No. 14 Article 6/July 6, 2012

Stress on Corn at Pollination

The good news for the 2012 Illinois corn crop is coming in small doses: it has rained in some places in recent weeks, and 46% of the crop was silking by July 1, which is far ahead of normal. Silks do seem to be abundant in many fields where there has been enough water to get plants to a height of 5 feet or more, and leaf color remains good in most of these fields.

But less positive factors continue to grow in importance, and as we continue without rain over most of the state, we expect this only to worsen. The percentage of the crop rated good-to-excellent dropped to 26% by July 1, and only about 10% of the acreage is considered to have adequate topsoil moisture. Temperatures remain high, and warm nights continue to push GDD accumulations above normal, though statewide the GDD accumulation since May 1 is only 135 above average, and most of that occurred in May, not June.

The worst news of all is that some fields, especially in southeastern Illinois, are now beyond any reasonable hope for producing any yield; plants in these fields are dead, are alive but past the pollination stage with very short plants and no kernels, or have lost most or all of their green color. Some fields have already been written off completely for insurance purposes, and some farmers are considering whether there's enough plant material to justify cutting failed fields for silage.

A great deal has been written about salvaging drought-damaged corn as a forage crop. I won't add much here, but questions that need to be answered include whether there is enough crop material present to justify harvest, whether there's a market, what the nutritive value of the forage will be, and how to avoid nitrate toxicity. Robert Bellm addresses nitrate in another article in this issue. Whether a failed crop is harvested for forage or not, much of the nutrient applied to the crop will be left in the field, and so less will have to be replaced for the next crop.

The early start to pollination is a very positive development, and the fact that even in dry areas the root system has managed to find enough water to get to this point bodes well for at least the pollination process itself. One novel aspect to pollination this year is that tassels are starting to shed pollen before they clear the uppermost (flag) leaf, in some cases when they are still barely visible. In such fields, it may be easier to stage pollination by appearance of the silks, which tend to emerge starting about the same time as first pollen shed.

Tassels are struggling to emerge due to lack of water for growth, as I have discussed in previous articles. The failure to emerge might have negative consequences as pollination tries to proceed. Pollen is shed as relative humidity drops, so exposed tassel branches start to shed as soon as temperatures start to rise, usually two to three hours after sunrise. If the tassel is wrapped inside two leaves, relative humidity stays high longer, and pollen shed is delayed by as much as several hours. I was in a field on July 2 from about 10:00 a.m. to 1:00 p.m., and pollen was much more abundant at the end of that time than at the beginning. The problem is that the temperature was above 90°F at 1:00 p.m. when pollen was being shed, and at such temperatures silks are often not be as receptive as they would have been at 70 to 75°F earlier in the day.

Under the current high temperatures, we can expect that pollen will be shed over fewer days than if it were cooler. We also expect that silk numbers may lag some due to shortage of water; silk emergence is generally considered to be among the growth processes most sensitive to lack of water, and if plants are struggling to take up enough water to push tassels out above the leaves, we can expect that silks may struggle as well, at least those (from the tip of the ear) that emerge late in the process.

It should come as no surprise, then, that even in fields showing silks and tassels now, the process of fertilizing kernels and keeping them going until grain fill begins may be less than fully successful in many of the drier fields. Pollination will be rapid under high temperatures, and by a week or so after we first see silks we can check to see how many kernels have been fertilized. Do this by removing husks and tugging gently on the silks; those that were fertilized successfully will detach easily, while those that have not been fertilized will remain attached.

Unfortunately, as we continue without rainfall in large areas of the state, the number of kernels fertilized may end up being considerably larger than the number that survive to fill and produce yield. That is, improvements in success of pollination and fertilization through breeding do not always translate into better yields. We remain optimistic that kernel numbers will be okay in many fields, but in each of the past two years there has been a great deal of kernel abortion, and there is every reason to expect a recurrence in 2012.

The factors that affect whether fertilized kernels abort are not entirely understood, but we know that the supply of sugars available to the developing ear is a primary determinant of success in keeping productive kernels. Sugars are produced by photosynthesis, and as we have been saying often this year, plants that are rolling their leaves for most of the afternoon are not doing a very good job of producing the necessary sugars. Here in east-central Illinois, most fields are showing at least some stress symptoms daily. This is reducing the amount of sugars available, and it's highly likely that the end result will be fewer kernels than normal by the time kernel filling starts several weeks from now.

As stress conditions continue, a reduction in kernel size often starts to develop at some point along the length of the cob. This indicates the start of the process of kernel loss from abortion. The abortion process isn't very well understood, but once initiated it is probably not reversible. All of this means that in the 2012 crop, the decreased number of kernels that end up filling is likely to be a primary yield barrier in many fields where stress continues through the pollination process.

If conditions improve during grain fill, the reduced number of kernels may be able to get a little larger than they normally would. But the potential to make larger kernels will depend heavily on having a healthy canopy. In any case, we would not expect an increase in kernel size to make up much of the yield loss that will come from reductions in kernel number.

In areas where stress has been present for weeks, the first sign of rapid deterioration in yield potential is often loss of canopy color. Plants that are struggling to take up water are also not taking up much nitrogen; this, along with the fact that leaves become physiologically less flexible with age, means that loss of canopy color after pollination is often not fully reversible. The daily wilting that is visible in many fields is taking a toll, primarily by diminishing the energy (sugar) required to keep leaf tissue healthy and to repair damage, but also by causing damage itself. Water loss helps cool the leaf, and so when leaves are not getting enough water to keep stomata open, leaf temperatures rise above air temperatures; when the air temperature is 100, leaf temperatures can be 105 to 110. In addition, sunlight energy that can't be used in photosynthesis when stomata are closed can cause direct damage to chlorophyll and the photosynthetic apparatus, leading eventually to loss of leaf color.

Many are pondering the question of how much corn yield potential has already been lost. For fields that are essentially destroyed at this point, we know the answer. In most fields--those that are pollinating now or will be pollinating in the next week under conditions of at least moderate stress--the first guess at yield potential will have to wait until we can count kernel numbers and get some idea of grain-filling conditions at stage R3 (roasting ear), during the last third of July.

Besides canopy color, light interception is an indicator of stress and photosynthetic capacity. As leaves roll in the afternoon (or morning, in some cases), light interception drops quickly. One of the best "drive-by" indicators of how a crop is doing at a given time during the day is the amount of light hitting the ground. In very stressed fields, well over half of the sunlight is getting to the ground, in which case virtually no photosynthesis is going on. Using this as an indicator, it appears that corn following corn is again struggling more than corn following soybean in 2012. This could be partly a result of incomplete recharge in some areas after last season, especially if the field was in first-year corn in 2011 and second-year corn in 2012.

I wish more optimism were possible--the start to the growing season was outstanding, and most producers "did everything right" to establish good yield potential, only to see low rainfall and high temperatures since. There's not much to be done other than to accept that the weather is beyond our control; trying to figure what we might have done differently will not be very productive.--Emerson Nafziger

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