AMENDMENT TO STOCKHOLDERS AGREEMENT

Exhibit 10.13

AMENDMENT TO STOCKHOLDERS AGREEMENT

This Amendment to Stockholders Agreement (the “Amendment”) is dated as of January 12, 2007, and is by and between Animal Health International, Inc., a Delaware corporation formerly known as Steer Parent Corporation and Walco International Holdings, Inc. (the “Company”), the Persons identified on the signature pages hereto as the Charlesbank Investors (the “Charlesbank Investors”) and certain of the Management Investors signatories hereto (the “Participating Management Investors”).

WITNESSETH

WHEREAS, the Company, the Charlesbank Investors, the Participating Management Investors and certain other Management Investors are party to that certain Amended and Restated Stockholders Agreement, dated as of September 8, 2005 (the “Agreement”);

WHEREAS, an amendment of the Agreement requires the consent of the Company, a majority-in-interest of the Management Investors (based on the number of Shares (as defined in the Agreement) held by each Management Investor) and a Charlesbank Majority Interest (as defined in the Agreement);

WHEREAS, the Participating Management Investors constitute a majority-in-interest of the Management Investors and the Charlesbank Investors constitute a Charlesbank Majority Interest; and

WHEREAS, the Company, the Charlesbank Investors and the Participating Management Investors wish to amend certain provisions of the Agreement.

NOW THEREFORE, in consideration of the premises and such other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged and agreed, the parties hereby agree as follows:

“In addition, this Agreement shall terminate, and cease to have any force and effect, upon the effective date of the relevant registration statement filed under the Securities Act in connection with an IPO.”

[SIGNATURE PAGE FOLLOWS]

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

CHARLESBANK INVESTORS (CONTINUED):