<PAGE>   1
                                                                     EXHIBIT 4.1

                SECOND AMENDMENT TO THE AMENDED AND RESTATED
                  REVOLVING CREDIT AND TERM LOAN AGREEMENT

                 THIS SECOND AMENDMENT TO THE AMENDED AND RESTATED REVOLVING
CREDIT AND TERM LOAN AGREEMENT (the "Amendment"), made and entered into as of
the 31st day of March, 1997, by and between DOANE PRODUCTS COMPANY, a Delaware
corporation ("Borrower"), MERCANTILE BANK NATIONAL ASSOCIATION, a national
banking association, SOUTHTRUST BANK OF ALABAMA, NATIONAL ASSOCIATION, a
national banking association, HARRIS TRUST AND SAVINGS BANK, an Illinois state
banking corporation, HIBERNIA NATIONAL BANK, a national bank, NBD BANK, and
BANK OF OKLAHOMA, N.A., a national banking association (collectively referred
to herein as the "Banks"), and MERCANTILE BANK NATIONAL ASSOCIATION, a national
banking association, as agent for the Banks (in such capacity, the "Agent").

                                  WITNESSETH:

                 WHEREAS, Borrower, Agent and Banks entered into an Amended and
Restated Revolving Credit and Term Loan Agreement dated February 28, 1996,
which agreement was amended on June 28, 1996 (as amended, the "Credit
Agreement"); and

                 WHEREAS, Borrower has requested that the Credit Agreement be
amended and modified as hereinafter set forth, to which said amendments and
modifications Agent and Banks are willing to agree;

                 NOW, THEREFORE, in consideration of the premises and the
mutual provisions and agreements hereinafter set forth, the parties hereto do
hereby mutually promise and agree as follows:

                                   AGREEMENTS

                 1.        In consideration of the amendments to the Credit
Agreement made herein, Borrower shall pay the Banks a fee equal to 15 basis
points on the revolver facility and outstanding term loan balance.

                 2.       Section 10.1(m)(i) of the Credit Agreement shall be
deleted in its entirety and in its place shall be substituted the following:

                 (i)      Minimum Consolidated EBITDA.  Borrower will have and
         maintain a Consolidated EBITDA, determined for the four-quarter period
         ending on the date of each such calculation hereunder, of at least:

<TABLE>
<CAPTION>                                           
          For Fiscal Quarter
         Ended On Or Between                                Minimum Consolidated EBITDA
         -------------------                                ---------------------------
         <S>                                                <C>
         March 31, 1997                                             $34,000,000
         June 30, 1997                                              $34,000,000
         September 30, 1997                                         $36,000,000

         December 31, 1997                                          $41,500,000
         March 31, 1998                                             $42,500,000
         June 30, 1998                                              $43,500,000
         September 30, 1998                                         $43,500,000

         December 31, 1998                                          $46,000,000
         March 31, 1999                                             $46,000,000
         June 30, 1999                                              $47,000,000
         September 30, 1999                                         $47,000,000

         December 31, 1999 through September 30, 2000               $48,500,000
</TABLE>
<PAGE>   2
                 3.       Section 10.1(m)(ii) of the Credit Agreement shall be
deleted in its entirety and in its place shall be substituted the following:

                 (ii)     Minimum Consolidated Cash Flow Coverage Ratio.
         Borrower will have and maintain a Consolidated Cash Flow Coverage
         Ratio of at least (i) 0.70 to 1.0 at all times during the period
         commencing July 1, 1996 and ending June 30, 1997, (ii) 0.75 to 1.0 at
         all times during the period commencing July 1, 1997 and ending
         September 30, 1997, (iii) .80 to 1.0 at all times during the period
         commencing October 1, 1997 and ending December 31, 1997,  (iv) 0.85 to
         1.0 at all times during the period commencing January 1, 1998 and
         ending September 30, 1998, (v) .90 to 1.0 at all times during the
         period commencing October 1, 1998 and ending September 30, 1999 and
         (vi) 1.0 to 1.0 during the period commencing October 31, 1999 and
         ending September 30, 2000.  For purposes of this Section, Capital
         Expenditures  included in the definition of "Consolidated Cash Flow
         Coverage Ratio" shall not include expenditures financed with direct or
         indirect proceeds from industrial revenue bonds or similar
         indebtedness.

                 4.       Section 10.1(m)(iii) of the Credit Agreement shall be
deleted in its entirety and in its place shall be substituted the following:

                 (iii)    Maximum Consolidated Total Debt to Consolidated
         EBITDA.  Borrower will at all times have and maintain a ratio of
         Consolidated Total Debt to Consolidated EBITDA which is less than or
         equal to the following:

<TABLE>
<CAPTION>
          For Fiscal Quarter                                        Maximum Consolidated Total
         Ended On Or Between                                        Debt to Consolidated EBITDA
         -------------------                                        ---------------------------
         <S>                                                               <C>
         March 31, 1997                                                      6.60 to 1.00
         June 30, 1997                                                       6.30 to 1.00
         September 30, 1997                                                  6.30 to 1.00

         December 31, 1997                                                   5.50 to 1.00
         March 31, 1998                                                      5.50 to 1.00
         June 30, 1998                                                       5.20 to 1.00
         September 30, 1998                                                  5.20 to 1.00

         December 31, 1998                                                   4.50 to 1.00
         March 31, 1999                                                      4.50 to 1.00
         June 30, 1999                                                       4.25 to 1.00
         September 30, 1999                                                  4.25 to 1.00

         December 31, 1999 through September, 30, 2000                       4.10 to 1.00

</TABLE>

                 5.       Notwithstanding anything to the contrary in the
Credit Agreement, amounts held by or on behalf of the Banks or other lenders
(including by collateral agents and indenture trustees) in reserve or cash
collateral accounts shall not be included (a) as "Consolidated Total Debt" for
purposes of Section 10.1(m)(iii), (b) as "Consolidated Senior Secured Debt" for
purposes of Section 10.1(m)(iv), or (c) as "outstanding" for purposes of
Section 10.2(a)(xiv).

                 6.       Section 10.2(a) is hereby amended by adding the
following as a new Section 10.2(a)(xv):

                 (xv)    In addition to any amounts permitted under clause (xiv)
         above, purchase money indebtedness (including in connection with
         industrial revenue bond financings) incurred after May 1, 1997 in an
         amount not to exceed $10,000,000 in the aggregate at any
<PAGE>   3
                 one time outstanding for Borrower and all Subsidiaries of
                 Borrower; provided, that if any portion of such Indebtedness
                 is borrowed from an Unrestricted Subsidiary, it shall be
                 subordinated in writing to the payment of Borrower's
                 Obligations in form and substance satisfactory to the Required
                 Banks.

                 7.       Borrower hereby represents and warrants to Agent and
Banks that:

                          (a)     The execution, delivery and performance by
Borrower of this Amendment are within the corporate powers of Borrower, have
been duly authorized by all necessary corporate action and require no action by
or in respect of, or filing with, any governmental or regulatory body, agency
or official.  The execution, delivery and performance by Borrower of this
Amendment do not conflict with, or result in a breach of the terms, conditions
or provisions of, or constitute a default under or result in any violation of,
and Borrower is not now in default under or in violation of, the terms of the
Certificate of Incorporation or Bylaws of Borrower, any applicable law, any
rule, regulation, order, writ, judgment or decree of any court or governmental
or regulatory agency or instrumentality, or any agreement or instrument to
which Borrower is a party or by which it is bound or to which it is subject;

                          (b)     This Amendment has been duly executed and
delivered and constitutes the legal, valid and binding obligation of Borrower
enforceable in accordance with its terms; and

                          (c)     As of the date hereof, all of the covenants,
representations and warranties of Borrower set forth in the Credit Agreement
are true and correct and no "Event of Default" (as defined therein) under or
within the meaning of the Credit Agreement has occurred and is continuing.

                 8.       All references in the Credit Agreement to "this
Credit Agreement" and any other references of similar import shall henceforth
mean the Credit Agreement as amended by the First Amendment and as further
amended by this Amendment.

                 9.       This Amendment shall be binding upon and inure to the
benefit of the parties thereto and their respective successors and assigns,
except the Borrower may not assign, transfer or delegate any of its rights or
obligations hereunder or thereunder.

                 10.      This Amendment shall be governed by and construed in
accordance with the internal laws of the State of Missouri.

                 11.      In the event of any inconsistency or conflict between
this Amendment and the Credit Agreement, the terms, provisions and conditions
of this Amendment shall govern and control.

                 12.      The Credit Agreement, as hereby amended and modified,
is and shall remain the binding obligation of Borrower and all of the
provisions, terms, stipulations, conditions, covenants and powers contained
therein shall stand and remain in full force and effect, except only as the
same are herein and hereby specifically varied or amended, and the same are
hereby ratified and confirmed.  This Amendment amends the Credit Agreement and
is not a novation thereof.  If any installment of principal or interest on any
of the Notes (as defined in the Credit Agreement) shall not be paid when due as
provided in the Notes, the holders of the Notes shall be entitled to and may
exercise all rights and remedies under the Notes and the Credit Agreement and
any of the other Transaction Documents (as defined in the Credit Agreement).

                 13.      ORAL AGREEMENTS OR COMMITMENTS TO LOAN MONEY, EXTEND
CREDIT OR TO FOREBEAR FROM ENFORCING REPAYMENT OF A DEBT, INCLUDING PROMISES TO
EXTEND OR RENEW SUCH DEBT, ARE NOT ENFORCEABLE.  TO PROTECT BORROWER, AGENT AND
BANKS FROM ANY MISUNDERSTANDING OR DISAPPOINTMENT, ANY AGREEMENTS REACHED BY
BORROWER, AGENT AND BANKS COVERING SUCH MATTERS ARE CONTAINED IN THE CREDIT
AGREEMENT, AS AMENDED BY THIS AMENDMENT, WHICH CONSTITUTES A COMPLETE AND
EXCLUSIVE STATEMENT OF THE AGREEMENTS BETWEEN BORROWER, AGENT AND BANKS.
EXCEPT AS BORROWER, AGENT AND BANKS MAY LATER AGREE IN WRITING TO MODIFY, THE
CREDIT AGREEMENT, AS AMENDED BY THIS AGREEMENT AND UNDERSTANDING BETWEEN THE





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<PAGE>   4
PARTIES HERETO AND SUPERSEDES ALL PRIOR AGREEMENTS AND UNDERSTANDINGS (ORAL OR
WRITTEN) RELATING TO THE SUBJECT MATTER HEREOF.

                 IN WITNESS WHEREOF, the parties hereto have executed this
instrument as of the date first written above.


                                  DOANE PRODUCTS COMPANY
                                  

                                  By: __________________________________________
                                          Thomas R. Heidenthal, Senior Vice
                                         President and Chief Financial Officer
                                  
                                  West 20th & State Line Road
                                  Joplin, Missouri  64804
                                  Telecopy number:  (417) 624-6327
                                  
                                  
                                  MERCANTILE BANK NATIONAL
                                  ASSOCIATION
                                  
                                  
                                  By: __________________________________________
                                          John A. Holland, Vice President
                                  
                                  721 Locust Street
                                  St. Louis, Missouri  63101
                                  Telecopy number:  (314) 425-3859
                                  
                                  
                                  SOUTHTRUST BANK OF ALABAMA,
                                  NATIONAL ASSOCIATION
                                  
                                  
                                  By: __________________________________________
                                          Lee Vaughn, Assistant Vice-President
                                  
                                  Metropolitan Lending
                                  420 North 20th Street
                                  Birmingham, Alabama  35203
                                  Telecopy number:  (205) 254-5911
                                  
                                  
                                  HARRIS TRUST AND SAVINGS BANK
                                  
                                  
                                  By: __________________________________________
                                            Emily L. Burt, Vice President
                                  
                                  111 West Monroe Street
                                  P.O. Box 755
                                  Chicago, Illinois  60690-0755
                                  Telecopy number:  (312) 461-2591
                                  




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<PAGE>   5
                                  HIBERNIA NATIONAL BANK
                                  
                                  
                                  By: __________________________________________
                                          Cheryl H. Denenea, Vice President
                                  
                                  313 Carondelet Street, 6th Floor
                                  New Orleans, Louisiana  70130
                                  Telecopy number:  (504) 533-2060
                                  
                                  
                                  BANK OF OKLAHOMA, N.A.
                                  
                                  
                                  By: __________________________________________
                                        Jane P. Faulkenberry, Vice President
                                  
                                  One Williams Center
                                  Tulsa, Oklahoma  74172
                                  Telecopy number:  (918) 588-6880
                                  
                                  
                                  NBD BANK
                                  
                                  
                                  By: __________________________________________
                                  Title:
                                  
                                  
                                  MERCANTILE BANK
                                  NATIONAL ASSOCIATION, as Agent
                                  
                                  
                                  By: __________________________________________
                                          John A. Holland, Vice President
                                  
                                  721 Locust Street
                                  St. Louis, Missouri  63101
                                  Telecopy number:  (314) 425-3859





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