General Motors Co., preparing to introduce new pickups in 2013, has scheduled 21 weeks without production at three U.S. full-size truck plants next year to update the factories for building the new models.

GM’s Fort Wayne assembly plant outside Fort Wayne is scheduled to suspend output for nine weeks from January to October, Chris Lee, a spokesman for the Detroit-based company, said in a telephone interview. The automaker plans to idle its Flint, Mich., factory for seven weeks from June to November. GM’s Arlington, Texas, plant is scheduled to shut for five weeks from June to December.

The largest U.S. automaker has said it boosted profit this year by building supply of Chevrolet Silverado and GMC Sierra trucks, two of its most profitable models. GM is idling pickup factories for updates and renovations so that they’re able to assemble the automaker’s next-generation trucks.

GM had 202,720 full-size pickups in inventory at the end of November, 105 days supply on a selling-day basis, the company said Dec. 1. The company has said it’s targeting year-end inventory of about 200,000 full-size pickups, or about 90 days supply.

Capacity in 2012 at GM’s U.S. full-size pickup plants will be about 640,000 units, according to an Aug. 4 slideshow presentation. That’s down from “normal” capacity of about 780,000 units, the automaker said.

Scheduled down time for GM’s plant in Silao, Mexico, that makes full-size vehicles will be finalized after Jan. 1, Lee said.