University of Illinois

No. 20/August 7, 1998

The 1998 Corn Crop: What's Ahead?

The USDA will release yield estimates next week, and my guess is that they'll show Illinois on track for a "trendline" yield, which is based on the shape of the line that graphs yields of the last decade or two. There is a lot of subjectivity (for example, throwing out years that "don't fit") when people draw trendlines, but this year appears to be set on a course to produce very good yields in many areas of the state, but fair or poor yields where the crop was planted very late and where flooding, dryness, or insects have taken a toll. When it's all said and done, I expect that statewide yield estimates will run in the middle or upper part of the 130 bushels per acre range--not great, not terrible, just a decent average for a year that's had a lot of ups and downs.

By now, the corn has completed pollination in almost all fields. Temperatures were about average during July, and growing-degree day (GDD) accumulations since planting have been about average. Rainfall records were somewhat mixed, with parts of the state still very wet and other parts needing rain. In most fields, however, pollination occurred successfully, with only a moderate amount of kernel abortion. At a population of about 30,000, some of the cornin our research area had 500 to 600 kernels per ear, after aborting about 50 to 100 kernels on the end of the ear. That amount of abortion (seen now as kernels on the end of the ear that are not filling) is more than we'd like to see and probably comes from the period of dry weather we experienced in mid-July. Still, the kernels present can probably accept most of the sugars that will be produced over the next month and compensate for reduced number by increasing in size.

By 2 to 3 weeks after pollination, rough yield estimates may be made in corn. From the example above, 30,000 ears times 550 kernels per ear equals 16,500,000 kernels per acre. We get that number by counting ears in a thousandth of an acre and counting kernels on three "representative" ears from the sample row. That estimate is not difficult to make (though it's often probably not very accurate); the bigger problem is that we don't have a good way to guess how large the kernels will get. The "sliderule" yield calculator that many people have used calculates yield by assuming that there will be 90,000 kernels per bushel. That's probably a bit on the conservative side-- kernels are often larger than that if filling conditions are good--but many people tend to take "optimistic" ear counts and samples, so this may improve the accuracy of the estimate by deflating yields. In any case, very good filling conditions may well result in kernels as large as 75,000 per bushel, while poor conditions or an early end to grainfill may result in kernels as small as 100,000 per bushel or more. When that happens, test weight is often low, both because kernels are shaped in a way that they don't fit together well, or because they have low density of the endosperm (starchy part of the kernel) because starch deposition was incomplete.

Once kernels have filled to the late-milk/early dough stage, where most of the corn that pollinated the first half of July is now, grainfilling is about 50 percent completed. Kernel number is more or less fixed at this stage, though any injury to the leaves will result in smaller kernels at maturity. Kernels in the early dough contain about 70 percent water. Later in the dough stage, the "milk line" will be seen where the zone of starch deposition meets the "milky" part of the kernel. This milk line will move down toward the base of the kernel, reaching the tip when the kernels are physiologically mature. At that time--projected to occur in late August for early planted, earlier-maturing hybrids--a "black layer" will be visible at the tip of the kernel, where it attaches to the cop. This indicates that no more material is moving into the kernel. It occurs when kernel moisture is 30 to 35 percent. Higher yields will result if maturity is reached "naturally," as the kernels fill to capacity. If anything such as frost or dryness prematurely stops normal photosynthesis by the leaves, kernel fill will end early, and smaller kernels will result. Kernels that do not fill to capacity for any reason will also form a black layer as the supply of sugars from the stalk through the cob ends.

The crop canopy is in good, but not outstanding shape at this point. If August temperatures stay moderate or are lower than average, and if enough rainfall occurs, kernel size could be above average. One of the features of the very good corn years of 1992, 1994, and 1996 was that August was cooler than normal, and September was warm and sunny (in most areas of Illinois, at least). This year does not appear to be on that trend, and it is likely that the crop will reach maturity before late September in most areas, meaning that September sunshine may not do the crop much good, except that it will help dry it down.

Emerson Nafziger (, Crop Sciences, (217)333-4424