listen, if you follow the source material, it's clear enough that this is a question without a clear answer. so, i'm currently compiling a list of best guesses, and i'm going to try to find a solution that takes them all into account.
here's another observation: if you check out this link, humans are very different than any other animal (except rats) in the cysteine:methionine ratio of their milk. we actually have more cysteine in our milk than methionine! again - is that a hint? i'm going to take it as one.
if the ratio of cysteine:methionine in human milk is about 3:2, maybe that's something to strive for, roughly, in the absence of better data. it's consistent with what i'm deducing, at least.
these other mammals, including cow and goat, are all leaning heavily towards methionine balance in the ratio. even cats - which are obligate carnivores and have very high taurine requirements - had higher levels of methionine. so, the obvious answer that cows eat grass and mothers eat all sorts of stuff doesn't really fly. it's kind of curious. but, it's a good hint...