DES MOINES, Iowa (AP) _ U.S. pork producers say they will at least get temporary relief from Canadian competition as a result of a ruling by the U.S. Commerce Department on an industry complaint about subsidized Canadian exports.

The government said Tuesday it will place a preliminary duty of about 3 cents a pound on Canadian shipments of fresh, chilled and frozen pork products to the United States.

The duty, in the form of a bond to be posted by importers, will take effect Friday and last until a final decision is announced sometime in the fall.

If U.S. pork producers prevail, the bond will go into the U.S. Treasury and a permanent duty will be imposed. If the Canadians prevail, the money will be returned.

But in the meantime, the bond requirements will make it more expensive to ship pork products from Canada to the United States.

''Hopefully, that will slow down the surge of pork products coming in,'' said Dave Mehlhaff, a spokesman for the National Pork Producers Council.

The Des Moines-based council said shipments of such products have increased by more than 50 percent since 1984 and totaled about 470 million pounds last year. It said a University of Missouri study indicates the subsidized competition cost U.S. pork producers $697 million in 1988, and that the cost to meatpackers was even greater.

The pork producers have said the Canadian government was paying its producers a subsidy of $19.50 a hog in the third quarter of 1988 and that it rose to $31 a hog in the fourth quarter.

The ruling was the second in a series of decisions in the case. The International Trade Commission issued its preliminary finding of subsidized competition in February.

The Commerce Department will hold a June 28 hearing and issue a final decision by July 17, said Sen. Charles Grassley, R-Iowa. The International Trade Commission then will have 45 days to determine whether the imports materially injure or threaten injury to the U.S. pork industry.

Don Gingerich of Parnell, president of the pork council, said he was delighted the government is interpreting the figures in the same way as the industry.

''These imports of pork products coming out of Canada, which have increased so dramatically since we put the countervailing duties on live animals, has really plagued our industry as far as the profits we are able to maintain from our herds,'' Gingerich said.