Rainy weather has resulted in major corn planting delays throughout Ohio, but according to Peter Thomison, an Ohio State University Extension corn specialist, farmers can still hold a sliver of hope that late planting won't put a big dent in yields at harvest time.

Thomison, who also is a scientist with the Ohio Agricultural Research and Development Center, and research associate Allen Geyer, examined trends related to planting dates and yields stretching back three decades. They reported their findings this week in the Crop Observation and Recommendation Network (CORN) newsletter.

As of May 15, the USDA’s National Agricultural Statistics Service reported that only 7% of Ohio's corn crop had been planted. That's 76% behind last year and 63% behind the five-year average. With 29% of the corn crop planted, Indiana farmers were faring a bit better – but that's still 56% behind last year and 37% behind the five-year average.

Because of the shorter growing season, greater disease and insect pressure and the potential threat of drought stress during the key stages of pollination and grain fill, it's usually estimated that farmers can expect yield losses of at least 1 bu./acre for every day's delay in planting after the first week of May. The expected loss grows to nearly 2 bu./day by the end of May.

However, the researchers examined what really happened to the corn crop in years when farmers experienced significant planting delays. What they found is that yields were generally down in those years. But, they also found that the data held a few surprises.

Since 1980, there were eight years when farmers experienced significant planting delays (1981, 1983, 1989, 1995, 1996, 2002, 2008 and 2009). In six of those years, the corn crop performed below the long-term yield trend, from a low of a loss of 5 bu./acre (in 1995) down to losing 56 bu./acre (in 2002, which saw a near record-low harvest of 88 bu.). One year, 1989, saw no variation from the long-term yield trend. And another year, 2009, saw a 15-bu./acre jump over the long-term yield trend. That year – when just 42% of the corn crop was planted by May 20 – saw a record state yield of 174 bu./acre.

"Weather conditions, both rainfall and temperatures, in July and August are probably the most important factors in determining yields," Thomison says. "In 2009, we had very favorable conditions after the late start, and we came through just fine."

However, if late planting is followed by severe dry weather during pollination and grain fill, corn yields will be severely affected, he says.