The most effective way to influence this as a consumer is 1) Don't buy these products (in any form) 2) Do buy products from people who are doing things differently (in ways you approve of) even if very expensive.

Vegetarians do #1, but not #2 - which doesn't encourage a change in practice at all, just a reduction of amounts.

It can be hard to stick to #2 when faced with a $20 roasting chicken, but that's the nature of this sort of change.

editing additions (above is unchanged) because I was obviously not clear enough.

I was responding to a post asking what can be done to change a market like that, not saying anything about the choice of being vegetarian itself.

If you decide to become vegetarian for whatever reasons, that is a market signal. However, it is not a market signal to change how meat is being processed, it is a market signal to reduce how much is being produced under any method.

DeusExMachina's point is fair, eggs are a valid (for some versions of "vegetarian" [1]. The rest of his comment is exactly the same as what I'm saying - by participating in the market but choosing to buy products that are more expensive because of the way they are raised, you are signalling a value for that practice. This is much more effective for change in how that market produces goods than simply not participating.

So to be clear - I'm not making a value judgment on choosing to be a vegetarian, I'm just pointing out something fairly obivous - that not participating in a market is not a very effective way of changing how that market produces things.

[1] in my lexicon vegetarians don't eat cheese or eggs either, which is different from vegan's who wouldn't eat honey for example - but that's not really important