By Nick Carey

CHICAGO (Reuters) - U.S. orders for heavy duty trucks in June were down 34 percent from the same month last year to a four-year low as trucking firms were holding off on buying new 18-wheelers amid a weak freight environment, according to preliminary data released by a freight transportation forecaster on Wednesday.

"The Class 8 market is stuck in a holding pattern, at the bottom end of this cycle," Don Ake, vice president for commercial vehicles at FTR said in a statement.

"Fleets are cautious as freight demand has cooled off this year," he said.

Preliminary data showed 13,000 units ordered in June, the lowest monthly total since July 2012 and the worst June since 2009.

FTR said that all truck manufacturers were equally affected by the month's weak order numbers.

Just last month, U.S. truck maker Navistar International Corp <NAV.N> lowered its sales forecast for the second half of 2016, blaming a weak market for heavy duty trucks.

FTR's Ake said that this low level of truck order activity could be expected for at least a few more months.

(Reporting by Nick Carey, editing by G Crosse)