University of Illinois

No. 22/August 29, 1997

Corn Moving Through Grainfill

Rainfall over much of the state in mid-August halted the deterioration in the Illinois corn crop, but the rains came too late to prevent substantial damage in many fields. Dry weather during the pollination period caused problems ranging from minor losses in kernel number to near-complete lack of kernels. The most severe losses in kernel number mostly occurred in fields that experienced problems such as insect damage in addition to lack of soil water.

Where kernels formed on the ears can provide clues to what caused kernel loss. If ears have a normal number of kernel rows (16 to 20), but rows are less than 30 to 35 kernels long, then it is likely that the later-emerging silks, which are those from kernels near the tip of the ear, may have failed to emerge, or may have emerged after pollen had all been shed. In other cases, substantial kernel abortion may have occurred due to continued dry weather after pollination. Such abortion shows up as "empty" kernels visible after abortion has occurred. Some kernel abortion usually occurs, but this year close to half of the kernels aborted in some fields.

Scattered kernels on the ear usually mean that insects actively fed on silks during pollination; or, in rare cases, the hybrid may have shed very little pollen due to high temperatures or dryness, or to genetic factors. If kernels are missing at the base of the ear, very dry weather early in the pollination process may have resulted in silks' being "nonreceptive," often as a result of waxy coating or other structural or physiological factors that make it difficult for pollen grains to adhere to the silk or germinate once they land on the silk.

What effect will a return to better soil moisture conditions have on the corn crop? That depends mainly on how many kernels have been set. If kernel number is low (fewer than 500 kernels per ear),then these kernels may well fill out to their maximum size, and yields could be limited by kernel number. What determines maximum kernel size is not very clear, but it is likely that conditions at or soon after pollination can affect the number of cells in the endosperm of the kernels and may limit the size of the cells as well. With normal kernel number (700 or so kernels per ear), such limitations may not mean much because yields are usually limited by the amount of photosynthesis during grainfill, and kernels do not reach their maximum size. When filling conditions are excellent, as they were in 1994 and 1996, kernel size increases more than normal, and high yields come from extra-large kernels.

Even with extra-large kernels in fields where kernel numbers are low, very high yields will probably not be possible this year. In fields with normal kernel numbers, very good yields are possible but will require that the canopy remain intact and active through mid-September. Relatively mild weather with a lot of sunshine and relatively infrequent rainfall will most favor high yields in these fields.

While some may be waiting until harvest to see how much their yield potential was affected by weather, others are interested in assessing the potential now. To do this, count the number of ears in 1/1,000th of an acre (17'5" in 30" rows), then take kernel counts on three ears in this row sample. Take the average number of kernels per ear, and multiply this number by the number of ears in the sample to get kernel number per 1/1,000th of an acre. Divide this figure by 90 to estimate bushels per acre. This assumes 90,000 kernels per bushel (zeroes dropped due to the count's being made on 0.001 acre). A major inaccuracy in this method comes from our inability to know what kernel size will be (size varies from less than 70,000 to more than 100,000 per bushel). Adjustments can be made in this factor (lower it for large kernels and increase it for small kernels) if you think that kernels will be other than normal in size.

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