Her due date is June 18, but she may not calve until after or before then.
How can you improve cattle breeds?
First of all, you have to start by picking your favorite breed. Then you improve that breed by breeding bulls of that breed to cows of the same breed together, culling out those animals that are inferior and keeping those that are superior. Cattle breeds can be further improved by the use of closed-herd breeding, which means incorporating in-breeding and line-breeding to bring out any undesirable genetic abnormalities that may exist within this breed. Any bull or cow that is a carrier to a type of genetic abnormality should be culled.
You can also improve a breed by selecting for different traits that this breed could be stronger in. For example, with Herefords, you could try to breed-up this breed to increase milking ability and the amount of milk cows give, while at the same time keeping (or improving) the breed's ability to have good feed efficiency and great mothering ability.
Can a black angus heifer be bred at 600 pounds?
No, unless the average weight of the main cowherd is 1000 lbs, which then the heifer is 60% of the average cowherd weight. But, if the average mature weight of your herd is a bit higher, then definitely not, as she is obviously too young and too light to be bred. If she's bred already, inject some Lutalyse in her NOW.