I depends on the breed of the cows and of the bull as they will all be different sizes at different ages. I have had bulls start breeding at seven months, now obviously that bull wouldn't be able to mount a fully grown cow of the common breeds anyway. The best thing to do is to let your bull mature to about sixteen to eighteen months before putting him into heavy service and even then he'll need a break and supplemented but, if your only gonna do a few cows then that's not so important but, I have seen as many as a dozen cows wear a bull plum out, started loosing weight and not looking so hot so bottom line is. I guess it depends on what your trying to accomplish and the breeds of cattle involved.