Considerations as the 2022 Soybean Crop Approaches Maturity

Planting of the 2022 Illinois soybean crop began a little later than normal, and ended ahead of normal (Figure 1). Although summer temperatures were a little higher than normal, the onset and pace of flowering (“blooming”) and podsetting lagged behind what we would have expected based on planting progress. This is unusual Given that there was unusually cool weather in July. The most likely explanation is that the dry weather held back both vegetative and reproductive development to some extent.

Leaves are beginning to drop is a few fields, and although the onset of the maturity sequence in soybeans is a little difficult to predict, it may follow a time course much like that of podsetting. That would indicate that podfilling (the time elapsed from podsetting to loss of leaf color) was extended to its normal duration, which signals that seeds are filling well. If that pattern holds, maturity of soybeans may lag that of corn this year, at least in fields that have had plentiful moisture. Soybeans don’t spend as much time drying in the field as corn, though, so harvest of the two crops is likely to overlap to a considerable extent in most areas.

Neither leaf color loss nor leaf drop is a precise signal for maturity. In a process that is not well understood, the decline in demand for sugars (as podfill slows) causes the plant to send a signal to the leaves to break down chlorophyll and leaf protein (most of which is present in the photosynthetic enzyme RuBisCO) and to send the resulting nitrogen to the seeds as they finish filling. This redistribution of N from leaves to seeds is an important process for soybeans; during the most rapid seedfilling phase (stage R6), soybean plants take up little N from the soil, but move a large amount of N from other parts of the plant into the seeds. As leaves lose their nitrogen, photosynthesis ends, leaves lose their color, and they soon detach from the stem.

Crop condition and potential

For the most part, the soybean crop overcame the slow growth and stress symptoms that were widespread in June and early July, and developed a good canopy with the dark green color that we associate with good yield potential. The US Drought Map released today (September 8) still shows areas of moderate drought and abnormally dry conditions in Illinois, but the crop likely has enough water in most fields to fill the seed that it set. One exception to that might be in late-planted fields in drier areas, which in some cases did not develop a very dark canopy color. These plants may have had thinner leaves and lower photosynthetic rates, so made less growth, took up (and fixed) less N, and likely have fewer pods and seeds. They may well lose color and drop leaves earlier than they would have with more water.

The August 1 estimate for Illinois soybean yield is 66 bushels per acre, which would be a new record for the state. The soybean condition rating has remained fairly stable the past two months – between 65 and 70% good+excellent – rising from about 60% during the dry weather in June. That’s similar to the ratings for the last two years.

One often-debated question is whether soybeans as a crop are more “drought-tolerant” than corn. Thirty or forty years ago we might have thought so, in part because soybean yield levels were not very high, and they showed less fluctuation than corn yields. Indeterminate soybeans can benefit from their extended period of flowering and podsetting, which enables them to recover from short periods of stress. But corn has been greatly improved in its ability to set kernels under somewhat stressful conditions, and this has lessened soybean’s advantage in this regard. The two crops are sensitive to dry soils at different times – in the drought year of 2012, rain arrived in time to help soybeans after the corn crop had set many fewer kernels than normal. We can probably call it a draw, concluding that the stress tolerance of both crops have been much improved by breeding. Both remain sensitive to stress, but they are differently sensitive to the timing of stress.

Plants as maturity and harvest approach

Dry conditions resulted in somewhat shorter soybean plants than normal in many soybean fields this year. This doesn’t mean that fewer nodes or pods formed, but percentages of pods on branches may be higher than usual in some fields. Having good pod numbers means more demand for sugars, which serves to prevent the development of green stem in soybeans. Having stems stay green after pods mature usually happens when the supply of sugar supply in the plant exceeds the amount of sugars that pods can accept, leaving plants unable to mobilize their N and to mature normally. Inputs like fertilizer N or foliar fungicide can enhance the amount of N (and sugars) in plants late, and can contribute to green stem. We don’t expect to see much of this in 2022, but it could happen if there are fields with low pod numbers.

At least in the drier areas of Illinois, there is little evidence this year of the “sagging” that soybean plants sometimes do as pods reach their maximum weight. Stems can bend when this happens, but they usually return to mostly upright once leaves drop and pods begin to dry down. We’ve come to see it as a sign of high yields developing, but its absence may just reflect the fact that stems are short and are sturdy enough to remain standing straight.

Soybeans that mature during warm weather dry quickly, and it pays to be ready to harvest them as soon as they reach 13% moisture. Once they get to 10 or 11 percent moisture, loss at the cutterbar and seed damage while passing through the combine increase. If elevators do not adjust bushels upward to for soybeans delivered with less than the standard moisture of 13%, that adds to the cost of having soybeans get too dry. Being able to switch combines quickly between corn and soybeans helps with harvesting both crops. Dry soybeans take on moisture as dew develops, so harvest in the morning may help lower harvest losses.