I was under the impression that cows have to produce a calf every year to keep giving milk. So in order to ensure as long as possible a lactation period they have always been pregnant throughout? It was certainly the case when my grandfather who was a small farmer was keeping milking cows in the 1950s. Other factors, mainly the widespread use of hormones in cattle in the US are much more likely to affect humans. The study was conducted in the States I think where they do not have our restrictions on the use of hormones. I admit I have been out of touch with the nuts of bolts of dairy farming for a bit, would anybody with up-to-date knowledge please give their opinion?