The state’s largest agricultural organization will oppose a proposed “split-roll” ballot measure that would increase commercial property taxes, saying it will cost farmers and consumers billions by mandating costly reassessments for California barns, wineries and processing plants.

Jamie Johansson, president of the 34,000-member California Farm Bureau Federation, said in an interview that it was highly unusual for his organization to oppose a ballot measure at this early stage, “but our board of directors is very concerned about the impact this initiative would have on rural California.”