What do 40,000 pounds of birdseed have in common with America's war on drugs?

Nothing, says Jean Laprise, an Ontario farmer who shipped the birdseed to his American customers only to have it seized when it crossed the United States-Canadian border.

Everything, say the United States Government and its critics, but for altogether different reasons.

The birdseed, nearly 20 tons of it, has been locked in a Detroit warehouse since Aug. 9, when it was impounded by the United States Customs Service. The reason: The seed consists of sterilized seeds processed from industrial hemp.

Mr. Laprise has found himself mired in one of the more bizarre episodes of Washington's campaign to curb illicit drug use. Hemp and marijuana are different varieties of the same plant species, Cannabis sativa, though the Government rarely distinguishes between them.

''They say it's a tractor-trailer full of drugs,'' Mr. Laprise said. ''We say it's a tractor-trailer full of birdseed.''