Dealers in farm equipment, whose business has been declining, now are being told that nearly three-fourths of Virginia's farmers expect to put off buying new tractors and mowers for another five years.

"Our dealers have had three very difficult years," said David Ottaway, executive vice president of the Virginia Farm and Industrial Equipment Dealers Association, an organization of 200 dealers.

"We're not forecasting doom, but it will be an additional period of very difficult times. And while we're not crying wolf, there are dealers up against some serious problems," Ottaway said.

Ottaway's association and Virginia Tech released recently initial results of a survey of 818 Virginia farmers showing that 71 percent don't expect to buy any new equipment for the next five years.

Reasons cited included soaring prices, high interest rates and a decline in farm income because of low prices despite generally bountiful harvests this year.

In the first eight months of 1981, Virginia dealers sold 29 percent fewer tractors than in the same period last year -- 1,067 versus 1,506 in January through August 1980.

Nationally, tractor sales declined 9.6 percent to 69,969 units comparing the same periods, the Farm and Industrial Equipment Institute of Chicago reported.

Sales of combines fell from 279 last year to 53 this year in the first eight months, down 81 percent in Virginia, while declining 12.7 percent to 13,079 units nationwide.

In Tidewater Virginia, farmers are buying only half as much equipment as normal, said David Stevenson of Stevenson Ford Tractor Inc. in Chesapeake.

"The tractor market is dead," Stevenson said.

And while sales have declined, prices have gone up.

The price of a 163-horsepower model, which Stevenson said is the workhorse of the region's large farms rose 60.4 percent this year. It costs $49,720 today against $31,000 last year.

Because of that steep price, cheaper Japanese models, especially for small tractors, are gaining in popularity, accounting for a quarter of Stevenson's sales.

A Japanese-made 40-horsepower tractor sells for $9,780 compared with $13,430 for a comparable American-made model, he said.

Equipment dealers in Suffolk, where this year's peanut crop recovered from last year's drought-caused failure, are generally better off, and are optimistic.

"We've had a good year," said Harold Blythe of Hobbs-Adams Engineering Co.