WASHINGTON — More than 800,000 acres were selected for enrollment in the Conservation Reserve Program during its 49th sign-up period. It was one of the most selective sign-up periods in the C.R.P.’s 30-year history, the U.S. Department of Agriculture said, with a record high Environmental Benefits Index cut-off and the lowest percentage of applications accepted. The nationwide enrollment limit is 24 million acres for fiscal 2017 and 2018. Farmer and rancher demand has been record high, the U.S.D.A. said. As of March 2016 there were 23.8 million acres enrolled, with 1.7 million set to expire this fall. The C.R.P. provides nearly $2 billion annually to land owners to offset costs of restoring, enhancing and protecting plant species that improve water quality, prevent soil erosion and strengthen wildlife habitat.