<PAGE>

                                                                  Exhibit 4.2.3

                        PHIBRO ANIMAL HEALTH CORPORATION

                                       and

                      PHILIPP BROTHERS NETHERLANDS III B.V.

                                   as Issuers

                                       and

                           the GUARANTORS named herein

                                       and

                       HSBC BANK USA, NATIONAL ASSOCIATION

                         as Trustee and Collateral Agent

                           ---------------------------

                          THIRD SUPPLEMENTAL INDENTURE

                           Dated as of March 10, 2005

                                       to

                                    INDENTURE

                          Dated as of October 21, 2003

                          ----------------------------

                                  127,491 Units

                             Each Unit Consisting of
       $809.5238095 principal amount of 13% Senior Secured Notes due 2007
                       of Phibro Animal Health Corporation
                                       and
                   $190.4761905 principal amount of 13% Senior
                        Secured Notes due 2007 of Philipp
                          Brothers Netherlands III B.V.


<PAGE>

     THIRD SUPPLEMENTAL INDENTURE ("Third Supplemental Indenture"), dated as of
March 10, 2005, among Phibro Animal Health Corporation (the "U.S. Issuer"),
Phillip Brothers Netherlands III B.V. (the "Dutch Issuer" and with the U.S.
Issuer, the "Issuers"), each of the Guarantors named herein, as guarantors, and
HSBC Bank USA, National Association, as Trustee (the "Trustee"). All capitalized
terms not otherwise defined herein shall have the meaning assigned to them in
the Indenture (as defined herein).

                              W I T N E S S E T H:

     WHEREAS the Issuers and the Guarantors named therein have heretofore
executed and delivered to the Trustee an Indenture, dated as of October 21,
2003, as amended by that certain First Supplemental Indenture dated as of June
25, 2004 and Second Supplemental Indenture, dated as of December 8, 2004 (as
such may be amended and supplemented from time to time, the "Indenture"),
providing for the issuance of 127,491 Units due 2007 (the "Units"), each Unit
consisting of $809.5238095 principal amount of 13% Senior Secured Notes due 2007
issued by Phibro Animal Health Corporation (the "U.S. Notes") and $190.4761905
principal amount of 13% Senior Secured Notes due 2007 issued by Philipp Brothers
Netherlands III B.V. (the "Dutch Notes" together, the "Notes"), including the
issuance of the Exchange Units; and

     WHEREAS, pursuant to the Registration Rights Agreement, the Issuers filed a
Registration Statement on Form S-4 (File No. 333-122063) with the Securities and
Exchange Commission (the "Commission") on January 14, 2005 under the Securities
Act of 1933, as amended (the "Act"), as amended by Amendment No. 1 thereto as
filed with the Commission on February 2, 2005, as amended by Post-Effective
Amendment No. 1 thereto as filed with the Commission on February 24, 2005 (such
Registration Statement, as so amended, being hereinafter referred to as the
"Registration Statement"), in connection with the Issuers' offer to exchange
(the "Exchange Offer") the aggregate principal amount of the unregistered
Initial Units for a like principal amount of Exchange Units, as registered under
the Act pursuant to the Registration Statement; and

     WHEREAS, the Registration Statement was declared effective by the
Commission on February 4, 2005, and Post-Effective Amendment No. 1 thereto was
declared effective by the Commission on March 2, 2005 and, pursuant to the
Registration Rights Agreement, the Exchange Offer was consummated on March 10,
2005; and

     WHEREAS, Section 9.01 of the Indenture provides that the Issuers and the
Trustee may execute this Third Supplemental Indenture without notice to or the
consent of any Holder to provide for issuance of the Exchange Units; and

     WHEREAS, the execution and delivery of this Third Supplemental Indenture
and the Exchange Units in the form of Exhibit B to the Indenture and the
Domestic, Dutch and Company Guaranties substantially in the form of Exhibits E,
F and G to the Indenture, respectively, have been authorized by resolution of
the respective Boards of Directors and shareholders of the Issuers and the
Guarantors; and

                                       2

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     WHEREAS, the Issuers and the Guarantors authorize the Trustee to cancel the
Initial Units and related Guarantees and to authenticate the Exchange Units and
related Guarantees in replacement therefor; and

     WHEREAS, all conditions and requirements necessary to make this Third
Supplemental Indenture and the Exchange Units the valid obligations of the
Issuers, and to make this Third Supplemental Indenture and the Guarantees
relating to the Exchange Units a valid and binding agreement of each of the
Guarantors, have been performed and fulfilled by the applicable parties hereto
and the execution and delivery hereof and thereof have been in all respects duly
authorized by the applicable parties hereto;

     NOW, THEREFORE, in consideration of the above premises, each party agrees,
for the benefit of the others and for the equal and ratable benefit of the
Holders of the Units, as follows:

          1. Definitions. (a) Capitalized terms used herein without definition
shall have the meanings assigned to them in the Indenture.

          (b) For all purposes of this Supplemental Indenture, except as
otherwise herein expressly provided or unless the context otherwise requires:
(i) the terms and expressions used herein shall have the same meanings as
corresponding terms and expressions used in the Indenture; and (ii) the words
"herein," "hereof" and "hereby" and other words of similar import used in this
Supplemental Indenture refer to this Supplemental Indenture as a whole and not
to any particular section hereof.

          2. Exchange Offer - Initial Units for Exchange Units. Promptly
following the execution and delivery of this Third Supplemental Indenture, the
Trustee shall, upon the written order of the Issuers, in the form of an
Officers' Certificate of the Issuers, authenticate and deliver Exchange Units
substantially in the form of Exhibit B to the Indenture in replacement of
Initial Units whose Holders have accepted the Exchange Offer and exchanged their
Units in accordance therewith, and upon delivery of certificates representing
the Initial Units to the Trustee for cancellation, the Trustee shall cancel such
Initial Units.

          3. Acceptance by Trustee. The Trustee accepts this Third Supplemental
Indenture and agrees to execute the trust created by the Indenture as hereby
supplemented, but only upon the terms and conditions set forth in the Indenture,
including the terms and provisions defining and limiting the liabilities and
responsibilities of the Trustee, which terms and provisions shall in like manner
define and limit its liabilities and responsibilities in the performance of the
trust created by the Indenture as hereby supplemented.

          4. Supplemental Indenture Part of Indenture; Ratification of
Indenture. This Third Supplemental Indenture is executed and shall be construed
as an indenture supplemental to the Indenture and, as provided in the Indenture,
this Third Supplemental Indenture forms a part thereof. Except as otherwise
expressly provided for in this Third Supplemental Indenture, all of the terms
and conditions of the Indenture are hereby ratified and shall remain unchanged
and continue in full force and effect.

                                       3

<PAGE>

          5. Trustees Makes No Representation. The recitals contained in this
     Third Supplemental Indenture shall be taken as the statements made solely
     by the Issuers and the Guarantors, and the Trustee shall have no liability
     or responsibility for their correctness, and, without limiting the
     generality of the foregoing, the Trustee shall not be responsible in any
     manner whatsoever for or with respect to any of (i) the validity or
     sufficiency of this Third Supplemental Indenture or any of the terms or
     provisions hereof, (ii) the proper authorization hereof by the Issuers and
     the Guarantors by corporate action or otherwise, (iii) the due execution
     hereof by the Issuers and the Guarantors or (iv) the consequences (direct
     or indirect and whether deliberate or inadvertent) of any amendment herein
     provided for, and the Trustee makes no representation with respect to any
     such matters.

          6. Effective Date. This Third Supplemental Indenture shall become
     effective upon the execution and delivery hereof by the Issuers, the
     Guarantors and the Trustee.

          7. Governing Law. THIS THIRD SUPPLEMENTAL INDENTURE SHALL BE GOVERNED
     BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK, AS
     APPLIED TO CONTRACTS MADE AND PERFORMED IN THE STATE OF NEW YORK, WITHOUT
     GIVING EFFECT TO APPLICABLE PRINCIPLES OF CONFLICTS OF LAW TO THE EXTENT
     THAT THE APPLICATION OF THE LAW OF ANOTHER JURISDICTION WOULD BE REQUIRED
     THEREBY.

          8. Counterparts. This Third Supplemental Indenture may be signed in
     any number of counterparts, each of which shall be an original, with the
     same effect as if the signatures thereto and hereto were upon the same
     instrument.

          9. Effect of Headings. The Section headings herein are for convenience
     only and shall not affect the construction thereof.



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                                       4

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                                   SIGNATURES

     IN WITNESS WHEREOF, the parties hereto have caused this Third Supplemental
Indenture to be duly executed, all as of the date written above.

                                      PHIBRO ANIMAL HEALTH CORPORATION

                                      By: /s/  Richard G. Johnson
                                          -------------------------------------
                                          Name:  Richard G. Johnson
                                          Title: Chief Financial Officer


                                      PHILIPP BROTHERS NETHERLANDS III B.V.

                                      By: /s/  Jack C. Bendheim
                                          -------------------------------------
                                          Name:  Jack C. Bendheim
                                          Title: Managing Director

                                      By: /s/  Joseph M. Katzenstein
                                          -------------------------------------
                                          Name:  Joseph M. Katzenstein
                                          Title: Managing Director


                                      DOMESTIC GUARANTORS:
                                      --------------------
                                      PRINCE AGRIPRODUCTS, INC.
                                      PHIBROCHEM, INC.
                                      PHIBRO ANIMAL HEALTH HOLDINGS, INC.
                                      PHIBRO CHEMICALS, INC.
                                      WESTERN MAGNESIUM CORP.
                                      C P CHEMICALS, INC.
                                      PHIBRO-TECH, INC.
                                      PHIBRO ANIMAL HEALTH U.S., INC.

                                      By: /s/  David C. Storbeck
                                          -------------------------------------
                                          Name:  David C. Storbeck
                                          Title: Vice President


                                      FOREIGN GUARANTOR:
                                      ------------------
                                      PHIBRO ANIMAL HEALTH SA

                                      By: /s/  Jack C. Bendheim
                                          -------------------------------------
                                          Name: Jack C. Bendheim
                                          Title: Managing Director


Accepted and Agreed to:
HSBC BANK USA, NATIONAL ASSOCIATION, as Trustee and Collateral Agent

By: /s/  Herawattee Alli
    --------------------------------
    Name:  Herawattee Alli
    Title: Assistant Vice President