How much does it need to rain in the Corn Belt?
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 two 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 three inches), and the sooner it falls, the better. The ideal would be perhaps two inches of well-distributed rain now, to restore depleted soil water in the upper two 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.