The National Cattlemen’s Assn. sued the Agriculture Department to halt the subsidized slaughter of nearly a million dairy cows, saying that the influx of beef is seriously depressing cattle markets. The suit, filed in U.S. District Court in Lubbock, Tex., means that the program to reduce dairy herds is now under fire on at least two fronts. A federal judge in Rochester, N.Y., has temporarily blocked the program because of complaints from the Humane Society about a requirement that cows destined for slaughter be branded on the face. Judge Michael Telesca extended his temporary order barring enforcement of the face branding through April 21.