I have 2 large barn with each 20 cows. The birth rate is estimated on 4. But every time the livestock has to born only 3 new cows appear.
The both barns have enough food in stock and also the gannary’s nex door has enough grain in stock. The highest fodder rate is 80 and the lowest 70 percent
Estimates aside, I don’t think I’ve ever seen more than 3 cows per year born from an Upgraded Barn with 20 cows in residence.
On the other hand, the average for a normal Barn with 10 cows is 2 born per year, so 4 would seem to be normal for twice that many adult cows, but in my experience, it isn’t - no matter how high the quality and quantity of fodder and grazing provided.