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Issue No. 21, Article 4/September 7, 2012

After the Rain

The remnants of Hurricane Isaac passed through Illinois between August 31 and September 2, dropping 2 to 6 inches of rain on the southern two-thirds of Illinois. While much of this area was among the hardest hit by drought in June and July, this rain came on top of rain in parts of the region earlier in August, so rainfall over the past six weeks has ranged from one to several inches above normal over much of the southern half of the state.

While the rain came too late to help corn yields, and too late to help soybean yields by much in most fields, it fell very nicely over three days, and most "disappeared without a trace" in many areas, with creeks still dry, no tile flow (yet, at least), and only moistened surface soil as evidence of precipitation.

In parts of the state that had been very dry, the rainfall in recent weeks has done much to restore soil water to more normal levels and to restore our confidence that it can still rain. Our probes at South Farms show that soil moisture in the top 40 inches was restored to near levels found in late May, depending on rotation. Soils under soybeans had more water than those under corn by late May, and while levels remain a little less than those in late May, there's no water shortage under soybeans now. Corn following corn appears to have taken in more water (possibly due to larger, deeper cracks), and according to the probe, soils in this rotation are now well moistened all the way down to 40 inches.

Accumulated precipitation (in.) August 30 to September 3. (Source: Midwest Regional Climate Center.)

While this rainfall has been useful in restoring the hope that we can go into 2013 with soil water at normal levels, it can do this year's crop some good only if there remain some green leaf area and kernels or seeds that were not yet fully filled. According to the latest NASS report, as of September 2, 63% of the corn crop was "mature" and 12% harvested, while 41% of the soybean crop was listed as "turning yellow" and 7% as "dropping leaves." It's not clear whether the second number in each case is included in the first number, but these numbers will continue to rise in the coming weeks, and there's no doubt that both crops are close to the end of adding yield, with corn a little ahead of soybeans.

Corn. Growing degree-day accumulations since May 1, when planting was nearly complete, have totaled some 2,700 in northern Illinois to 3,000 in the southern part of the state. Hybrids grown in Illinois range in their GDD requirements from perhaps 2,500 for very early hybrids to 2,800 for late-maturing hybrids. This means that nearly every field has had enough GDDs to mature the corn planted there, and we're mostly waiting for the corn to dry down some more before harvest.

Some corn fields died early due to drought, but those that still have some green should be close to maturity (black layer) as well. As I've noted before, the advantages to getting the crop harvested tend to outweigh the disadvantages as corn grain moisture approaches 20%. Crop insurance complicates this decision some, but low-cost drying offered by some elevators, weak stalks that may stay standing only until the next moderate wind, grain quality that can go only go down as the crop stands in the field, and the increase in harvest (header) loss as grain dries below 18% to 20% moisture all weigh on the side of early harvest.

Factors that point toward waiting to harvest include the fact that elevator shrink (typically 1.4% per point of moisture loss) exceeds actual shrink (about 1.2% per point), meaning that fewer bushels are sold than are actually harvested. This "loss" diminishes as grain dries and is only about 1 percent at 20% moisture. Some elevators offer a few cents more per bushel, which can cancel out some of this. Lastly, with soils near field capacity and hence highly "compactible" in some areas where there has been a lot of rain, it may pay to wait for some drying before driving a heavy combine on the soil.

Soybean. Soybean harvest has yet to get underway in most areas, and we have had no reports of yields so far. There are some fields that still have a lot of green color; in some cases this is because pods are still filling, but in other cases plants may be staying green because they have few pods to fill. Soybean plants sometimes get caught in a bind, where they still have photosynthetic capacity (green leaf area) but no place for the sugars to go, and there are not enough seeds to signal that leaves should start to lose their green color and nutrients (largely protein) to the seeds and to start the process of senescence that makes the crop harvestable. There is not a lot of this in 2012, but if you have a still-green field that was under a lot of stress early, you might check to see if it is filling a good number of pods.

While we know that drought during the growing season can sometimes bring on early senescence even when there aren't many pods, the process by which a crop with a green canopy decides when to end the seed-filling process is not very well known. The cool nights we had in mid-August might have signaled this process to begin. Shortening days may play a part as well. And such a signal might be sent when seeds reach a certain size, perhaps in response to slowing the rate of sugar movement into the pods.

In any case, soybean yellowing started rather uniformly from north to south this year in Illinois; as of September 2, the percentage turning yellow ranged from the 40s in the north to the 50s in the south, with no correlation to the percentage of corn rated as mature. That implies that soybean senescence and maturity are more related to weather or day length than to the amount of stress, though stress in August was not uniformly distributed in Illinois.

While the official estimates and the crop tours in recent weeks tend to indicate relatively low soybean yields, good numbers on yield will probably come only after harvest. It is difficult to predict yields in soybeans, in part because we tend to take samples in one or a few very small places in the field. We do this because soybeans tend to look uniform, which gives us confidence that almost any spot to check yields will work, when in fact (as yield monitors show) yields are often not as uniform as crop appearance would suggest. We also take very small samples--only a few feet of row for plant counts, then a few plants of those for pod counts. This is truly a case of "aiming a long stick with a short end" and can get us very far from an accurate yield estimate. Of course, taking enough samples for more accuracy would have us in the field for hours, and for many people it's not worth the time and effort.

In addition to taking inadequate samples, we often find a lot of variability among plants in pod counts. This means a lot of variability in counts of seeds per plant, so less of a sense of having reached an accurate yield estimate. There is no reason we can't estimate yields for soybean like we do for corn: figure out how many seeds are filling per acre, then divide by the number of seeds we think will make up a bushel at harvest. But when we do this the predicted yield is often higher than we know we will harvest, which shakes our confidence. My only advice is that if your counts and calculations say the field will yield 70 bushels and you think from experience that it will only yield 50, your experience should win out. A corollary might be that it makes more sense to just guess than to try to do pod counts and calculations at all.

More subjective ways to assess soybean yields include looking at the "wall" of soybean pods when you push a section of row sideways or, even easier, looking at pod numbers once leaves drop. There's an art to this, and experience counts a lot, but in general stems that look like they're "bristling" with pods (3 to 5 pods per node on 6 to 8 nodes on two feet or so of stem, with pods sticking out away from stems) and have pods filled at the top of the stem will indicate good yields, while two or three pods per node that do not stand out away from the stem (hence aren't as well filled) and on fewer nodes will mean lower yields. A good place to practice this is in strip trials, where you'll find out actual yields after harvest.

Wheat. Questions remain about wheat planting following failed corn fields and about the need for fertilizer, especially nitrogen, in such fields. While we have not yet taken many soil samples to check N levels, it's certain that the rainfall over southern Illinois has moved much of the soil nitrate down, at least below the zone where the wheat roots would reach it early. Many producers are chopping or even tilling in or rolling down the corn residue, putting it in contact with the surface soil. With warm soils and a month remaining before wheat planting should begin in southern Illinois, chances are increased that even more of the surface soil N will be tied up as corn residue starts to break down. This means that our earlier suggestion that wheat following corn shouldn't need N at planting in dry areas this year is probably no longer operative, now that most such areas are no longer dry. This diminishes the advantage that wheat following corn might have in retrieving some of the leftover soil N, and in some cases it may make it more advantageous to plant wheat following soybean instead of following corn. In that case, or even if sticking with planting wheat following corn, using normal MAP/DAP applications to supply some nitrogen and phosphorus makes sense, especially in fields with modest soil test P levels.

If it stays dry over the winter we might be able to adjust spring N applications, perhaps based on soil test nitrate in March, but chances of that would appear to be small at this point, and with more rain they will get even smaller. In other words, the rain we've gotten means that we should manage wheat as we normally would: around 1.4 to 1.6 million seeds per acre, planted after October 3-5 in south-central and southwestern Illinois and after October 7-8 in the southernmost parts, with serious attention to uniformity of seed placement regardless of tillage or previous crop.--Emerson Nafziger

Emerson Nafziger

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