The good news is that most of the Midwest's late-planted corn crop escaped serious heat or drought stress during the critical pollination period and, to date, much of the important grain-filling period, says Bob Nielsen. The discouraging news, Purdue University's corn specialist says, is that the unusually cool 2009 growing season continues to put the brakes on the development of the crop.

According to the most recent weekly USDA crop progress report, the majority of his hometown state’s crop has progressed through the dough stage of development (R4) and is moving toward the dent stage (R5); but, it now lags nearly 3 weeks behind the 5-year average progress. If the current rate of crop progress continues for the remainder of the season, quite a bit of the state’s crop will mature in October rather than September, Nielsen says.

"Recent night-time low temperatures in the low to mid-40’s have certainly increased growers’ concerns about the prospects for successfully maturing this crop and whether these unusually cool temperatures will impact grain yield," Nielsen says. "Unfortunately, the effects of such an unusually cool grain-filling period on corn maturity dates and yield in the central Corn Belt are not well known, partly because the historical occurrence of such unusually cool grain-filling periods is so infrequent."

Nielsen says the years 1992, 2002 and 2008 bare similarities to this year's growing season. Though crop progress in those three growing seasons were similarly delayed, the end result for grain yields varied dramatically.

"In my judgement, the 2009 growing season is more similar to the 1992 and 2008 growing seasons than to the disastrous 2002 growing season," he says. "Drought stress accompanied the delayed crop development in 2002 and contributed strongly to the large decrease from trend yield that year."

Indiana’s corn crop has not experienced such widespread drought stress in 2009, Nielsen says, adding that the USDA-NASS certainly believes that yields will be good this year, according to their first yield estimate released Aug. 12 that pegs Indiana’s 2009 corn crop at 163 bushels per acre, or 5.6% above trend yield.

Nevertheless, Nielsen says the recent weeks of cool weather accentuated with the recent nights of temperatures in the low to mid-40’s have fueled vigorous debates about how the crop will respond. Moderate temperatures and adequate moisture during the grain-fill period are generally favorable for kernel set success and kernel weight development.

However, it is true that temperatures as low as 50 F or lower can be detrimental to the photosynthetic processes, Nielsen says. Canadian researchers documented that photosynthetic rates in corn decreased by 18% to 30% the day following a cold temperature stress of about 40 F during grain filling.

"The more important question is whether multiple days of cold temperature stress during grain filling can cause longer-term reductions in photosynthetic rates that may actually lead to a premature senescence and development of kernel black layer," Nielsen says. "There is limited research that addresses this question.

"Observations over the years, though, lead me to believe that there comes a point late in grain fill where extended periods of cool temperatures cause the plant to slowly shut down even though no actual frost injury has occurred."

Nielsen says these observations are in agreement with those of Daynard (1972), who suggested that extended periods of cool temperatures, not frost, were a more probably cause of what he characterized as “premature” black layer development. He noted that kernel black formation occurred shortly after cold spells when the average daily maximum temperatures were 54 F or cooler.

"The good news, to date, is that we have yet to experience such low daily maximum temperatures," Nielsen says.

He says Purdue research from 1992 offers a hint of what to expect on the calendar timing of kernel black layer formation (i.e., physiological maturity) relative to the silking date.

For planting dates where silking occurred toward late July, kernel black layer formation occurred by Sept. 21. Where silking occurred in early August, kernel black layer occurred by Oct. 11. Where silking occurred about mid-August, kernel black layer formation occurred by Oct. 27, but occurred 10 to 14 days after a killing freeze event.

All of the earlier silking dates (late July and early August) successfully reached kernel black layer prior to a killing freeze. Given the similarities between 1992 and 2009, Nielsen suggests that these data represent something of a crystal ball for corn growers to gaze into for this year’s crop.

Regarding a fall freeze risk to this year’s crop, Nielsen says that USDA-NASS estimated that 76% of Indiana’s corn crop had silked by Aug. 2. Previous Purdue research suggests that most of that should black layer no later than early October.

Another 13% of the crop had silked by Aug. 9 and that may black layer by approximately Oct. 11. Much of the remainder of the state’s crop (8% to 11%) had silked by Aug. 16 or later. That portion of the crop may not mature until late October to early November and will likely experience a killing fall freeze prior to normal kernel black layer formation.

"Assuming that the tail end of this year’s crop will at least make it to the half-milkline stage of development prior to a killing freeze, the potential yield loss for an individual field due to premature plant death would be no more than 12%," Nielsen says.