General Motors Corp. (GM) on Thursday said it will invest about $20 million in its Shreveport, La., truck assembly plant to increase production of the Hummer H3 sport utility vehicle and Chevrolet Colorado (search) and GMC Canyon mid-size pickups.

The plant will add a third production shift to its paint shop in the first quarter of 2006 to prepare for increased production, the world's largest automaker said.

Sales of the Colorado and Canyon (search) have fallen steadily over the last three months, with Colorado sales slumping 41.7 percent in October and Canyon sales off 33.1 percent.

Still, on a year-to-date basis, Colorado sales have risen 17.6 percent while Canyon sales have soared 42.6 percent.

GM's (search) total recent investments in Shreveport for all three vehicles top more than $1 billion, the company said in a statement.