(Reuters) - The U.S. Census said on Wednesday it had 573,779 workers on its payroll in mid-May, suggesting a huge boost in Friday's U.S. employment report. The agency hired hundreds of thousands of temporary workers to help with the once-a-decade Census, and economists have been expecting that to provide a large, if short-lived, lift to the still-suffering U.S. job market. It was not clear exactly how those jobs would be counted in Friday's closely watched U.S. employment report for May. Economists polled by Reuters forecast an overall gain of 513,000, including a sizable contribution from Census hiring.