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I've seen the following sort of blurb on a lot of milk products, saying:
"Our pledge: Milk comes from cows not treated with the rBST growth hormone"
If they indicate this, does that mean there's some sort of downside with treating a cow with recombinant bovine somatotropin (rBST) growth hormones? Are there any reported instances where non-rBST treated cow's milk are better than rBST treated cow's milk?
From calsunshine.com:
The United States Department of Agriculture (USDA) determined that because rBST is a species-specific growth hormone, milk from rBST treated cows is identical to that of cows not injected with the hormone.
However, the US Court of Appeals for the 6th Circuit has held in Int’l Dairy Foods Ass’n et al. v. Boggs (pp. 9-10):
[A] compositional difference does exist between milk from untreated cows and conventional milk.
[...]
As detailed by the amici parties seeking to strike down the Rule, the use of rbST in milk production has been shown to elevate the levels of insulin-like growth factor 1 (IGF-1), a naturally-occurring hormone that in high levels is linked to several types of cancers, among other things
[...]
According to these studies, milk produced during this stage is considered to be low quality due to its increased fat content and its decreased level of proteins. The amici further note that milk from treated cows contains higher somatic cell counts, which makes the milk turn sour more quickly and is another indicator of poor milk quality. This evidence precludes us from agreeing with the district court’s conclusion that there is no compositional difference between the two types of milk.
tl;dr There are differences, but none which ought to have or have been shown to have a meaningful impact on health.
Relatively current research indicates that fatty acid composition varies a small amount between cows treated with rBST and those that are not (and note that this study was on produced milk, so other practices that correlate with rBST usage may actually be causal). The differences are of a few percentage points at most, and are quite unlikely to be significant factors in health. A broader look at various compounds found very little difference in practice between the composition of milk (on average) from sources that used rBST and those that did not.
Also, most of the rBST itself that made it into the milk is destroyed during pasteurization.
So the answer is tentatively "yes there are detectable differences, but not any that ought to impact health".
Note however that rBST causes health complications in cows. So you might want to avoid rBST milk because you prefer not to obtain milk from cows that are even less comfortable than normal, but it is probably not rational to avoid it for health reasons unless you habitually alter behavior to avoid even low-risk events.
