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Issue No. 12, Article 4/June 13, 2008

Getting the Rest of the Crop Planted

The Illinois soybean crop was listed as 66% planted as of June 8. Corn was 95% planted, but with 14% of the corn crop rated as poor or very poor, it is likely that some of the planted corn will still need to be replanted, in addition to fields that have not yet been planted the first time.

I previously (May 29) talked about prospects for corn planted in mid-June or later, so I will not dwell on that topic here. One enduring subject, however, has to do with switching hybrid to one with earlier maturity. According to work done by Bob Nielsen at Purdue and Peter Thomison at Ohio State University, late planting diminishes the growing degree day (GDD) requirement for a given corn hybrid, by perhaps as much as 300 GDD if planting is delayed into mid-June. As an observation, this decrease in GDD is related to the decrease in yield typical for late-planted corn--it's not a "free gift" for planting late.

If we assume that a mid-season hybrid planted on time requires about 2,700 GDD to reach maturity, then planting this hybrid in mid-May might require only 2,400 GDD to reach maturity. According to the maps in the Illinois Agronomy Handbook (chapter 1), corn planted on June 10 has about a 75% chance of accumulating 2,400 GDD before first frost in central Illinois, and an 85% to 95% chance of doing this in southern Illinois. Adding a week to the planting date will diminish these chances, but they might still be reasonably high in the southern third of Illinois. This means that a 111-112-day hybrid may still be okay to plant in that area. Between I-64 and I-70, however, where most fields still to be planted are located, it might be prudent to switch to a hybrid with a relative maturity rating of 105 to 108 days to increase its chances. The tradeoff from using a shorter-season hybrid is that the hybrid will not have been tested as well for the area. This might mean more foliar disease pressure and less ability to tolerate drought stress near pollination.

Another enduring topic, for those who have large ponds in planted fields, is how long seeds or plants can live while submerged. The answer, now that temperatures have returned to normal or above-normal levels: not very long. In almost all cases where water has to drain away through the soil (that is, slowly), seeds or plants will be dead by the time the water dries up enough to provide some oxygen to the seeds or roots. You can often see this as death of seeds even at the edge of ponds that dry up the earliest. Roots or parts of roots will often die with flooding, due to lack of oxygen and/or pathogens, so even if plants survive they may grow poorly. Loss of nitrogen in low-lying areas may contribute to yield loss, but plant damage by itself is usually severe enough to assure low yields in such areas.

The situation with soybean is becoming more critical, with many acres still to plant and some fields still very wet. We have also heard about emergence problems, and we expect replanting to be necessary in such cases. As I noted earlier, our experience with double-cropping in the most-affected areas of Illinois at least gives an idea of what to expect. We would not replace a soybean variety of normal (Group 4) maturity in southern Illinois with an earlier one, even if planting is delayed past June 25. This is because shorter-maturing varieties tend to flower for a shorter period, and this can be detrimental when flowering is relatively early, as explained below.

The start of flowering in soybean is related to the crop's responsiveness to changes in day length, and late planting tends to reduce the importance of this phenomenon. As nights get longer (days get shorter) after the longest day of the year (June 20 in 2008), a biochemical conversion that takes place in a plant hormone during the night turns the hormone into its "active" form, which results in the start of flowering. This process cannot be completed when the night is too short, and it is slower when night temperatures are cool. Warm nights can override the requirement for longer nights, and so move flowering up. Later-maturing varieties need longer nights and hence tend to flower later.

The flowering process also can begin only after soybean plants have reached the V3 stage (with three expanded trifoliolate leaves.) Before that, the plants are too immature to support flowering. This year, most of the planted soybeans are still small, and they will likely reach V3 only in late June, by which time they also will likely be "triggered" to flower if temperatures stay high. Those planted later than June 15 will need about 15 to 20 days of growth to reach V3. In most cases, the start to flowering of the soybean crop will depend on when the plants reach V3, since the trigger related to night length and night temperature will already have been activated.

Soybean plants that start to flower early tend to flower for a relatively shorter time, meaning that they often end up shorter in height than earlier-planted ones. Wet weather in late July or early August can extend the flowering period and allow plants to grow taller, which can help overcome the late and abbreviated start that we are seeing this year. With so much of this year's crop planted so late, only through continued rainfall are yields likely to be even average.

A note on wheat: The warm temperatures over the past two weeks have brought the wheat crop along very quickly, and the crop is starting to lose its color in many places in southern Illinois. We have been surprised before by how well the crop can fill under similar circumstances, but the very late heading this year means that much of the crop has had less than three weeks of grain filling by now. It is hard to imagine that it can fill grains fully in such a short period, but we'll hold out hope that kernels will be close to average in size by the time of maturity. Remember that the green heads and stems can help fill grain as well, so grain fill is not complete as long as there is some green color on plants. The June estimate raised the May wheat yield number up by 4 bushels, so we might have some reason for optimism.--Emerson Nafziger

Author:
Emerson Nafziger

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