WASHINGTON — U.S. winter wheat crop condition ratings continued to improve slightly from a week earlier, while corn and soybean plantings were still well behind the five-year average.

In the 18 major growing states, 48% of the winter wheat crop was rated good to excellent as of May 17, compared with 46% the previous week and 45% at the same time last year, the U.S. Department of Agriculture said in its weekly Crop Progress report. The crop was rated 26% poor to very poor, compared with 27% the previous week and 24% last year.

Winter wheat headed in the 18 states was 56%, compared with 40% a week earlier, 47% last year and an average of 60% over 2004-08.

Planting of spring wheat in the six leading states as of May 17 was 50% completed, the U.S.D.A. said, up from 35% a week earlier but far behind 92% a year ago and 90% as the five-year average for the date. Spring wheat emerged in those states was at 21%, up from 13% the previous week but below last year’s 50% or the 59% average.

Corn planting was 62% completed in the 18 major states, up from 48% the previous week but still behind last year’s 70% and the 2004-08 average of 85%. Planting in top-producing Iowa was 90% completed, above the last year’s 73% and consistent with the average. In comparison, planting in Illinois, the second-largest corn growing state, was only 20% completed as of May 17, well behind 73% last year and 92% as the average for the date.

Corn emerged was at 30%, up from 14% a week earlier and above last year’s 24% but below the five-year average of 49%.

Soybean planting in the 18 major states, at 25% completed as of May 17, was equal to what it was at this time last year, but it was behind the five-year average of 44% for the date, the U.S.D.A. said.