Fertility is determined by Body Condition and Hormonal Balance. Body Condition is determined by Genetics and Nutrition.
Increasing fertility through Selection is highly cost effective.  Increasing fertility via supplementation is also cost effective, but only up to a point.

Is increasing calving rate from 80 pc to 90 pc through extra supplementation cost effective?  That extra supplement required to allow an additional 10 pc cows to become pregnant is also fed to 80 pc of cows who do not require it.  That extra 10 pc conception comes at too high a price.

Both cows pictured above are hormonally balanced (academically fertile).  Only the cow on the left is practically fertile (hormonal balance and inherently good body condition.  In order for the cow on the right to be able to calve annually she has to be subsidized by the cow on the left.  Breed more cows like the one on the left and feed less cows like the one on the right.

Feed for 80 pc calving rate and breed for 100 pc calving rate.