Here's the veto reason:
Last week, House and Senate negotiators on the Farm Bill agreed to the terms of the legislation, and the final bill (called a conference report) is slated for an up-or-down vote in the House and Senate this week. It's controversial for a number of reasons, mainly its $300-billion price tag and the subsidies that wealthy farmers, including sugar growers, are slated to receive at a time when they are already reaping record profits. For that reason, President Bush is considering a veto. It appears though that the measure has broad, bipartisan support in Congress; there's something in it for everyone (kind of like a Department of Defense authorizing and appropriations bill with a nugget for every district and major player), and if Bush vetoes it, both chambers may override.
http://hsus.typepad.com/wayne/2008/05/farm-bill.html
This is a conflict for me, as I do not believe we should continue to subsidise our farmers. It's a practice that has failed IMO. If they wanted to really help the animals, they should propose a clean bill and not muddy it up with other interests.