fat and protein level
fat and protein levels in the milk will change when the growth habits of the pasture alters at different times of the year. I am a herd manager on a large dairy in Australia so I am not familiar with your growing seasons. The fat content of the milk is dependent mainly on the amount of fibre in the diet. When the pasture is very soft for example if the weather is cold and the pasture rotation too short to get the grass to more than 1.75 leaf stage, the fat level will drop - so adding in a bit of hay will improve it.
Protein is a bit trickier to change. It may be that you need to add a bit more protein to the cows ration at this time of the year, for example some lupin or canola meal.
hope this helps