I have one that I bred at 10 1/2 mos. She weighed 1100 pounds. She's still here growing big calves every year. She will have produced 5 calves by the time she's 5 1/2 yrs old. Not many animals out there can say that. However she is the exception, not the rule. That all adds up to an extra calf every 6 yrs. That will more than pay for the flushing she will get this year.
Now the rule at our farm. Heifers get bred at 15 mos. or they get gone. 2 doses of AI at the most. Angus heifers are more than fertile at 12 mos. as a general rule. But, others have provided some sound advice in waiting until they reach the right weight. We like ours at 800#'s before breeding but thats not a rule.
Sizmic