Welcome to the forum and to the wonderful world of cattle!

Do you know where these calves are coming from and if they are started on colostrum etc?
Also it helps to know if the heifers a twin or not. Twins can very often be freemartin (sterile) if twin to a bull calf. Since your planning on breeding it's helpful to know this.

Keep vitamin B and probiotics on hand. Also keep a thermometer set aside for the calves. Temperature can tell you A LOT!
Stay away from soy replacers. Stick with milk based replacers.
Less is more when feeding calves. Don't over feed. You want them begging for more when they finish their bottle.
Have a good calf starter in front of the babies asap along with a small stemmed hay.
Read lots (which your doing)

Ask lots of questions (we're happy to answer) and enjoy them babies!
