Emerson Nafziger and Giovani Preza Fontes
Department of Crop Sciences, University of Illinois Urbana-Champaign
The article was published at https://farmdoc.illinois.edu/field-crop-production/keeping-a-close-eye-on-the-2024-soybean-crop.html on August 30, 2024. You can also read this article in Portuguese and Spanish.
Ratings for the 2024 Illinois soybean crop have been high most of the season, and the August 1 estimate released by NASS is for yield at average 66 bushels per acre. That would be a new record for Illinois, 2 bushels higher than the 2021 yield, and 4.9 bushels higher than the 2024 trendline yield.
While the soybean yield prospects are good, this has been an unusual year, and this lets us watch how the crop’s response to the weather is playing out in the field. The 50%-planted date of May 16 was slightly earlier than the average over the past decade, but planting had “a long tail,” with only about 80% of the crop planted by May 31. Dry June weather lowered plant growth rates, but high temperatures caused flowering to begin early: 25% of the crop flowered by June 30. Above-average rainfall in July caused rapid growth, and soybeans in many fields ended up a little taller than normal, with a very good crop canopy and canopy color by early August.
The photos in Figure 1 were taken on August 9 in a field of 15-inch rows that was planted in mid-April. The crop was not taller than normal, but the heavy canopy meant that little light was reaching the lower nodes. Those nodes had few pods, and most of the leaves that would have been attached there had dropped off. Flowers would have formed on the lower nodes, but with little nourishment coming from the shaded leaves there, pods either failed to develop or dropped off.
This loss of lower pods and leaves on soybean plants is probably the result of shading that developed during rapid vegetative growth in July. Could the loss of lower leaves and pods offset the advantage that a dark green canopy provides during podding and seedfilling? Probably not – while high pod numbers per plant are important, having those pods develop at nodes that also have healthy leaves provides the best chance for seed to reach their full weight. Figure 2, which was taken in the same field on August 21, shows that pod numbers are concentrated at higher nodes on the stem, and that green leaves are attached at these nodes.
About 20 percent of the Illinois soybean crop (including doublecrop after wheat harvest) was planted in June this year. In some areas, growth of late-planted soybeans was slowed by dry soils in June, with the crop still in early vegetative growth stages by early July. These fields also grew rapidly during the July rains, and while they generally developed good canopy cover, they did not grow as tall or have as much internal shading as the early-planted fields had. Figure 3 shows the field and plants of soybeans planted on June 4 near Champaign. By late August, these plants had much less leaf area than early-planted soybeans.
While cooler than normal temperatures in August likely slowed soybean growth a little, the pod numbers are good in this late-planted crop, and the lower leaves are mostly intact and receiving some sunlight, which should help fill pods on the lower nodes. With a lot of seedfilling still ahead, and with less leaf area than early-planted soybeans, we can expect fair to good yields from the late-planted crop, as long as good weather continues into mid-September.
Weather and maturity
One unusual feature of the 2024 season has been the cool temperatures during parts of August. At Champaign, nine days in August (Aug. 9-12 and 18-22) had daytime highs in the 70s (average of 76) and nighttime lows in the 50s, averaging 56. This lowered water use rates during these periods, which helped prolong the supply of water. High temperatures and uneven rainfall this week have lowered soil moisture reserves across much of the state: the US Drought Monitor shows that 29% of Illinois, mostly around the edges of northern and southern Illinois, is “abnormally dry” – the lowest level of deficiency. Water use rates are slowing considerably for the early-planted crop, and cooler temperatures in September will help, but water stress could hasten maturity and lower yields, especially of the late-planted crop, in some areas.
Cool temperatures during seedfilling, especially at night, may sometimes – at least temporarily – have a negative effect on soybean plants. On warm, sunny days with good soil moisture, the photosynthetic rates in soybean can exceed the rate at which sugars are transported out of the leaves. That’s especially the case when the “sink strength” – the overall demand for sugars in the plant – is low, as it can be during vegetative growth or when pod numbers are low. This excess sugar can accumulate in leaves in the form of starch. This starch normally converts back to sugar and moves out of leaves during the night. Cool night temperatures slow this process, and starch that remains in the leaves can slow photosynthesis the following day. A high demand from developing seeds can accelerate sugar movement out of the leaf during the day, resulting in less starch accumulation.
During the cool periods in August this year, lower daytime temperatures and high sunshine intensity offset one another, but daily photosynthesis rates were probably lowered a little, so less starch might have accumulated. Most fields were filling pods, so sink strength was good. So we don’t think the net effect of the cool weather was negative, especially since warm temperatures prevailed between the cool stretches.
Watching to see when the soybean leaves begin to lose green color can provide a clue about how well the crop was able to fill its pods. One factor that triggers maturity is the decreasing demand for sugars as seeds approach maturity. This may send some sort of signal to leaves to begin to break down their protein and send the nitrogen, along with sugars, to complete the seedfilling process. Evidence for this includes the fact that plants with few or no pods often stay green much longer than plants with high pod numbers; in fact, low pods numbers and “staygreen” are often found together. When scattered plants stay green, they typically are plants with few or no pods. When whole fields have green stems after pods are dry, pod and seed numbers are often low, which may weaken the signal that initiates maturity.
According to crop progress as reported by NASS, flowering and podsetting were both a little early this year, and the start of leaf drop (6% on August 25) is in contrast with the 5-year average, which showed no leaf drop by this date. While most leaf drop is likely happening in early-planted fields –26% of Illinois soybeans were planted by April 28 – it is possible that dryness in some areas is bringing the crop to an early end. In such cases, leaves may shrivel without losing all of their green color, and seeds may end up smaller than normal.
The 2024 Illinois soybean crop built its base for high yields – good pod numbers and outstanding canopy color – by the end of July, especially in early-planted fields. While scouting fields into August is always a useful exercise, a canopy like the crop had by early August this year is one of the best indicators of a good crop to come. It is also an indicator that the crop likely needs little help from fungicide, insecticide, or nutrients in order to get to good yields. It’s not certain that yield will reach the 66 bushels per acre forecast, of course, but it certainly appears to have the potential to do so.