Agriculture Secretary Bob Bergland yesterday took a retrospective look at last month's farmer demonstrations in Washington and summed up their impact on farm policy in a single word: none.

"I don't think there'll be any legislative changes, or any policy changes, because of this year's demonstrations," Bergland said in an appearance at the National Press Club.

"Given the record of 25 percent higher farm prices [in 1978]... the record $27.3 billion in farm export sales chalked up last year, and the prospects for another good year in 1979, we see no need for any basic changes in this administration's basic farm program," the secretary said.

Assessing last month's demonstrations, in which 3,500 farmers and hundreds of tractors occupied the Mall, and, occasionally, the city's major thoroughfares, Bergland came closer than he had before to blaming them on a predecessor. Earl Butz.

"Generally speaking," he said, "most of the people who were demonstrating this year... had taken on a heavy burden in 1974-75. Now, I've gone back and reread Earl Butz's speeches... The signal went out to these people, that they could invest, and Washington would take care of things. And a lot of persons went out and overinvested."

It was those heavily indebted young farmers, Bergland said, who came here last month to protest federal farm policies. The secretary repeated his view that the angry demonstrators represented a small minority of the farm population. The majority, he said, is enjoying higher income and is thus satisfied with federal policy.

For 1979, Bergland said that a continued worldwide shortage will keep beef prices high, but that increased swine and poultry production will assure that "we will not have a meat shortage."

And he said that last year's corn surplus will lead to a historic first this year for the once-lowly soybean. As farmers take acreage out of corn, he said, "our country will have more acres devoted to soybeans than to any other crop, for the first time ever."

Conceding that the record farm income he predicts for 1979 will mean record food prices for consumers, Bergland sought to deflect popular dissatisfaction with the high cost of groceries away from the farmer.

The real culprits, he said, are the food industry and consumers themselves, who have increased the share of "convenience" foods -- prepared, packaged products -- in the average market basket.

Recent data show, he said, that the share of each food dollar that goes for preparation and packaging now exceeds the portion that pays for the raw foodstuffs.