SYDNEY: Mining giant BHP Billiton reported a slump in copper output Wednesday as strike action at the world’s largest mine for the metal dented production, while flagging the sale of some US shale assets. The world’s biggest miner said copper production slid 44 percent to 227,000 tonnes in the first quarter of 2017 compared to the same period last year, revising down its annual forecast to between 1.33 million and 1.36 million tonnes. It blamed a 43-day strike at the Escondida mine in Chile where BHP has a 57.5 percent stake. Workers ended their lengthy stand-off last month, triggering a law that pushed back pay negotiations by 18 months.