The Loudoun County Board of Supervisors, distressed by the effect of rapid growth on the county's open spaces, passed by a 5-to-3 vote yesterday a controversial measure designed to blunt development in rural areas.

The measure on transferable development rights (TDR) is designed to channel new growth to sections of Loudoun already developed while placing permanent restrictions on growth in farming areas.

Because counties are not allowed home rule in Virginia, the unorthodox TDR proposal must be approved by the General Assembly, traditionally reluctant to approve unusual measures affecting land use, before it takes effect.

The board's vote came at a meeting at which Supervisor James F. Brownell, a Republican, was elected to succeed Democrat Frank Raflo as board chairman. Brownell, the only full-time farmer on the board, edged out Supervisor Andrew R. Bird III, also a Republican, by a 6-to-2 vote.

Both votes represented triumphs for the pro-agricultural wing that has long dominated Loudoun politics. Many said that dominance may end in the near future as the county's political center shifts toward the suburbs of eastern Loudoun.

The impending shift heightened the importance of yesterday's victory for the measure's supporters such as Raflo, who has maintained that it represents one of the last chances the county has to control development.

Debate yesterday typified the vehemence of opinion that has greeted transferable development rights since they were first proposed in 1984. Supervisor Thomas S. Dodson said the concept offered the county "something other than wall-to-wall developments."

Supervisor Frank I. Lambert, however, countered that TDR is "not a grass-roots program; [it] is an elitist concept imported into the county by . . . elements seeking bargain-priced estates. Our citizens have much to lose and little to gain from the enactment of this scheme."

TDR would work by allowing a farmer to sell for cash the right to develop on his property. The purchaser, a developer, could then use these development rights elsewhere in the county to build at a higher level of density than would otherwise be allowed by zoning rules.

Once a landowner has sold his development rights, all construction would be permanently restricted on the property at any density greater than one house every 50 acres.

Likewise, according to the county's land use plans, no developer could build at the maximum allowed density in Loudoun unless he purchased development rights.

Once development rights have been sold for a property, the restriction on building could be lifted only after 25 years and only after the landowner purchased new development rights from some other location.

Loudoun's TDR program would be the only one in Virginia and one of a small number in the nation. Montgomery County is one of the jurisdictions that has implemented a TDR program and the concept has been a source of controversy there for several years. In July a Maryland state judge upheld the TDR concept by approving a $7 million golf course and 850-unit housing complex on Avenel Farm near the Potomac River.

Raflo, like other TDR supporters, maintains that the concept achieves a happy balance between development and agriculture, which remains Loudoun's largest industry. "High quality" developments, such as the Xerox Corp.'s multibillion-dollar residential and commercial project, are attracted to Loudoun because of its open spaces and pastoral setting, supporters say.

Opponents of the measure, however, argue that it unfairly concentrates growth on the eastern area of the county, lowering the quality of life there. Furthermore, they say, it reduces the county's ability to exact other concessions from developers, such as the construction of roads and sewers.

Raflo, however, yesterday repeated his claim that the measure's chances for passage are good. Voting for TDR yesterday were Democratic Supervisors Raflo, Dodson, Betty W. Tatum and Ann B. Kavanagh, joined by Republican Brownell. Opposing the measure were Republicans Bird and Lambert as well as Supervisor Steve W. Stockman, who because he is a federal employe, must serve as an independent.

A proposal to install Bird as chairman was rejected by the same voters who passed the measure.