No. 23 Article 9/October 9, 2009

Corn and Frost, 2009

The "extended" 2009 growing season is about to come to an end, at least in northern Illinois, with frost or freeze predicted for this weekend for most of the area north of I-80, and frost possible into the central portions of the state. Light frost has already occurred in some of northern Illinois, where low temperatures reached the mid-30s on one or more days over the past week. Frost normally occurs in the second week of October in this part of the state, so it's not coming particularly early this year.

Unfortunately, the 2009 corn crop in many fields in the northern half of the state is not yet--as it normally would be--at the stage where frost will have no effect on yield. By now we all know the painful story of slow planting progress in Illinois in 2009; the 50% corn-planted date was about May 22, about three weeks later than normal. It was June before we reached 90% planted. Progress was a little faster than average in some pockets, but no single crop reporting district (CRD) reached 50% planted during the first half of May.

Late planting had little effect on corn yield in Illinois in 2008, because July and August temperatures were near normal, September was a good month, and the crop continued to fill into October, aided by the late frost. September was a good month to fill grain in both years, but in 2009 the cool weather in July and August and frost's coming at its normal time make this year different. The yield prediction for the state (179 bushels per acre) is the same as in 2008, so the effect of late planting on yield will be minimized again this year. But maturity is lagging far behind normal now, and frost will end the 2009 season earlier than we might have hoped in some fields.

The departure from normal GDD accumulations is striking (Table 3). As an example, a 105-day hybrid that needs 2,550 GDD from planting to maturity in northern Illinois would normally mature in late September, but in 2009 it would not mature until the second week of October, or close to the average date of first frost. The same hybrid planted in late May would be nearly mature by now under normal temperatures, but in early October this year, it still needs some 300 GDD to mature. It will not get that.

The GDD accumulation in May was about 10 GDD per day, so we can approximate GDD accumulations for corn planted before the end of May by adding 10 GDD for each day earlier than May 31 that the crop was planted. For example, we can add about 100 GDD to the numbers in the third GDD column in Table 3 for corn planted on May 20.

In general, the GDD-based predictions of maturity have held up fairly well this year, meaning that it took all of September in many fields for the crop to reach maturity. Late planting did not reduce the GGD requirement by much, if any. As the GDD numbers would suggest, the situation is a little better for late-planted corn in central and southern Illinois in 2009, but the hybrids grown in these areas typically need 2,700 to 2,800 GDD to mature. The late-planted crop thus has not quite reached maturity in many fields in central Illinois.

Statewide, only 5% of the crop has been harvested. Most harvest reports from central Illinois are from early-planted fields, where harvest moistures have been in the low to mid-20s. Some later-planted corn has been harvested, but at grain moisture percentages in the high 20s to 30. Harvest progress has been much faster in southern Illinois, where about 30% of the crop was harvested by October 4.

What will frost do to the crop? It has been a long time since a substantial amount of the Illinois corn crop was still not mature before frost arrives. Here are some points to consider as we anticipate frost or freeze in the next week:

The amount of yield loss from frost coming before the crop is mature will be directly related to the amount of fill the kernels have left to do at the time of frost. Watching kernel development over the past month has been a major sport for producers and agronomists this year, bringing the level of excitement normally associated with watching grass grow or paint dry. We have searched anxiously for the "black layer"--the layer of cells that collapses and turns dark at the base of the kernels, signaling the end of active transport of sugars into the kernel. Kernels are at their maximum weight when this happens, and so it is the point at which the crop is safe from yield loss due to frost.

Even in fields where enough GDD accumulated to reach maturity, black layer does not seem to have formed as distinctly as it usually does. In some fields, leaves died early, bringing an end to photosynthesis and cutting off the supply of sugars to the kernels before they were completely filled. When this happens, the cells that make up the black layer may not die or darken normally, so the black layer may be less distinct.

For much of the crop that still has green leaf area and so is producing sugars, total GDD accumulation since planting is still less than the number required for that hybrid to reach maturity, so we would not expect a black layer yet. A good way to track kernel fill while it's happening is by watching the "milk line"--the juncture between hard starch that forms starting at the crown of the kernel and the soft, liquid ("milk")-containing part of the kernel at the base. Here is a guideline to use when monitoring kernel fill, adapted from Crop Insights, Vol. 19, No. 13 (Pioneer Hi-Bred International):

The good news here is that the potential loss of yield due to a premature end to grain filling (such as we will get from frost in some fields) decreases fairly quickly as the milk line migrates down; the plant is more efficient (in terms of GDD) at increasing kernel dry weight early in the maturation process, so potential loss amounts decrease with time as grain filling slows.

The dry weight numbers in the list above represent what yield we might expect if grain filling came to a sudden halt at the stage indicated. There are actually few instances in the field where grain filling comes to a sudden halt, whether from lack of moisture, frost, or even hail. That's because the plant has some sugars stored in the stalk and can continue to move some of these sugars into the kernels even when there is no more green (stems or leaves) on the plant and so no more sugar production. The amount of such "post-brown" sugar movement depends on whether the stalk and shank stay alive to enable sugar transport to continue. Temperatures no lower than 30 or 31 for several hours will often not kill the stalk tissue; in such cases, as much as half the "missing" dry weight can still move into the kernels.

There may be some fields, though, where there have been enough GDDs to reach maturity but the black layer is still not visible. It may be that the extended filling period and the slow pace of filling in recent weeks have meant an interruption of the normal end to grain fill and that the black layer cells simply aren't collapsing or forming the black pigment we usually see. In such cases, use the milk line method, and don't worry about the black layer. When the milk line is no longer visible and there is no liquid at the base of the kernels, then the kernel is as heavy as it will get, black layer or not.

An additional observation that many have made this year is that kernel size seems to be unusually large, even in (if not especially in) later-planted fields. This is a very positive development; we often get so involved in the woes of weather and other problems that we forget to look at what the crop is actually doing in the field. So even in those fields with the milk line only halfway down, kernels may already be close to normal dry weights. In such cases frost will reduce yield, but yield will still be good.

For example, Lyle Paul, agronomist at the U of I research center near DeKalb, has been tracking kernel dry weight in corn planted early and late. On September 29, corn planted at the end of May already had kernel weights of 90,000 to 100,000 kernels per bushel, even though the kernels were less than 1/4 milk line. Kernels from early-planted corn were only at 80,000 to 90,000 per bushel, so with another week of fill, actual yields from the late-planted corn might be close to those from the early-planted corn, even though it will not fill completely. This is an example of looking at what we have rather than what we're losing; from early reports, I expect yields to be very good from most fields in most areas this year, even where the crop was planted late and came to a premature end due to frost.

The October 4 crop development report for Illinois shows an unusual lack of correlation between percentages of the crop rated as being in dent stage and the percentage rated as mature. In the Northwest CRD, only 86% of the crop is listed as in dent and 40% as mature, while in the Northeast, 93% is listed as in dent and only 10% as mature. The central and southern CRDs show the more typical (positive) correlations between percentages in these two stages. Even so, less than 90% of the crop is rated as being in the dent stage in the central districts (W, C, E). This is much less than we would have expected. One possible reason is that some hybrids do not show the typical indentation ("dent") in the crown of their kernels, even when they are fully mature, and so they are not rated as being in dent stage. As in the case with "missing" black layer, the milk line is the best indicator of actual grain-filling progress. If a milk line is visible, then the kernels are definitely in "dent" stage whether the crown is rounded or indented.

Effects of frost coming before maturity on stalk quality are a little difficult to predict, but I do not anticipate much problem from this. While movement of sugars from the stalk to the ear following a premature end to photosynthesis normally means more stalk quality problems, I expect that stalks strengthened (deposited lignins) relatively well in most fields over the past month. Having adequate lignin is important for stalk strength after freezing; stalk cells die after hard frost, and so maintaining stalk sugar content should be less important for stalk strength. Most fields have stood well up to now, and unless we get (or got) strong winds, lodging should not be a big factor this year. An exception might be fields that were planted very late and are only in dough stage; stalks in these fields may not have much lignin, and freezing might seriously weaken them.

Another issue of considerable concern is what effect frost on immature corn might have on grain dry-down rates and grain quality, especially test weights. Kernels that stop filling when they still have a milk line visible have sugars at the tip. Sugars tend to "hold on to" water more than starch, so such kernels dry more slowly. Cobs are also wetter, which means slower drying. On the other hand, frost that kills husks tends to make them dry faster, and if they loosen when they dry (not always a given), such kernels will probably be drier several weeks from now than if the weather had stayed warm and they had continued to fill.

Sugars remaining in the base of corn kernels typically darken as sugars caramelize with high-temperature drying. Tips on such kernels often shrink back, often changing the shape of the kernels. This, sometimes along with slightly lower kernel starch density, may reduce test weight. Discolored kernels with lower test weight may be subject to dockage at the time of sale. Even though neither factor has much effect on corn's feed quality, ethanol yields can be affected by the lower starch percentage.

In summary, most corn fields in the central and southern parts of Illinois are close enough to maturity that frost around October 10 should have minimal effect on yield, standability, or grain quality. In northern Illinois, fields planted late (about 10% of the acreage was planted after June 1) and with later-maturing hybrids are likely in early dent, at which stage a frost may reduce yield by 20% to 25%, and a hard freeze (28 degrees or less for several hours) may reduce yields by a third or more.

On the positive side, the corn crop has done a good job of filling grain up to now, and all indications are that yields will be good even in those fields where frost will prematurely put an end to grain filling. Frost will also kill some green weeds and, even though we would prefer that the crop make yield longer, it will start the grain dry-down process earlier than normal, and so may allow earlier harvest and reduce drying costs.--Emerson Nafziger

Close this window