Just an FYI as there's been discussion of libel, lawsuits, etc.
Online postings and libel/defamation:
http://www.reputationhawk.com/onlinedefamation.html
Of interest:
"Much of the difficulty that surrounds successfully winning an online defamation case is brought about because the plaintiff must first prove who the publisher or writer of the statement is and, secondly, that the statement is false and written with the intention of causing damage."
Yes, ISP's and Forum/server operators will most likely end up giving up your IP address if a Court Order is supplied.
However its the 'intention of causing damage' that is the key.
If you have a point of view, with some kind of data to back up your claims, it'd be rather difficult to prove someone here was trying to damage Manf. X versus opining to a fellow Sheltie owner on a better food product.
"Personal Opinions
It should be noted that personal opinion on any topic can be posted on blogs, websites, and other sources but according to defamation laws and statutes, libelous comments cannot be thinly veiled as personal opinion and judges and courts will consider each statement or comment accordingly to ascertain whether or not it truly is an expression of personal opinion or is, in fact, a defamatory comment.
The Statute Of Limitations
Another point of note is that a statute of limitations does exist for online content, and while these statutes do differ in some states, the most common statute lasts for a period of twelve months. This period starts from when the comment is first posted online since claims that publishing on the Internet is the equivalent of continuous publication have already been rejected by most courts."
Considering that there are actual facts available as to what comprises some of basic ingredients of Dog foods in this case, I sincerely doubt any Manf. would ever consider taking this course of action except in the most dire of situations.
Going around posting Dog Food X will kill your dog might just be enough to make someone fire up of the Corporate Lawyers Lear Jet.
Posting that Dog Food X is crap, made of saw dust, or useless non-digestible plant fiber; is not going to make any waves whatsoever. They know you're going to show up in court, and simply read from the label. I doubt they really want to explain what 4D's is, or get into what meal and rendering is...
So, I wouldn't worry too much about posting anything reasonable, or even somewhat unreasonable. Assuming you're in the US, and not breaking forum TOS. The UK and probably Canada are a different kettle of fish.