Richard E. Lyng and William G. Lesher, key figures in the Reagan administration's handling of farm policy during the last four years, have decided to set up a Washington consulting partnership on agriculture policy.

Both announced last year that they would not stay at the Agriculture Department -- Lyng giving up his post as deputy secretary, the USDA's day-to-day manager, and Lesher resigning as assistant secretary for economics.

Lesher said that he and Lyng will deal with agribusinesses and trade groups seeking insights on agricultural policy but that they will do no lobbying, which federal law would prohibit in any case.

"We will leave any lobbying to the clients themselves -- some of them already have representation here -- but we see ourselves in a role as advising as to what policy issues might be pursued and how their ideas might dovetail with current farm policy," Lesher said.

The departure of Lyng and Lesher is viewed by most of Washington's agricultural cognoscenti as a critical loss to the administration as it tries to take control of the farm bill debate, make major budget cuts in farm programs and give farm policy a "free market" orientation.

Another departure, that of Lesher's former deputy assistant secretary, J. Dawson Ahalt, to be U.S. agricultural attache in Argentina, leaves one more vital gap in Secretary John R. Block's legislative lobbying team.

Block relied heavily on all three men to present the administration's point of view during the 1981 farm bill debate and in later dealings with Congress over farm programs. This time , the secretary faces the prospect of trying to make an even more difficult policy case without experienced aides at his elbow.

The administration has not named a successor to Lesher. John Norton, a western cotton and vegetable farmer with no federal management experience, is expected to be nominated to fill Lyng's No. 2 slot. On the economics side, only Randy Russell, Ahalt's successor as deputy assistant secretary, has lobbying experience from 1981. BUT WHO WOULD WANT IT? . . .

Charles Shuman's resignation as head of the beleaguered Farmers Home Administration (FmHA) caught farm-state legislators by surprise and left them with no special political friend at home to promote for the job.

The inside quip around the USDA, however, is that only someone whose senses have gone on leave would want the job at the FmHA, which became one of the most controversial and most criticized agencies in government under the Reagan-Block-Shuman rubric.

Shuman's handling of farm and rural-housing loan policies generated lawsuits, farmer protests and tractorcades and frequent salvos of criticism from Capitol Hill. The agency's activities even inspired Hollywood to produce "Country," a movie that depicts the FmHA as a heartless ogre.

More of the same seems to be the order of the day, with reports of continuing deterioration of the farm economy and with more and more farmers turning to the FmHA for financial assistance that may or may not be there.

All that aside, sources indicate that Block and the administration probably will look within the department or turn to the cadre of politically appointed state FmHA directors to replace Shuman.

One hat already in the ring is that of Dale N. Richey, state director in Alabama and 1980 campaign manager for Sen. Jeremiah Denton (R-Ala.). He has told members of Congress that he has the support of some state directors in the South. But Richey has been controversial in his own right, and strong opposition is expected from farmers, home builders and even some Republicans who didn't want him to have the state post. A critic tells of a sign in his office that said, "I Don't Get Mad, I Just Get Even."