A number of folks have been wondering lately, "How much rain does it take to end a drought?" Most areas of Illinois have received some respectable rainfall in the past 2 weeks, and some, mostly in the northern part of the state, are truly hoping that the rains slack off soon, before crop damage from too much water increases further. Even in areas that have received rainfall, the "drought mentality" has set in to the point that many are still worried about the prospects for the corn crop, even if soils have a good supply of stored water at this point. |
The recent weather pattern has not cooperated with earlier predictions of a drought setup, which was supposed to include high pressure dominating over the southeastern states, stopping the flow of moist air into the Corn Belt. Rainfall in May was variable, ranging from several inches below normal in parts of west-central Illinois to several inches above normal in other places. Overall, though, rainfall in May was adequate to restore soil water to some extent, and the crop at the end of the month was not suffering seriously from lack of water anywhere in the state.
How much is the crop at risk from lack of water as we move toward pollination? Probably no more than average for this time of year. The fact that the crop was planted early and got off to a fast start is a real benefit, in that it will likely pollinate a week or so earlier than normal except where planting was delayed. In most fields in Illinois, there is adequate stored soil water to take the crop at least to tasseling, if not well into pollination. As the crop nears full canopy development in the next weeks, the rate of water loss will increase, approaching 1.5 inches per week if the temperatures are average, and even more if it's hotter than usual. Well-supplied soils can store as much as 8 to 10 inches of water in the rooting zone, and so can carry the crop for several weeks even without rainfall.
There is little evidence that the crop has been held back by lack of water to this point; even in the area east of Quincy where rainfall has been considerably below normal, the crop was 3 to 4 feet tall at the end of last week. It also has excellent color, and even though rainfall has been low and soils appeared to be dry, it was clear that the crop had not suffered greatly. That area has received some rain since then.
It is useful to "let the crop tell us" how it has fared through dry periods. The main response we would see from very dry soils up to now would be shorter-than-normal plants; plant cells do not compete well for water with the air, and if soil water is inadequate, cells don't elongate as much, and so the stem does not grow as much. In this way, plant height (and, to a lesser extent, leaf size) represents an accumulation of effects of dry soils on growth. On a daily basis, the earlier in the day the plant leaves start to roll, the more deficient in water the soil is. If leaves curl only in late afternoon and then relax in the evening, they are being affected by dryness, but not very severely. If they curl before noon and don't relax until well after sundown, then moisture deficits are having more effect. This year, despite reports of dry soils, few fields showed much leaf curling at all. That means that they had roots tapping into soil moisture adequately to meet the water needs of the plant.
While the crop looks very good at this point in most areas, it is certainly too early to conclude that water supply is assured for this crop. As recently as 1997, the crop ran out of water abruptly in July in some areas, with large yield losses in many cases. A clear advantage this year is that the roots have been able to grow well in soil that was less compacted than normal this spring, and the dry surface soils in some areas have encouraged root development at greater depths. But we still need rainfall in July for this crop to reach what looks to be outstanding potential at this point. And with drier soils now than last year at this stage of development in some areas, the need for August rainfall might be greater than it was last year.
Back to the present: corn planted in late April here at Urbana is now about 36 inches tall, in stage V9, while corn planted in late March is about 10 inches taller and at stage V10 to V11. All of the crop has very good color, and there are few obvious problems at this point. Corn that is V10 now should show tassels by the end of June if temperatures remain normal or above.--Emerson Nafziger