CHICAGO (AP) _ Grain futures prices retreated Friday on the Chicago Board of Trade after an influential forecaster released an optimistic spring plantings outlook. Soybean futures finished mixed.

Wheat and corn futures retreated after the private firm Sparks Cos. of Memphis, Tenn., predicted spring plantings will be larger than expected, increasing the chances of plentiful harvests.

Sparks said 1997 U.S. corn acreage will come in at 81.5 million, up from the 79.487 million acres last year that produced a large harvest.

Spring wheat plantings were estimated at 23.512 million acres, about the same as the 23.636 million acres last year. But the total wheat crop was projected to come in at 2.44 billion bushels, up from 2.282 billion in 1996.

Soybean futures were mixed amid expectations for widespread rain this weekend in the Rio Grande do Sul region of southern Brazil. The competing Brazilian crops have been languishing under drought-like conditions, but large amounts of rain could help just as the plants are entering the crucial flowering stage.

Wheat for March delivery fell 6 3/4 cents to $3.80 1/2 a bushel; March corn fell 1/4 cent to $2.73 1/4 a bushel; March oats fell 1 cent to $1.57 3/4 a bushel; March soybeans were unchanged at $7.46 a bushel.

Beef futures were mixed on the Chicago Mercantile Exchange, while pork futures were lower.

February live cattle fell .12 cent to 65.60 cents a pound; January feeder cattle rose .02 cent to 69.07 cents a pound; February live hogs fell .55 cent to 75.75 cents a pound; February frozen pork bellies fell .67 cent to 74.20 cents a pound.