Weston A Price, in his book "Nutrition and Physical Degeneration" says that the dairy products (especially the butter) generated by cows in spring are the most high-vitamin products. When the grass is growing, young and green, this is when the volume of nutrients in it are most high.
As an aside, have you noticed how many people are drinking wheatgrass shots these days? It's better for cows. They are the ones who are designed to digest the nutrients in fresh young wheatgrass!
So the dairy from cows in the spring and summer is more high-vitamin than the dairy from cows in the fall and winter. Check out the chart on this page called "Why Grassfed Butter Is Better."
The same goes for the meat!
There's a chart on that same page called "Afternoon hay may be better than morning hay, but fresh grass is best!"
Here's a quote:
Once the grass is cut and dried, there is a dramatic decline in vital nutrients, including omega-3 fatty acids, vitamin E, and the carotenes (beta-carotene and related antioxidant vitamins.) ...typically, the animals are harvested in the early fall when the animals are fresh from green pasture. This insures that their meat will have its full allotment of health-enhancing vitamins