Call it magic or whatever you like, but when Ralph Nader points a finger, the Department of Agriculture turns temporarily speechless.

Last January, Nader issued a long and harsh critique of the USDA's meat and poultry inspection program.

The USDA didn't have much to say at the time, except that Nader was wrong.

Less than a month later, however, the department's Food Safety and Inspection Service (FSIS) cranked out a 25-page riposte that took longer and more vigorous issue with Nader's allegations.

It turns out, however, that it wasn't exactly the USDA that answered. A Lexington, Mass., consulting firm known as Temple, Barker & Sloan Inc. was hired by the department to produce the response at a cost to taxpayers of $20,140.

It further turns out that the chief consultant was Thomas P. Grumbly, who during the Carter administration was an executive assistant to Donald Houston, administrator of the FSIS.

Grumbly is remembered at the USDA as one of the architects of changes in meat inspection that the Reagan administration followed through on and which Nader sharply criticizes. Grumbly also has done consulting work for the American Meat Institute, the trade group that Nader said has captured control of the USDA.

Nader had a new bone to pick with the department when he learned of the Grumbly report.

He whipped off a letter to Secretary John R. Block last week, asking about the "deplorable waste" of USDA money to hire a mouthpiece.

A USDA spokesman said that department people actually wrote most of the response to Nader and that Grumbly's role was "to put it all together for us."