Sioux City, Ia (AP)- Nurses have authorized their union leaders to call a strike at MercyOne Siouxland Medical Center in Sioux City. The union must give the hospital at least 10 days’ notice before picketing could start. The nurses also rejected MercyOne’s most recent contract offer. The two sides have been negotiating since at least June. The local’s current contract expired Sept. 1. The nurses union last authorized a strike in June 2010 but settled with the hospital before a walkout began.