Corn in much of the Midwest was sailing along again after an early drought, fueled by ample moisture from July into early August. Then, Mother Nature turned the faucet off and dialed up the thermometer. Most agronomists agree that’s less than ideal for the back half of grain fill.

Indeed, Dan Quinn, Purdue Extension corn specialist, reports several cases of “top dieback,” where the top several leaves on the corn plant begin to senescence early. In fact, he documented this near Lafayette, Ind., in early September. What is not known is how much it will impact yield, he says.

Impact on ears

Meanwhile, Dave Nanda, director of genetics for Seed Genetics Direct, suggests there will likely be a more direct impact on kernel size and plumpness due to the very dry late grain fill period. Nanda recently revisited a field in central Indiana to make observations.

“The impact of dry weather appeared to vary considerably by hybrid,” he says. “I examined ears from two hybrids, planted close together. With one hybrid, kernel depth was still good, but the kernels weren’t plump. The ears just had a lighter feel than I would have expected from earlier visits.

“Plus, some kernels near the tip had aborted since my previous visit. There were yellow-colored skeletons remaining, but the plant obviously pulled the plug on them to finish off other kernels. All plants care about is making viable babies — progeny. It probably amounted to a couple kernels aborted on each kernel row at the tip.”

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However, Nanda saw something different on ears from the other hybrid. “They had a heavier feel, there weren’t aborted kernels at the tip, and when I broke the ear in half, I still found deep, fairly plump kernels forming,” he says. “In both cases, these hybrids were about 10 days from reaching black layer, which is physiological maturity.

“I concluded that there was a significant difference in how these two hybrids handled and reacted to late-season stress during grain fill caused by hot, dry weather.”

Yield implications

Here are two quick examples showing how Nanda’s observations impact yield estimates:

Hybrid 1 was significantly impacted by dry weather. Suppose in mid-August, the estimate for Hybrid 1 looked like this: 32 ears per 1/1,000 acre times 16 kernel rows times 40 kernels per row equals 20,480 divided by 80 fudge factor, based on modern hybrids. Estimated yield is 256 bushels per acre.

Now, using Nanda’s updated report, the estimate becomes: 32 ears times 16 kernel rows times 38 kernels per row equals 19,456 divided by 90 (higher number for lighter kernels) equals 216 bushels per acre. That’s a potential serious yield hit.

Hybrid 2 was minimally impacted by dry weather. The mid-August estimate for Hybrid 2 was the same, at 256 bushels per acre. The new estimate would be 32 ears times 16 rows times 40 kernels (no impact) equals 20,480 divided by 82.5 fudge factor (slight change for possible decrease in kernel size) equals 248 bushels per acre.

How will the field fare? It may depend upon how much of each hybrid was planted across the field, Nanda concludes.