For the last five years, production of milk replacement heifers has been on the rebound in the United States. But US born replacements may decline in coming years, for the same reason they’ve been rising, Canada, long prohibited from supplying the US dairy cow market, is back in business. Imports of Canadian breeding cattle were barred on May 20, 2003, the day the Canadian Food Inspection Agency revealed a native-born cow had tested positive for bovine spongiform encephalopathy, better known as mad cow disease.