Good idea to separate smaller heifers
M a k e a p l a n fo r s u c c e s s f u l breeding of your replacement heifers.
Do you have a minimum weight for your breeding heifers? It should relate to the weight of your mature cows.
Given the great grass year that 2017 was, most heifers did quite well on grass and were housed in great order.
It would be a good idea to separate out the smaller heifers intended for the bull now. You can increase the feeding level to them now, and keep them as a separate group for a while, before you commence breeding. By doing this, you can feed additional concentrates to them right up to breeding, to achieve target breeding weights. A wise man once suggested to me that “if you have to weigh your heifers to decide if they are fit for breeding, then they are not big enough to bull!”.