BALTIMORE (AP) - Heavy truck maker AB Volvo said it will lay off 600 workers next year at its Hagerstown engine-and-transmission plant.The Swedish company said the layoffs reflect expectations for an industry wide decline in demand for heavy-duty trucks. The Hagerstown plant, which employs about 1,770 people, is one of the largest employers in the western Maryland city of 38,000. The plant produces parts for Volvo and Mack trucks. ''While these reductions are necessary for the responsible management of our operations through the downturn, we regret the impact they will have on our employees, their families, and the Washington County community,'' said Sten-Ake Aronsson, senior vice president of Volvo Powertrain North America. The layoffs, expected in the first half of 2007, are Volvo's second announcement of large-scale layoffs in the United States this month. A week earlier, Volvo said it would lay off 450 workers at its truck assembly plant in Macungie, Pa., cutting the work force there by 40 percent. Volvo predicted in its third-quarterly earnings report Tuesday that the American market for heavy trucks will fall by 40 percent in the first half of 2007. Volvo, the world's second-largest truck maker, said its third-quarter profit rose 34 percent, to $538 million, on rising European and North American demand and greater profitability at its Mack Trucks unit.