A 758-acre ranch located in the heart of the county's premier dairy belt is likely to be the next piece of property protected by the county's Open Space District.

The Board of Supervisors is expected today to pay $2.3 million for development rights on the Roblar Ranch, located in the Two Rock Valley west of Petaluma.

"It's going to be a very good acquisition," said Supervisor Mike Kerns, who represents the south county area.

The ranch, consisting mostly of pasture land, is part of a larger, 3,800-acre swath of dairy production and grazing land targeted for protection by the Sonoma County Agricultural Preservation and Open Space District.

The six square miles of land located on Petaluma's western outskirts "is the prime dairy ranch and pasture land area in the county," said Stuart Martin, a land acquisition specialist with the district.

He said the Roblar Ranch will be "one of the larger conservation easements in the area."

Of the 3,800 acres, only 400 so far are protected by conservation easements, although Martin said talks with other landowners about purchasing their development rights are continuing.

The ranch is owned by John Barella, who owns North Bay Construction Inc.

Martin said Barella plans to sell the 758 protected acres to two adjoining dairy ranchers to expand their operations.

Barella will retain ownership of another 198 acres that contain the ranch's home and other farm buildings.

While that land will not be covered by the conservation easement, Martin said existing zoning will prevent further subdivision of the property.

Martin said the development potential of the 758 acres would be about six homes.

The $2.3 million will be paid with funds from a quarter-cent sales tax approved by county voters in 1990. The purpose of the tax is to preserve open space, but also to protect agriculture by reducing the pressure to urbanize farmlands.

"It gives those in agriculture a chance to continue in agriculture," said Kerns.

Martin said that since its inception, the tax has protected about 58,000 acres countywide, or more than 90 square miles of land.

That includes more than 32,000 acres zoned for agricultural use, including 10,000 acres sandwiched between Petaluma and the coast.

The tax, set to expire in 2011 unless reauthorized by voters, raises about $17 million a year.