Maybe it’s just a coincidence, and maybe it isn’t.
Of the ten or a dozen dairy cows we have owned, two stand out as difficult to milk, not for any deficiency in teat size or location (we’ve experienced both of those, more than once or twice) but for inadequate let-down reflex. These two make plenty of milk, lovely, yellow high-fat content milk; but instead of pushing it out, as any good cow does, when you help her along with some scientific squeezing action, these two make you wring it out of them. Takes twice as long as milking any other cow, or maybe longer yet. Maybe there’s no connection, but these two also happen to be the only two in the bunch which were once-upon-a-time milked with machines. Is there something about the compressor-produced suction of the milking machine that makes let-down reflex unnecessary, and, supposing there is, would an unnecessary reflex diminish over time?
We don’t know, but you won’t catch us buying a machine milked cow again in a hurry.