I keep mine out year round.but if you are putting it out for the good of the does and young,and for the bucks for horn growth.
You need to have it out as soon as the does are breed, if not before,she needs it in her system when the young start to form inside her.
As for the bucks , spring is fine. as long as you don't wait till they start putting on the horns.they also need it in there system ahead of time.
as for helping the All around health of both bucks and does you should start in the late winter befor spring gets here.JMHO.
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