WASHINGTON — National prices of key dairy products fluctuated modestly in the week ended May 25 compared to the previous week. Sales in pounds over the past month gyrated and sometimes showed fairly dramatic changes, said the U.S. Department of Agriculture in its May 30 National Dairy Products Sales Report.

Butter, which experienced a 33% decline in weekly sales to 3 million lbs from 4.5 million lbs as of April 27, averaged $1.61 per lb for the week ended May 25, a per-lb decrease of 3.1c from the previous week, the U.S.D.A. said.

Cheddar cheese as US 40-lb blocks averaged $1.84 per lb for the week ended May 25, down 4.3c from the previous week. Sales for the week ended May 25, about 12 million lbs, saw a 24% rise from 9.7 million lbs for the week ended April 27, the U.S.D.A. said.

US 500-lb barrels adjusted to 38% moisture averaged $1.78 per lb, up 1.5c from the week ended May 18. Sales for the week ended May 25 were 9.8 million lbs, down 3% from 10 million lbs for the week ended April 27, the U.S.D.A. said.

Dry whey prices averaged 57.4c per lb for the week ended May 25, marginally higher than the previous week, the U.S.D.A. said. Sales were slightly higher than a month ago at about 9.2 million lbs.