Is it safe to pull the placenta out from the heifer?

If you have never had a cow with a retained placenta, you should probably call your veterinarian and work with him/her on this.

The answer is, it depends on how attached the placenta is. If the cotyledon/caruncle interfaces have all been broken down and the placenta is just "hung up" on one caruncle, then it is safe to gently apply some traction to the placenta to help it fall out of the uterus. If the placenta is still attached, it is very dangerous to pull on the placenta because it will rip, leaving some of the placenta in the uterus where it will likely set up an infection and could cause the heifer to become sterile.

You can try to remove the placenta by very gently pulling down (towards the ground) on it - you should be using about the pressure you would use to pull three helium balloons toward you. If the placenta doesn't move, leave it alone - cows can take up to a week to completely pass the placenta and it doesn't seem to cause any permanent damage to the uterus. However, this cow should be watched for signs of milk fever (hypocalcemia, causes weakness and can be life-threatening), ketosis (negative energy balance, usually happens at 4-6 weeks after calving) and any signs of "ain't doing right (ADR)" such as lower feed consumption. A retained placenta seems to be an indication that something isn't quite right with the cow and these other problems are more likely in a cow that has retained a placenta.