No. All offspring from first-calf heifers should be sold because they are just going to be too small to be any good as replacements. I'd wait until your first-calvers are older, like after they've had their second calf to consider keeping any replacements from them. You may have an opportunity to keep some heifers as replacements from second calvers, but you're better off keeping heifers from cows that have proven time and time again to produce good keeper-quality calves.
First-calvers are still growing when they have their second calf, and loose their baby teeth after this time. This growth and teeth-replacement period is quite stressful on both mom and baby, and as a result the calf isn't going to be gaining well because of these stressors. Besides, this first-calf heifer isn't a proven dam yet, so it is better to ship her offspring and give her a second (or third) chance to redeem herself and produce a better calf. By the time she has weaned her first and had her second, her adult teeth would've grown back and she would've finally reached maturity. Mature cows tend to give bigger, and better calves than first-calf heifers do.
Since yearling bulls are the best to use on heifers, and one yearling bull will breed from 15 to 20 heifers, you should have 4 bulls for your herd of 70 heifers.
Betsie and Beatrice
Breeding season for cows and heifers should be from 45 to 90 days long. Sixty days is considered optimum.
When the heifers are at least 14 months old, you can put yearling bulls (15-20 heifers per bull for yearlings) in with them. Don't put your big mature bulls in with your heifers as this could cause problems later on.
Heifers should be vaccinated for Bang's Disease (or Brucellosis) when they are between 2 and 10 months of age.
Heifers are still growing and maturing themselves, so a lot of resources being put into the calf's growth still need to be put into the heifer to keep her growing as well.
Bulls can reach puberty by the time they are 8 months of age, but most are put in with the breeding herd by the time they are 12 months of age. Heifers reach puberty by the time they are 10 to 12 months of age, but most heifers are not bred until they are 15 months old. Of course the breed of the bulls and heifers depends on when they reach puberty and/or when they should be used for producing offspring.
Your heifers weight should be 60% of the cow herd's. But, they should also be around 15 months of age, though a couple months plus or minus isn't going to hurt either.
A yearling bull, which is best used on heifers, can breed from 10 to 20 heifers in a breeding season.
Most dairy heifers should be at least 15 months of age before they can be bred.
Usually it is better to butcher a beef steer but heifers are okay.
Angus, Red Angus, Texas Longhorn, Hereford, Shorthorn, Red Poll, Devon, Galloway, or any other British breed that will put more muscle and growth in the calves but keep them small for the heifers to calve out.