Dave Chalton writes

With respect to cows, if that was what you were asking, supplements in
the form of concentrate are not fed to ease gestation, calving or
lactation. They are fed to keep the cow in good condition if - and
this is the central point - their intake of whatever their main food
is contains insufficient dry-matter.

They are fed because otherwise many cows loose a *lot* of condition.

They are also fed as a carrier to mineral and vitamin supplements,
although the quantities here need not be large.

It's also true that either excessive feed or inadequate feed are not
conducive to a high survival rate at and immediately following calving.

--
Oz
This post is worth absolutely nothing and is probably fallacious.
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