Navistar, the US-based heavy-duty truck manufacturer, has reported a net loss of $56 million, or 93 cents a share, in the quarter ended Jan. 31. That compares with a year-earlier loss of $35 million, or 58 cents. Navistar has been battling with tough market conditions for freight trucks in the US over the past year. The US market is suffering from a glut of nearly new trucks and inventory at all the main truck makers has been high, compounding the adverse impact on the industry and market of a slowing economy.