DETROIT - General Motors Co. (GM) will extend a temporary shutdown at its full-size truck plant in Indiana and add shifts to a pickup factory in Michigan to improve the balance of inventory on dealer lots, according to Bloomberg.

Fort Wayne Assembly, which makes Chevrolet Silverado and GMC Sierra pickups in Roanoke, Ind., will close the first week of January in addition to the last week of December, Rich LeTourneau, shop chairman at United Auto Workers Local 2209, according to Bloomberg. Saturday shifts at the plant have been canceled for the rest of the year, he said.

The automaker is adjusting its mix of trucks as it reduces inventory of the vehicles. GM is targeting 90 days' supply of Silverado and Sierra full-size trucks by the end of the year, from 115 days at the end of July, Don Johnson, GM's vice president of U.S. sales, said on an Aug. 2 conference call.

GM will add shifts on two Saturdays in September and three in October at Flint Assembly, which builds bigger, four-door crew cab Silverado and Sierra pickups that aren't made in Fort Wayne, according to UAW Local 598's website.

The automaker is trimming truck inventory "through production adjustments rather than through incentives," Jim Cain, a spokesman for Detroit-based GM, said in a phone interview.