WASHINGTON — Prices of dairy products such as Grade AA butter, cheddar cheese blocks and barrels, dry whey and nonfat dry milk were mixed in the U.S. Department of Agriculture’s May 15 National Dairy Products Sales Report.

Butter graded AA averaged $1.68 a lb as of May 11, 2013, down 3.8c from the previous week and the lowest since April 13, when butter was posted at $1.66 a lb, the U.S.D.A. said.

 Cheddar cheese US 40 lb blocks averaged $1.89 a lb in the same period, up 3.1c from the previous week. Since April 13 when cheddar blocks averaged $1.69 a lb, block prices have been moving higher each week, the U.S.D.A. said.

The price of US 500 lb barrels adjusted to 38% moisture averaged $1.74 a lb as of May 11, down 2.1c from the previous week. The most current price reflects a decline from the recent high of $1.78 a lb posted on April 27, 2013.

Dry whey was only slightly lower in the latest week, the U.S.D.A. said. Supplies that met U.S.D.A. Extra Grade standards averaged 57.8c in the latest week, down 0.2c from the previous week. Dry whey prices have been mostly stable since April 13, varying only slightly from week to week.