Quick version:They give it to cows to increase milk production. It's developed by Monsanto and therefore immediately suspicious. Because it increases milk production and dairy farming is such a low margin business, it is basically impossible to find guaranteed non-rBGH milk, which means that you are drinking it every day. This makes some people cranky, while others don't care.
Recombinant Bovine Growth Hormone (aka rBGH, Recombinant Bovine Somatotropin, and rBST) is something that it is impossible to get a balanced opinion on. Go ahead and search the internet - you will find people claiming that only a hug from Jesus Christ himself could be better for the consumer, counterbalanced by claims that rBGH will seduce your grandmother IN A VERY BAD WAY. I tried to find out about this stuff for a report I gave in my one required college Biology course, and here is a quick summary of what I found...
rBGH doesn't really affect the milk. It looks and acts exactly like the real Bovine Growth Hormone that cows create all of the time in order to make themselves grow and produce milk. If you could somehow drain the naturally occurring hormone from one cow and give it to the next one, then the effects would be similar. This point is a pretty easy point in favor of rBGH. If it's basically the exact same stuff as what naturally occurs, then what's the problem? Well, there are lots of ways of looking at this, but I think the best one is to look at it as the hormones that humans inject themselves with.
pro: 1 con: 0
Testosterone is great if you want big muscles, but it also makes you impotent when you take massive doses. Estrogen in quantity seems to be correlated with breast cancer. So things that are naturally occurring are not always good for you in all amounts. This point is important, and will be scored in the anti-rBGH column to counterbalance that first "it's natural" claim.
pro: 1 con: 1
Since we've now had a strawman on each side, we should start looking at what rBST does, both to cows and to people. In the short term, rBST seems harmless to people. In the long term, we have no freaking clue. This sounds very bad, but we have no clue about the long term affects of most things that you deal with every day. I'm going to go out on a limb here and say that intaking a little extra rBST every day does absolutely nothing in the long term. It's a very easily destroyed protein, and your digestive system is very good at breaking things down. A glass of milk's worth of recombinant bovine somatotropin every day is basically noise in the sea of crap that we ingest every day of our lives. This might not be true if you are a hyper-organic ovo-lacto vegetarian ascetic living on a farm in some pristine location. But it's true enough for everyone who has bought sufficiently into modern society to own a computer, connect it to the internet, and read this. The primary effects of the drug on us are therefore pretty negligible.
pro: 2 con: 1
Since the primary effects are negligible, we're going to have to look at the secondary effects if we want to find something for the con side. Fortunately, this is pretty easy to do. rBGH increases milk production, and it causes cows to need to eat more to make more milk. But it doesn't increase their body's defenses against the stuff they are consuming at a now more rapid rate. Therefore it increases the milk's "macrobiotic cell count". Put bluntly, you get a lot more pus in rBST milk than you get in non-rBST milk. It does not increase the amount of pus beyond USDA limits, and the pus count can be brought back down by giving the cows stuff that assists their immune systems, antibiotics. But since we either get pus in our milk or increased antibiotic resistance, this one goes right in the con column.
pro: 2 con: 2
Now we're going to get into more subjective territory, and it will basically all be my opinions from here on out. You can decide which column you want to put each point in, but I mostly put the following right in the con bucket. rBST was marketed to dairy farmers as a magic drug that made cows produce more milk. They said "great" and started using it, not realizing that it also required them to feed their cows more and have a different cow lifestyle. Thus, rBST has significantly decreased cow quality of life, which wasn't that great to begin with. This is starting to get better over time as farmers wise up, but initial usage always seems to cause malnutrition and other bad effects. Not only that, but in the US, dairy farming is so heavily subsidized that we actually pay people to throw milk away. rBST then creates a direct economic incentive for farmers to create more milk at an artificially high price, and then throw it away. We don't need more milk, we need fewer dairy farmers. Therefore, my final conclusion really has to be that rBGH is stupid, not evil. You should try and buy happy cow milk not because the rBST monster will kill you and your children, but because the whole dairy creation, distribution, and animal-treatment system is pretty fucked and rBST is only helping it get more so. But it does make the milk cheaper by a few cents per gallon!
pro: 2 + slightly cheaper milk
con: 2 + support crappy system
In Jongleur math, that comes out to:
con: a jillion