre and 3,000 seeds per pound, yield in bushels is the number of seeds per plant divided by 1.5. At 2.5 seeds per pod, each pod per plant would mean 1.67 bushels per acre, and 30 pods per plant would mean 50 bushels per acre.

Will the Aug. 1 NASS estimate of 55 bushels per acre for Illinois soybeans hold up? I don’t have a basis to judge, but there are both some very good and some very poor soybeans in Illinois fields.

Having a stretch of warm, sunny weather in September would help to fill the green pods on green plants in many of the late-planted fields. But pod numbers are not going to increase, and that means many fields will not produce yields this year as high as those we saw in many areas in 2018.