These data comprise 19 weekly measurements of the protein content of milk samples from each of 79 cows. Cows 1-25 were fed a barley diet, cows 26-52 a mixed diet of barley and lupins, and cows 53-79 a diet of lupins alone. Time is measured in weeks since calving. This dataset is notable because the experiment terminated at a fixed date, and therefore (since calving did not occur at the same time for all cows) there are fewer than 19 observations on some if the cows.