Corn and soybean crops limp towards the finish line

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.

Corn

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. Soil compaction was certainly an issue this year, but the smaller root systems and drying soils meant that plants weren’t as able to get access to water deeper in the soil as they would have been with earlier planting, even into compacted soils. Soil compaction is always present after tillage and planting using heavy equipment, but roots of early-planted corn can usually make connections to tap water from deeper in the soil even when there is compaction.

Temperatures this season have tracked very close to normal: from May 1 through September 1, the statewide GDD accumulation was about 2,580, 15 GDDs above normal. This total ranges from about 2,300 in northern Illinois, to 2,500 in central Illinois, and to more than 2,700 GDD in the southern part of Illinois. Had the crop been planted at the normal time, some fields in southern Illinois would be starting to dry down by now, and those in central Illinois would be getting close to black layer. But corn planted on June 1 instead of May 1 in northern and central Illinois lost about 350 and 450 GDDs, respectively, and so accumulated only about 1,950 and 2,050 GDDs by September 1. Corn planted on June 15 lost an additional 250 GDDs or so, and so accumulated only about 1,700 and 1,800 GDDs by September 1 in northern and central Illinois, respectively.

If we assume for simplicity that hybrids normally grown in northern and central Illinois require 2,550 and 2,700 GDDs from planting to maturity, corn planted on June 1 would need to accumulate roughly 600 and 650 GDDs, respectively, from early September through maturity. Normal GDD accumulations in September in northern and central Illinois are about 450 and 500, respectively, and accumulating the number of GDDs needed to reach maturity would, with normal temperatures, take until about October 20 in northern Illinois and about October 15 in central Illinois. September temperatures have been above normal in four of the last five years, but we can’t count on that in 2019. Corn planted after June 10 requires more GDD to mature than it is likely to get before the average date of first frost, which is around October 20.

What if the corn doesn’t get enough GDDs to mature fully? According to the Iowa State University publication Corn Growth and Development (PMR 1009), dry matter accumulation slows considerably near the end of the grainfilling period: it takes 380 GDDs to accumulate the last 10% of kernel dry weight, and 205 GDDs to accumulate the last 3% of dry weight. So having the corn stop filling with 200 GDD yet to go should not cost a lot of yield. That depends somewhat on how grainfill ends, though: a hard freeze (28 degrees or less) stops grainfill and starch formation in the kernels quickly, while slow deterioration of the leaf area before grainfill ends allows more sugar to move into the kernels and be converted to starch to add dry weight. Kernels that don’t fill completely tend to have a constricted base where they attach to the cob, and that can mean lower test weight. If frost stops the conversion of sugars to starch, kernels remain unfilled to the tip and also accumulate sugar there, which can slow field drying and can make kernels discolor more easily during heated-air drying.

In the drier areas of eastern Illinois we’re seeing a plants starting to die in patches, with ear tips dropping and leaves drying up. Such patches are typically where soils hold less water, and in some fields also in low areas where there was damage from standing water early or perhaps more compaction at planting. If other areas in the field are still green, we expect patches where the crop died early to show lower kernel weights and yields.

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.

Soybeans

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.

One of the difficult-to-predict differences between early- and late-planted soybeans is the timing of crop maturity. We gauge the end of seedfilling by when the crop canopy loses its color, which signals that the leaves have exported their nitrogen to the seed with the last of the sugars. That happens quickly—often over only two days or so—and we don’t have a very good way to guess when it’s going to happen. We think that the signal to end seedfilling originates in the pods—plants without pods stay green, and even the leaf that feeds a single node without pods may stay green. It’s possible that the timing of this signal is related to how many pods are on the plant and to what extent the seeds in these pods have filled.

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.

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 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.