<PAGE>

                                                                 Exhibit 10.83.3
                                                                 ---------------

                      AMENDMENT TO RETENTION PLAN AGREEMENT

     This Amendment (the "Amendment") is entered into as of January 4, 2002
between Synbiotics Corporation ("Synbiotics") and the undersigned employee of
Synbiotics or its subsidiary ("Employee").

     A. Synbiotics and Employee are parties to a written retention plan
agreement, as amended (the "Retention Plan Agreement"), under which Synbiotics
promised, on the day following the sale of Synbiotics' animal heath business, to
(i) make a cash bonus payment to Employee and (ii) accelerate the vesting and
provide a fixed exercisability period of all stock options issued to Employee.

     B. Synbiotics is working toward entering into a Stock Purchase Agreement
with Redwood West Coast, LLC ("Redwood"), under which Redwood would make an
investment in Synbiotics stock (the "Stock Purchase Agreement"). Synbiotics has
determined that a closing under the Stock Purchase Agreement would constitute
the sale of Synbiotics' animal health business and entitle the Employee to a
cash payment of $102,749.94 (the "Retention Plan Cash Amount") under the
Retention Plan Agreement.

     C. Redwood requires Employee and Synbiotics to enter into this Amendment
before Redwood will enter into the Stock Purchase Agreement.

     D. Synbiotics and Employee desire to amend the Retention Plan Agreement to
provide that Synbiotics will issue to Employee 570,833 fully vested, fully paid
and nonassessable shares of common stock of Synbiotics (the "New Shares")
pursuant to the Synbiotics 1995 Stock Option/Stock Issuance Plan, as amended
(the "1995 Plan") instead of paying the Retention Plan Cash Amount.

     E. Employee is willing separately to agree to forfeit any and all of
Employee's options to purchase stock of Synbiotics, in exchange for Synbiotics
making (and not seeking reimbursement from Employee for) certain tax withholding
amounts for and on behalf of Employee.

     NOW, THEREFORE:

     1. Synbiotics and Employee hereby agree that if and only if Synbiotics and
Redwood enter into the Stock Purchase Agreement, the Retention Plan Agreement is
hereby amended (as of immediately before the closing under the Stock Purchase
Agreement (the "Stock Purchase Closing")) by:

        1.1 deleting the paragraph captioned "Cash Bonus" in its entirety and
replacing such paragraph with the following:

        "The Company will issue to you on or before May 15, 2002, 570,833 fully
vested, fully paid and nonassessable shares of common stock of the Company
pursuant to the Company's 1995 Stock Option/Stock Issuance Plan, as amended."

        1.2 deleting the paragraph captioned "Restricted Stock and Stock
Options" in its entirety.

     2. Employee represents and warrants to Synbiotics as follows:

        2.1 Employee is acquiring the New Shares for Employee's own account and
not with a view to or for sale in connection with any distribution of the New
Shares.

        2.2 Employee acknowledges that Employee has received a copy of the
Prospectus of Synbiotics dated as of January 4, 2002. Employee has received all
the information Employee considers necessary or appropriate for deciding whether
to sign this Amendment and acquire the New Shares.

<PAGE>

     3. The value of the New Shares will be taxable income to Employee.
Withholding taxes must be paid to Federal and State tax authorities. Synbiotics
agrees to pay any and all tax withholding amounts required to be paid by
Synbiotics and/or by Employee arising out of or in connection with the issuance
of the New Shares (except for "secondary" employee-side withholdings, if any,
associated with the "primary" employee-side withholding payments made by
Synbiotics on behalf of Employee), and without any requirement for Employee to
repay Synbiotics for the withholding. Withholdings paid by Synbiotics on behalf
of Employee count as "taxes paid" on Employee's 2002 Federal and State income
tax returns. In exchange for this tax withholding benefit, Employee agrees to
and hereby does (effective immediately before the Stock Purchase Closing)
terminate and forfeit, without the need for further action by any party, any and
all of Employee's options to purchase capital stock of Synbiotics. Synbiotics
has no obligation with regard to any further or other taxes of Employee.

     4. Even if Employee's employment is terminated for any reason prior to May
15, 2002, Employee will still be entitled to receive the New Shares on May 15,
2002.

     5. All of the terms and provisions of the Retention Plan Agreement as
amended hereby remain in full force and effect. However, Employee confirms that
upon Redwood's January 2002 investment under the Stock Purchase Agreement, the
Retention Plan Agreement (as amended hereby) is terminated except for
Synbiotics' obligations arising from such January 2002 Redwood investment.

                                      EMPLOYEE

                                      /s/ Robert D. Buchanan
                                      ----------------------------------------
                                      Name:  Robert D. Buchanan
                                             ---------------------------------


                                      SYNBIOTICS CORPORATION


                                      By: /s/ Paul A. Rosinack
                                          ------------------------------------
                                          Paul A. Rosinack, President & C.E.O.