For artificial insemination to succeed, it must be timed to fall within the oestrus cycle so that

Artificial insemination (AI) must be timed to coincide with the oestrus (heat) period of the female animal so that fertilisation will take place. During oestrus, the female ovulates (releases a mature egg from the ovary), and the reproductive tract undergoes physiological changes that facilitate sperm transport toward the egg. If semen is deposited at this precise time, the sperm can reach the egg in the oviduct and fertilisation can occur.

The timing has nothing to do with semen virility; the quality of semen is determined at the time of collection and processing, not by when it is deposited. Sperm survival in the female tract is important, but it is a means to an end, not the objective itself. While it is true that a female in oestrus is more receptive, the question asks about why AI must be timed within the oestrus cycle, and the fundamental reason is to ensure that viable sperm and a mature egg are present simultaneously for fertilisation.

In practice, AI is typically performed 12-18 hours after the onset of standing heat in cattle, which aligns with the expected time of ovulation.