I notice a difference in yields due to differences in lactation stages of the cows whose milk I use. They don't all calve in the spring necessarily, but during later stages they have more solids/milkfat yielding more cheese/gallon. During winter when they are on hay, I use a little CaCl2 to help with coagulation.
The flavor is said to be better when they are in green pasture, but I can't honestly say that I notice the difference.