LOL...we have a Jersey cow that we milk by hand every day. Last summer, she got out of our pasture and into a patch of weeds in the neighbors field. Ate her fill. That night when we milked her, it was sour, and stayed sour for almost two weeks. Perfectly healthy cow. Free range. No hormones, antibiotics, no other changes in diet or water. No additional stresses (she was perfectly happy moseying around in the neighbors field, not mooing, or trying to get back. Just...eating and chewing her cud)
If it wasn't the weeds that made her milk sour, what was it? Seriously. We had to throw out milk for almost two full weeks. She was not ill. No runny nose. Eyes clear. Good appetite. Regular vigor. *shrugs*
Just curious what you might think. That's all.
As open and shut as this might seem on the surface, it is still an anecdote, a series of one, and retrospective at that. I know a dairy farmer who when he was a teenager, his brother would sneak into the barn while he was milking and blare a compressed air horn, startling everything and everyone in the barn, in an effort to play a prank on his brother. After a couple days of this the milk tasted off and could not be sold to the local dairy. Niether are science and are not applicable to all creatures, not even all cattle. There are too many variables. Eating things one is not used to when one is otherwise on a very regimented diet is certainly a stressor. In countries where bovines are milked but not fenced, they don't seem to report these things. Maybe it does happen, but they just put the milk into yogurt or cheese and that covers the off flavors? Put a kid in a candy store unsupervised, and he gorges on confections. 2 hours later he seems happy, but over the next several days he will have all possibilities of GI problems. The candy is out of his system by then, but he's still not feeling well. My guess is the vamp would walk past that kid without an inkling of feeding.
When you say, "if cows eat weeds they have sour milk;" it is very different than saying, "if a cow who is normally manger fed on commercial feed pellets, suddenly eats four wheelbarrows of weeds in one sitting then that cow might have sour milk."
In a similar way "Falling will almost always kill you"; is not the same as Falling off the roof of a scyscraper will almost always kill you."
I doubt it's spcifically and only the weeds. I think it's highly likely that any radical change in the animal's daily schedule, diet, sleep, typical habits, things we might not even think about, like if it is the usual person is doing the milking and are they wearing suddenly different looking or smelling clothes? Often things taken to the extreme become wrong because the part does not always resemble the whole.
Add to that, that cattle are ruminants. Their biology of digestion is different. They use fermentation. When making wines it is possible to produce ethers and esters, nice small molecules that will get into the blood and have all sorts of breakdown products, which probably would get into milk. If that is what's going on, then it might not be the weeds per se but that the cow had a high sugar load and managed to get a variety of yeast and compatable bacteria to cause a chain reaction.
To test this you could mix some distillers yeast into a 5 gallon bucket of corn syrup and a large supply of water. Leave it out for the cow as free choice and then taste the milk over the next week while keeping a journal. Oh and to make it statistically significant you will need at least twenty cattle. (It's obvious why nobody wants to test this, because cattle are expensive, and milking them is real work. You'd proabably need a government grant to even study it so as to offset the risk in case it kills the cows)
I'm not saying that these anecdotes are worthless, far from it. I'm just saying that the whole issue is too complex to ignore all but one variable.