Daylight in the stall.
In nature, cows mated during summer and were calving next spring. This is when grass is at its best for optimal milkproduction and so for the growth of their calves.
This is what the cow is used to, so she is evolved.
That is why it is important to simulate for the milking cows the “May-Situation” year-round through mimicking daylight hours in de stall and give them the best, fresch, tasty “May-quality” feed.
In terms of light in the barn: dairy cows need about 16 hours daylight while dry cows and highly pregnant heifers should have only 8-12 hours a day. The former simulates the summer while the latter represents the winter setting.
Young cattle also need 16 hours a day for growing and to be more fertile!
A good check of what is good daylight is: “Can you read your newspaper in the entire barn ?”