Corn and soybean traders are questioning the old saying that "rain makes grain" with the Midwest already wet and Wednesday weather forecasts calling for excessive rains across some key production regions the end of last week.

However, some forecasters warned the weather could bring much heavier rains to key growing areas in Illinois and Iowa that are already too wet. This could mean extended planting delays in eastern Iowa and western Illinois as well as worsening flooding that could potentially force widespread replanting.

Flooding and soil erosion have already been reported in many areas of Iowa, the state office of the National Agricultural Statistics Board said in its weekly crop update. The NASS report noted Iowa farmers were concerned about standing water after precipitation received during the weekend. Topsoil moisture was rated surplus across 54% of the state, the highest reading since June 2010.

While USDA estimated that U.S. corn planting had reached 86% complete as of May 26, only 4 points behind the five-year average, that still left more than 13.6 million acres of intended corn area to be planted, based on the March planting intentions of 97.3 million acres.

Many producers will likely continue to plant corn past their final planting dates for crop insurance and accept a reduced insurance guarantee rather than switching crops or claiming prevented planting, but such late-planted corn figures to produce significantly reduced yields in the event of normal summer and fall weather.

It will take near-perfect conditions throughout the growing season for late-planted corn to yield well, similar to what occurred in 2009, when U.S. corn production was record high. Of course that was also the year of the "harvest from hell" and that large crop was fraught with quality problems.

The late planting and emergence pattern means that the second half of July through early August is shaping up to be the key summer period for crop production.

Producers made better-than-expected progress on soybean planting the week before last with USDA estimating 44% of the U.S. crop planted was planted of May 26, up 20 points from a week earlier, although still nearly a week behind the five-year average pace of 61% complete. U.S. soybean emergence at 14% was less than half the five-year average of 30%.

The concern is that Iowa was the furthest behind normal on soybean planting with 40% of the statewide crop planted versus an average pace of 83%. In north-central Iowa, only 20% of the crop had been planted.

The final soybean planting date in Iowa for crop insurance purposes is June 15. Iowa producers can continue to plant soybeans into early July, but fields planted past early June will have significantly lower yield potential.

While the weather forecasts are concerning, however, it would be wise to remember that ordinarily, corn and soybean price rallies based on too much rainfall are short-lived in nature. There may be further near-term upside, but the rally could fall apart as quickly as it sprouted.

Editor’s note: Richard Brock, Corn & Soybean Digest's marketing editor, is president of Brock Associates, a farm market advisory firm, and publisher of The Brock Report.