No. 19 Article 5/August 3, 2007

Staging Corn Kernel Development

Though the condition of the Illinois corn crop remains good, there is some concern that the crop is reaching kernel stages earlier than expected. Because the extent to which kernels reach their final weight depends on both the quality of the filling period in terms of sunlight and adequate soil water, as well as on the health of the canopy, an early end to filling usually means lower yield.

Using the corn growing degree day (GDD) calculator, I constructed Table 2 to show where we are with regard to GDD at Champaign in 2007. The dates used are actual dates of planting in a planting date study we have here at Urbana. Note that GDD accumulations are only about 100 or so ahead of normal for the earlier planting dates, while GDD since the latest date is close to normal. This difference is coming from the warm May, and GDD accumulations in June and July were close to normal for those two months. Accumulations in June were a little higher than normal and in July a little less than normal; these two months produced nearly the same GDD in 2007.

We are using Pioneer 34A20 in this study, which is described by the seed company as 109 days relative maturity, with 1,370 GDD to silking and 2,650 to physiological maturity. The Illinois Agronomy Handbook (page 23) has a table indicating that, for a 2,700-GDD hybrid, silking (R1) occurs at about 1,400 GDD, dough stage (R4) at about 1,925, and dent stage at 2,450. These are approximations derived from research by others. In "How a Corn Plant Develops" (Iowa State University Special Report No. 48), milk stage (R3) is given as 18 to 22 days after silking, dough (R4) as 24 to 28 days after silking, and dent (R5) as 35 to 42 days after silking.

Is the corn crop in Illinois moving too quickly through these stages, and will kernels end up smaller than normal? The National Agricultural Statistics Service report (Weather and Crops) indicated that 48% of Illinois corn was in "dough" stage and 5% in "dent" on July 29, compared to the average (of the past 5 years) of 35% and 5%, respectively. The faster rate of reaching the dough stage is expected, since the crop pollinated earlier than normal. While an average percentage of the crop is reaching the dent stage, cold weather damage in some southern Illinois river bottoms this year may mean that more dented corn is farther north than usual.

Kernels from both of the April plantings appear to be in early dent stage and those from the May 10 planting in milk stage; those from the May 30 planting have just completed pollination. While the predicted GDD accumulations to different kernel stages are only approximations, kernels from the early plantings do seem to have advanced to the dent stage earlier this year than either days or GDD accumulations would have predicted. In contrast, those planted on May 10 and May 30 have reached milk and "blister" (R2) stages at close-to-predicted GDD numbers.

Ears on August 1 from corn planted (left to right) on April 2, April 20, May 10, and May 30 at Urbana.

Part of the difficulty in matching kernel stages to GDD or days is that the stages differ so much in length, with the early stages averaging only about a week per stage and the dough and dent stages each lasting two or three weeks. In our study, it is easy to see that kernels, while they show the characteristic dimpling at the crown that we call "denting," are still quite "doughy" in consistency. We are not seeing here the shallow kernels that some have reported for corn that is denting earlier than expected.

Photo of the germ face of kernels from ears shown in previous photo. The top row shows ears planted April 2 (left) and April 20 (right), and the bottom row shows ears from May 10 (left) and May 30 (right) planting dates.

Another problem with identification of the dent stage is that not all hybrids show denting to an equal extent. In fact, some hybrids have kernels that are rounded rather than distinctly dented, even at maturity. Denting is the result of starch accumulating in the crown of the kernel first, and as the starch granules in the endosperm cells "pack" into their final configuration, accompanied by loss of moisture from those cells, the cell volumes decrease and the dent forms as a consequence. This is less exact than the description of the dent stage would seem to suggest, and it is possible that high rates of starch accumulation this year might have kernels showing "dent" when their endosperm remains "doughy" in consistency.

If kernel numbers are average for the plant population in a field and they start to dent when the kernel depth (point to crown distance) is less than 1/4 inch, starch accumulation may be starting to wind down early, and kernels could end up smaller than normal. The key to whether this might be happening is usually whether the canopy, stalks, and roots are still healthy and functioning. Corn plants in August have little place to send their sugars except into kernels to be converted to starch, so it's unlikely that healthy, functioning plants are failing to fill kernels as fast as possible.

Finally, GDD accumulations for the early plantings to date indicate that this hybrid needs only about 500 more GDD to reach black layer. That is less than normal for August, so we may well see physiological maturity taking place in early-planted fields in central Illinois by the end of this month. If the weather stays no warmer than average and we get some rainfall before midmonth, yields should be good. Watch canopies closely before then, as loss of color will signal a quick end to grain-filling.--Emerson Nafziger

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