**2. Evolution of dairy milk production**

In the last decades, industrial intensive milk production system uses the Holstein-Friesian breed, known as highest milk producing cow in the world. It is well documented that, with almost no exceptions, there has been a continuous increase in milk yield per cow. In all countries milk production and milk composition evolved over the years, due to a higher genetic merit and better management of the cows [31–32]. For example, from 2002 to 2013, in Denmark, all but two years showed a significant increase in the milk production compared to the previous year [33]. In São Miguel island, Azores, the same evolution was observed [34].

Also, in all countries, seasonal variations in milk production and composition were observed, both in intensive [35, 36] and pasture-based systems [37]. Previous studies have also proven that milk production varies with parity. It is generally observed a progressive increase in milk production in the first three to four parities and then a progressive decrease [38, 39].
