Can a heifer be a bull?

No. Heifers are female, bulls are male. Heifers cannot change their sex like some other creatures can. However, heifers that were born with a twin brother and shared the same placenta with her twin brother can develop bull-like characteristics. These are called Freemartins or Hermaphrodites. Hermaphrodites are 100% sterile, and a hermaphrodite heifer is a heifer that has both male and female sexual characteristics but can not fully become a bull like REAL bulls are.