Mack Trucks Inc. will begin laying off workers at its Hagerstown, Md., plant in September in a further attempt to cut costs, company officials said yesterday. They said that up to 500 of the 3,100 employes may be furloughed by next spring.

The nation's second-largest truck maker and Washington County's biggest employer, Mack posted losses of $14 million for the first quarter of 1986.

The number of workers laid off will depend on production levels, plant general manager Ross Rhoads said in a letter posted on company bulletin boards yesterday.

The Allentown, Pa.-based manufacturer announced earlier this year that it planned to cut costs in Hagerstown by contracting with outside firms for some parts. The plant makes engines, transmissions and rear-axle carriers for big rigs.

Mack officials said that they broke off contract negotiations with the United Auto Workers in February because the union had not offered sufficient cost-saving concessions. UAW officials could not be reached for comment yesterday.

Rhoads' letter was posted yesterday "to stop all the rumors that we were going to lay off about 2,000 workers," said company spokesman Dave Cole. "We wanted to inform our employes and keep them updated."

Hagerstown city administrator William Breichner yesterday called the layoff plans "very unfortunate."

" . . . Naturally, a layoff of any kind does not sit well with the city," he said.

In asking for substantial cuts in wages and benefits, the company said it was pressed by the shrinking truck market and competition from foreign manufacturers.

A UAW proposal for cutting costs and saving jobs was rejected by the company.

"Our union made every effort they thought they possibly could to meet the cost saving Mack said they had to achieve to keep our work" in Hagerstown, UAW spokesman Karl Mantyla said in February.