During my uni student days I worked as a waiter which included a lot of barista work. Here in Adelaide we definitely noticed differences in the milk during the year. During the cooler/wetter times of year it was much easier to texture the milk and produce rich microfoam. Towards the end of a lot hot summer we will dread it when a table would order 6 caps as it was a real challenge getting decent foam. I guess it just comes down to the amount of grass/feed available to the cows.