KANSAS CITY — While generally good corn planting progress was made last week, traders’ ideas of progress vary widely ahead of the U.S. Department of Agricultures’ May 20 Crop Progress report. And, what progress was made came to a temporary halt in many areas early this week with widespread heavy rainfall across much of the Midwest Sunday night into Monday.

“Traders expect (corn) planting to range from 55% to 70% completed,” a report on the LSN newswire said, “that’s off from the 81% long-term average for this period.”

The newswire also said traders expected soybean planting to advance to 25% to 30% completed as May 19, which also would be below the 42% average for the date.

Anecdotal reports indicated very active planting across the Midwest late into the weekend, but also a considerable number of fields yet untended.

In its May 13 Crop Progress report, the U.S.D.A. said corn planting in the 18 major states was 28% completed as of May 12, far behind 85% a year earlier and 65% as the 2008-12 average for the date.

Soybean planting as of May 12 was only 6% completed compared with 43% a year earlier and 24% as the five-year average. The U.S.D.A. will issue its latest weekly Crop Progress report at 3:00 p.m. Central Time on May 20.

An agricultural economist from Kansas State University said late planted corn may yield 6% to 8% less than average, and forecast that U.S. farmers may switch from 2 million to 4 million acres from corn to soybeans.

A University of Illinois study indicated average yields in that state tend to decline by 8% for corn planted after May 10, by 15% after May 20 and by 25% after June 1.

Late last week consultant Informa Economics forecast 2013 U.S. corn planted area at 96.8 million acres, down about 500,000 acres from the U.S.D.A.’s most recent projection of 97.3 million acres. Informa forecast U.S. soybean area at 78.3 million acres, up 1.2 million acres from the latest U.S.D.A. projection of 77.1 million acres.

New crop corn and soybean futures prices at the CME Group were slightly lower in overnight trading early May 20.