Issue No. 18, Article 7/July 28, 2006
Check Corn and Soybean Fields Carefully
While there seem to be relatively few serious problems in Illinois corn and soybean fields so far in 2006, this does not mean that every field is a lock for high yields. This is the time of year when unexpected problems might develop, and failure to see such problems now can mean real disappointment at harvest, especially if the problems were ones that could have been alleviated if found in time. As in most good crop years, the view from the road is good to excellent, and we need to walk out into the field to see how the crop there is doing.
One problem that has affected some areas of southern Illinois is wind damage from the storms last week. When looking at such fields, note whether plants have broken lower stalks or the roots have pulled out of the soil and stalks are still intact. Having roots pull loose on one side usually causes less yield loss than when stalks are broken over, because the stem remains capable of transporting water and nutrients, and such plants often have more of their leaf area still capable of functioning. Roots that pull out of the ground are much less effective, however, but if even a third of the root system remains in the soil, it can do a reasonable job of taking up water and nutrients if the soil doesn't dry out too much. Water use is starting to decline in the more advanced corn fields, and lodged plants use less water as well.
Stalks that break over often have nearly all of their upper leaves lying on the ground, and if the ear is in contact with the ground as well, such plants may produce little yield. Breakage at a stalk node is generally much more serious than "kinking" at an internode, but both reduce the ability of the stalk to transport water and nutrients. Regardless of whether the stem is intact or broken, lodged corn plants cannot intercept normal amounts of sunlight, and yield reductions of at least 15 to 25 percent should be expected for corn that lodged at kernel growth stages R2 (blister) or R3 (milk). Harvest will usually be difficult, and harvest losses will often add to physiological yield loss. Once they're past pollination, corn plants rapidly lose their ability to respond to lodging by "goosenecking" partially upright, so there may be little improvement in harvestability in many affected fields.
The first thing to note in most fields is how successful pollination and kernel set have been. At a plant population of 30,000 per acre and assuming (conservatively) an average final kernel weight of 90,000 kernels per bushel, 3 kernels per ear means one bushel per acre. Thus ears need to fill at least 600 kernels for a yield of 200 bushels per acre. This isn't quite as exact as it sounds, since more kernels on an ear often end up smaller and fewer kernels per ear end up larger, but it's a starting point to assess potential. If filling conditions remain favorable, then kernel number might in fact limit yield, even though kernels might grow to be considerably larger than average.
This is the prime time to look at the corn crop canopy to see how well the crop is taking in sunlight. If kernels numbers are reasonably high (15 to 20 million per acre), then sunlight interception by the canopy is often the limiting factor for yield. A subjective way to look at the canopy is to look down the row at about eye level, noting how densely the leaves tend to overlap across the rows. Then, look at the soil surface underneath the canopy at about midday when the sun is shining, and see how many patches of sunlight there are on the soil surface. While there is always some sunlight hitting the soil surface, the patches of light should be small and randomly scattered. If they are numerous and lined up in the center between rows, it's likely that plant population might be too low, row spacing might be too wide, or plants might not have developed to their normal size due to some sort of stress.
It's unusual for insects to defoliate large corn plants to the extent that light interception is compromised. Hail is a more common cause of leaf area loss, and for corn just past pollination, yield loss from hail is almost directly proportional to the reduction in light interception. Light interception is not proportional to leaf area loss, since some leaf area can be lost before light interception starts to drop. Leaf diseases don't necessarily reduce light interception, but they do cause intercepted light to be used poorly, and the net effect is often more damaging than leaf area loss because diseased leaves do not let much light pass through to be intercepted by lower leaves.
Soybean plants have responded to favorable moisture this season by continuing to make considerable vegetative growth as they flower and start to set pods. Most early-planted soybeans are now at stage R3 (beginning podset) or perhaps R4 (full pod), though some have not yet reached R3, which requires that a small pod be present on one of the four uppermost nodes. Flowering is slightly delayed in Illinois this year, likely due to July rainfall and moderate temperatures most of this month, but also due to late planting, including some replanting and doublecropping. We normally think that earlier flowering and podsetting is favorable, but unless it turns dry in August, the later flowering this year might not be a problem.
While it's usually advantageous to avoid moisture stress as we move into podsetting, we would also prefer that there not be heavy rainfall and a lot of cloudy weather, both of which can lead to tall plants and large leaves. We saw a great deal of this in 2003, and when it turned dry in August that year, the crop deteriorated quickly. Compared to that year, we have not had in 2006 the excessive rainfall that led to compromised root systems in 2003. So the soybean crop is not as vulnerable this year, but tall plants and large leaves still cause a great deal of internal shading within the soybean canopy, and this can reduce seed numbers and seed size. A heavy canopy also retains moisture better, and this can cause problems with diseases such as white mold. There's no reason to worry too much about excessive canopy growth at this point, but it is worth noting that very high soybean yields are more likely when plant size is average than when plants are much shorter or much taller than normal.
Like corn, soybean fills seeds well only if it has adequate seed numbers and a fully functioning canopy, capable of taking in most of the sunlight that fall on the crop. The period in which this needs to happen has not yet started in most fields, but it will begin once pod numbers are near their maximum and seeds start to fill, in early August. Soybeans tend to have more leaf area than they need, so small amounts of defoliation may not affect yield. Leaf diseases and diseases such as brown stem rot and sudden death syndrome will reduce the amount of sugars moving to the seeds, though, and so will reduce yield in most cases. Be on the watch as well for insects like aphid that can reduce sugar flow and for insects or diseases that attack pods and seeds.--Emerson Nafziger