No. 16 Article 5/July 8, 2005

Corn Enters Critical Stages

While there were numerous rainfall events in Illinois during the past week, many areas continue to experience marginal to severe dryness. While rain in variable amounts fell in many places on July 4, the report through July 3 was that over 90% of Illinois topsoil was deficient in moisture. Corn crop condition had even worsened slightly from the previous week; about 3/4 of the crop is listed in fair to very poor condition. Rainfall might have improved that marginally, but rain amounts were not generally much more than enough to relieve deficits for several days. Lower wind speed and cooler nights this week will help prolong the relief some, but most crops continue on the edge of, if not in the center of, water deficiency.

While the crop condition report may suggest that corn is in slightly worse shape than it really is, the fact that about 20% of the crop is now silking means that we are well into its most critical time. It takes some water to get tassels to emerge, and as I've noted before, it takes even more water to get silks to emerge. We will need to watch fields very closely to make sure plants are actually pushing silks after tassels emerge, but the fact that silking progress is being made is a positive sign that the crop isn't as "dead" as some might suggest. Under such marginal conditions, however, it becomes even more important to protect the crop against silk clipping or anything that damages leaves unless it's clear that the crop prospect is so bad that spending more money on it isn't justified. Few Illinois fields are in such bad shape, at least at the moment, but prospects for rainfall are not very promising.

A direct way to assess what the water supply has been in a particular field is to note plant height. With a full supply of water, the warm temperatures of the past month would have resulted in corn plant height being above normal. The extent to which corn has been shortened is a direct indicator of its water supply since it began rapid growth in early June. Some fields are starting to show tassels on short plants. This usually means that they had enough water to push tassels but only after stem internodes were fixed at a length shorter than normal. The uppermost two or three internodes elongate to push the tassel out, but the internodes below that will not get any longer, so plants that are short at tasseling will stay short. Good yields from short plants are possible, but short plants usually do not intercept all of the sunlight and so do not yield as much as if they were taller, even if they form normal numbers of kernels.

The timing and uniformity of pollination events will be a large question this year, especially as insects continue to put pressure on the crop and decisions about crop protection need to be made. With normal water supplies and temperatures, 1/4 to 1/3 of the total number of silks might emerge during the night on the first day that silks appear. Silks continue to emerge at declining rates over about 5 days. Ideally, pollen will be shed each morning, starting before first silks emerge, peaking a couple of days after silks first emerge, and lasting past the time the last silks emerge. Under water stress, pollen shed is likely to begin at the normal time (a few days after full tassel emergence), but silks may not grow or emerge as quickly as normal. This can prolong the number of days that silks emerge, perhaps to 7 or 8, meaning that pollen shed might end before the last silks are out. If water deficiency continues to worsen during this period, silks may stop emerging altogether. Both late silking and reduced silk numbers can thus result in formation of fewer than normal numbers of kernels.

Along with possible effects on prolonging silk emergence and limiting kernel number, uneven soil water supply in many fields might cause differences in timing and success of pollination across the field. It is important to scout such fields more thoroughly than normal, in order to know whether the whole field or only some of the field is worth protecting against pests, and also to know when protective measures need to be taken.

Some think the soybean crop is in worse shape than the corn crop because it started flowering earlier. Actually, both crops started flowering earlier than normal this year, though not as early as in 2004. This is typically positive, but only if there is water to help plants develop normally. Soybean plants growing in dry soils are struggling to set pods from their early flowers, many of which are dropping off the plants without starting to develop pods. Flowers that open later can set pods if the water supply has increased, but such flexibility will not last for many more weeks. Early-maturing varieties will lose the ability to set pods from new flowers sooner than late-maturing varieties.

The prospects for the corn crop in most Illinois fields will become much clearer in the next 2 weeks or so, while soybean has 3 to 4 weeks to recover much--though a declining percentage--of its full yield potential. The period when relief of water deficiency will bring back high yield potential in Illinois is starting to run out for both crops as they struggle through these weeks of dry weather. Please forgive the understatement, but we really do need rain.--Emerson Nafziger

Close this window