To be honest I never gave this phrase much thought before, but its meaning of "don't get so worked up" is actually kind of funny when applied to animal rights, because that's exactly what we should be doing in my opinion. If enough people DO get worked up, then perhaps real change will eventually take place. And these days when I say don't have a cow, I tend to mean it literally, as in, don't eat 'em! So I'm wondering if vegans can somehow reclaim or sloganize this phrase, snort.
The cow in the photo above by the way is not just some stock image, but a real cow living in Germany that a friend of mine (hi Krissa!) has gotten to know. And she will attest to the fact that this lovely creature is very friendly and will run up to greet her. Sadly though we continue to deny that the emotional and social lives of farm animals are just as real and complex as that of their perhaps cuter or cuddlier counterparts, and maybe in part that explains why some people still feel they have the right to eat these non-human animals. Consuming cows just doesn't get most non-vegan folk as worked up (okay, I SO want to make a joke here, but I'll spare you) as the eating of dogs and cats, but I think that particular indignation (and now I feel like joking about indigestion!) can still be used as a starting point. People love companion animals, and if we can get them to see that eating them is not fundamentally any different from eating farmed animals, then that's one possible way of leading them to veganism.
Incidentally, when I Googled the title of this post, I found an interview with Jeffrey Moussaieff Masson (of When Elephants Weep fame) that's definitely worthwhile reading.