SIXTH AMENDMENT TO CREDIT AGREEMENT

Exhibit 10.15D

SIXTH AMENDMENT TO CREDIT AGREEMENT

Parties:

Recitals:

     A. CoBank (in its capacity as the Administrative Agent (“Agent”) and as a Syndication Party) and Borrower have entered into that certain Amended and Restated Credit Agreement dated as of December 16, 2003, and that certain First Amendment to 2003 Amended and Restated Credit Agreement dated December 15, 2005, that certain Second Amendment to 2003 Amended and Restated Credit Agreement dated June 30, 2006, that certain Third Amendment to 2003 Amended and Restated Credit Agreement dated December 13, 2006, and that certain Fourth Amendment to 2003 Amended and Restated Credit Agreement dated September 17, 2007 and that certain Fifth Amendment to 2003 Amended and Restated Credit Agreement dated as of November 2, 2007 (as amended, and as further amended, modified, or supplemented from time to time, the “Credit Agreement”) pursuant to which CoBank and any entity which becomes a “Syndication Party” has extended certain credit facilities to Borrower under the terms and conditions set forth in the Credit Agreement.

     B. Borrower has requested that the Agent and the Syndication Parties extend the maturity of the 2-Year Facility, which the Agent and the Syndication Parties are willing to do under the terms and conditions as set forth in this Sixth Amendment to Credit Agreement (“Sixth Amendment”). Borrower, the Agent and the Syndication Parties acknowledge and agree that the “2-Year Facility” is now a facility that will terminate and mature in approximately one year, further acknowledge and agree that the references to a “2-Year Facility” and other various references including the term “2-Year” have been left in the Credit Agreement to avoid a significant redrafting thereof and to avoid redrafting of other related documents, and further acknowledge and agree that the references to a “2-Year Facility” and other various references including the term “2-Year”shall not express or imply that the term or maturity of the facility is other than that agreed to in this Amendment.

Agreement :

     Now, therefore, in consideration of the mutual covenants and agreements herein contained and other good and valuable consideration, the receipt and adequacy of which are hereby acknowledged, the parties hereto hereby agree as follows:

     1.  Amendments to Credit Agreement . The Credit Agreement is amended as of the Effective Date as follows:

          1.1 The following Sections of Article 1 are hereby amended in their entirety to read as follows:

          1.11 Base Rate : means a rate per annum announced by the Administrative Agent on the first Business Day of each week, which shall be the sum of (a) the higher of (i) 150 basis points greater than the highest of the one week, or one month LIBO Rate or (ii) the Prime Rate, plus (b) 25 basis points.

          1.18 Committed Letter of Credit Fee : shall mean a fee equal to 125.0 basis points multiplied by the face amount of the Committed Letter of Credit.

          1.20 Commitment Fee Factor : means 25.0 basis points per annum.

          1.48 LIBO Rate : means (a) with respect to each day during each LIBO Rate Period applicable to a LIBO Rate Loan, the per annum rate for the LIBO Rate Period selected by Borrower, determined by the Administrative Agent (which shall be the LIBOR rate in effect two Business Days prior to the LIBO Rate Loan) rounded up to the 1/100th of 1% per annum, or (b) with respect to the determination of the Base Rate, the LIBOR rate, determined by the Administrative Agent for the determination period relating thereto, on the date of determination, in each case divided a percentage equal to 100% minus the stated maximum rate of all reserve requirements (including, without limitation, any marginal, emergency, supplemental, special or other reserves) applicable on such date to any member bank of the Federal Reserve System in respect of “Eurocurrency liabilities” as defined in Regulation D (or any successor category of liabilities under Regulation D).

          1.49 LIBOR Margin : means 125.0 basis points per