I will pass on what my AI trainer told us when I went through the class about 30 years ago. cows will have a larger cervix than the heifers, but the heifers will not have any scar tissue or damage from the birthing process. heifers also tend to have a higher conception rate. As a general rule of thumb, most farmers that he talks to say that the cows are generally easier. my experience is that I also prefer cows; not that they are necessarily easier, but I know before hand that they have already been pregnant once so all the parts are in the right place. It is just as likely the AI gun will slide right through a heifer as easily as a cow. I do think it is easier to find the cervix opening on a cow vs a heifer just because of the larger cervix. I don't notice a difference in the softness of the cervix between the two. If you have been breeding your cows, don't let the heifers intimidate you. good luck