The pace of America's corn harvest slowed from a tiptoe to a crawl last week, falling further behind average, as farmers used windows of dry weather to focus on soybeans.

The proportion of the US corn crop in the silo hit 20% by Sunday, up just three percentage points in the week, Washington data showed.

In a typical year, 58% of the harvest is completed by now, at a rate of more than 10% of the crop a week.

The harvest - the most delayed since records began in 1985 - is proving particularly slow in some of the key Midwest producing states, including Illinois and Iowa.

Soybean preference 

The delays reflect in part persistent rain which has prompted Mississippi State University to report three times its normal rainfall.

However, farmers appear to have used let-ups from precipitation last week to focus on soybeans, which are viewed as more susceptible to damage and yield loss through being left standing in the field.

The proportion of soybeans harvested jumped from 30% to 44% over the week, with Illinois farmers making particular headway.

They had 33% of their soybeans in store by Sunday, compared with 13% a week before.

The report also showed steady headway with US planting of winter wheat, of which 76% was sown by Sunday.

While lower than the average of 85%, it represented an improvement of seven percentage points over the week.