The Reagan administration endorsed legislation yesterday that could open 51.4 million acres of national forests to mining, logging and other development, provoking a cry of protest from wilderness advocates.

Richard E. Lyng, deputy secretary of agriculture, told a Senate subcommittee that after 10 years of study and litigation, "we can no longer afford further delays" in development of millions of acres of national forest lands.

Specifically, Lyng endorsed a bill sponsored by Sen. S.I. Hayakawa (R-Calif.) that would open 36 million acres of forest to development immediately.

The bill also would force Congress to meet strict deadlines for deciding whether an additional 15.4 million acres should be set aside permanently as wilderness or opened to development.

Reflecting the administration's pro-development view, Lyng also endorsed a facet of the bill that would effectively prohibit the U.S. Forest Service from planning new wilderness areas.

Ultimately, only Congress can designate wilderness protection areas, where vehicles, logging and other development are prohibited.

Lyng said the Reagan administrations is skeptical of any new wilderness designations that would slow the "availability of resources including timber, minerals, oil and gas, geothermal . . . recreation anf forage production," or logging.

William Turnage, executive director of the Wilderness Society, assailed the Hayakawa proposal as "a narrow-special interest bill for the benefit of the corporate purchasers of federal timber."