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Issue No. 13, Article 8/June 22, 2007

How Soon and How Much Does It Need to Rain?

Rain fell in widely variable amounts over a large part of Illinois on June 18, ranging from none in parts of southern Illinois to as much as 2 inches near the Wisconsin border. One of the real frustrations in years of marginal rainfall is the reminder that rain in such years usually falls in thunderstorms and that such storms produce uneven distributions. We all understand this fact, but looking at rainfall maps that show an inch of rainfall in the same county where I got none still isn't easy. It also hasn't helped that some have gotten into a radar-watching mode, with results on the ground often not matching up with what the radar shows.

Let's put "spotty" rainfall into perspective. Here in Champaign we got about one-third of an inch of rain. The corn looked good in the afternoon on June 18, but leaves were starting to curl again on June 20. Water use on June 19 in fields with larger plants was about half the amount of rain that fell on June 18. By the end of June 20, the water that fell on June 18 will be back in the atmosphere. If we had received one inch, it would have provided enough water to the crop for three more days. So the deficit and associated stress symptoms will likely reappear if we continue without rain, even in areas that received more rain.

We think, though, that such rainfall has a "priming" effect, by giving the plant greater ability to produce more roots to explore deeper soil layers. Hence an inch of rain might be much more than three times as valuable as a third of an inch. A corn crop that is under water stress from midmorning, as many fields have been in recent weeks, produces very little photosynthate (sugar) during that day. That means that the plant has little ability to produce more growth, whether the growth is roots or leaves and stem. The symptom of this that we can see is the reduction in top growth that has been evident in the most stressed fields in the past week. If top growth is reduced, root growth potential is reduced as well, probably in proportion to the reduction in top growth, if not more. This failure of roots to grow rapidly means compromised ability of roots to grow into deeper soil layers to extract more water.

The question of how soon it "needs" to rain for the crop is a common one, but it is not a question with a direct answer. A similar question is how much rain needs to fall, presumably to put the crop back on track for top yields. The answer to both is that the more rain that falls (within reason--probably not more than 3 inches), and the sooner it falls, the better. The ideal would be perhaps 2 inches of well-distributed rain now, to restore depleted soil water in the upper 2 feet of the soil, and also to restore photosynthesis and growth rates to normal.

Except in those fields where there has been some death of leaf tissue as a result of a long period of dry weather, loss in yield potential has been relatively minor so far. But in many ways, the reduction in photosynthesis resulting from reduced growth (in leaf area) and leaf curling that accompanies water stress means that the crop is losing valuable sunlight hours here at the longest days of the year. Such plants may well turn out much the same as late-planted corn. In other words, good yields are still possible, but for this potential to be realized, above-average growing conditions will be required, including a return to good rainfall amounts and distribution, favorable conditions into late September, and lack (or control) of insect and disease attack.

Despite the setbacks, the early-planted corn in central and southern Illinois has continued to make rapid growth, and we will see tassels in some fields as early as this weekend. Leaf stages advance very rapidly in the final week before tassel emergence, at least when there is enough water. The height of our earliest-planted corn near Urbana seems about normal, and the top leaves are starting to show the upright orientation that signals the growth of the tassel into the whorl.

While tassels might appear early, there is no guarantee at this point that the plants will be able to muster enough water to produce silks on time. I'll discuss this more next week, but given the water shortages so far this year, you will want to be extra diligent to get into fields as soon as tassels appear and to note when pollen shed begins. Silks should appear within a day or two of the start of pollen shed. Because the rate of appearance of silks is likely to be slowed in fields with dry soils, and because late appearance of silks means less pollen available, it will be critical to watch fields to see if insects are eating silks. They might land in tassels and eat pollen as well, but the real danger is that they eat silks off to prevent pollination.

While soybean seems to be suffering a great deal in dry fields, and stands tend to be somewhat spotty in places, the soybean crop continues to show moderate growth in most areas. As I stated before, the crunch time for soybean is still several weeks away, so we can afford to wait and worry about this crop after we worry about pollination in corn. Continuing high temperatures will mean an early start to flowering. Once we are past the longest day of the year (June 21), high temperature, especially at night, will greatly speed up flowering, and we might well see flowers on earlier varieties before July. This can be an advantage, but only if we get enough rainfall to extend the flowering period and increase the number of flowers that become pods.

Wheat harvest is progressing rapidly and is likely to be nearly complete in Illinois by the end of June. Yield reports to date have been variable, but there is some very good wheat. The variety trial at Brownstown was harvested on June 14, and yields of varieties ranged from 54 to 95 bushels per acre, with an average of 78. That may turn out to be one of the higher-yielding locations this year, but it represents the ability of this crop to bounce back from early problems. Soils in most wheat fields are too dry to germinate double-crop soybean seed now.--Emerson Nafziger

Emerson Nafziger

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