Issue No. 13, Article 8/June 17, 2005
Heading into the Main Event
Even though heavy rainfall finally provided much-needed moisture to Champaign-Urbana on June 13 and 14, we can assume that crops in some places in Illinois still struggle with decreasing soil moisture in the rooting zone, with increasingly severe symptoms of inadequate water to keep plants functioning well. For places that did receive rainfall, the added surface soil moisture should help provide a spurt of plant growth, including better root growth that will help plants maintain growth during periods of dryness later in the season. While a dry June is usually a positive for corn and soybean in that it encourages root growth, dryness that starts soon after emergence can restrict root growth early enough that roots can't penetrate to deeper moisture. We hope that most crops in Illinois are well anchored with a healthy root system now and so are prepared to function well as we move into the critical middle part of the growing season.
The visible stress and slow growth in many cornfields up until recent rains have brought forth a number of attempts to describe how corn plants "set" their yield potential and how stress might or might not affect yield potential during vegetative growth. There seems to be a fair amount of confusion on various fine points of how yields "happen" in corn, and I'll try to clear some of this up. Be warned, though, that a lot of this has not been very thoroughly investigated in field-grown plants, and it is almost impossible to be precise about how weather and soils interact with plant development as these events take place.
Keeping in mind this warning, here are some points that might help illuminate how corn plants develop yield. Much of this is derived from the University of Illinois Experiment Station Bulletin no. 721, published by Professor O. T. Bonnett in 1966, and from How a Corn Plant Develops, from Iowa State University.
- By about V5 (5 leaf collars visible), the plant has stopped initiating leaves at the growing point (triangular tip of the stem, usually just below the soil surface at V5) and has started to differentiate the tassel at the tip of the stem.
- By V6, the buds on the lower eight or nine nodes start to differentiate ear shoots, one per node. Tiny ear shoots look much like tiny tassels when they first start to form, but they are in axillary positions (on the side of the stem), while the tassel is at the terminus of the stem. Some "positional dominance" takes place here, in which the uppermost ear shoot suppresses (through sending a chemical signal) the growth of lower ear shoots; but until that happens, lower ear shoots will develop just as well as, and even faster than, upper ones. The shoots at the lowermost node or two might develop into tillers ("suckers") instead of ear shoots, especially on some hybrids and if growing conditions are very good.
- As is true with most early developmental events in the plant, stress conditions (which I define generically as inadequate levels of sugars in the plant due to decreased photosynthetic rates) usually do not inhibit the differentiation of tissue such as ear shoots, but they can inhibit the growth of plant organs once initiated. In other words, a plant would have to be practically dead before it stopped trying to develop as its genetic programming tells it to.
- Ear shoots develop their number of kernel rows soon after they are differentiated, with lateral division meaning that rows always occur in pairs. It's possible that all ear shoots in a given hybrid have the same number of kernel rows but that stress can abort one or two pairs of rows. When this might happen and whether it's as common as many believe it to be isn't clear, but the ear shoot develops surrounded by a lot of plant tissue, and it's likely that this helps insulate it somewhat from direct stress.
- Kernel growth gets its start first at the base of the developing ear, and kernel "bumps" (called "initials" before lateral division, "spikelet primordials" afterward) continue to appear toward the tip of the ear, while kernels at the base start to develop silks at their tips. This addition of "potential" kernels at the tip of the ear is a continuous process over a period of perhaps a week or so on an individual ear. With patience and care, it is possible to dissect out the uppermost ear shoot from a plant at about V8 and to see that potential kernels go all the way out to the tip of the ear. Even so, it's not easy to know if the number of kernels per ear is "fixed" by then or if there are still more to come. Thus the question about when kernel number per row (and hence per ear) is at its maximum is not easy to answer. It may not be a very important question, though, as we'll see later.
- With patience, you can remove an upper ear at about V10 (corn waist high or so, depending on temperatures and moisture supply during growth), peel off the husks carefully, and count the number of potential kernels. By then, and especially by a few leaf stages later, this number is typically in the range of 1,000 or so, which is nearly always more kernels than will develop and fill on the plant.
- At about the time the tassel appears, the tip (husks) of the ear shoot appears above the collar of the "ear" leaf (usually the sixth or seventh leaf down from the top leaf). A day or two after (or sometimes at the same time or even before) the first pollen is shed by the tassel, silks appear from the end of the husks that cover the ear. (Pollen grains can be seen falling in the morning, or pollen shed is indicated by the appearance of "dangling" anthers after they drop out of the tassel spikelets.) The first silks to appear are from the base of the ear. Silks do almost all of their elongating at night, so a fresh "crop" of silks appear each morning, usually for four to six mornings, with a few stragglers before and after. Pollen is shed as the relative humidity starts to drop, and a small percentage of pollen grains fall on silks. Silks that have been pollinated previously become "nonreceptive" to new pollen.
- Pollen grains germinate on the surface of silks and grow down all the way into the kernel, where they release genetic material that fertilizes the egg cells in the kernel. A potential kernel only becomes a "real" kernel once it's been fertilized and cells start to divide and grow, eventually becoming recognizable parts of the seed. The potential kernels at the very tip of the ear usually don't receive pollen, either because pollen shed ends before they emerge or because they don't emerge at all. Kernels may also fail to be fertilized because their silks are clipped or, in rare cases, because there isn't enough pollen to go around. In addition, fertilized kernels can abort, meaning that they fail to continue to develop, usually in response to stress. Thus the actual kernel number is always less than potential kernel number, meaning that our attempts to determine potential kernel number are usually just an academic exercise.
- Kernels develop a maximum potential size, based on how many cells they produce following fertilization and how many starch granules (for example) there are per cell in the endosperm. Thus stress during the 2 weeks after pollination can both abort kernels and also restrict the maximum kernel size. In one study, we found that removing neighboring plants 2 weeks after pollination usually increased the yield of remaining plants, which suggests that kernel size in the field seldom reaches its maximum potential. If kernels do not reach their maximum size under field conditions, then it's logical that the number of kernels that develop on each ear does not limit yield.
- We think that yield is limited not by the number of kernels per ear or per acre but rather by the total amount of photosynthesis that takes place to fill kernels during the 8 weeks or so after pollination. Having a complete canopy of healthy leaves should thus be our management goal, along with choosing hybrids that have the ability to stay standing while allocating the lion's share of sugars to the kernels to increase yield.
- Exceptions to this scenario can occur when stress severely limits the number of kernels that develop, though when the weather pattern fosters continuing stress, having low kernel numbers might still not limit yield. But there have been instances when things like silk clipping, silk balling, and other phenomena have lowered kernel numbers enough to hurt yield potential. Much more common are things like dryness or leaf loss from hail, insects, or disease that directly reduce photosynthetic rates and so limit the fill of existing kernels. Only in rare cases do weather and stress conditions before pollination limit the size, health, and duration of the crop canopy, and hence limit final yield.
Soybean plants have benefited from the rainfall where it's occurred and continue to grow slowly where the surface soil remains dry. There will probably be very little pre-June 21 (longest day) flowering this year compared to 2004, mostly because plants did not get large enough by the time nights reached the minimum flower-triggering length last week. Average temperatures and adequate soil moisture will result in rapid growth over the next few weeks, and we would like the crop to develop a full canopy by the time it flowers, which should be about the second week of July, or earlier if it's hot. Wide rows will have more difficulty filling their canopy, which is the main reason they tend to yield less than narrower rows. Any soybean planted from now on, including double cropped, should be in narrow rows for this reason.
Wheat development has been greatly speeded up by warm temperatures, and this, along with dry soils, has helped bring grain fill to a rapid end in the southern part of Illinois. This might be good for early planting of double-cropped soybean, but it may have reduced wheat yields by a few bushels. The June estimate is for wheat yield in Illinois to average 59 bushels per acre, which is good but not a record. The dry weather since heading should mean relatively few diseases. This, and the abundant sunshine, should translate into good test weights, even if yields don't break records. On the other hand, as experienced double-croppers know, there is very little surface soil moisture following a stretch of dry weather during wheat grain filling, so getting soybean seed to germinate following wheat harvest might be a challenge.--Emerson Nafziger