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.

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

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