Dry July could have capped crop yield estimates; be wary of yield formulas, MU agronomist warns

COLUMBIA, Mo. – Crop yields are easily overestimated when dry weather hits in July. The kernel count is there, but without rain the corn doesn’t fill.

Bill Wiebold, University of Missouri agronomist, warned that “You can’t see yields and won’t know until harvest.”

“I don’t want to be Doctor Doom, but you must be cautious in estimating yields on how the crop looks now.”

Above average rains fell in June across Missouri. That was followed by dry weather in July. Spotty rains left some areas with less than an inch of water. A strong start led to vigorous growth with lots of leaves.

“The crop looks good,” the MU Extension specialist said in a weekly teleconference, Aug. 5. “When weather turns dry, too much vegetation hurts.”

“Corn needs rain in July. Three-quarters of an inch won’t do it, no matter how deep your soils,” Wiebold said. “If forecasts for rain come true, this warning can be forgotten.”

Pat Guinan, MU Extension climatologist, gave hope. Wednesday night and Thursday, thunderstorms developing in Nebraska are expected to move southeast across Missouri. Rains of 1.5 to 3 inches are forecast.

“That could bring 3 inches of rain to St. Louis, which had only 0.7 inch of rain in July,” Guinan said.

In the first days of August, scattered rains fell in northwest Missouri bringing relief to corn and soybean crops.

Brent Myers, MU corn specialist, said 80 percent of the Missouri corn crop is at dough stage, the kernel filling time. “Once, kernels reach this stage, yield can increase 80 percent,” Myers said. “That is, if water is present.”

“Don’t shut off the irrigation,” Myers said. By this time of year, all kernels will be in place, but they won’t increase in size without water.

It is kernel count and then kernel size that determine yields. Size is still in doubt.

Wiebold said formulas for estimating corn yield are based on number of rows of kernels and number of kernels in a row on a cob. Those formulas are based on years with average rainfall. “This has not been an average year.”

Guinan said cool temperatures alleviated the rain shortage. “Preliminary numbers (for Missouri) show July was second coolest on record, going back 120 years. Only 1950 was cooler.”

“Cool weather slows evaporation from plants and soil.”

Mid-August temperature forecasts are moderate, not above normal.

Wiebold noted that even in cool weather, there is evaporation. “It just takes longer to empty the bank account,” he said.

“July rain is critical for corn.  No rain then can cause permanent damage,” Wiebold said. “For the soybean crops, August rain determines yields.”

“Double-crop soybean fields have been hammered,” Wiebold said.

The crop planted after wheat harvest just didn’t start. “Few people plant double crop now,” he added. “But for those who do, it’s devastating.”

“We were headed to extremely large yields,” Wiebold said. “If lack of rain cut yields 10 percent, Missouri farmers can still have record crops – if we get rains now.”

Myers said studies of corn development, show lots of variation late in the season. Rain after dough stage can add a lot to dry matter.

“There are a lot of days left for kernel fill, if we get the rain.”

Wiebold said soybean-yield formulas are almost worthless. “I don’t know if you’ve pulled a soybean plant and tried to count pods and beans in the pods. You have to look and guess.”

“There are so many ways to make a mistake in soybean estimates.” The best clue on lower yield is if you see leaves dying, he said.

“If rain arrives, the crop yield still depends on photosynthesis,” Wiebold said. “Sunlight and water allow leaves to return to work.”