USG Corp. urged its stockholders Monday to reject a slate of board candidates recommended by Desert Partners LP and repeated its opposition to the Texas group's $1.9 billion hostile takeover attempt. In a letter accompanying its proxy statement for the May 11 annual meeting, USG asked its stockholders to elect the slate of candidates nominated by the USG board to fill six spots on the 15-member panel. Desert Partners, meanwhile, announced it had extended until April 8 its $42-a-share cash tender offer for up to 21.5 million shares of USG stock. The offer was set to expire Monday at 11:59 p.m. EST. Desert Partners, a limited partnership led by Texas oilmen Jack E. Brown and Cyril Wagner Jr., had also offered to buy the remainder of USG's outstanding shares with securities valued at $42 a share. Chicago-based USG, a leading producer of gypsum wallboard and other building materials, told its stockholders that Desert Partners had reserved the right to cancel its tender offer unless all of the many conditions set forth in Securities and Exchange Commission regulations were met. If Desert Partners were to win the proxy fight, then drop its tender offer, ``Desert Partners would have obtained control of 40 percent of USG's Board without paying anything to USG stockholders or committing to do so in the future,'' wrote Robert J. Day, USG's chairman and chief executive officer. Day stated that Brown and Wagner had proposed takeovers of at least three other public companies in the past but had not yet acquired any of them. Day's letter said Desert Partners' proxy materials did not disclose whether its board candidates had any experience in overseeing a large public company ``and did not disclose what the Desert candidates would do if elected other than support the Desert Partners takeover campaign.'' Desert Partners announced the extension of its cash tender offer in a statement. The Texas group also said it was negotiating for additional financing in hopes of increasing the number of shares in the cash tender offer from 21.5 million to 39 million. Desert Partners owned a 9.9 percent stake in USG when it launched the tender offer, and said a ``nominal number'' of shares had been tendered through the end of last week.