Farmers from North Dakota to Iowa buckled down for some of the coldest weather in a generation on Wednesday, throwing extra rations to pigs or building igloos for chickens before hunkering down for a day of sub-zero temperatures and bone-chilling winds. Some, like Kurt Line in Indiana, joked about the forecast. Line said he looked forward to working on his tax returns, something he usually puts off, rather than braving the cold to load corn for a local processing plant. But temperatures expected to plunge in some areas as low as minus 40 degrees, the point at which Fahrenheit and Celsius converge, are no laughing matter for an industry dependent on the elements. The brutal chill was caused by the polar vortex, a mass of freezing air that normally spins around the North Pole but has made its way south to the United States. Cattle ranchers Joey Myers and her …