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 the 2019 cropping season moves into its final stages.
To no one’s surprise, various crop tours in recent weeks have confirmed that corn yields in parts of Illinois are likely to be disappointing. If there is a positive, it’s that the crop may look a little better than we thought it would by now after more than half of it was planted after June 1. While canopy cover and color in early July were a little better than expected, lack of rainfall and a less vigorous root system on late-planted corn meant that water stress began to show up in July. In areas where the dryness continued through August, some fields now show little green leaf area, and ear tips have dropped in drier parts of fields.
The driest parts of the state are the counties around the Quad Cities and in east central Illinois, with rainfall totals in July and August only about half of normal in these areas. This region shows up as being abnormally dry or in moderate drought on the U.S. drought map. Much of northeastern and southern Illinois received at least normal rainfall amounts over the past two months, and a band from St. Louis east along I-70 in south central Illinois shows rainfall totals of 150% or more of normal. Although late planting has gotten most of the attention, rain amounts, including lack of rain in some areas, will be a big part of the 2019 cropping story. That would have been the case even if planting had been early.
Late planting made the lack of adequate water a bigger problem. Many fields showed early water stress symptoms, and ended up with shorter-than-normal plants; both point to soil compaction as a major issue.
The USDA-NASS will issue the September 1 crop yield estimates on September 12. The August 1 estimate was 181 bushels per acre for Illinois corn, which is down 29 bushels (14%) from the 2018 Illinois corn yield. The corn crop ratings are not very high: 19, 35, and 44 percent of acres were rated as poor or very poor, fair, and good or excellent in the September 1 report. In 2017, only 55% of the crop rated as good or excellent in early September. That year, the Illinois yield estimate went from 188 in August to a final of 201 bushels per acre. This year is not a lot like 2017 (or any other year in the last 40), so we’ll need to wait to see what the new estimate turns out to be.
With 80 percent 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 as well. 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 September 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: 1) late canopy formation, which likely limited the supply of sugars needed to set pods; 2) less favorable (lighter green) canopy color, at least in some fields; 3) lower than normal numbers of nodes with pods, especially in dry areas where plants are short; and 4) 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. Many plants have only two or three pods per node, and only 10 to 12 nodes with pods. There appear to be more productive (pod-bearing) branches than normal in some fields, possibly because main stem growth was limited so branches had more resources. In some plants I’ve seen, a third to half of the pods are on branches. We don’t know if this affects yields compared to having most or all of the pods on the main stem.
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 is because, compared to August, days in September are shorter and average temperatures are lower, meaning that the amount of daily photosynthesis is lower. 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 September 1 to September 30. Most of this is due to lower temperatures.
We’re now starting to see the loss of leaf color in early-maturity varieties, even those that were planted late. We know, of course, that early varieties mature earlier than later ones, both because the period of pod formation is shorter in early varieties, and because seedfilling starts earlier. 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 that fields that mature later may not have great yields, either.
Will the August 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 that many fields will not produce yields this year as high as those we saw in many areas in 2018.
Source: Illinois State University