Shares of Tyson Foods Inc. plunged as the US's largest meatpacker again cut its outlook for the fiscal year, signaling the beef industry's pains amid a shrinking US cattle herd are far from over. Tyson said the change was driven by significant margin compression amid a severe cattle shortage and the expected impact of lower cattle prices on the value of live cattle inventories. The revised outlook — which is below the low point of its prior forecast — shows that the company's drastic moves to try to rightsize the business with plant closures are still coming up short.