While a whopping 43% of corn was planted across the Midwest in the brief dry window from May 13-19, the return of extremely wet weather last week means the market will be watching the “tail” of planting to see how much of the final 14% of expected corn acreage farmers ultimately plant, how many will choose to collect crop insurance, and how many will switch to later-planted soybeans in June. It will be critical in determining the final size of U.S. 2013 corn and soybean crops.

Not only did widespread rainfall across America’s heartland during the Memorial Day weekend slam the planting window shut, flooding may have damaged some already-emerged corn and soil crusting may impede emergence as fields dry.

In its May 28 Crop Progress report, the U.S. Department of Agriculture estimated corn planting in the 18 largest growing states was 86% completed as of May 26, compared with 90% as the 2008-12 average for the date and 99% at the same time last year when planting was exceptionally early due to the warm spring.

That means roughly 14% of the crop, or about 13.6 million acres, was unplanted as of May 26, based on the U.S.D.A.’s March 28 Prospective Plantings report, which said farmers intended to plant 97.3 million acres of corn in 2013. That is not an overwhelming amount considering growers may plant more than 40% of the crop in a week. The question is, will farmers choose to still plant corn considering the late date, and what will be the effect on yields.

Some analysts and agronomists said farmers may plant corn until June 15, while others would “throw in the towel earlier” to meet crop insurance deadlines. Some farmers also may opt to plant short season corn, if seed is available, which tends to yield less than full-season corn. Some saw the “drop dead” date for planting corn as June 1, with northern areas more limited.

“It doesn’t look good for much additional planting in Iowa, Minnesota, Nebraska, Missouri or eastern parts of the Dakotas,” said David Salmon, president of Weather Derivatives, a Belton, Mo.-based energy and agricultural weather consulting service. He sees a break in the wet weather this week, but only “a day or two” of dry enough conditions that will allow more planting in the region.

Mr. Salmon said he expects about 2 million acres of intended corn area not to be planted.

“From this point forward, the biggest concern is yields,” Mr. Salmon said.

There’s no question the prime planting period for corn is past. Most see May 15-20 as the optimum time to have corn planting completed across the Midwest. Normally, planting by that date means the crop will be in its critical, yield-determining pollination period ahead of the hottest weeks of summer.

A University of Illinois study showed average corn yields in Illinois, the nation’s second-largest corn growing state after Iowa, tend to decline by 15% after May 20 and by 25% after June 1. The effects of late planting are similar in other states of similar latitude. Corn planting in Illinois was 74% completed by May 19 and 89% by May 26. Planting in Iowa was 71% completed by May 19 and 85% by May 26.

In its May 10 World Agricultural Supply and Demand Estimates, the U.S.D.A. projected 2013 U.S. corn production at 14,140 million bus, based on the Prospective Plantings number of 97.3 million acres planted, 8% average abandonment resulting in harvested area of 89.5 million acres, and an average yield of 158 bus an acre, slightly below the expected trend yield because some late planting already was anticipated.

On an anecdotal basis, assuming 95.3 million acres of corn are planted, and 87.7 million acres are harvested (8% abandonment, which tends to increase in years of unusual weather), using the U.S.D.A. yield of 158 bus an acre, the top end potential for the crop would be 13,857 million bus, down 2% from the U.S.D.A.’s May 10 projection. But, a yield of 158 bus an acre becomes less likely as planting drags on. Using the University of Illinois numbers, again anecdotally, one may estimate 71% of the crop planted by May 19 yielding 158 bus an acre, 15% more planted by May 26 yielding 135 bus and the final 14% yielding 120 bus, resulting in a low-end estimated production of 13,088 million bus, down 7% from the initial U.S.D.A. projection.

“It’s not going to be a 14 billion bus crop,” Mr. Salmon said, “but it’s not going to be a disaster like last year either.”

And of course that depends on the weather “cooperating” by delivering an “average” or better growing season. As he said earlier in the year, Mr. Salmon iterated that he expects weather during the rest of the growing season to be “just as screwed up” as it has been so far. June is expected to warm up quickly (6 to 8 degrees above average), which may help the crop “catch up” but may not bode so well it continues into the corn pollination period in July, he said.

“On average we’ll be fine,” Mr. Salmon said, “but no one will be average.”