In the specific case of cattle, a lot has to do with the size of the calf.
Grass has less energy and protein than prepared foods, so the calves may be
smaller and easier to bear.

The breed also affects this. Some beef breeds are bred for heavy bones and
muscle so the calves will be larger in the shoulders and harder to bear.
Farmers who breed a large breed bull to a smaller breed cow have to watch
the cows carefully and assist the birth sometimes.

I wonder if the exercise of the field cattle has an effect. We used to keep
our breeding hogs on pasture. The exercise was supposed to be good for them.



> This phenomenon is even apparent in animals. The ranch hands of the Adams
> Ranch in Florida said that they leave their grass-fed pregnant cows to
> handle births on their own out in the fields and there is rarely if ever a
> problem, whereas grain-fed cattle have much higher risks of complications
> and tend to require having a farmer or vet at hand during the birth, in
> case
> there are problems. Every wild animal birth I have seen on nature shows was
> amazingly quick and easy, whereas I have seen a grain-fed, barn-cooped cow
> in tremendous distress from her pregnancy.
>
>