The vaunted agricultural research establishment that has helped make U.S. farmers the most productive in history is in organizational disarray, seriously underfinanced and drifting without national policy direction, according to a congressional report.

The study, released last month by the Office of Technology Assessment, a congressional research and policy arm, said that ample future world food supplies hinge, to a large degree, on major changes in U.S. approaches to research.

By implication, the OTA report put a major share of responsibility for research problems on the Department of Agriculture. The OTA said , however, that the Reagan administration has taken some recent and tentative steps toward improving research management at the USDA.

But the study said that unless other major changes are ordered by Congress and the executive branch the research establishment will continue drifting and will decline in scientific facilities that have not kept pace with needs.

Sen. Ted F. Stevens (R-Alaska), chairman of the OTA board, welcomed the study but warned that federal budget constraints mean "we are not going to be able to do everything we'd like to do." But Rep. Morris K. Udall (D-Ariz.), vice chairman, cautioned that the study's warnings cannot be ignored if U.S. and world agriculture is expected to continue to meet global food demands.

A House agriculture research subcommittee chaired by Rep. George E. Brown Jr. (D-Calif.) is planning a round of oversight hearings on some of the issues raised by the OTA study.

A key theme of the study was that public spending on research--which it described as an area that cannot be left to profit-oriented private industry--has not kept pace with the need for improved agricultural technology.

The OTA said there is little challenge to the justification for public funding of food and agricultural research. Producers as well as consumers have benefited from scientific developments, but the report said that increased state-level spending means that taxpayers in food-surplus states have subsidized consumers in food-deficit areas--an imbalance that more federal aid could correct.

But the OTA said the USDA lags far behind other federal agencies in spending on major research. It said that in 1978 the USDA spent 1.5 percent of its budget on research. Defense, in contrast, spent 45 percent on research and development; Energy, 16 percent; Health, Education and Welfare, 12 percent.

"It appears that the primary responsibility for this decline and low level of federal research funding for agriculture has been USDA's," the report said. "USDA leadership has not had much appreciation for the value of research and has not given it high priority . . . Up to 1980, the executive budgets have not shown the needed increases in agricultural research."

But the study stressed that other steps can be taken, apparently at little cost, to rejuvenate and strengthen the research system by eliminating overlap, by better organization, by clearly defining national goals and by establishing policies to meet them.

Some other points raised by the OTA:

USDA's structure hinders its ability to manage and to conduct nationally focused research and to react to U.S. agricultural needs. The report said the department's research has been conducted more like a series of regional programs.

There is no satisfactory long-term means of evaluating existing and potential research work or development of research priorities. The report said basic decisions are made on an "ad hoc basis" with inadequate coordination between federal and state agencies.

USDA and state agricultural experiment stations seem to be working on an "indistinguishable problem." The report said considerable overlap and duplication exists, with concern among many scientists that national research issues do not get adequate attention.